Should we MAC-then-encrypt or encrypt-then-MAC?

Question

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?

Answer 1

I'm assuming you actually know all of this better than I do... anyway, this paper neatly summarises all these approaches and what level of security they do or don't provide. I shall paraphrase it in English, rather than Mathematical notation, as I understand it, here:

• Encrypt-then-MAC:
  • Provides integrity of Ciphertext. Assuming the MAC shared secret has not been compromised, we ought to be able to deduce whether a given ciphertext is indeed authentic or has been forged; for example, in public key cryptography anyone can send you messages. EtM ensures you only read valid messages.
  • Plaintext integrity.
  • If the cipher scheme is malleable we need not be so concerned, since the MAC code will filter out this invalid ciphertext.
  • The MAC does not provide any information on the plaintext since, assuming the output of the cipher appears random, so does the MAC. In other words, we haven't carried any structure from the plaintext into the MAC code.
• MAC-then-Encrypt:
  • Does not provide any integrity on the ciphertext, since we have no way of knowing until we decrypt the message whether it was indeed authentic or spoofed.
  • Plaintext integrity.
  • If the cipher scheme is malleable it may be possible to alter the message to appear valid and have a valid MAC code. This is a theoretical point, of course, since practically speaking the MAC secret should provide protection.
  • Here, the MAC cannot provide any information on the plaintext either, since it is encrypted.
• Encrypt-and-MAC:
  • No integrity on the ciphertext again, since the MAC is taken against the plaintext. This opens the door to some chosen-ciphertext attacks on the cipher, as shown in section 4 of Breaking and provably repairing the SSH authenticated encryption scheme: A case study of the Encode-then-Encrypt-and-MAC paradigm.
  • Integrity of the plaintext can be verified
  • If the cipher scheme is malleable, the contents of the ciphertext could well be altered, but on decryption we ought to find the plaintext is invalid. Of course, any implementation error that can be exploited in the decryption process has been by that point.
  • May reveal information about the plaintext in the MAC. Theoretical, of course, but a less than ideal scenario. This occurs if the plaintext messages are repeated, and the MACed data does not include a counter (it does in the SSH 2 protocol, but only as a 32-bit counter, so you should take care to rekey before it overflows).

In short, Encrypt-then-MAC is the most ideal scenario. Any modifications to the ciphertext that do not also have a valid MAC code can be filtered out before decryption, protecting against any attacks on the implementation. The MAC cannot, also, be used to infer anything about the plaintext. MAC-then-Encrypt and Encrypt-and-MAC both provide different levels of security, but not the complete set provided by Encrypt-then-MAC.

Answer 2

@Ninefingers answers the question quite well; I just want to add a few details.

Encrypt-then-MAC is the mode which is recommended by most researchers. Mostly, it makes it easier to prove the security of the encryption part (because thanks to the MAC, a decryption engine cannot be fed with invalid ciphertexts; this yields automatic protection against chosen ciphertext attacks) and also avoids any trouble to confidentiality from the MAC (since the MAC operates on the encrypted text, it cannot reveal anything about the plaintext, regardless of its quality). Note that the padding oracle attacks, which have been applied in the field to ASP.NET, are chosen ciphertext attacks.

Ferguson and Schneier, in their book Practical Cryptography, have argued the opposite: that MAC-then-encrypt (or MAC-and-encrypt) is the "natural" order and that encrypt-then-MAC is overly complex. The sore point of encrypt-then-MAC is that you have to be careful about what you MAC: you must not forget the IV, or (in case the protocol allows algorithm flexibility) the unambiguous identifier for the encryption algorithm; otherwise, the attacker could change either, inducing a plaintext alteration which would be undetected by the MAC. To prove their point, Ferguson and Schneier describe an attack over an instance of IPsec in which the encrypt-then-MAC was not done properly.

So while encrypt-then-MAC is theoretically better, it is also somewhat harder to get right.

Answer 3

Hugo Krawczyk has a paper titled The Order of Encryption and Authentication for Protecting Communications (or: How Secure Is SSL?). It identifies 3 types of combining authentication (MAC) with encryption:

1. Encrypt then Authenticate (EtA) used in IPsec;
2. Authenticate then Encrypt (AtE) used in SSL;
3. Encrypt and Authenticate (E&A) used in SSH.

It proves that EtA is the secure way to use, and both AtE and E&A are subject to attacks, unless the encryption method is either in CBC mode or it is a stream cipher.

The abstract says everything; I emphasized important parts by bolding them:

We study the question of how to generically compose symmetric encryption and authentication when building “secure channels” for the protection of communications over insecure networks. We show that any secure channels protocol designed to work with any combination of secure encryption (against chosen plaintext attacks) and secure MAC must use the encrypt-then-authenticate method. We demonstrate this by showing that the other common methods of composing encryption and authentication, including the authenticate-then-encrypt method used in SSL, are not generically secure. We show an example of an encryption function that provides (Shannon’s) perfect secrecy but when combined with any MAC function under the authenticate-then-encrypt method yields a totally insecure protocol (for example, finding passwords or credit card numbers transmitted under the protection of such protocol becomes an easy task for an active attacker). The same applies to the encrypt-and-authenticate method used in SSH.

On the positive side we show that the authenticate-then-encrypt method is secure if the encryption method in use is either CBC mode (with an underlying secure block cipher) or a stream cipher (that xor the data with a random or pseudorandom pad). Thus, while we show the generic security of SSL to be broken, the current practical implementations of the protocol that use the above modes of encryption are safe.

Answer 4

There is no property of a MAC that states that information about the input should not be leaked. As such, you should encrypt the message first, then apply a MAC. This way, even if the MAC leaks information, all that is leaked is ciphertext.

Answer 5

If the MAC is encrypted - it will not leak information. MAC then encrypt typically encrypts the appended MAC.