What alternative term can be used to describe asymmetric cryptographic algorithms?

1. INTRODUCTION

In the early-1990s, when the commercial Internet was still young [!], security was taken seriously by most users. Many thought that increased security provided comfort to paranoid people while most computer professionals realized that security provided some very basic protections that we all needed? Cryptography for the masses barely existed at that time and was certainly not a topic of common discourse. By the turn of the century, of course, the Internet had grown in size and importance so as to be the provider of essential communication between billions of people around the world and is the ubiquitous tool for commerce, social interaction, and the exchange of an increasing amount of personal information — and we even have a whole form of currency named for cryptography!

Security and privacy impacts many applications, ranging from secure commerce and payments to private communications and protecting health care information. One essential aspect for secure communications is that of cryptography. But it is important to note that while cryptography is necessary for secure communications, it is not by itself sufficient. The reader is advised, then, that the topics covered here only describe the first of many steps necessary for better security in any number of situations.

This paper has two major purposes. The first is to define some of the terms and concepts behind basic cryptographic methods, and to offer a way to compare the myriad cryptographic schemes in use today. The second is to provide some real examples of cryptography in use today. [See Section A.4 for some additional commentary on this...]

DISCLAIMER: Several companies, products, and services are mentioned in this tutorial. Such mention is for example purposes only and, unless explicitly stated otherwise, should not be taken as a recommendation or endorsement by the author.

2. BASIC CONCEPTS OF CRYPTOGRAPHY

Cryptography — the science of secret writing — is an ancient art; the first documented use of cryptography in writing dates back to circa 1900 B.C. when an Egyptian scribe used non-standard hieroglyphs in an inscription. Some experts argue that cryptography appeared spontaneously sometime after writing was invented, with applications ranging from diplomatic missives to war-time battle plans. It is no surprise, then, that new forms of cryptography came soon after the widespread development of computer communications. In data and telecommunications, cryptography is necessary when communicating over any untrusted medium, which includes just about any network, particularly the Internet.

There are five primary functions of cryptography:

1. Privacy/confidentiality: Ensuring that no one can read the message except the intended receiver.
2. Authentication: The process of proving one's identity.
3. Integrity: Assuring the receiver that the received message has not been altered in any way from the original.
4. Non-repudiation: A mechanism to prove that the sender really sent this message.
5. Key exchange: The method by which crypto keys are shared between sender and receiver.

In cryptography, we start with the unencrypted data, referred to as plaintext. Plaintext is encrypted into ciphertext, which will in turn [usually] be decrypted back into usable plaintext. The encryption and decryption is based upon the type of cryptography scheme being employed and some form of key. For those who like formulas, this process is sometimes written as:

C = Ek[P]
P = Dk[C]

      where P = plaintext, C = ciphertext, E = the encryption method, D = the decryption method, and k = the key.

Given this, there are other functions that might be supported by crypto and other terms that one might hear:

• Forward Secrecy [aka Perfect Forward Secrecy]: This feature protects past encrypted sessions from compromise even if the server holding the messages is compromised. This is accomplished by creating a different key for every session so that compromise of a single key does not threaten the entirely of the communications.
• Perfect Security: A system that is unbreakable and where the ciphertext conveys no information about the plaintext or the key. To achieve perfect security, the key has to be at least as long as the plaintext, making analysis and even brute-force attacks impossible. One-time pads are an example of such a system.
• Deniable Authentication [aka Message Repudiation]: A method whereby participants in an exchange of messages can be assured in the authenticity of the messages but in such a way that senders can later plausibly deny their participation to a third-party.

In many of the descriptions below, two communicating parties will be referred to as Alice and Bob; this is the common nomenclature in the crypto field and literature to make it easier to identify the communicating parties. If there is a third and fourth party to the communication, they will be referred to as Carol and Dave, respectively. A malicious party is referred to as Mallory, an eavesdropper as Eve, and a trusted third party as Trent.

Finally, cryptography is most closely associated with the development and creation of the mathematical algorithms used to encrypt and decrypt messages, whereas cryptanalysis is the science of analyzing and breaking encryption schemes. Cryptology is the umbrella term referring to the broad study of secret writing, and encompasses both cryptography and cryptanalysis.

3. TYPES OF CRYPTOGRAPHIC ALGORITHMS

There are several ways of classifying cryptographic algorithms. For purposes of this paper, they will be categorized based on the number of keys that are employed for encryption and decryption, and further defined by their application and use. The three types of algorithms that will be discussed are [Figure 1]:

• Secret Key Cryptography [SKC]: Uses a single key for both encryption and decryption; also called symmetric encryption. Primarily used for privacy and confidentiality.
• Public Key Cryptography [PKC]: Uses one key for encryption and another for decryption; also called asymmetric encryption. Primarily used for authentication, non-repudiation, and key exchange.
• Hash Functions: Uses a mathematical transformation to irreversibly "encrypt" information, providing a digital fingerprint. Primarily used for message integrity.

FIGURE 1: Three types of cryptography: secret key, public key, and hash function.

3.1. Secret Key Cryptography

Secret key cryptography methods employ a single key for both encryption and decryption. As shown in Figure 1A, the sender uses the key to encrypt the plaintext and sends the ciphertext to the receiver. The receiver applies the same key to decrypt the message and recover the plaintext. Because a single key is used for both functions, secret key cryptography is also called symmetric encryption.

With this form of cryptography, it is obvious that the key must be known to both the sender and the receiver; that, in fact, is the secret. The biggest difficulty with this approach, of course, is the distribution of the key [more on that later in the discussion of public key cryptography].

Secret key cryptography schemes are generally categorized as being either stream ciphers or block ciphers.

A] Self-synchronizing stream cipher. [From Schneier, 1996, Figure 9.8]

B] Synchronous stream cipher. [From Schneier, 1996, Figure 9.6]

FIGURE 2: Types of stream ciphers.

Stream ciphers operate on a single bit [byte or computer word] at a time and implement some form of feedback mechanism so that the key is constantly changing. Stream ciphers come in several flavors but two are worth mentioning here [Figure 2]. Self-synchronizing stream ciphers calculate each bit in the keystream as a function of the previous n bits in the keystream. It is termed "self-synchronizing" because the decryption process can stay synchronized with the encryption process merely by knowing how far into the n-bit keystream it is. One problem is error propagation; a garbled bit in transmission will result in n garbled bits at the receiving side. Synchronous stream ciphers generate the keystream in a fashion independent of the message stream but by using the same keystream generation function at sender and receiver. While stream ciphers do not propagate transmission errors, they are, by their nature, periodic so that the keystream will eventually repeat.

FIGURE 3: Feistel cipher. [Source: Wikimedia Commons]

A block cipher is so-called because the scheme encrypts one fixed-size block of data at a time. In a block cipher, a given plaintext block will always encrypt to the same ciphertext when using the same key [i.e., it is deterministic] whereas the same plaintext will encrypt to different ciphertext in a stream cipher. The most common construct for block encryption algorithms is the Feistel cipher, named for cryptographer Horst Feistel [IBM]. As shown in Figure 3, a Feistel cipher combines elements of substitution, permutation [transposition], and key expansion; these features create a large amount of "confusion and diffusion" [per Claude Shannon] in the cipher. One advantage of the Feistel design is that the encryption and decryption stages are similar, sometimes identical, requiring only a reversal of the key operation, thus dramatically reducing the size of the code or circuitry necessary to implement the cipher in software or hardware, respectively. One of Feistel's early papers describing this operation is "Cryptography and Computer Privacy" [Scientific American, May 1973, 228[5], 15-23].

Block ciphers can operate in one of several modes; the following are the most important:

• Electronic Codebook [ECB] mode is the simplest, most obvious application: the secret key is used to encrypt the plaintext block to form a ciphertext block. Two identical plaintext blocks, then, will always generate the same ciphertext block. ECB is susceptible to a variety of brute-force attacks [because of the fact that the same plaintext block will always encrypt to the same ciphertext], as well as deletion and insertion attacks. In addition, a single bit error in the transmission of the ciphertext results in an error in the entire block of decrypted plaintext.
• Cipher Block Chaining [CBC] mode adds a feedback mechanism to the encryption scheme; the plaintext is exclusively-ORed [XORed] with the previous ciphertext block prior to encryption so that two identical plaintext blocks will encrypt differently. While CBC protects against many brute-force, deletion, and insertion attacks, a single bit error in the ciphertext yields an entire block error in the decrypted plaintext block and a bit error in the next decrypted plaintext block.
• Cipher Feedback [CFB] mode is a block cipher implementation as a self-synchronizing stream cipher. CFB mode allows data to be encrypted in units smaller than the block size, which might be useful in some applications such as encrypting interactive terminal input. If we were using one-byte CFB mode, for example, each incoming character is placed into a shift register the same size as the block, encrypted, and the block transmitted. At the receiving side, the ciphertext is decrypted and the extra bits in the block [i.e., everything above and beyond the one byte] are discarded. CFB mode generates a keystream based upon the previous ciphertext [the initial key comes from an Initialization Vector [IV]]. In this mode, a single bit error in the ciphertext affects both this block and the following one.
• Output Feedback [OFB] mode is a block cipher implementation conceptually similar to a synchronous stream cipher. OFB prevents the same plaintext block from generating the same ciphertext block by using an internal feedback mechanism that generates the keystream independently of both the plaintext and ciphertext bitstreams. In OFB, a single bit error in ciphertext yields a single bit error in the decrypted plaintext.
• Counter [CTR] mode is a relatively modern addition to block ciphers. Like CFB and OFB, CTR mode operates on the blocks as in a stream cipher; like ECB, CTR mode operates on the blocks independently. Unlike ECB, however, CTR uses different key inputs to different blocks so that two identical blocks of plaintext will not result in the same ciphertext. Finally, each block of ciphertext has specific location within the encrypted message. CTR mode, then, allows blocks to be processed in parallel — thus offering performance advantages when parallel processing and multiple processors are available — but is not susceptible to ECB's brute-force, deletion, and insertion attacks.

A good overview of these different modes can be found at CRYPTO-IT.

Secret key cryptography algorithms in use today — or, at least, important today even if not in use — include:

• Data Encryption Standard [DES]: One of the most well-known and well-studied SKC schemes, DES was designed by IBM in the 1970s and adopted by the National Bureau of Standards [NBS] [now the National Institute of Standards and Technology [NIST]] in 1977 for commercial and unclassified government applications. DES is a Feistel block-cipher employing a 56-bit key that operates on 64-bit blocks. DES has a complex set of rules and transformations that were designed specifically to yield fast hardware implementations and slow software implementations, although this latter point is not significant today since the speed of computer processors is several orders of magnitude faster today than even twenty years ago. DES was based somewhat on an earlier cipher from Feistel called Lucifer which, some sources report, had a 112-bit key. This was rejected, partially in order to fit the algorithm onto a single chip and partially because of the National Security Agency [NSA]. The NSA also proposed a number of tweaks to DES that many thought were introduced in order to weaken the cipher; analysis in the 1990s, however, showed that the NSA suggestions actually strengthened DES, including the removal of a mathematical back door by a change to the design of the S-box [see "The Legacy of DES" by Bruce Schneier [2004]]. In April 2021, the NSA declassified a fascinating historical paper titled "NSA Comes Out of the Closet: The Debate over Public Cryptography in the Inman Era" that appeared in Cryptologic Quarterly, Spring 1996.

  DES was defined in American National Standard X3.92 and three Federal Information Processing Standards [FIPS], all withdrawn in 2005:

  • FIPS PUB 46-3: DES [Archived file]
  • FIPS PUB 74: Guidelines for Implementing and Using the NBS Data Encryption Standard
  • FIPS PUB 81: DES Modes of Operation

  Information about vulnerabilities of DES can be obtained from the Electronic Frontier Foundation.

  Two important variants that strengthen DES are:

  • Triple-DES [3DES]: A variant of DES that employs up to three 56-bit keys and makes three encryption/decryption passes over the block; 3DES is also described in FIPS PUB 46-3 and was an interim replacement to DES in the late-1990s and early-2000s.

  • DESX: A variant devised by Ron Rivest. By combining 64 additional key bits to the plaintext prior to encryption, effectively increases the keylength to 120 bits.

  More detail about DES, 3DES, and DESX can be found below in Section 5.4.

• Advanced Encryption Standard [AES]: In 1997, NIST initiated a very public, 4-1/2 year process to develop a new secure cryptosystem for U.S. government applications [as opposed to the very closed process in the adoption of DES 25 years earlier]. The result, the Advanced Encryption Standard, became the official successor to DES in December 2001. AES uses an SKC scheme called Rijndael, a block cipher designed by Belgian cryptographers Joan Daemen and Vincent Rijmen. The algorithm can use a variable block length and key length; the latest specification allowed any combination of keys lengths of 128, 192, or 256 bits and blocks of length 128, 192, or 256 bits. NIST initially selected Rijndael in October 2000 and formal adoption as the AES standard came in December 2001. FIPS PUB 197 describes a 128-bit block cipher employing a 128-, 192-, or 256-bit key. AES is also part of the NESSIE approved suite of protocols. [See also the entries for CRYPTEC and NESSIE Projects in Table 3.]

  The AES process and Rijndael algorithm are described in more detail below in Section 5.9.

• CAST-128/256: CAST-128 [aka CAST5], described in Request for Comments [RFC] 2144, is a DES-like substitution-permutation crypto algorithm, employing a 128-bit key operating on a 64-bit block. CAST-256 [aka CAST6], described in RFC 2612, is an extension of CAST-128, using a 128-bit block size and a variable length [128, 160, 192, 224, or 256 bit] key. CAST is named for its developers, Carlisle Adams and Stafford Tavares, and is available internationally. CAST-256 was one of the Round 1 algorithms in the AES process.

