Problem - Kevin Tafuro

For example, here’s a wrapper for the all-in-one SHA1 interface discussed in Recipe 6.6:
#define SPC_SHA1_DGST_LEN (20)
/* Include anything else you need. */
void spc_extended_sha1(unsigned char *message, unsigned long n,unsigned char *md) {
unsigned char tmp[SPC_SHA1_DGST_LEN];
SHA1(message, n, tmp);
SHA1(tmp, sizeof(tmp), md);
}
Note that this solution does not protect against birthday attacks. When using SHA1,
birthday attacks are generally considered totally impractical. However, to be conservative,
you can use a nonce to protect against such attacks, as discussed in Recipe 6.8.
See Also
Recipes 6.6, 6.8
6.8 Using a Nonce to Protect Against Birthday
Attacks
Problem
You want to harden a hash function against birthday attacks instead of switching to
an algorithm with a longer digest.
Solution
Use a nonce or salt before and after your message (preferably a securely generated
random salt), padding the nonce to the internal block size of the hash function.
Discussion
Hash functions are not secure by themselves—not for a password system,
not for message authentication, not for anything! If you do need a
hash function by itself, be sure to at least protect against length extension
attacks, as described in Recipe 6.7.
In most cases, when using a nonce or salt with a hash function, where the nonce is as
large as the output length of the hash function, you double the effective strength of
the hash function in circumstances where a birthday attack would apply. Even
smaller nonces help improve security.
270 | Chapter 6: Hashes and Message Authentication
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