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Message Authentication Code (MAC)
A MAC takes advantage of the irreversibility property of hash functions. A MAC is the hash of a message concatenated with a key.
mac = hash(message + key)
As with a checksum, a mac can be sent along with the message in a communication. A man in the middle won't be able to alter the message secretly, because they can't produce a new valid mac without access to the secret key.
- mac =
hash(message + key) - send mac
- send message
- if
mac == hash(message + key)then the message is valid
The disadvantage of a MAC is that the receiver will also need a copy of the secret key. This is why MACs are generally only used when the sender and the receiver are the same entity.
Assignment
Fix the macMatches function. It currently uses a plain checksum. Concatenate the key to the end of the message before hashing it.