What is message digest? Explain HMAC algorithm?

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written 8.4 years ago by

teamques10 ★ 68k

A hash function accepts a variable size message M as input and produces a fixed sized output, referred to as Hash code H (M).
(Unlike a MAC) Hash code doesn’t use any key but it is a function of only input message.
The hash code is also referred to as message digest or hash value.
Hash code is a function of all the bits of message digest or hash value.
Hash code is a a function of all the bits of message and provides an error detection capability.
A change in any bit or bits in the message results in change of the hash code.
The following figure shows various ways in which a hash code can be used to provide message authentication.

Authentication
Only has code encrypted using symmetric encryption.
Reduces processing burden for those applications that do not require confidentiality.

Only hash functions code is encrypted using public key encryption and using sender’s private key.
This provides authentication and digital signature.

Message plus the private key encrypted hash code is encrypted hash code is encrypted using symmetric secret key.
Provides authentication (Only A and B know secret key )
Confidentiality and digital signature (Entire message is used at encryption)

The technique assumes that the 2 communication parties share a common secret value S.
It computes hash value over the concentration of M and S (M || S) and appends resulting value to M.
Because B possesses S it can recomputed hash value to verify.
Because the secret value itself is not sent, an opponent can’t modify an intercepted message and hence can’t generate false message.
Provides authentication (Only A and B share S)

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