First off: I know that encrypt-then-authenticate is better, however I want to understand the following for authenticate-then-encrypt. The underlying cipher is AES-CTR or AES-CBC for this consideration.
When using encrypt-then-authenticate, the authenticity of the of the cipher text can be proven, because both parties have the key for the authentication algorithm, e.g. HMAC.
When using authenticate-then-encrypt, one party can't prove the authenticity of the cipher text, as it has to decrypt it first. If it decrypts and then checks the authenticity digest and gets a match, then it knows, that the message has authenticity.
And here is my question: does an algorithm for authenticity, e.g. HMAC, that uses a key, provide anything more than the integrity a simple checksum like CRC-xx can also provide? If a party decrypts a message and gets a match on the checksum, that should mean, that the correct key was used, that only the two parties have and therefore authenticate the message.
Thanks in advance and BR.