• International Data Encryption Algorithm [IDEA]: Secret-key cryptosystem written by Xuejia Lai and James Massey, in 1992 and patented by Ascom; a 64-bit SKC block cipher using a 128-bit key.

• Rivest Ciphers [aka Ron's Code]: Named for Ron Rivest, a series of SKC algorithms.

  • RC1: Designed on paper but never implemented.

  • RC2: A 64-bit block cipher using variable-sized keys designed to replace DES. It's code has not been made public although many companies have licensed RC2 for use in their products. Described in RFC 2268.

  • RC3: Found to be breakable during development.

  • RC4: A stream cipher using variable-sized keys; it is widely used in commercial cryptography products. An update to RC4, called Spritz [see also this article], was designed by Rivest and Jacob Schuldt. More detail about RC4 [and a little about Spritz] can be found below in Section 5.13.

  • RC5: A block-cipher supporting a variety of block sizes [32, 64, or 128 bits], key sizes, and number of encryption passes over the data. Described in RFC 2040.

  • RC6: A 128-bit block cipher based upon, and an improvement over, RC5; RC6 was one of the AES Round 2 algorithms.

• Blowfish: A symmetric 64-bit block cipher invented by Bruce Schneier; optimized for 32-bit processors with large data caches, it is significantly faster than DES on a Pentium/PowerPC-class machine. Key lengths can vary from 32 to 448 bits in length. Blowfish, available freely and intended as a substitute for DES or IDEA, is in use in a large number of products.

• Twofish: A 128-bit block cipher using 128-, 192-, or 256-bit keys. Designed to be highly secure and highly flexible, well-suited for large microprocessors, 8-bit smart card microprocessors, and dedicated hardware. Designed by a team led by Bruce Schneier and was one of the Round 2 algorithms in the AES process.

• Threefish: A large block cipher, supporting 256-, 512-, and 1024-bit blocks and a key size that matches the block size; by design, the block/key size can grow in increments of 128 bits. Threefish only uses XOR operations, addition, and rotations of 64-bit words; the design philosophy is that an algorithm employing many computationally simple rounds is more secure than one employing highly complex — albeit fewer — rounds. The specification for Threefish is part of the Skein Hash Function Family documentation.

• Anubis: Anubis is a block cipher, co-designed by Vincent Rijmen who was one of the designers of Rijndael. Anubis is a block cipher, performing substitution-permutation operations on 128-bit blocks and employing keys of length 128 to 3200 bits [in 32-bit increments]. Anubis works very much like Rijndael. Although submitted to the NESSIE project, it did not make the final cut for inclusion.

• ARIA: A 128-bit block cipher employing 128-, 192-, and 256-bit keys to encrypt 128-bit blocks in 12, 14, and 16 rounds, depending on the key size. Developed by large group of researchers from academic institutions, research institutes, and federal agencies in South Korea in 2003, and subsequently named a national standard. Described in RFC 5794.

• Camellia: A secret-key, block-cipher crypto algorithm developed jointly by Nippon Telegraph and Telephone [NTT] Corp. and Mitsubishi Electric Corporation [MEC] in 2000. Camellia has some characteristics in common with AES: a 128-bit block size, support for 128-, 192-, and 256-bit key lengths, and suitability for both software and hardware implementations on common 32-bit processors as well as 8-bit processors [e.g., smart cards, cryptographic hardware, and embedded systems]. Also described in RFC 3713. Camellia's application in IPsec is described in RFC 4312 and application in OpenPGP in RFC 5581. Camellia is part of the NESSIE suite of protocols.

• CLEFIA: Described in RFC 6114, CLEFIA is a 128-bit block cipher employing key lengths of 128, 192, and 256 bits [which is compatible with AES]. The CLEFIA algorithm was first published in 2007 by Sony Corporation. CLEFIA is one of the new-generation lightweight block cipher algorithms designed after AES, offering high performance in software and hardware as well as a lightweight implementation in hardware.

• FFX-A2 and FFX-A10: FFX [Format-preserving, Feistel-based encryption] is a type of Format Preserving Encryption [FPE] scheme that is designed so that the ciphertext has the same format as the plaintext. FPE schemes are used for such purposes as encrypting social security numbers, credit card numbers, limited size protocol traffic, etc.; this means that an encrypted social security number, for example, would still be a nine-digit string. FFX can theoretically encrypt strings of arbitrary length, although it is intended for message sizes smaller than that of AES-128 [2128 points]. The FFX version 1.1 specification describes FFX-A2 and FFX-A10, which are intended for 8-128 bit binary strings or 4-36 digit decimal strings.

• GSM [Global System for Mobile Communications, originally Groupe Spécial Mobile] encryption: GSM mobile phone systems use several stream ciphers for over-the-air communication privacy. A5/1 was developed in 1987 for use in Europe and the U.S. A5/2, developed in 1989, is a weaker algorithm and intended for use outside of Europe and the U.S. Significant flaws were found in both ciphers after the "secret" specifications were leaked in 1994, however, and A5/2 has been withdrawn from use. The newest version, A5/3, employs the KASUMI block cipher. NOTE: Unfortunately, although A5/1 has been repeatedly "broken" [e.g., see "Secret code protecting cellphone calls set loose" [2009] and "Cellphone snooping now easier and cheaper than ever" [2011]], this encryption scheme remains in widespread use, even in 3G and 4G mobile phone networks. Use of this scheme is reportedly one of the reasons that the National Security Agency [NSA] can easily decode voice and data calls over mobile phone networks.

• GPRS [General Packet Radio Service] encryption: GSM mobile phone systems use GPRS for data applications, and GPRS uses a number of encryption methods, offering different levels of data protection. GEA/0 offers no encryption at all. GEA/1 and GEA/2 are proprietary stream ciphers, employing a 64-bit key and a 96-bit or 128-bit state, respectively. GEA/1 and GEA/2 are most widely used by network service providers today although both have been reportedly broken. GEA/3 is a 128-bit block cipher employing a 64-bit key that is used by some carriers; GEA/4 is a 128-bit clock cipher with a 128-bit key, but is not yet deployed.

• KASUMI: A block cipher using a 128-bit key that is part of the Third-Generation Partnership Project [3gpp], formerly known as the Universal Mobile Telecommunications System [UMTS]. KASUMI is the intended confidentiality and integrity algorithm for both message content and signaling data for emerging mobile communications systems.

• KCipher-2: Described in RFC 7008, KCipher-2 is a stream cipher with a 128-bit key and a 128-bit initialization vector. Using simple arithmetic operations, the algorithms offers fast encryption and decryption by use of efficient implementations. KCipher-2 has been used for industrial applications, especially for mobile health monitoring and diagnostic services in Japan.

• KHAZAD: KHAZAD is a so-called legacy block cipher, operating on 64-bit blocks à la older block ciphers such as DES and IDEA. KHAZAD uses eight rounds of substitution and permutation, with a 128-bit key.

• KLEIN: Designed in 2011, KLEIN is a lightweight, 64-bit block cipher supporting 64-, 80- and 96-bit keys. KLEIN is designed for highly resource constrained devices such as wireless sensors and RFID tags.

• Light Encryption Device [LED]: Designed in 2011, LED is a lightweight, 64-bit block cipher supporting 64- and 128-bit keys. LED is designed for RFID tags, sensor networks, and other applications with devices constrained by memory or compute power.

• MARS: MARS is a block cipher developed by IBM and was one of the five finalists in the AES development process. MARS employs 128-bit blocks and a variable key length from 128 to 448 bits. The MARS document stresses the ability of the algorithm's design for high speed, high security, and the ability to efficiently and effectively implement the scheme on a wide range of computing devices.

• MISTY1: Developed at Mitsubishi Electric Corp., a block cipher using a 128-bit key and 64-bit blocks, and a variable number of rounds. Designed for hardware and software implementations, and is resistant to differential and linear cryptanalysis. Described in RFC 2994, MISTY1 is part of the NESSIE suite.

• Salsa and ChaCha: Salsa20 is a stream cipher proposed for the eSTREAM project by Daniel Bernstein. Salsa20 uses a pseudorandom function based on 32-bit [whole word] addition, bitwise addition [XOR], and rotation operations, aka add-rotate-xor [ARX] operations. Salsa20 uses a 256-bit key although a 128-bit key variant also exists. In 2008, Bernstein published ChaCha, a new family of ciphers related to Salsa20. ChaCha20, originally defined in RFC 7539 [now obsoleted], is employed [with the Poly1305 authenticator] in Internet Engineering Task Force [IETF] protocols, most notably for IPsec and Internet Key Exchange [IKE], per RFC 7634, and Transaction Layer Security [TLS], per RFC 7905. In 2014, Google adopted ChaCha20/Poly1305 for use in OpenSSL, and they are also a part of OpenSSH. RFC 8439 replaces RFC 7539, and provides an implementation guide for both the ChaCha20 cipher and Poly1305 message authentication code, as well as the combined CHACHA20-POLY1305 Authenticated-Encryption with Associated-Data [AEAD] algorithm.

• Secure and Fast Encryption Routine [SAFER]: A series of block ciphers designed by James Massey for implementation in software and employing a 64-bit block. SAFER K-64, published in 1993, used a 64-bit key and SAFER K-128, published in 1994, employed a 128-bit key. After weaknesses were found, new versions were released called SAFER SK-40, SK-64, and SK-128, using 40-, 64-, and 128-bit keys, respectively. SAFER+ [1998] used a 128-bit block and was an unsuccessful candidate for the AES project; SAFER++ [2000] was submitted to the NESSIE project.

• SEED: A block cipher using 128-bit blocks and 128-bit keys. Developed by the Korea Information Security Agency [KISA] and adopted as a national standard encryption algorithm in South Korea. Also described in RFC 4269.

• Serpent: Serpent is another of the AES finalist algorithms. Serpent supports 128-, 192-, or 256-bit keys and a block size of 128 bits, and is a 32-round substitution–permutation network operating on a block of four 32-bit words. The Serpent developers opted for a high security margin in the design of the algorithm; they determined that 16 rounds would be sufficient against known attacks but require 32 rounds in an attempt to future-proof the algorithm.

• SHACAL: SHACAL is a pair of block ciphers based upon the Secure Hash Algorithm [SHA] and the fact that SHA is, at heart, a compression algorithm. As a hash function, SHA repeatedly calls on a compression scheme to alter the state of the data blocks. While SHA [like other hash functions] is irreversible, the compression function can be used for encryption by maintaining appropriate state information. SHACAL-1 is based upon SHA-1 and uses a 160-bit block size while SHACAL-2 is based upon SHA-256 and employs a 256-bit block size; both support key sizes from 128 to 512 bits. SHACAL-2 is one of the NESSIE block ciphers.

• Simon and Speck: Simon and Speck are a pair of lightweight block ciphers proposed by the NSA in 2013, designed for highly constrained software or hardware environments. [E.g., per the specification, AES requires 2400 gate equivalents and these ciphers require less than 2000.] While both cipher families perform well in both hardware and software, Simon has been optimized for high performance on hardware devices and Speck for performance in software. Both are Feistel ciphers and support ten combinations of block and key size:

• Skipjack: SKC scheme proposed, along with the Clipper chip, as part of the never-implemented Capstone project. Although the details of the algorithm were never made public, Skipjack was a block cipher using an 80-bit key and 32 iteration cycles per 64-bit block. Capstone, proposed by NIST and the NSA as a standard for public and government use, met with great resistance by the crypto community largely because the design of Skipjack was classified [coupled with the key escrow requirement of the Clipper chip].

• SM4: Formerly called SMS4, SM4 is a 128-bit block cipher using 128-bit keys and 32 rounds to process a block. Declassified in 2006, SM4 is used in the Chinese National Standard for Wireless Local Area Network [LAN] Authentication and Privacy Infrastructure [WAPI]. SM4 had been a proposed cipher for the Institute of Electrical and Electronics Engineers [IEEE] 802.11i standard on security mechanisms for wireless LANs, but has yet to be accepted by the IEEE or International Organization for Standardization [ISO]. SM4 is described in SMS4 Encryption Algorithm for Wireless Networks [translated by Whitfield Diffie and George Ledin, 2008] and at the SM4 [cipher] page. SM4 is issued by the Chinese State Cryptographic Authority as GM/T 0002-2012: SM4 [2012].

• Tiny Encryption Algorithm [TEA]: A family of block ciphers developed by Roger Needham and David Wheeler. TEA was originally developed in 1994, and employed a 128-bit key, 64-bit block, and 64 rounds of operation. To correct certain weaknesses in TEA, eXtended TEA [XTEA], aka Block TEA, was released in 1997. To correct weaknesses in XTEA and add versatility, Corrected Block TEA [XXTEA] was published in 1998. XXTEA also uses a 128-bit key, but block size can be any multiple of 32-bit words [with a minimum block size of 64 bits, or two words] and the number of rounds is a function of the block size [~52+6*words], as shown in Table 1.

TABLE 1. Tiny Encryption Algorithm [TEA] options.Block Size
2nKey Size
mnWord Size
nKey Words
mRounds
T

32	64	16	4	32
48	72 96	24	3 4	36 36
64	96 128	32	3 4	42 44
96	96 144	48	2 3	52 54
128	128 192 256	64	2 3 4	68 69 72

• TWINE: Designed by engineers at NEC in 2011, TWINE is a lightweight, 64-bit block cipher supporting 80- and 128-bit keys. TWINE's design goals included maintaining a small footprint in a hardware implementation [i.e., fewer than 2,000 gate equivalents] and small memory consumption in a software implementation.

