The use of cryptographic hash functions like MD5 or SHA for message authentication has
become a standard approach in many Internet applications and protocols. Though very easy to
implement, these mechanisms are usually based on ad hoc techniques that lack a sound security
analysis.
We present new constructions of message authentication schemes based on a cryptographic
hash function. Our schemes, NMAC and HMAC, are proven to be secure as long as the un-
derlying hash function has some reasonable cryptographic strengths. Moreover we show, in a
quantitative way, that the schemes retain almost all the security of the underlying hash function.
In addition our schemes are efficient and practical. Their performance is essentially that of the
underlying hash function. Moreover they use the hash function (or its compression function) as
a black box, so that widely available library code or hardware can be used to implement them
in a simple way, and replaceability of the underlying hash function is easily supported.

Keying Hash Functions for Message Authentication