# 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).

# Implement a SHA-1 keyed MAC (cont.)

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.