the cryptopals crypto challenges

Implement a SHA-1 keyed MAC

Find a SHA-1 implementation in the language you code in.

Don't cheat. It won't work.

Do not use the SHA-1 implementation your language already provides (for instance, don't use the "Digest" library in Ruby, or call OpenSSL; in Ruby, you'd want a pure-Ruby SHA-1).

Write a function to authenticate a message under a secret key by using a secret-prefix MAC, which is simply: 

SHA1(key || message)

Verify that you cannot tamper with the message without breaking the MAC you've produced, and that you can't produce a new MAC without knowing the secret key.

Cryptography Services | NCC Group