Although not an SKC scheme, check out Section 5.17 about Shamir's Secret Sharing [SSS].

There are several other references that describe interesting algorithms and even SKC codes dating back decades. Two that leap to mind are the Crypto Museum's Crypto List and John J.G. Savard's [albeit old] A Cryptographic Compendium page.

3.2. Public Key Cryptography

Public key cryptography has been said to be the most significant new development in cryptography in the last 300-400 years. Modern PKC was first described publicly by Stanford University professor Martin Hellman and graduate student Whitfield Diffie in 1976. Their paper described a two-key crypto system in which two parties could engage in a secure communication over a non-secure communications channel without having to share a secret key.

PKC depends upon the existence of so-called one-way functions, or mathematical functions that are easy to compute whereas their inverse function is relatively difficult to compute. Let me give you two simple examples:

1. Multiplication vs. factorization: Suppose you have two prime numbers, 3 and 7, and you need to calculate the product; it should take almost no time to calculate that value, which is 21. Now suppose, instead, that you have a number that is a product of two primes, 21, and you need to determine those prime factors. You will eventually come up with the solution but whereas calculating the product took milliseconds, factoring will take longer. The problem becomes much harder if we start with primes that have, say, 400 digits or so, because the product will have ~800 digits.
2. Exponentiation vs. logarithms: Suppose you take the number 3 to the 6th power; again, it is relatively easy to calculate 36 = 729. But if you start with the number 729 and need to determine the two integers, x and y so that logx 729 = y, it will take longer to find the two values.

While the examples above are trivial, they do represent two of the functional pairs that are used with PKC; namely, the ease of multiplication and exponentiation versus the relative difficulty of factoring and calculating logarithms, respectively. The mathematical "trick" in PKC is to find a trap door in the one-way function so that the inverse calculation becomes easy given knowledge of some item of information.

Generic PKC employs two keys that are mathematically related although knowledge of one key does not allow someone to easily determine the other key. One key is used to encrypt the plaintext and the other key is used to decrypt the ciphertext. The important point here is that it does not matter which key is applied first, but that both keys are required for the process to work [Figure 1B]. Because a pair of keys are required, this approach is also called asymmetric cryptography.

In PKC, one of the keys is designated the public key and may be advertised as widely as the owner wants. The other key is designated the private key and is never revealed to another party. It is straight-forward to send messages under this scheme. Suppose Alice wants to send Bob a message. Alice encrypts some information using Bob's public key; Bob decrypts the ciphertext using his private key. This method could be also used to prove who sent a message; Alice, for example, could encrypt some plaintext with her private key; when Bob decrypts using Alice's public key, he knows that Alice sent the message [authentication] and Alice cannot deny having sent the message [non-repudiation].

Public key cryptography algorithms that are in use today for key exchange or digital signatures include:

• RSA: The first, and still most common, PKC implementation, named for the three MIT mathematicians who developed it — Ronald Rivest, Adi Shamir, and Leonard Adleman. RSA today is used in hundreds of software products and can be used for key exchange, digital signatures, or encryption of small blocks of data. RSA uses a variable size encryption block and a variable size key. The key-pair is derived from a very large number, n, that is the product of two prime numbers chosen according to special rules; these primes may be 100 or more digits in length each, yielding an n with roughly twice as many digits as the prime factors. The public key information includes n and a derivative of one of the factors of n; an attacker cannot determine the prime factors of n [and, therefore, the private key] from this information alone and that is what makes the RSA algorithm so secure. [Some descriptions of PKC erroneously state that RSA's safety is due to the difficulty in factoring large prime numbers. In fact, large prime numbers, like small prime numbers, only have two factors!] The ability for computers to factor large numbers, and therefore attack schemes such as RSA, is rapidly improving and systems today can find the prime factors of numbers with more than 200 digits. Nevertheless, if a large number is created from two prime factors that are roughly the same size, there is no known factorization algorithm that will solve the problem in a reasonable amount of time; a 2005 test to factor a 200-digit number took 1.5 years and over 50 years of compute time. In 2009, Kleinjung et al. reported that factoring a 768-bit [232-digit] RSA-768 modulus utilizing hundreds of systems took two years and they estimated that a 1024-bit RSA modulus would take about a thousand times as long. Even so, they suggested that 1024-bit RSA be phased out by 2013. [See the Wikipedia article on integer factorization.] Regardless, one presumed protection of RSA is that users can easily increase the key size to always stay ahead of the computer processing curve. As an aside, the patent for RSA expired in September 2000 which does not appear to have affected RSA's popularity one way or the other. A detailed example of RSA is presented below in Section 5.3.

• Diffie-Hellman: After the RSA algorithm was published, Diffie and Hellman came up with their own algorithm. Diffie-Hellman is used for secret-key key exchange only, and not for authentication or digital signatures. More detail about Diffie-Hellman can be found below in Section 5.2.

• Digital Signature Algorithm [DSA]: The algorithm specified in NIST's Digital Signature Standard [DSS], provides digital signature capability for the authentication of messages. Described in FIPS PUB 186-4.

• ElGamal: Designed by Taher Elgamal, ElGamal is a PKC system similar to Diffie-Hellman and used for key exchange. ElGamal is used in some later version of Pretty Good Privacy [PGP] as well as GNU Privacy Guard [GPG] and other cryptosystems.

• Elliptic Curve Cryptography [ECC]: A PKC algorithm based upon elliptic curves. ECC can offer levels of security with small keys comparable to RSA and other PKC methods. It was designed for devices with limited compute power and/or memory, such as smartcards and PDAs. More detail about ECC can be found below in Section 5.8. Other references include the Elliptic Curve Cryptography page and the Online ECC Tutorial page, both from Certicom. See also RFC 6090 for a review of fundamental ECC algorithms and The Elliptic Curve Digital Signature Algorithm [ECDSA] for details about the use of ECC for digital signatures.

• Identity-Based Encryption [IBE]: IBE is a novel scheme first proposed by Adi Shamir in 1984. It is a PKC-based key authentication system where the public key can be derived from some unique information based upon the user's identity, allowing two users to exchange encrypted messages without having an a priori relationship. In 2001, Dan Boneh [Stanford] and Matt Franklin [U.C., Davis] developed a practical implementation of IBE based on elliptic curves and a mathematical construct called the Weil Pairing. In that year, Clifford Cocks [GCHQ] also described another IBE solution based on quadratic residues in composite groups. RFC 5091: Identity-Based Cryptography Standard [IBCS] #1 describes an implementation of IBE using Boneh-Franklin [BF] and Boneh-Boyen [BB1] Identity-based Encryption. More detail about Identity-Based Encryption can be found below in Section 5.16.

• Public Key Cryptography Standards [PKCS]: A set of interoperable standards and guidelines for public key cryptography, designed by RSA Data Security Inc. [These documents are no longer easily available; all links in this section are from archive.org.]

• PKCS #1: RSA Cryptography Standard [Also RFC 8017]
• PKCS #2: Incorporated into PKCS #1.
• PKCS #3: Diffie-Hellman Key-Agreement Standard
• PKCS #4: Incorporated into PKCS #1.
• PKCS #5: Password-Based Cryptography Standard [PKCS #5 V2.1 is also RFC 8018]
• PKCS #6: Extended-Certificate Syntax Standard [being phased out in favor of X.509v3]
• PKCS #7: Cryptographic Message Syntax Standard [Also RFC 2315]
• PKCS #8: Private-Key Information Syntax Standard [Also RFC 5958]
• PKCS #9: Selected Attribute Types [Also RFC 2985]
• PKCS #10: Certification Request Syntax Standard [Also RFC 2986]
• PKCS #11: Cryptographic Token Interface Standard
• PKCS #12: Personal Information Exchange Syntax Standard [Also RFC 7292]
• PKCS #13: Elliptic Curve Cryptography Standard
• PKCS #14: Pseudorandom Number Generation Standard is no longer available
• PKCS #15: Cryptographic Token Information Format Standard

• Cramer-Shoup: A public key cryptosystem proposed by R. Cramer and V. Shoup of IBM in 1998.

• Key Exchange Algorithm [KEA]: A variation on Diffie-Hellman; proposed as the key exchange method for the NIST/NSA Capstone project.

• LUC: A public key cryptosystem designed by P.J. Smith and based on Lucas sequences. Can be used for encryption and signatures, using integer factoring.

• McEliece: A public key cryptosystem based on algebraic coding theory.

For additional information on PKC algorithms, see "Public Key Encryption" [Chapter 8] in Handbook of Applied Cryptography, by A. Menezes, P. van Oorschot, and S. Vanstone [CRC Press, 1996].

A digression: Who invented PKC? I tried to be careful in the first paragraph of this section to state that Diffie and Hellman "first described publicly" a PKC scheme. Although I have categorized PKC as a two-key system, that has been merely for convenience; the real criteria for a PKC scheme is that it allows two parties to exchange a secret even though the communication with the shared secret might be overheard. There seems to be no question that Diffie and Hellman were first to publish; their method is described in the classic paper, "New Directions in Cryptography," published in the November 1976 issue of IEEE Transactions on Information Theory [IT-22[6], 644-654]. As shown in Section 5.2, Diffie-Hellman uses the idea that finding logarithms is relatively harder than performing exponentiation. And, indeed, it is the precursor to modern PKC which does employ two keys. Rivest, Shamir, and Adleman described an implementation that extended this idea in their paper, "A Method for Obtaining Digital Signatures and Public Key Cryptosystems," published in the February 1978 issue of the Communications of the ACM [CACM], [21[2], 120-126]. Their method, of course, is based upon the relative ease of finding the product of two large prime numbers compared to finding the prime factors of a large number.

Diffie and Hellman [and other sources] credit Ralph Merkle with first describing a public key distribution system that allows two parties to share a secret, although it was not a two-key system, per se. A Merkle Puzzle works where Alice creates a large number of encrypted keys, sends them all to Bob so that Bob chooses one at random and then lets Alice know which he has selected. An eavesdropper [Eve] will see all of the keys but can't learn which key Bob has selected [because he has encrypted the response with the chosen key]. In this case, Eve's effort to break in is the square of the effort of Bob to choose a key. While this difference may be small it is often sufficient. Merkle apparently took a computer science course at UC Berkeley in 1974 and described his method, but had difficulty making people understand it; frustrated, he dropped the course. Meanwhile, he submitted the paper "Secure Communication Over Insecure Channels," which was published in the CACM in April 1978; Rivest et al.'s paper even makes reference to it. Merkle's method certainly wasn't published first, but he is often credited to have had the idea first.

An interesting question, maybe, but who really knows? For some time, it was a quiet secret that a team at the UK's Government Communications Headquarters [GCHQ] had first developed PKC in the early 1970s. Because of the nature of the work, GCHQ kept the original memos classified. In 1997, however, the GCHQ changed their posture when they realized that there was nothing to gain by continued silence. Documents show that a GCHQ mathematician named James Ellis started research into the key distribution problem in 1969 and that by 1975, James Ellis, Clifford Cocks, and Malcolm Williamson had worked out all of the fundamental details of PKC, yet couldn't talk about their work. [They were, of course, barred from challenging the RSA patent!] By 1999, Ellis, Cocks, and Williamson began to get their due credit in a break-through article in WIRED Magazine. And the National Security Agency [NSA] claims to have knowledge of this type of algorithm as early as 1966. For some additional insight on who knew what when, see Steve Bellovin's "The Prehistory of Public Key Cryptography."

3.3. Hash Functions

Hash functions, also called message digests and one-way encryption, are algorithms that, in essence, use no key [Figure 1C]. Instead, a fixed-length hash value is computed based upon the plaintext that makes it impossible for either the contents or length of the plaintext to be recovered. Hash algorithms are typically used to provide a digital fingerprint of a file's contents, often used to ensure that the file has not been altered by an intruder or virus. Hash functions are also commonly employed by many operating systems to encrypt passwords. Hash functions, then, provide a mechanism to ensure the integrity of a file.

Hash functions are also designed so that small changes in the input produce significant differences in the hash value, for example:

Hash string 1: The quick brown fox jumps over the lazy dog
Hash string 2: The quick brown fox jumps over the lazy dog.

MD5 [hash string 1] = 37c4b87edffc5d198ff5a185cee7ee09
MD5 [hash string 2] = 0d7006cd055e94cf614587e1d2ae0c8e

SHA1 [hash string 1] = be417768b5c3c5c1d9bcb2e7c119196dd76b5570
SHA1 [hash string 2] = 9c04cd6372077e9b11f70ca111c9807dc7137e4b

RIPEMD160 [hash string 1] = ee061f0400729d0095695da9e2c95168326610ff
RIPEMD160 [hash string 2] = 99b90925a0116c302984211dbe25b5343be9059e

Let me reiterate that hashes are one-way encryption. You cannot take a hash and "decrypt" it to find the original string that created it, despite the many web sites that claim or suggest otherwise, such as CrackStation, Hashes.com, MD5 Online, md5thiscracker, OnlineHashCrack, and RainbowCrack.

Note that these sites search databases and/or use rainbow tables to find a suitable string that produces the hash in question but one can't definitively guarantee what string originally produced the hash. This is an important distinction. Suppose that you want to crack someone's password, where the hash of the password is stored on the server. Indeed, all you then need is a string that produces the correct hash and you're in! However, you cannot prove that you have discovered the user's password, only a "duplicate key."

