Problem - Kevin Tafuro

void DGST_Update(DGST_CTX *ctx, unsigned char *m, size_t len);
void DGST_Final(unsigned char *tag. DGST_CTX *ctx);
Some hash function implementations won’t have an explicit reset implementation, in
which case you can implement the reset functionality by calling DGST_Init( ) again.
Even though OpenSSLalready has an HMAC implementation, here is an example of
binding the previous HMAC implementation to OpenSSL’s SHA1 implementation:
typedef SHA_CTX DGST_CTX;
#define DGST_BLK_SZ 64
#define DGST_OUT_SZ 20
#define DGST_Init(x) SHA1_Init(x)
#define DGST_Reset(x) DGST_Init(x)
#define DGST_Update(x, m, l) SHA1_Update(x, m, l)
#define DGST_Final(o, x) SHA1_Final(o, x)
See Also
Recipes 5.26, 6.3, 6.4, 6.9
6.11 Using OMAC (a Simple Block Cipher–Based
MAC)
Problem
You want to use a simple MAC based on a block cipher, such as AES.
Solution
Use the OMAC implementation provided in the “Discussion” section.
Discussion
Be sure to look at our generic recommendations for using a MAC (see
Recipe 6.9).
OMAC is a straightforward message authentication algorithm based on the CBCencryption
mode. It fixes some security problems with the naïve implementation of a
MAC from CBC mode (CBC-MAC). In particular, that MAC is susceptible to lengthextension
attacks, similar to the ones we consider for cryptographic hash functions
in Recipe 6.7.
280 | Chapter 6: Hashes and Message Authentication
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