Problem - Kevin Tafuro

To postprocess, we encrypt the hash output with one AES key, encrypt the nonce
with the other AES key, then XOR the two ciphertexts together. This gives us provable
security with good assumptions, plus the additional benefits of a nonce (see
Recipe 6.12).
The core MAC127 data type is SPC_MAC127_CTX. There are only two functions: one to
initialize a context, and one to MAC a message. The initialization function has the
following signature:
void spc_mac127_init(SPC_MAC127_CTX *ctx, unsigned char *key);
This function has the following arguments:
ctx
Context object that holds key material so that several messages may be MAC’d
with a single key.
key
Buffer that contains a 16-byte key.
To MAC a message, we use the function spc_mac127( ):
void spc_mac127(SPC_MAC127_CTX *ctx, unsigned char *m, size_t l,
unsigned char *nonce, unsigned char *out);
This function has the following arguments:
ctx
Context object to be used to perform the MAC.
m
Buffer that contains the message to be authenticated.
l
Length of the message buffer in octets.
nonce
Buffer that contains a 16-byte value that must not be repeated.
out
Buffer into which the output will be placed. It must be at least 16 bytes in size.
No more than 16 bytes will ever be written to it.
Here is our implementation of MAC127:
#include
#ifndef WIN32
#include
#include
#include
#else
#include
#include
#endif
#include
Using a MAC That’s Optimized for Software Speed | 289
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