Hash algorithms in common use today include:

• Message Digest [MD] algorithms: A series of byte-oriented algorithms that produce a 128-bit hash value from an arbitrary-length message.

  • MD2 [RFC 1319]: Designed for systems with limited memory, such as smart cards. [MD2 has been relegated to historical status, per RFC 6149.]

  • MD4 [RFC 1320]: Developed by Rivest, similar to MD2 but designed specifically for fast processing in software. [MD4 has been relegated to historical status, per RFC 6150.]

  • MD5 [RFC 1321]: Also developed by Rivest after potential weaknesses were reported in MD4; this scheme is similar to MD4 but is slower because more manipulation is made to the original data. MD5 has been implemented in a large number of products although several weaknesses in the algorithm were demonstrated by German cryptographer Hans Dobbertin in 1996 ["Cryptanalysis of MD5 Compress"]. [Updated security considerations for MD5 can be found in RFC 6151.]

• Secure Hash Algorithm [SHA]: Algorithm for NIST's Secure Hash Standard [SHS], described in FIPS PUB 180-4 The status of NIST hash algorithms can be found on their "Policy on Hash Functions" page.

  • SHA-1 produces a 160-bit hash value and was originally published as FIPS PUB 180-1 and RFC 3174. SHA-1 was deprecated by NIST as of the end of 2013 although it is still widely used.

  • SHA-2, originally described in FIPS PUB 180-2 and eventually replaced by FIPS PUB 180-3 [and FIPS PUB 180-4], comprises five algorithms in the SHS: SHA-1 plus SHA-224, SHA-256, SHA-384, and SHA-512 which can produce hash values that are 224, 256, 384, or 512 bits in length, respectively. SHA-2 recommends use of SHA-1, SHA-224, and SHA-256 for messages less than 264 bits in length, and employs a 512 bit block size; SHA-384 and SHA-512 are recommended for messages less than 2128 bits in length, and employs a 1,024 bit block size. FIPS PUB 180-4 also introduces the concept of a truncated hash in SHA-512/t, a generic name referring to a hash value based upon the SHA-512 algorithm that has been truncated to t bits; SHA-512/224 and SHA-512/256 are specifically described. SHA-224, -256, -384, and -512 are also described in RFC 4634.

  • SHA-3 is the current SHS algorithm. Although there had not been any successful attacks on SHA-2, NIST decided that having an alternative to SHA-2 using a different algorithm would be prudent. In 2007, they launched a SHA-3 Competition to find that alternative; a list of submissions can be found at The SHA-3 Zoo. In 2012, NIST announced that after reviewing 64 submissions, the winner was Keccak [pronounced "catch-ack"], a family of hash algorithms based on sponge functions. The NIST version can support hash output sizes of 256 and 512 bits.

• RIPEMD: A series of message digests that initially came from the RIPE [RACE Integrity Primitives Evaluation] project. RIPEMD-160 was designed by Hans Dobbertin, Antoon Bosselaers, and Bart Preneel, and optimized for 32-bit processors to replace the then-current 128-bit hash functions. Other versions include RIPEMD-256, RIPEMD-320, and RIPEMD-128.

• eD2k: Named for the EDonkey2000 Network [eD2K], the eD2k hash is a root hash of an MD4 hash list of a given file. A root hash is used on peer-to-peer file transfer networks, where a file is broken into chunks; each chunk has its own MD4 hash associated with it and the server maintains a file that contains the hash list of all of the chunks. The root hash is the hash of the hash list file.

• HAVAL [HAsh of VAriable Length]: Designed by Y. Zheng, J. Pieprzyk and J. Seberry, a hash algorithm with many levels of security. HAVAL can create hash values that are 128, 160, 192, 224, or 256 bits in length. More details can be found in "HAVAL - A one-way hashing algorithm with variable length output" by Zheng, Pieprzyk, and Seberry [AUSCRYPT '92].

• The Skein Hash Function Family: The Skein Hash Function Family was proposed to NIST in their 2010 hash function competition. Skein is fast due to using just a few simple computational primitives, secure, and very flexible — per the specification, it can be used as a straight-forward hash, MAC, HMAC, digital signature hash, key derivation mechanism, stream cipher, or pseudo-random number generator. Skein supports internal state sizes of 256, 512 and 1024 bits, and arbitrary output lengths.

• SM3: SM3 is a 256-bit hash function operating on 512-bit input blocks. Part of a Chinese National Standard, SM3 is issued by the Chinese State Cryptographic Authority as GM/T 0004-2012: SM3 cryptographic hash algorithm [2012] and GB/T 32905-2016: Information security techniques—SM3 cryptographic hash algorithm [2016]. More information can also be found at the SM3 [hash function] page.

• Tiger: Designed by Ross Anderson and Eli Biham, Tiger is designed to be secure, run efficiently on 64-bit processors, and easily replace MD4, MD5, SHA and SHA-1 in other applications. Tiger/192 produces a 192-bit output and is compatible with 64-bit architectures; Tiger/128 and Tiger/160 produce a hash of length 128 and 160 bits, respectively, to provide compatibility with the other hash functions mentioned above.

• Whirlpool: Designed by V. Rijmen [co-inventor of Rijndael] and P.S.L.M. Barreto, Whirlpool is one of two hash functions endorsed by the NESSIE competition [the other being SHA]. Whirlpool operates on messages less than 2256 bits in length and produces a message digest of 512 bits. The design of this hash function is very different than that of MD5 and SHA-1, making it immune to the types of attacks that succeeded on those hashes.

Readers might be interested in HashCalc, a Windows-based program that calculates hash values using a dozen algorithms, including MD5, SHA-1 and several variants, RIPEMD-160, and Tiger. Command line utilities that calculate hash values include sha_verify by Dan Mares [Windows; supports MD5, SHA-1, SHA-2] and md5deep [cross-platform; supports MD5, SHA-1, SHA-256, Tiger, and Whirlpool].

A digression on hash collisions. Hash functions are sometimes misunderstood and some sources claim that no two files can have the same hash value. This is in theory, if not in fact, incorrect. Consider a hash function that provides a 128-bit hash value. There are, then, 2128 possible hash values. But there are an infinite number of possible files and ∞ >> 2128. Therefore, there have to be multiple files — in fact, there have to be an infinite number of files! — that have the same 128-bit hash value. [Now, while even this is theoretically correct, it is not true in practice because hash algorithms are designed to work with a limited message size, as mentioned above. For example, SHA-1, SHA-224, and SHA-256 produce hash values that are 160, 224, and 256 bits in length, respectively, and limit the message length to less than 264 bits; SHA-384 and all SHA-256 variants limit the message length to less than 2128 bits. Nevertheless, hopefully you get my point — and, alas, even if you don't, do know that there are multiple files that have the same MD5 or SHA-1 hash values.]

The difficulty is not necessarily in finding two files with the same hash, but in finding a second file that has the same hash value as a given first file. Consider this example. A human head has, generally, no more than ~150,000 hairs. Since there are more than 7 billion people on earth, we know that there are a lot of people with the same number of hairs on their head. Finding two people with the same number of hairs, then, would be relatively simple. The harder problem is choosing one person [say, you, the reader] and then finding another person who has the same number of hairs on their head as you have on yours.

This is somewhat similar to the Birthday Problem. We know from probability that if you choose a random group of ~23 people, the probability is about 50% that two will share a birthday [the probability goes up to 99.9% with a group of 70 people]. However, if you randomly select one person in a group of 23 and try to find a match to that person, the probability is only about 6% of finding a match; you'd need a group of 253 for a 50% probability of a shared birthday to one of the people chosen at random [and a group of more than 4,000 to obtain a 99.9% probability].

What is hard to do, then, is to try to create a file that matches a given hash value so as to force a hash value collision — which is the reason that hash functions are used extensively for information security and computer forensics applications. Alas, researchers as far back as 2004 found that practical collision attacks could be launched on MD5, SHA-1, and other hash algorithms and, today, it is generally recognized that MD5 and SHA-1 are pretty much broken. Readers interested in this problem should read the following:

• AccessData. [2006, April]. MD5 Collisions: The Effect on Computer Forensics. AccessData White Paper.
• Burr, W. [2006, March/April]. Cryptographic hash standards: Where do we go from here? IEEE Security & Privacy, 4[2], 88-91.
• Dwyer, D. [2009, June 3]. SHA-1 Collision Attacks Now 252. SecureWorks Research blog.
• Gutman, P., Naccache, D., & Palmer, C.C. [2005, May/June]. When hashes collide. IEEE Security & Privacy, 3[3], 68-71.
• Kessler, G.C. [2016]. The Impact of MD5 File Hash Collisions on Digital Forensic Imaging. Journal of Digital Forensics, Security & Law, 11[4], 129-138.
• Kessler, G.C. [2016]. The Impact of SHA-1 File Hash Collisions on Digital Forensic Imaging: A Follow-Up Experiment. Journal of Digital Forensics, Security & Law, 11[4], 139-148.
• Klima, V. [2005, March]. Finding MD5 Collisions - a Toy For a Notebook.
• Lee, R. [2009, January 7]. Law Is Not A Science: Admissibility of Computer Evidence and MD5 Hashes. SANS Computer Forensics blog.
• Leurent, G. & Peyrin, T. [2020, January]. SHA-1 is a Shambles: First Chosen-Prefix Collision on SHA-1 and Application to the PGP Web of Trust. Real World Crypto 2020.
• Leurent, G. & Peyrin, T. [2020, January]. SHA-1 is a Shambles: First Chosen-Prefix Collision on SHA-1 and Application to the PGP Web of Trust. [paper]
• Stevens, M., Bursztein, E., Karpman, P., Albertini, A., & Markov, Y. [2017]. The first collision for full SHA-1.
• Stevens, M., Karpman, P., & Peyrin, T. [2015, October 8]. Freestart collision on full SHA-1. Cryptology ePrint Archive, Report 2015/967.
• Thompson, E. [2005, February]. MD5 collisions and the impact on computer forensics. Digital Investigation, 2[1], 36-40.
• Wang, X., Feng, D., Lai, X., & Yu, H. [2004, August]. Collisions for Hash Functions MD4, MD5, HAVAL-128 and RIPEMD.
• Wang, X., Yin, Y.L., & Yu, H. [2005, February 13]. Collision Search Attacks on SHA1.

Readers are also referred to the Eindhoven University of Technology HashClash Project Web site. for For additional information on hash functions, see David Hopwood's MessageDigest Algorithms page and Peter Selinger's MD5 Collision Demo page. For historical purposes, take a look at the situation with hash collisions, circa 2005, in RFC 4270.

In October 2015, the SHA-1 Freestart Collision was announced; see a report by Bruce Schneier and the developers of the attack [as well as the paper above by Stevens et al. [2015]]. In February 2017, the first SHA-1 collision was announced on the Google Security Blog and Centrum Wiskunde & Informatica's Shattered page. See also the paper by Stevens et al. [2017], listed above. If this isn't enough, see the SHA-1 is a Shambles Web page and the Leurent & Peyrin paper, listed above.

For an interesting twist on this discussion, read about the Nostradamus attack reported at Predicting the winner of the 2008 US Presidential Elections using a Sony PlayStation 3 [by M. Stevens, A.K. Lenstra, and B. de Weger, November 2007].

Finally, note that certain extensions of hash functions are used for a variety of information security and digital forensics applications, such as:

• Hash libraries, aka hashsets, are sets of hash values corresponding to known files. A hashset containing the hash values of all files known to be a part of a given operating system, for example, could form a set of known good files, and could be ignored in an investigation for malware or other suspicious file, whereas as hash library of known child pornographic images could form a set of known bad files and be the target of such an investigation.
• Rolling hashes refer to a set of hash values that are computed based upon a fixed-length "sliding window" through the input. As an example, a hash value might be computed on bytes 1-10 of a file, then on bytes 2-11, 3-12, 4-13, etc.
• Fuzzy hashes are an area of intense research and represent hash values that represent two inputs that are similar. Fuzzy hashes are used to detect documents, images, or other files that are close to each other with respect to content. See "Fuzzy Hashing" by Jesse Kornblum for a good treatment of this topic.

3.4. Why Three Encryption Techniques?

So, why are there so many different types of cryptographic schemes? Why can't we do everything we need with just one?

The answer is that each scheme is optimized for some specific cryptographic application[s]. Hash functions, for example, are well-suited for ensuring data integrity because any change made to the contents of a message will result in the receiver calculating a different hash value than the one placed in the transmission by the sender. Since it is highly unlikely that two different messages will yield the same hash value, data integrity is ensured to a high degree of confidence.

Secret key cryptography, on the other hand, is ideally suited to encrypting messages, thus providing privacy and confidentiality. The sender can generate a session key on a per-message basis to encrypt the message; the receiver, of course, needs the same session key in order to decrypt the message.

Key exchange, of course, is a key application of public key cryptography [no pun intended]. Asymmetric schemes can also be used for non-repudiation and user authentication; if the receiver can obtain the session key encrypted with the sender's private key, then only this sender could have sent the message. Public key cryptography could, theoretically, also be used to encrypt messages although this is rarely done because secret key cryptography algorithms can generally be executed up to 1000 times faster than public key cryptography algorithms.

FIGURE 4: Use of the three cryptographic techniques for secure communication.

Figure 4 puts all of this together and shows how a hybrid cryptographic scheme combines all of these functions to form a secure transmission comprising a digital signature and digital envelope. In this example, the sender of the message is Alice and the receiver is Bob.

