RSA Security 5.2.2 manual Block Ciphers, Padding, Ciphers in Crypto-C, RC2 RC5 RC6

Cryptography Overview

Block Ciphers

Block ciphers encrypt data block-by-block. They can encrypt each block separately as in ECB mode, or they can use other modes to make the cipher less vulnerable to attacks based on regular patterns. A mode of operation usually combines the underlying cipher with feedback and other simple operations. The security remains a function of the cipher and not of the mode. See “Modes of Operation” on page 41 for more information.

Padding

When you encrypt a message using a block cipher, usually your message length will not be a multiple of the block size. Some modes can deal with variable size blocks, but others require the message be a multiple of the block size. For these modes, padding provides a solution to this problem. To pad, you add a regular pattern of bytes to the end of the last block to make it a complete block. With padding, the actual number of bytes encrypted can be as much as one block more than the original data.

Ciphers in Crypto-C

Crypto-C implements the following block ciphers:

•DES

•Triple DES

•DESX

•RC2

•RC5

•RC6

•AES

DES

The Digital Encryption Standard, DES, is a commercial encryption US standard that has been available for over 15 years. The federal standard document FIPS PUB 46-2 describes the algorithm.

For DES, the block size is eight bytes. Therefore, the input must be a multiple of eight bytes, or else it must be padded to be a multiple of eight bytes for DES to operate in CBC or ECB modes properly. The key consists of 56 random bits and 8 parity bits, forming a 64-bit, or 8-byte, key.

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