c28.py

from pals.sha1 import sha1

prefix = b'foo'

def mac(data):
    return sha1(prefix + data)

def validate(hash, data):
    return hash == mac(data)

a = mac(b'bar')

print(validate(a, b'bar'))

'''
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.
'''