A digital envelope comprises an encrypted message and an encrypted session key. Alice uses secret key cryptography to encrypt her message using the session key, which she generates at random with each session. Alice then encrypts the session key using Bob's public key. The encrypted message and encrypted session key together form the digital envelope. Upon receipt, Bob recovers the session secret key using his private key and then decrypts the encrypted message.

The digital signature is formed in two steps. First, Alice computes the hash value of her message; next, she encrypts the hash value with her private key. Upon receipt of the digital signature, Bob recovers the hash value calculated by Alice by decrypting the digital signature with Alice's public key. Bob can then apply the hash function to Alice's original message, which he has already decrypted [see previous paragraph]. If the resultant hash value is not the same as the value supplied by Alice, then Bob knows that the message has been altered; if the hash values are the same, Bob should believe that the message he received is identical to the one that Alice sent.

This scheme also provides nonrepudiation since it proves that Alice sent the message; if the hash value recovered by Bob using Alice's public key proves that the message has not been altered, then only Alice could have created the digital signature. Bob also has proof that he is the intended receiver; if he can correctly decrypt the message, then he must have correctly decrypted the session key meaning that his is the correct private key.

This diagram purposely suggests a cryptosystem where the session key is used for just a single session. Even if this session key is somehow broken, only this session will be compromised; the session key for the next session is not based upon the key for this session, just as this session's key was not dependent on the key from the previous session. This is known as Perfect Forward Secrecy; you might lose one session key due to a compromise but you won't lose all of them. [This was an issue in the 2014 OpenSSL vulnerability known as Heartbleed.]

The system described here is one where we basically encrypt the secret session key with the receiver's public key. While this generic scheme works well, it causes some incompatibilities in practice. RFC 9180, released in early 2022, describes a new approach to building a Hybrid Public Key Encryption [HPKE] process. HPKE was designed specifically to be simple, reusable, and future-proof. A nice description of the process can be found in a blog posting titled, "HPKE: Standardizing Public-Key Encryption [Finally!]" [C. Wood].

3.5. The Significance of Key Length

In a 1998 article in the industry literature, a writer made the claim that 56-bit keys did not provide as adequate protection for DES at that time as they did in 1975 because computers were 1000 times faster in 1998 than in 1975. Therefore, the writer went on, we needed 56,000-bit keys in 1998 instead of 56-bit keys to provide adequate protection. The conclusion was then drawn that because 56,000-bit keys are infeasible [true], we should accept the fact that we have to live with weak cryptography [false!]. The major error here is that the writer did not take into account that the number of possible key values double whenever a single bit is added to the key length; thus, a 57-bit key has twice as many values as a 56-bit key [because 257 is two times 256]. In fact, a 66-bit key would have 1024 times more values than a 56-bit key.

But this does bring up the question — "What is the significance of key length as it affects the level of protection?"

In cryptography, size does matter. The larger the key, the harder it is to crack a block of encrypted data. The reason that large keys offer more protection is almost obvious; computers have made it easier to attack ciphertext by using brute force methods rather than by attacking the mathematics [which are generally well-known anyway]. With a brute force attack, the attacker merely generates every possible key and applies it to the ciphertext. Any resulting plaintext that makes sense offers a candidate for a legitimate key. This was the basis, of course, of the EFF's attack on DES.

Until the mid-1990s or so, brute force attacks were beyond the capabilities of computers that were within the budget of the attacker community. By that time, however, significant compute power was typically available and accessible. General-purpose computers such as PCs were already being used for brute force attacks. For serious attackers with money to spend, such as some large companies or governments, Field Programmable Gate Array [FPGA] or Application-Specific Integrated Circuits [ASIC] technology offered the ability to build specialized chips that could provide even faster and cheaper solutions than a PC. As an example, the AT&T Optimized Reconfigurable Cell Array [ORCA] FPGA chip cost about $200 and could test 30 million DES keys per second, while a $10 ASIC chip could test 200 million DES keys per second; compare that to a PC which might be able to test 40,000 keys per second. Distributed attacks, harnessing the power of up to tens of thousands of powerful CPUs, are now commonly employed to try to brute-force crypto keys.

TABLE 2. Minimum Key Lengths for Symmetric Ciphers.Type of AttackerBudgetToolTime and Cost
Per Key RecoveredKey Length Needed
For Protection
In Late-199540 bits56 bits

Pedestrian Hacker	Tiny	Scavenged computer time	1 week	Infeasible	45
$400	FPGA	5 hours [$0.08]	38 years [$5,000]	50
Small Business	$10,000	FPGA	12 minutes [$0.08]	18 months [$5,000]	55
Corporate Department	$300K	FPGA	24 seconds [$0.08]	19 days [$5,000]	60
ASIC	0.18 seconds [$0.001]	3 hours [$38]
Big Company	$10M	FPGA	7 seconds [$0.08]	13 hours [$5,000]	70
ASIC	0.005 seconds [$0.001]	6 minutes [$38]
Intelligence Agency	$300M	ASIC	0.0002 seconds [$0.001]	12 seconds [$38]	75

Table 2 — from a 1996 article discussing both why exporting 40-bit keys was, in essence, no crypto at all and why DES' days were numbered — shows what DES key sizes were needed to protect data from attackers with different time and financial resources. This information was not merely academic; one of the basic tenets of any security system is to have an idea of what you are protecting and from whom are you protecting it! The table clearly shows that a 40-bit key was essentially worthless against even the most unsophisticated attacker. On the other hand, 56-bit keys were fairly strong unless you might be subject to some pretty serious corporate or government espionage. But note that even 56-bit keys were clearly on the decline in their value and that the times in the table were worst cases.

So, how big is big enough? DES, invented in 1975, was still in use at the turn of the century, nearly 25 years later. If we take that to be a design criteria [i.e., a 20-plus year lifetime] and we believe Moore's Law ["computing power doubles every 18 months"], then a key size extension of 14 bits [i.e., a factor of more than 16,000] should be adequate. The 1975 DES proposal suggested 56-bit keys; by 1995, a 70-bit key would have been required to offer equal protection and an 85-bit key necessary by 2015.

A 256- or 512-bit SKC key will probably suffice for some time because that length keeps us ahead of the brute force capabilities of the attackers. Note that while a large key is good, a huge key may not always be better; for example, expanding PKC keys beyond the current 2048- or 4096-bit lengths doesn't add any necessary protection at this time. Weaknesses in cryptosystems are largely based upon key management rather than weak keys.

Much of the discussion above, including the table, is based on the paper "Minimal Key Lengths for Symmetric Ciphers to Provide Adequate Commercial Security" by M. Blaze, W. Diffie, R.L. Rivest, B. Schneier, T. Shimomura, E. Thompson, and M. Wiener [1996].

The most effective large-number factoring methods today use a mathematical Number Field Sieve to find a certain number of relationships and then uses a matrix operation to solve a linear equation to produce the two prime factors. The sieve step actually involves a large number of operations that can be performed in parallel; solving the linear equation, however, requires a supercomputer. Indeed, finding the solution to the RSA-140 challenge in February 1999 — factoring a 140-digit [465-bit] prime number — required 200 computers across the Internet about 4 weeks for the first step and a Cray computer 100 hours and 810 MB of memory to do the second step.

In early 1999, Shamir [of RSA fame] described a new machine that could increase factorization speed by 2-3 orders of magnitude. Although no detailed plans were provided nor is one known to have been built, the concepts of TWINKLE [The Weizmann Institute Key Locating Engine] could result in a specialized piece of hardware that would cost about $5000 and have the processing power of 100-1000 PCs. There still appear to be many engineering details that have to be worked out before such a machine could be built. Furthermore, the hardware improves the sieve step only; the matrix operation is not optimized at all by this design and the complexity of this step grows rapidly with key length, both in terms of processing time and memory requirements. Nevertheless, this plan conceptually puts 512-bit keys within reach of being factored. Although most PKC schemes allow keys that are 1024 bits and longer, Shamir claims that 512-bit RSA keys "protect 95% of today's E-commerce on the Internet." [See Bruce Schneier's Crypto-Gram [May 15, 1999] for more information.]

It is also interesting to note that while cryptography is good and strong cryptography is better, long keys may disrupt the nature of the randomness of data files. Shamir and van Someren ["Playing hide and seek with stored keys"] have noted that a new generation of viruses can be written that will find files encrypted with long keys, making them easier to find by intruders and, therefore, more prone to attack.

Finally, U.S. government policy has tightly controlled the export of crypto products since World War II. Until the mid-1990s, export outside of North America of cryptographic products using keys greater than 40 bits in length was prohibited, which made those products essentially worthless in the marketplace, particularly for electronic commerce; today, crypto products are widely available on the Internet without restriction. The U.S. Department of Commerce Bureau of Industry and Security maintains an Encryption FAQ web page with more information about the current state of encryption registration.

Without meaning to editorialize too much in this tutorial, a bit of historical context might be helpful. In the mid-1990s, the U.S. Department of Commerce still classified cryptography as a munition and limited the export of any products that contained crypto. For that reason, browsers in the 1995 era, such as Internet Explorer and Netscape, had a domestic version with 128-bit encryption [downloadable only in the U.S.] and an export version with 40-bit encryption. Many cryptographers felt that the export limitations should be lifted because they only applied to U.S. products and seemed to have been put into place by policy makers who believed that only the U.S. knew how to build strong crypto algorithms, ignoring the work ongoing in Australia, Canada, Israel, South Africa, the U.K., and other locations in the 1990s. Those restrictions were lifted by 1996 or 1997, but there is still a prevailing attitude, apparently, that U.S. crypto algorithms are the only strong ones around; consider Bruce Schneier's blog in June 2016 titled "CIA Director John Brennan Pretends Foreign Cryptography Doesn't Exist." Cryptography is a decidedly international game today; note the many countries mentioned above as having developed various algorithms, not the least of which is the fact that NIST's Advanced Encryption Standard employs an algorithm submitted by cryptographers from Belgium. For more evidence, see Schneier's Worldwide Encryption Products Survey [February 2016].

On a related topic, public key crypto schemes can be used for several purposes, including key exchange, digital signatures, authentication, and more. In those PKC systems used for SKC key exchange, the PKC key lengths are chosen so as to be resistant to some selected level of attack. The length of the secret keys exchanged via that system have to have at least the same level of attack resistance. Thus, the three parameters of such a system — system strength, secret key strength, and public key strength — must be matched. This topic is explored in more detail in Determining Strengths For Public Keys Used For Exchanging Symmetric Keys [RFC 3766].

4. TRUST MODELS

Secure use of cryptography requires trust. While secret key cryptography can ensure message confidentiality and hash codes can ensure integrity, none of this works without trust. In SKC, Alice and Bob had to share a secret key. PKC solved the secret distribution problem, but how does Alice really know that Bob is who he says he is? Just because Bob has a public and private key, and purports to be "Bob," how does Alice know that a malicious person [Mallory] is not pretending to be Bob?

There are a number of trust models employed by various cryptographic schemes. This section will explore three of them:

• The web of trust employed by Pretty Good Privacy [PGP] users, who hold their own set of trusted public keys.
• Kerberos, a secret key distribution scheme using a trusted third party.
• Certificates, which allow a set of trusted third parties to authenticate each other and, by implication, each other's users.

Each of these trust models differs in complexity, general applicability, scope, and scalability.

4.1. PGP Web of Trust

Pretty Good Privacy [described more below in Section 5.5] is a widely used private e-mail scheme based on public key methods. A PGP user maintains a local keyring of all their known and trusted public keys. The user makes their own determination about the trustworthiness of a key using what is called a "web of trust."

FIGURE 5: GPG keychain.

Figure 5 shows a PGP-formatted keychain from the GNU Privacy Guard [GPG] software, an implementation of the OpenPGP standard. This is a section of my keychain, so only includes public keys from individuals whom I know and, presumably, trust. Note that keys are associated with e-mail addresses rather than individual names.

In general, the PGP Web of trust works as follows. Suppose that Alice needs Bob's public key. Alice could just ask Bob for it directly via e-mail or download the public key from a PGP key server; this server might a well-known PGP key repository or a site that Bob maintains himself. In fact, Bob's public key might be stored or listed in many places. [My public key, for example, can be found at //www.garykessler.net/pubkey.html or at several public PGP key servers, including //keys.openpgp.org.] Alice is prepared to believe that Bob's public key, as stored at these locations, is valid.

Suppose Carol claims to hold Bob's public key and offers to give the key to Alice. How does Alice know that Carol's version of Bob's key is valid or if Carol is actually giving Alice a key that will allow Mallory access to messages? The answer is, "It depends." If Alice trusts Carol and Carol says that she thinks that her version of Bob's key is valid, then Alice may — at her option — trust that key. And trust is not necessarily transitive; if Dave has a copy of Bob's key and Carol trusts Dave, it does not necessarily follow that Alice trusts Dave even if she does trust Carol.

The point here is that who Alice trusts and how she makes that determination is strictly up to Alice. PGP makes no statement and has no protocol about how one user determines whether they trust another user or not. In any case, encryption and signatures based on public keys can only be used when the appropriate public key is on the user's keyring.

4.2. Kerberos

Kerberos is a commonly used authentication scheme on the Internet. Developed by MIT's Project Athena, Kerberos is named for the three-headed dog who, according to Greek mythology, guards the entrance of Hades [rather than the exit, for some reason!].

Kerberos employs a client/server architecture and provides user-to-server authentication rather than host-to-host authentication. In this model, security and authentication will be based on secret key technology where every host on the network has its own secret key. It would clearly be unmanageable if every host had to know the keys of all other hosts so a secure, trusted host somewhere on the network, known as a Key Distribution Center [KDC], knows the keys for all of the hosts [or at least some of the hosts within a portion of the network, called a realm]. In this way, when a new node is brought online, only the KDC and the new node need to be configured with the node's key; keys can be distributed physically or by some other secure means.

