Question

I’m looking to authenticate that a particular message is coming from a particular place.

Example: A repeatedly sends the same message to B. Lets say this message is "helloworld" which is encrypted to "asdfqwerty".

How can I ensure that a third party C doesn’t learn that B always receives this same encrypted string, and C starts sending "asdfqwerty" to B?

How can I ensure that when B decrypts "asdfqwerty" to "helloworld", it is always receiving this "helloworld" from A?

Thanks for any help.

Answer 1

For the former, you want to use a Mode of Operation for your symmetric cipher that uses an Initialization Vector. The IV ensures that every encrypted message is different, even if it contains the same plaintext.

For the latter, you want to sign your message using the private key of A(lice). If B(ob) has the public key of Alice, he can then verify she really created the message.

Finally, beware of replay attacks, where C(harlie) records a valid message from Alice, and later replays it to Bob. To avoid this, add a nonce and/or a timestamp to your encrypted message (yes, you could make the IV play double-duty as a nonce).