Most of the time, when some data must be encrypted, it must also be protected with a MAC, because encryption protects only against passive attackers. There are some nifty encryption modes which include a MAC (EAX, GCM...) but let's assume that we are doing old-style crypto, so we have a standalone encryption method (e.g. AES with CBC chaining and PKCS#5 padding) and a standalone MAC (e.g. HMAC with SHA-256). How should we assemble the encryption and the MAC?
- Compute the MAC on the cleartext, append it to the data, and then encrypt the whole? (That's what TLS does)
- Compute the MAC on the cleartext, encrypt the cleartext, and then append the MAC at the end of the ciphertext?
- Encrypt the cleartext, then compute the MAC on the ciphertext, and append it to the ciphertext? (In that case, we do not forget to include the IV and the encryption method identifier into the MACed data.)
The first two options are often called "MAC-then-encrypt" while the third is "encrypt-then-MAC". What are the arguments for or against either?