FIGURE 6: Kerberos architecture.

The Kerberos Server/KDC has two main functions [Figure 6], known as the Authentication Server [AS] and Ticket-Granting Server [TGS]. The steps in establishing an authenticated session between an application client and the application server are:

1. The Kerberos client software establishes a connection with the Kerberos server's AS function. The AS first authenticates that the client is who it purports to be. The AS then provides the client with a secret key for this login session [the TGS session key] and a ticket-granting ticket [TGT], which gives the client permission to talk to the TGS. The ticket has a finite lifetime so that the authentication process is repeated periodically.
2. The client now communicates with the TGS to obtain the Application Server's key so that it [the client] can establish a connection to the service it wants. The client supplies the TGS with the TGS session key and TGT; the TGS responds with an application session key [ASK] and an encrypted form of the Application Server's secret key; this secret key is never sent on the network in any other form.
3. The client has now authenticated itself and can prove its identity to the Application Server by supplying the Kerberos ticket, application session key, and encrypted Application Server secret key. The Application Server responds with similarly encrypted information to authenticate itself to the client. At this point, the client can initiate the intended service requests [e.g., Telnet, FTP, HTTP, or e-commerce transaction session establishment].

The current version of this protocol is Kerberos V5 [described in RFC 1510]. While the details of their operation, functional capabilities, and message formats are different, the conceptual overview above pretty much holds for both. One primary difference is that Kerberos V4 uses only DES to generate keys and encrypt messages, while V5 allows other schemes to be employed [although DES is still the most widely algorithm used].

4.3. Public Key Certificates and Certificate Authorities

Certificates and Certificate Authorities [CA] are necessary for widespread use of cryptography for e-commerce applications. While a combination of secret and public key cryptography can solve the business issues discussed above, crypto cannot alone address the trust issues that must exist between a customer and vendor in the very fluid, very dynamic e-commerce relationship. How, for example, does one site obtain another party's public key? How does a recipient determine if a public key really belongs to the sender? How does the recipient know that the sender is using their public key for a legitimate purpose for which they are authorized? When does a public key expire? How can a key be revoked in case of compromise or loss?

The basic concept of a certificate is one that is familiar to all of us. A driver's license, credit card, or SCUBA certification, for example, identify us to others, indicate something that we are authorized to do, have an expiration date, and identify the authority that granted the certificate.

As complicated as this may sound, it really isn't. Consider driver's licenses. I have one issued by the State of Florida. The license establishes my identity, indicates the type of vehicles that I can operate and the fact that I must wear corrective lenses while doing so, identifies the issuing authority, and notes that I am an organ donor. When I drive in other states, the other jurisdictions throughout the U.S. recognize the authority of Florida to issue this "certificate" and they trust the information it contains. When I leave the U.S., everything changes. When I am in Aruba, Australia, Canada, Israel, and many other countries, they will accept not the Florida license, per se, but any license issued in the U.S. This analogy represents the certificate trust chain, where even certificates carry certificates.

For purposes of electronic transactions, certificates are digital documents. The specific functions of the certificate include:

• Establish identity: Associate, or bind, a public key to an individual, organization, corporate position, or other entity.
• Assign authority: Establish what actions the holder may or may not take based upon this certificate.
• Secure confidential information [e.g., encrypting the session's symmetric key for data confidentiality].

Typically, a certificate contains a public key, a name, an expiration date, the name of the authority that issued the certificate [and, therefore, is vouching for the identity of the user], a serial number, any pertinent policies describing how the certificate was issued and/or how the certificate may be used, the digital signature of the certificate issuer, and perhaps other information.

FIGURE 7: VeriSign Class 3 certificate.

A sample abbreviated certificate is shown in Figure 7. This is a typical certificate found in a browser, in this case, Mozilla Firefox [MacOS]. While this is a certificate issued by VeriSign, many root-level certificates can be found shipped with browsers. When the browser makes a connection to a secure Web site, the Web server sends its public key certificate to the browser. The browser then checks the certificate's signature against the public key that it has stored; if there is a match, the certificate is taken as valid and the Web site verified by this certificate is considered to be "trusted."

The most widely accepted certificate format is the one defined in International Telecommunication Union Telecommunication Standardization Sector [ITU-T] Recommendation X.509. Rec. X.509 is a specification used around the world and any applications complying with X.509 can share certificates. Most certificates today comply with X.509 Version 3 and contain the following information:

• Version number
• Certificate serial number
• Signature algorithm identifier
• Issuer's name and unique identifier
• Validity [or operational] period
• Subject's name and unique identifier
• Subject public key information
• Standard extensions
• Certificate appropriate use definition
• Key usage limitation definition
• Certificate policy information
• Other extensions
• Application-specific
• CA-specific

Certificate authorities are the repositories for public keys and can be any agency that issues certificates. A company, for example, may issue certificates to its employees, a college/university to its students, a store to its customers, an Internet service provider to its users, or a government to its constituents.

When a sender needs an intended receiver's public key, the sender must get that key from the receiver's CA. That scheme is straight-forward if the sender and receiver have certificates issued by the same CA. If not, how does the sender know to trust the foreign CA? One industry wag has noted, about trust: "You are either born with it or have it granted upon you." Thus, some CAs will be trusted because they are known to be reputable, such as the CAs operated by AT&T Services, Comodo, DigiCert [formerly GTE Cybertrust], EnTrust, Broadcom [formerly Symantec, formerly VeriSign], and Thawte. CAs, in turn, form trust relationships with other CAs. Thus, if a user queries a foreign CA for information, the user may ask to see a list of CAs that establish a "chain of trust" back to the user.

One major feature to look for in a CA is their identification policies and procedures. When a user generates a key pair and forwards the public key to a CA, the CA has to check the sender's identification and takes any steps necessary to assure itself that the request is really coming from the advertised sender. Different CAs have different identification policies and will, therefore, be trusted differently by other CAs. Verification of identity is just one of many issues that are part of a CA's Certification Practice Statement [CPS] and policies; other issues include how the CA protects the public keys in its care, how lost or compromised keys are revoked, and how the CA protects its own private keys.

As a final note, CAs are not immune to attack and certificates themselves are able to be counterfeited. One of the first such episodes occurred at the turn of the century; on January 29 and 30, 2001, two VeriSign Class 3 code-signing digital certificates were issued to an individual who fraudulently claimed to be a Microsoft employee [CERT/CC CA-2001-04 and Microsoft Security Bulletin MS01-017 - Critical]. Problems have continued over the years; good write-ups on this can be found at "Another Certification Authority Breached [the 12th!]" and "How Cybercrime Exploits Digital Certificates." Readers are also urged to read "Certification Authorities Under Attack: A Plea for Certificate Legitimation" [Oppliger, R., January/February 2014, IEEE Internet Computing, 18[1], 40-47].

As a partial way to address this issue, the Internet Security Research Group [ISRG] designed the Automated Certificate Management Environment [ACME] protocol. ACME is a communications protocol that streamlines the process of deploying a Public Key Infrastructure [PKI] by automating interactions between CAs and Web servers that wish to obtain a certificate. More information can be found at the Let's Encrypt Web site, an ACME-based CA service provided by the ISRG.

4.4. Summary

The paragraphs above describe three very different trust models. It is hard to say that any one is better than the others; it depends upon your application. One of the biggest and fastest growing applications of cryptography today, though, is electronic commerce [e-commerce], a term that itself begs for a formal definition.

PGP's web of trust is easy to maintain and very much based on the reality of users as people. The model, however, is limited; just how many public keys can a single user reliably store and maintain? And what if you are using the "wrong" computer when you want to send a message and can't access your keyring? How easy it is to revoke a key if it is compromised? PGP may also not scale well to an e-commerce scenario of secure communication between total strangers on short-notice.

Kerberos overcomes many of the problems of PGP's web of trust, in that it is scalable and its scope can be very large. However, it also requires that the Kerberos server have a priori knowledge of all client systems prior to any transactions, which makes it unfeasible for "hit-and-run" client/server relationships as seen in e-commerce.

Certificates and the collection of CAs will form a PKI. In the early days of the Internet, every host had to maintain a list of every other host; the Domain Name System [DNS] introduced the idea of a distributed database for this purpose and the DNS is one of the key reasons that the Internet has grown as it has. A PKI will fill a similar void in the e-commerce and PKC realm.

While certificates and the benefits of a PKI are most often associated with electronic commerce, the applications for PKI are much broader and include secure electronic mail, payments and electronic checks, Electronic Data Interchange [EDI], secure transfer of Domain Name System [DNS] and routing information, electronic forms, and digitally signed documents. A single "global PKI" is still many years away, that is the ultimate goal of today's work as international electronic commerce changes the way in which we do business in a similar way in which the Internet has changed the way in which we communicate.

5. CRYPTOGRAPHIC ALGORITHMS IN ACTION

The paragraphs above have provided an overview of the different types of cryptographic algorithms, as well as some examples of some available protocols and schemes. Table 3 provides a list of some other noteworthy schemes and cryptosystems employed — or proposed — for a variety of functions, most notably electronic commerce and secure communication. The paragraphs below will show several real cryptographic applications that many of us employ [knowingly or not] everyday for password protection and private communication. Some of the schemes described below never were widely deployed but are still historically interesting, thus remain included here. This list is, by no means, exhaustive but describes items that are of significant current and/or historic importance [a subjective judgement, to be sure].

TABLE 3. Other Crypto Algorithms and Systems of Note.

