Problem - Kevin Tafuro

CMAC is the message-integrity component of the CWC encryption mode. It is based
on a universal hash function that is similar to hash127. It requires an 11-byte nonce
per message. The Zork implementation has the following API:
int cmac_init(cmac_t *ctx, unsigned char key[16]);
void cmac_mac(cmac_t *ctx, unsigned char *msg, u_int32 msglen,
unsigned char nonce[11], unsigned char output[16]);
void cmac_cleanup(cmac_t *ctx);
void cmac_update(cmac_t *ctx, unsigned char *msg, u_int32 msglen);
void cmac_final(cmac_t *ctx, unsigned char nonce[11], unsigned char output[16]);
The cmac_t type keeps track of state and needs to be initialized only when you key
the algorithm. You can then make messages interchangeably using the all-in-one API
or the incremental API.
The all-in-one API consists of the cmac_mac( ) function. It takes an entire message and
a nonce as arguments and produces a 16-byte output. If you want to use the incremental
API, cmac_update( ) is used to pass in part of the message, and cmac_final( )
is used to set the nonce and get the resulting tag. The cmac_cleanup( ) function
securely erases the context object.
To use the CMAC API, just copy the cmac.h and cmac.c files, and compile and link
against cmac.c.
See Also
• The CMAC home page: http://www.zork.org/cmac/
• Recipe 6.9
6.14 Using a MAC That’s Optimized for Software
Speed
Problem
You want to use the MAC that is fastest in software.
Solution
Use a MAC based on Dan Bernstein’s hash127, as discussed in the next section. The
hash127 library is available from http://cr.yp.to.
Using a MAC That’s Optimized for Software Speed | 287
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