RSA Security 5 manual AIRC4WithMAC, Pointer to a BRC4WITHMACPARAMS structure

AI_RC4WithMAC

AI_RC4WithMAC

Purpose:

This AI implements a stream cipher with a simple tamper-detection message authentication code based on AI_MAC. When applied to a plaintext buffer of n bytes, it produces a ciphertext of n bytes using the same algorithm as AI_RC4, and then it appends a MAC of macLen bytes. You can find a description of AI_RC4 in B. Schneier's Applied Cryptography. For more information about AI_MAC, see page 96.

Type of information this allows you to use:

the RC4 With MAC encryption algorithm. The MAC is computed using AI_MAC by first passing the key to AI_MAC, then the plaintext, and finally a block of macLen zero bytes. The resulting value from AI_MAC is appended to the ciphertext. For decryption, the MAC value is checked.

The key passed to both AI_RC4 and AI_MAC is created by appending the salt bytes to the end of the key passed to B_EncryptInit or B_DecryptInit. That is, for this AI, the RC4 key depends on the salt as well as the key object passed to the Init routine.

Format of info supplied to B_SetAlgorithmInfo:

pointer to a B_RC4_WITH_MAC_PARAMS structure:

typedef struct	{
ITEM	salt;	/* variable-length salt */
unsigned int	macLen;	/* length to use for MAC value */

} B_RC4_WITH_MAC_PARAMS;

The salt ITEM supplies the salt value that is appended to the key, where the ITEM’s data points to an unsigned byte array and the ITEM’s len gives its length. If the length is zero, no salt is appended to the key, and the ITEM’s data is ignored. macLen has a minimum of 2 and maximum of 16.

Format of info returned by B_GetAlgorithmInfo:

pointer to a B_RC4_WITH_MAC_PARAMS structure (see above).

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