Posted by arlene

This is by no means an exhaustive list of current symmetric cryptosystems, but is rather a quick list of interesting and commercially important systems.

DES

The Data Encryption Standard (DES) is a block cipher that uses a 56-bit key to encrypt a 64-bit plaintext block into a 64-bit ciphertext block. DES is defined by Federal Information Processing Standard (FIPS) 46, published in November 1976. The most common mode of operation of DES is CBC. In this mode of operation, each output block of ciphertext is exclusive-ORed with the next plaintext block to form the next input to the DES algorithm. The process begins with a 64-bit IV. Without CBC, individual blocks of ciphertext can be replaced by an attacker to disrupt communications. Also, because 8-character blocks of 8-bit data such as ASCII fall naturally on 64-bit boundaries, a cryptanalyst can identify when blocks are repeated and can gain substantial insight into the plaintext. Although DES has been in widespread use for more than 20 years, withstanding intense scrutiny from cryptanalysts, its 56-bit key length is too small to resist brute force attacks by modem computers. Consequently, DES should not be used in new systems, and it should be retired from older systems.

Triple DES

A recent variation of DES, called Triple DES or 3DES, uses three 56-bit DES keys to encrypt each block. In a Triple DES encryption operation, the data block is encrypted with the first key, decrypted with the second key,³ and encrypted again using the third key. The middle operation is a decryption, so that if the three keys are chosen to be the same, then Triple DES reduces to ordinary DES. In its three-key mode, Triple DES requires a 168-bit key. A second variant, in which the first and third keys are the same, requires a 112-bit key.

IDEA

The International Data Encryption Algorithm (IDEA) is a block cipher developed by Lai and Massey in 1991. IDEA uses 128-bit keys to encrypt 64-bit blocks. IDEA was used as the bulk cipher in older versions of Pretty Good Privacy (PGP).

RC4

RC4 is a variable key length stream cipher designed by Ron Rivest for RSA Security. RC4 is widely used on the Internet as the bulk encryption cipher in SSL/TLS, with key lengths in the range of 40 bits to 128 bits. RC4 is a proprietary cipher of RSA Security, but the source code of a program that appears to duplicate the function of RC4 was published anonymously on the Internet in 1995. As a consequence, the RC4 algorithm is receiving much more cryptographic scrutiny.

Blowfish

Blowfish is a variable key length block cipher designed by Bruce Schneier of Counterpane Systems. Blowfish is freely available and is very fast, running nearly three times faster than a DES implementation on an Intel Pentium processor. Blowfish is widely used in file encryption applications for personal computers, secure tunneling applications, and others. The key length is variable from 32 bits to 448 bits, which makes it interesting for variable security applications.

CAST

CAST is a variable key length block cipher designed by Carlisle Adams and Stafford Tavares for Entrust Technologies. The key length is variable from 40 bits to 128 bits. CAST is the standard bulk encryption cipher in some recent of versions of POP. In January 1997, Entrust released a version of CAST for free use in both commercial and noncommercial applications.

AES

The Advanced Encryption Standard (AES) is an effort of the National Institute of Standards and Technology (NIST) to develop and standardize a replacement for DES. The effort was launched on January 2, 1997. On October 2, 2000, NIST announced that it had selected the Rijndael algorithm. Rijndael is an iterated block cipher whose block length and key length can be independently set to 128 bits, 192 bits, or 256 bits. NIST published FIPS 197 for AES in 2001.

Unless there are compelling reasons to choose a specific algorithm, AES is now an especially good choice for new applications. Because it is now a standard, it is receiving careful scrutiny by cryptographers, and it continues to resist attack.

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