Message authentication check
From Hackepedia
A message authentication check, or MAC is a cryptographic check that a message is from a given origin.
Most MACs are constructed from keyed one way hash functions. The method is for the sender and receiver to agree on a symmetric key, and to then calculate:
C = hash(K_a | message)
IPsec uses the HMAC methods, which actually calculate:
C = hash('55555555', hash(K_a | message | 'uuuuuuuuu'))
this usage makes HMAC-MD5 and HMAC-SHA1 immune to recently discovered birthday attacks on MD5 and SHA1.
