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HASH FUNCTION, MESSAGE DIGEST AND HMAC 155
Register output
t A B C D E
77 8243ecda 6240e72c 67bb0c15 21095e62 c76126c5
78 a8342af0 8243ecda 189039cb 67bb0c15 21095e62
79 e1426096 a8342af0 a090fb36 189039cb 67bb0c15
After all 512-bit blocks have been processed, the output represented by the five words,
H0, H1, H2, H3 and H4 is the 160-bit message digest as shown below:
H0: 48878397
H1: 9801d679
H2: 394bd834
H3: 28c28e41
H4: 2b8dee05
The 160-bit message digest is then the data concatenation of {Hi}:
H0||H1||H2||H3||H4 = 488783979801d679394bd83428c28e412b8dee05
As discussed previously, the digitised document or message of any length can create a
160-bit message digest which is produced using the SHA-1 algorithm.
Any change to a digitised message in transit results in a different message digest. In
fact, changing a single bit of the data modifies at least half of the resulting digest bits.
Furthermore, it is computationally impossible to find two meaningful messages that have
the same 160-bit digest. On the other hand, given a 160-bit message digest, it is also
impossible to find a meaningful message with that digest.
4.5 Hashed Message Authentication Codes (HMAC)
The keyed-hashing Message Authentication Code (HMAC) is a key-dependent one-way
hash function which provides both data integrity and data origin authentication for files
sent between two users. HMACs have the same properties as the one-way hash functions
discussed earlier in this chapter, but they also include a secret key. HMACs can be
used to authenticate data or files between two users (data authentication). They can also
be used by a single user to determine whether or not his files have been altered (data
integrity).
To evaluate HMAC over the message or file, the following expression is required
to compute:
HMAC = H [(K ⊕ opad)||H [(K ⊕ ipad)||M]]
where ipad = inner padding
= 0 x 36 (repeated b times)