RSA Security 5.2.2 manual RC5

Cryptography Overview

it is proprietary to RSA Security. The RC2 cipher has an eight-byte block size. Therefore, the input must be a multiple of eight bytes, or be padded to be a multiple of eight bytes, for the RC2 cipher to operate properly in CBC or ECB modes.

The RC2 input key can be of any length from 1 to 128 bytes. The algorithm uses the input key to generate an effective key that is actually used for encryption purposes. Internally, the algorithm builds a key table based on the bits of the key data; the chosen number of effective key bits limits the number of possible key tables. The effective key size is variable and takes values from one bit up to 1024 bits.

Note: Control over your effective key size benefits you, because you can generate up to 128 bytes of key data and set the algorithm to use a smaller number of effective bits, such as 80. Then, in the future, if you want to increase the number of effective key bits, you do not have to change the code that generates the key data, only the effective key bit parameter.

RC5

The RC5 cipher was developed by Ronald Rivest as an alternative to DES encryption; it is proprietary to RSA Security. It is a block cipher with the block being either 4 bytes, 8 bytes, or 16 bytes, depending on the word size. The input must be a multiple of the block size, or it must be padded to a multiple of the block size for the RC5 cipher to operate properly. The RC5 cipher’s speed and security are dependent on input parameters determined by the user. These parameters are:

•word size

•rounds

•key size (in bytes)

Word size generally refers to the size of a hardware register. For hardware implementations of the RC5 cipher, developers can take advantage of larger registers to increase speed. On chips with smaller registers, the word size can be emulated in software. Version 1.0 of the RC5 cipher accepts word sizes of 16, 32, or 64 bits. Crypto- C accepts a word size of 32 or 64 bits. The block size is twice the word size. For a word size of 32 bits, the block size is 64 bits, or 8 bytes, the same as for DES and the RC2 cipher. For a word size of 64 bits, the block size is 128 bits, or 16 bytes.

The number of rounds is the number of times the operation employs the inner encryption function. Varying the number of rounds allows developers to make a tradeoff between speed and security. The greater the number of rounds, the greater the security, but the slower the execution. The number of rounds can be anywhere from 0 (zero) to 255. For the RC5 cipher with a 32-bit word size, RSA Security recommends at least 16 rounds for applications; while no practical attacks are known

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