Bitmessage	A decentralized, encrypted, peer-to-peer, trustless communications protocol for message exchange. The decentralized design, outlined in "Bitmessage: A Peer-to-Peer Message Authentication and Delivery System" [Warren, 2012], is conceptually based on the Bitcoin model.
Capstone	A now-defunct U.S. National Institute of Standards and Technology [NIST] and National Security Agency [NSA] project under the Bush Sr. and Clinton administrations for publicly available strong cryptography with keys escrowed by the government [NIST and the Treasury Dept.]. Capstone included one or more tamper-proof computer chips for implementation [Clipper], a secret key encryption algorithm [Skipjack], digital signature algorithm [DSA], key exchange algorithm [KEA], and hash algorithm [SHA].
Challenge-Handshake Authentication Protocol [CHAP]	An authentication scheme that allows one party to prove who they are to a second party by demonstrating knowledge of a shared secret without actually divulging that shared secret to a third party who might be listening. Described in RFC 1994.
Chips-Message Robust Authentication [CHIMERA]	A scheme proposed for authenticating navigation data and the spreading code of civilian signals in the Global Positioning System [GPS]. This is an anti-spoofing mechanism to protect the unencrypted civilian signals; GPS military signals are encrypted.
Clipper	The computer chip that would implement the Skipjack encryption scheme. The Clipper chip was to have had a deliberate backdoor so that material encrypted with this device would not be beyond the government's reach. Described in 1993, Clipper was dead by 1996. See also EPIC's The Clipper Chip Web page.
Cryptography Research and Evaluation Committees [CRYPTEC]	Similar in concept to the NIST AES process and NESSIE, CRYPTEC is the Japanese government's process to evaluate algorithms submitted for government and industry applications. CRYPTEX maintains a list of public key and secret key ciphers, hash functions, MACs, and other crypto algorithms approved for various applications in government environments.
Derived Unique Key Per Transaction [DUKPT]	A key management scheme used for debit and credit card verification with point-of-sale [POS] transaction systems, automated teller machines [ATMs], and other financial applications. In DUKPT, a unique key is derived for each transaction based upon a fixed, shared key in such a way that knowledge of one derived key does not easily yield knowledge of other keys [including the fixed key]. Therefore, if one of the derived keys is compromised, neither past nor subsequent transactions are endangered. DUKPT is specified in American National Standard [ANS] ANSI X9.24-1:2009 [Retail Financial Services Symmetric Key Management Part 1: Using Symmetric Techniques] and can be purchased at the ANSI X9.24 Web page.
ECRYPT Stream Cipher Project [eSTREAM]	The eSTREAM project came about as a result of the failure of the NESSIE project to produce a stream cipher that survived cryptanalysis. eSTREAM ran from 2004 to 2008 with the primary purpose of promoting the design of efficient and compact stream ciphers. As of September 2008, the eSTREAM suite contains seven ciphers.
Escrowed Encryption Standard [EES]	Largely unused, a controversial crypto scheme employing the SKIPJACK secret key crypto algorithm and a Law Enforcement Access Field [LEAF] creation method. LEAF was one part of the key escrow system and allowed for decryption of ciphertext messages that had been intercepted by law enforcement agencies. Described more in FIPS PUB 185 [archived; no longer in force].
Federal Information Processing Standards [FIPS]	These computer security- and crypto-related FIPS PUBs are produced by the U.S. National Institute of Standards and Technology [NIST] as standards for the U.S. Government. Current Federal Information Processing Standards [FIPS] related to cryptography include: FIPS PUB 140-2: Security Requirements for Cryptographic Modules FIPS PUB 180-4: Secure Hash Standard [SHS] FIPS PUB 186-4: Digital Signature Standard [DSS] FIPS PUB 197: Advanced Encryption Standard [AES] FIPS PUB 198-1: The Keyed-Hash Message Authentication Code [HMAC] FIPS PUB 202: SHA-3 Standard: Permutation-Based Hash and Extendable-Output Functions
Fortezza	A PCMCIA card developed by NSA that implements the Capstone algorithms, intended for use with the Defense Messaging Service [DMS]. Originally called Tessera.
GOST	GOST is a family of algorithms defined in the Russian cryptographic standards. Although most of the specifications are written in Russian, a series of RFCs describe some of the aspects so that the algorithms can be used effectively in Internet applications: RFC 4357: Additional Cryptographic Algorithms for Use with GOST 28147-89, GOST R 34.10-94, GOST R 34.10-2001, and GOST R 34.11-94 Algorithms RFC 4490: Using the GOST 28147-89, GOST R 34.11-94, GOST R 34.10-94, and GOST R 34.10-2001 Algorithms with Cryptographic Message Syntax [CMS] RFC 4491: Using the GOST R 34.10-94, GOST R 34.10-2001, and GOST R 34.11-94 Algorithms with the Internet X.509 Public Key Infrastructure Certificate and CRL Profile RFC 5830: GOST 28147-89: Encryption, Decryption, and Message Authentication Code [MAC] Algorithms RFC 6986: GOST R 34.11-2012: Hash Function Algorithm RFC 7091: GOST R 34.10-2012: Digital Signature Algorithm [Updates RFC 5832: GOST R 34.10-2001] RFC 7801: GOST R 34.12-2015: Block Cipher "Kuznyechik" RFC 7836: Guidelines on the Cryptographic Algorithms to Accompany the Usage of Standards GOST R 34.10-2012 and GOST R 34.11-2012 RFC 8891: GOST R 34.12-2015: Block Cipher "Magma"
IP Security [IPsec]	The IPsec protocol suite is used to provide privacy and authentication services at the IP layer. An overview of the protocol suite and of the documents comprising IPsec can be found in RFC 2411. Other documents include: RFC 4301: IP security architecture. RFC 4302: IP Authentication Header [AH], one of the two primary IPsec functions; AH provides connectionless integrity and data origin authentication for IP datagrams and protects against replay attacks. RFC 4303: IP Encapsulating Security Payload [ESP], the other primary IPsec function; ESP provides a variety of security services within IPsec. RFC 4304: Extended Sequence Number [ESN] Addendum, allows for negotiation of a 32- or 64- bit sequence number, used to detect replay attacks. RFC 4305: Cryptographic algorithm implementation requirements for ESP and AH. RFC 5996: The Internet Key Exchange [IKE] protocol, version 2, providing for mutual authentication and establishing and maintaining security associations. IKE v1 was described in three separate documents, RFC 2407 [application of ISAKMP to IPsec], RFC 2408 [ISAKMP, a framework for key management and security associations], and RFC 2409 [IKE, using part of Oakley and part of SKEME in conjunction with ISAKMP to obtain authenticated keying material for use with ISAKMP, and for other security associations such as AH and ESP]. IKE v1 is obsoleted with the introduction of IKEv2. RFC 4307: Cryptographic algorithms used with IKEv2. RFC 4308: Crypto suites for IPsec, IKE, and IKEv2. RFC 4309: The use of AES in CBC-MAC mode with IPsec ESP. RFC 4312: The use of the Camellia cipher algorithm in IPsec. RFC 4359: The Use of RSA/SHA-1 Signatures within Encapsulating Security Payload [ESP] and Authentication Header [AH]. RFC 4434: Describes AES-XCBC-PRF-128, a pseudo-random function derived from the AES for use with IKE. RFC 2403: Describes use of the HMAC with MD5 algorithm for data origin authentication and integrity protection in both AH and ESP. RFC 2405: Describes use of DES-CBC [DES in Cipher Block Chaining Mode] for confidentiality in ESP. RFC 2410: Defines use of the NULL encryption algorithm [i.e., provides authentication and integrity without confidentiality] in ESP. RFC 2412: Describes OAKLEY, a key determination and distribution protocol. RFC 2451: Describes use of Cipher Block Chaining [CBC] mode cipher algorithms with ESP. RFCs 2522 and 2523: Description of Photuris, a session-key management protocol for IPsec. In addition, RFC 6379 describes Suite B Cryptographic Suites for IPsec and RFC 6380 describes the Suite B profile for IPsec. IPsec was first proposed for use with IP version 6 [IPv6], but can also be employed with the current IP version, IPv4. [See more detail about IPsec below in Section 5.6.]
Internet Security Association and Key Management Protocol [ISAKMP/OAKLEY]	ISAKMP/OAKLEY provide an infrastructure for Internet secure communications. ISAKMP, designed by the National Security Agency [NSA] and described in RFC 2408, is a framework for key management and security associations, independent of the key generation and cryptographic algorithms actually employed. The OAKLEY Key Determination Protocol, described in RFC 2412, is a key determination and distribution protocol using a variation of Diffie-Hellman.
Kerberos	A secret key encryption and authentication system, designed to authenticate requests for network resources within a user domain rather than to authenticate messages. Kerberos also uses a trusted third-party approach; a client communications with the Kerberos server to obtain "credentials" so that it may access services at the application server. Kerberos V4 used DES to generate keys and encrypt messages; Kerberos V5 uses DES and other schemes for key generation. Microsoft added support for Kerberos V5 — with some proprietary extensions — in Windows 2000 Active Directory. There are many Kerberos articles posted at Microsoft's Knowledge Base, notably "Kerberos Explained."
Keyed-Hash Message Authentication Code [HMAC]	A message authentication scheme based upon secret key cryptography and the secret key shared between two parties rather than public key methods. Described in FIPS PUB 198 and RFC 2104. [See Section 5.19 below for details on HMAC operation.]
Message Digest Cipher [MDC]	Invented by Peter Gutman, MDC turns a one-way hash function into a block cipher.
MIME Object Security Services [MOSS]	Designed as a successor to PEM to provide PEM-based security services to MIME messages. Described in RFC 1848. Never widely implemented and now defunct.
Mujahedeen Secrets	A Windows GUI, PGP-like cryptosystem. Developed by supporters of Al-Qaeda, the program employs the five finalist AES algorithms, namely, MARS, RC6, Rijndael, Serpent, and Twofish. Also described in Inspire Magazine, Issue 1, pp. 41-44 and Inspire Magazine, Issue 2, pp. 58-59. Additional related information can also be found in "How Al-Qaeda Uses Encryption Post-Snowden [Part 2]."
New European Schemes for Signatures, Integrity and Encryption [NESSIE]	NESSIE was an independent project meant to augment the work of NIST during the AES adoption process by putting out an open call for new cryptographic primitives. The NESSIE project ran from about 2000-2003. While several new block cipher, PKC, MAC, and digital signature algorithms were found during the NESSIE process, no new stream cipher survived cryptanalysis. As a result, the ECRYPT Stream Cipher Project [eSTREAM] was created.
NSA Suite B Cryptography	An NSA standard for securing information at the SECRET level. Defines use of: Advanced Encryption Standard [AES] with key sizes of 128 and 256 bits, per FIPS PUB 197 for encryption The Ephemeral Unified Model and the One-Pass Diffie Hellman [referred to as ECDH] using the curves with 256 and 384-bit prime moduli, per NIST Special Publication 800-56A for key exchange Elliptic Curve Digital Signature Algorithm [ECDSA] using the curves with 256 and 384-bit prime moduli, per FIPS PUB 186-3 for digital signatures Secure Hash Algorithm [SHA] using 256 and 384 bits, per FIPS PUB 180-3 for hashing RFC 6239 describes Suite B Cryptographic Suites for Secure Shell [SSH] and RFC 6379 describes Suite B Cryptographic Suites for Secure IP [IPsec]. RFC 8423 reclassifies the RFCs related to the Suite B cryptographic algorithms as Historic, and it discusses the reasons for doing so.
Pretty Good Privacy [PGP]	A family of cryptographic routines for e-mail, file, and disk encryption developed by Philip Zimmermann. PGP 2.6.x uses RSA for key management and digital signatures, IDEA for message encryption, and MD5 for computing the message's hash value; more information can also be found in RFC 1991. PGP 5.x [formerly known as "PGP 3"] uses Diffie-Hellman/DSS for key management and digital signatures; IDEA, CAST, or 3DES for message encryption; and MD5 or SHA for computing the message's hash value. OpenPGP, described in RFC 2440, is an open definition of security software based on PGP 5.x. The GNU Privacy Guard [GPG] is a free software version of OpenPGP. [See more detail about PGP below in Section 5.5.]
Privacy Enhanced Mail [PEM]	An IETF standard for secure electronic mail over the Internet, including provisions for encryption [DES], authentication, and key management [DES, RSA]. Developed by the IETF but never widely used. Described in the following RFCs: RFC 1421: Part I, Message Encryption and Authentication Procedures RFC 1422: Part II, Certificate-Based Key Management RFC 1423: Part III, Algorithms, Modes, and Identifiers RFC 1424: Part IV, Key Certification and Related Services
Private Communication Technology [PCT]	Developed by Microsoft for secure communication on the Internet. PCT supported Diffie-Hellman, Fortezza, and RSA for key establishment; DES, RC2, RC4, and triple-DES for encryption; and DSA and RSA message signatures. Never widely used; superseded by SSL and TLS.
Secure Electronic Transaction [SET]	A communications protocol for securing credit card transactions, developed by MasterCard and VISA, in cooperation with IBM, Microsoft, RSA, and other companies. Merged two other protocols: Secure Electronic Payment Protocol [SEPP], an open specification for secure bank card transactions over the Internet developed by CyberCash, GTE, IBM, MasterCard, and Netscape; and Secure Transaction Technology [STT], a secure payment protocol developed by Microsoft and Visa International. Supports DES and RC4 for encryption, and RSA for signatures, key exchange, and public key encryption of bank card numbers. SET V1.0 is described in Book 1, Book 2, and Book 3. SET has been superseded by SSL and TLS.
Secure Hypertext Transfer Protocol [S-HTTP]	An extension to HTTP to provide secure exchange of documents over the World Wide Web. Supported algorithms include RSA and Kerberos for key exchange, DES, IDEA, RC2, and Triple-DES for encryption. Described in RFC 2660. S-HTTP was never as widely used as HTTP over SSL [https].
Secure Multipurpose Internet Mail Extensions [S/MIME]	An IETF secure e-mail scheme superseding PEM, and adding digital signature and encryption capability to Internet MIME messages. S/MIME Version 3.1 is described in RFCs 3850 and 3851, and employs the Cryptographic Message Syntax described in RFCs 3369 and 3370. [More detail about S/MIME can be found below in Section 5.15.]
Secure Sockets Layer [SSL]	Developed in 1995 by Netscape Communications to provide application-independent security and privacy over the Internet. SSL is designed so that protocols such as HTTP, FTP [File Transfer Protocol], and Telnet can operate over it transparently. SSL allows both server authentication [mandatory] and client authentication [optional]. RSA is used during negotiation to exchange keys and identify the actual cryptographic algorithm [DES, IDEA, RC2, RC4, or 3DES] to use for the session. SSL also uses MD5 for message digests and X.509 public key certificates. SSL was found to be breakable soon after the IETF announced formation of group to work on TLS and RFC 6176 specifically prohibits the use of SSL v2.0 by TLS clients. SSL version 3.0 is described in RFC 6101. All versions of SSL are now deprecated in favor of TLS; TLS v1.0 is sometimes referred to as "SSL v3.1." [More detail about SSL can be found below in Section 5.7.]
Server Gated Cryptography [SGC]	Microsoft extension to SSL that provided strong encryption for online banking and other financial applications using RC2 [128-bit key], RC4 [128-bit key], DES [56-bit key], or 3DES [equivalent of 168-bit key]. Use of SGC required a Windows NT Server running Internet Information Server [IIS] 4.0 with a valid SGC certificate. SGC was available in 32-bit Windows versions of Internet Explorer [IE] 4.0; support for Mac, Unix, and 16-bit Windows versions of IE was planned, but never materialized, and SGC was made moot when browsers started to ship with 128-bit encryption.
ShangMi [SM] Cipher Suites	A suite of authentication, encryption, and hash algorithms from the People's Republic of China. SM2 Cryptography Algorithm: A public key crypto scheme based on elliptic curves. An overview of the specification, in Chinese, can be found in GM/T 0009-2012. Additional specifications can be found in: GB/T 32918.1-2016, Part 1: General GB/T 32918.2-2016, Part 2: Digital signature algorithm GB/T 32918.3-2016, Part 3: Key exchange protocol GB/T 32918.4-2016, Part 4: Public key encryption algorithm GB/T 32918.5-2017, Part 5: Parameter definition SM3 Cryptographic Hash Algorithm: A hash algorithm operating on 512-bit blocks to produce a 256-bit hash value. Described in GB/T 32905-2016. SM4 Block Cipher Algorithm: A Feistel block cipher algorithm with a block length and key length of 128 bits, and 32 rounds. Described in GB/T 32907-2016. An application of the ShangMi Cipher Suites in TLS can be found in RFC 8998.
Signal Protocol	A protocol for providing end-to-end encryption for voice calls, video calls, and instant messaging [including group chats]. Employing a combination of AES, ECC, and HMAC algorithms, it offers such features as confidentiality, integrity, authentication, forward/future secrecy, and message repudiation. Signal is particularly interesting because of its lineage and widespread use. The Signal Protocol's earliest versions were known as TextSecure, first developed by Open Whisper Systems in 2013. TextSecure itself was based on a 2004 protocol called Off-the-Record [OTR] Messaging, designed as an improvement over OpenPGP and S/MIME. TextSecure v2 [2014] introduced a scheme called the Axolotl Ratchet for key exchange and added additional communication features. After subsequent iterations improving key management [and the renaming of the key exchange protocol to Double Ratchet], additional cryptographic primitives, and the addition of an encrypted voice calling application [RedPhone], TextSecure was renamed Signal Protocol in 2016. The Ratchet key exchange algorithm is at the heart of the power of this system. Most messaging apps employ the users' public and private keys; the weakness here is that if the phone falls into someone else's hands, all of the messages on the device — including deleted messages — can be decrypted. The Ratchet algorithm generates a set of so-called "temporary keys" for each user, based upon that user's public/private key pair. When two users exchange messages, the Signal protocol creates a secret key by combining the temporary and permanent pairs of public and private keys for both users. Each message is assigned its own secret key. Because the generation of the secret key requires access to both users' private keys, it exists only on their two devices. The Signal Protocol is/has been employed in: WhatsApp [introduced 2014] G Data Software's Secure Chat [introduced 2015; service discontinued 2018] Google's Allo app [introduced 2016; discontinued in favor of Messages app, 2019] Facebook Messenger [introduced 2016] Skype's Private Conversations mode [introduced 2018] All of Google's Rich Communication Services [RCS] on Android systems [introduced 2020] A reasonably good writeup of the protocol can be found in "Demystifying the Signal Protocol for End-to-End Encryption [E2EE]" by Kozhukhovskaya, Mora, and Wong [2017].
Simple Authentication and Security Layer [SASL]	A framework for providing authentication and data security services in connection-oriented protocols [a la TCP], described in RFC 4422. It provides a structured interface and allows new protocols to reuse existing authentication mechanisms and allows old protocols to make use of new mechanisms. It has been common practice on the Internet to permit anonymous access to various services, employing a plain-text password using a user name of "anonymous" and a password of an email address or some other identifying information. New IETF protocols disallow plain-text logins. The Anonymous SASL Mechanism [RFC 4505] provides a method for anonymous logins within the SASL framework.
Simple Key-Management for Internet Protocol [SKIP]	Key management scheme for secure IP communication, specifically for IPsec, and designed by Aziz and Diffie. SKIP essentially defines a public key infrastructure for the Internet and even uses X.509 certificates. Most public key cryptosystems assign keys on a per-session basis, which is inconvenient for the Internet since IP is connectionless. Instead, SKIP provides a basis for secure communication between any pair of Internet hosts. SKIP can employ DES, 3DES, IDEA, RC2, RC5, MD5, and SHA-1. As it happened, SKIP was not adopted for IPsec; IKE was selected instead.
SM9	Chinese Standard GM/T0044-2016 SM9 [2016] is the Chinese national standard for Identity Based Cryptography. SM9 comprises three cryptographic algorithms, namely the Identity Based Digital Signature Algorithm, Identity Based Key Agreement Algorithm, and Identity Based Key Encapsulation Algorithm [allowing one party to securely send a symmetric key to another party]. The SM9 scheme is also described in The SM9 Cryptographic Schemes [Z. Cheng].
Telegram	Telegram, launched in 2013, is a cloud-based instant messaging and voice over IP [VoIP] service, with client app software available for all major computer and mobile device operating systems. Telegram allows users to exchange messages, photos, videos, etc., and supplies end-to-end encryption using a protocol called MTProto. stickers, audio and files of any type. MTProto employs 256-bit AES, 2048-bit RSA, and Diffie-Hellman key exchange. There have been several controversies with Telegram, not the least of which has to do with the nationality of the founders and the true location of the business, as well as some operation issues. From a cryptological viewpoint, however, one cautionary tale can be found in "On the CCA [in]security of MTProto" [Jakobsen & Orlandi, 2015], who describe some of the crypto weaknesses of the protocol; specifically, that "MTProto does not satisfy the definitions of authenticated encryption [AE] or indistinguishability under chosen-ciphertext attack [IND-CCA]" [p. 1].
Transmission Control Protocol [TCP] encryption [tcpcrypt]	As of 2019, the majority of Internet TCP traffic is not encrypted. The two primary reasons for this are [1] many legacy protocols have no mechanism with which to employ encryption [e.g., without a command such as STARTSSL, the protocol cannot invoke use of any encryption] and [2] many legacy applications cannot be upgraded, so no new encryption can be added. The response from the IETF's TCP Increased Security Working Group was to define a transparent way within the transport layer [i.e., TCP] with which to invoke encryption. The TCP Encryption Negotiation Option [TCP-ENO] addresses these two problems with an out-of-band, fully backward-compatible TCP option with which to negotiate use of encryption. TCP-ENO is described in RFC 8547 and tcpcrypt, an encryption protocol to protect TCP streams, is described in RFC 8548.
Transport Layer Security [TLS]	TLS v1.0 is an IETF specification [RFC 2246] intended to replace SSL v3.0. TLS v1.0 employs Triple-DES [secret key cryptography], SHA [hash], Diffie-Hellman [key exchange], and DSS [digital signatures]. TLS v1.0 was vulnerable to attack and updated by v1.1 [RFC 4346], which is now classified as an HISTORIC specification. TLS v1.1 was replaced by TLS v1.2 [RFC 5246] and, subsequently, by v1.3 [RFC 8446]. TLS is designed to operate over TCP. The IETF developed the Datagram Transport Layer Security [DTLS] protocol to operate over the User Datagram Protocol [UDP]. DTLS v1.3 is described in RFC 9147. [See more detail about TLS below in Section 5.7.]
TrueCrypt	Open source, multi-platform cryptography software that can be used to encrypt a file, partition, or entire disk. One of TrueCrypt's more interesting features is that of plausible deniability with hidden volumes or hidden operating systems. The original Web site, truecrypt.org, suddenly went dark in May 2014. The current fork of TrueCrypt is VeraCrypt. [See more detail about TrueCrypt below in Section 5.11.]
X.509	ITU-T recommendation for the format of certificates for the public key infrastructure. Certificates map [bind] a user identity to a public key. The IETF application of X.509 certificates is documented in RFC 5280. An Internet X.509 Public Key Infrastructure is further defined in RFC 4210 [Certificate Management Protocols] and RFC 3647 [Certificate Policy and Certification Practices Framework].

