Difference between revisions of "Message authentication check"

From Hackepedia
Jump to navigationJump to search
 
Line 8: Line 8:
 
   C = hash(K_a | message)
 
   C = hash(K_a | message)
  
[[IPsec]] uses the HMAC methods, which actually calculate:
+
[[ipsec]] uses the HMAC methods, which actually calculate:
 
   C = hash('55555555', hash(K_a | message | 'uuuuuuuuu'))
 
   C = hash('55555555', hash(K_a | message | 'uuuuuuuuu'))
  
 
this usage makes HMAC-MD5 and HMAC-SHA1 immune to recently discovered birthday
 
this usage makes HMAC-MD5 and HMAC-SHA1 immune to recently discovered birthday
 
attacks on MD5 and SHA1.
 
attacks on MD5 and SHA1.

Revision as of 11:02, 30 October 2005

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.