5.1. Password Protection

Nearly all modern multiuser computer and network operating systems employ passwords at the very least to protect and authenticate users accessing computer and/or network resources. But passwords are not typically kept on a host or server in plaintext, but are generally encrypted using some sort of hash scheme.

A] /etc/passwd file

 root:Jbw6BwE4XoUHo:0:0:root:/root:/bin/bash
 carol:FM5ikbQt1K052:502:100:Carol Monaghan:/home/carol:/bin/bash
 alex:LqAi7Mdyg/HcQ:503:100:Alex Insley:/home/alex:/bin/bash
 gary:FkJXupRyFqY4s:501:100:Gary Kessler:/home/gary:/bin/bash
 todd:edGqQUAaGv7g6:506:101:Todd Pritsky:/home/todd:/bin/bash
 josh:FiH0ONcjPut1g:505:101:Joshua Kessler:/home/webroot:/bin/bash

B.1] /etc/passwd file [with shadow passwords]

 root:x:0:0:root:/root:/bin/bash
 carol:x:502:100:Carol Monaghan:/home/carol:/bin/bash
 alex:x:503:100:Alex Insley:/home/alex:/bin/bash
 gary:x:501:100:Gary Kessler:/home/gary:/bin/bash
 todd:x:506:101:Todd Pritsky:/home/todd:/bin/bash
 josh:x:505:101:Joshua Kessler:/home/webroot:/bin/bash

B.2] /etc/shadow file

 root:AGFw$1$P4u/uhLK$l2.HP35rlu65WlfCzq:11449:0:99999:7:::
 carol:kjHaN%35a8xMM8a/0kMl1?fwtLAM.K&kw.:11449:0:99999:7:::
 alex:1$1KKmfTy0a7#3.LL9a8H71lkwn/.hH22a:11449:0:99999:7:::
 gary:9ajlknknKJHjhnu7298ypnAIJKL$Jh.hnk:11449:0:99999:7:::
 todd:798POJ90uab6.k$klPqMt%alMlprWqu6$.:11492:0:99999:7:::
 josh:Awmqpsui*787pjnsnJJK%aappaMpQo07.8:11492:0:99999:7:::

FIGURE 8: Sample entries in Unix/Linux password files.

Unix/Linux, for example, uses a well-known hash via its crypt[] function. Passwords are stored in the /etc/passwd file [Figure 8A]; each record in the file contains the username, hashed password, user's individual and group numbers, user's name, home directory, and shell program; these fields are separated by colons [:]. Note that each password is stored as a 13-byte string. The first two characters are actually a salt, randomness added to each password so that if two users have the same password, they will still be encrypted differently; the salt, in fact, provides a means so that a single password might have 4096 different encryptions. The remaining 11 bytes are the password hash, calculated using DES.

As it happens, the /etc/passwd file is world-readable on Unix systems. This fact, coupled with the weak encryption of the passwords, resulted in the development of the shadow password system where passwords are kept in a separate, non-world-readable file used in conjunction with the normal password file. When shadow passwords are used, the password entry in /etc/passwd is replaced with a "*" or "x" [Figure 8B.1] and the MD5 hash of the passwords are stored in /etc/shadow along with some other account information [Figure 8B.2].

Windows NT uses a similar scheme to store passwords in the Security Access Manager [SAM] file. In the NT case, all passwords are hashed using the MD4 algorithm, resulting in a 128-bit [16-byte] hash value [they are then obscured using an undocumented mathematical transformation that was a secret until distributed on the Internet]. The password password, for example, might be stored as the hash value [in hexadecimal] 60771b22d73c34bd4a290a79c8b09f18.

Passwords are not saved in plaintext on computer systems precisely so they cannot be easily compromised. For similar reasons, we don't want passwords sent in plaintext across a network. But for remote logon applications, how does a client system identify itself or a user to the server? One mechanism, of course, is to send the password as a hash value and that, indeed, may be done. A weakness of that approach, however, is that an intruder can grab the password off of the network and use an off-line attack [such as a dictionary attack where an attacker takes every known word and encrypts it with the network's encryption algorithm, hoping eventually to find a match with a purloined password hash]. In some situations, an attacker only has to copy the hashed password value and use it later on to gain unauthorized entry without ever learning the actual password.

An even stronger authentication method uses the password to modify a shared secret between the client and server, but never allows the password in any form to go across the network. This is the basis for the Challenge Handshake Authentication Protocol [CHAP], the remote logon process used by Windows NT.

As suggested above, Windows NT passwords are stored in a security file on a server as a 16-byte hash value. In truth, Windows NT stores two hashes; a weak hash based upon the old LAN Manager [LanMan] scheme and the newer NT hash. When a user logs on to a server from a remote workstation, the user is identified by the username, sent across the network in plaintext [no worries here; it's not a secret anyway!]. The server then generates a 64-bit random number and sends it to the client [also in plaintext]. This number is the challenge.

Using the LanMan scheme, the client system then encrypts the challenge using DES. Recall that DES employs a 56-bit key, acts on a 64-bit block of data, and produces a 64-bit output. In this case, the 64-bit data block is the random number. The client actually uses three different DES keys to encrypt the random number, producing three different 64-bit outputs. The first key is the first seven bytes [56 bits] of the password's hash value, the second key is the next seven bytes in the password's hash, and the third key is the remaining two bytes of the password's hash concatenated with five zero-filled bytes. [So, for the example above, the three DES keys would be 60771b22d73c34, bd4a290a79c8b0, and 9f180000000000.] Each key is applied to the random number resulting in three 64-bit outputs, which comprise the response. Thus, the server's 8-byte challenge yields a 24-byte response from the client and this is all that would be seen on the network. The server, for its part, does the same calculation to ensure that the values match.

There is, however, a significant weakness to this system. Specifically, the response is generated in such a way as to effectively reduce 16-byte hash to three smaller hashes, of length seven, seven, and two, respectively. Thus, a password cracker has to break at most a 7-byte hash. One Windows NT vulnerability test program that I used in the past reported passwords that were "too short," defined as "less than 8 characters." When I asked how the program knew that passwords were too short, the software's salespeople suggested to me that the program broke the passwords to determine their length. This was, in fact, not the case at all; all the software really had to do was to look at the last eight bytes of the Windows NT LanMan hash to see that the password was seven or fewer characters.

Consider the following example, showing the LanMan hash of two different short passwords [take a close look at the last 8 bytes]:

AA:	89D42A44E77140AAAAD3B435B51404EE
AAA:	1C3A2B6D939A1021AAD3B435B51404EE

Note that the NT hash provides no such clue:

AA:	C5663434F963BE79C8FD99F535E7AAD8
AAA:	6B6E0FB2ED246885B98586C73B5BFB77

It is worth noting that the discussion above describes the Microsoft version of CHAP, or MS-CHAP [MS-CHAPv2 is described in RFC 2759]. MS-CHAP assumes that it is working with hashed values of the password as the key to encrypting the challenge. More traditional CHAP [RFC 1994] assumes that it is starting with passwords in plaintext. The relevance of this observation is that a CHAP client, for example, cannot be authenticated by an MS-CHAP server; both client and server must use the same CHAP version.

5.2. Diffie-Hellman Key Exchange

Diffie and Hellman introduced the concept of public key cryptography. The mathematical "trick" of Diffie-Hellman key exchange is that it is relatively easy to compute exponents compared to computing discrete logarithms. Diffie-Hellman allows two parties — the ubiquitous Alice and Bob — to generate a secret key; they need to exchange some information over an unsecure communications channel to perform the calculation but an eavesdropper cannot determine the shared secret key based upon this information.

Diffie-Hellman works like this. Alice and Bob start by agreeing on a large prime number, N. They also have to choose some number G so that G