Assuming I am using different secrets/keys for each MAC pass, does running both MtE and EtM creates any vulnerability?

This is what I mean about doing MtEtM:

K0 ≠ K1 ≠ K2
addMAC(msg, K) = msg || MAC(msg, K)
ciphertext = addMAC(encrypt(addMAC(msg, K1), K0), K2)

If I understood correctly, EtM has the advantage of ensuring that the recipient doesn't even use the decryption key when a message is tampered. But MtE is harder to forge.

Intuitively it sounds that combining both would make the system at least a bit more secure, but I am worried about some sort of timing attack.

Also, is the use of HMACs instead of generic MACs here a potential problem?

  • 1
    $\begingroup$ What makes you think that EtM is easier to forge than MtE? $\endgroup$
    – MechMK1
    Nov 30 '20 at 12:08

If the MAC is theoretically secure, but its implementation has a side channel (like by Differential Power Analysis, maybe even timing) leaking information about the MACed message (but not the key), and the encryption is secure including it's implementation, then neither MtE nor MtEtM are safe (because the side channel leaks about the message), but EtM is secure (because the MAC is applied to ciphertext, which can safely leak).

Thus EtM can be secure when MtEtM is not. A better reason to prefer EtM to MtEtM is Occam's razor.

  • 1
    $\begingroup$ Not just Occam's razor, also performance and (depending on the exact construction) the need for multiple passes disabling the ability to do streaming encryption. EG if you MAC, then encrypt the MAC || Plaintext, then MAC again, you'll need at least 2 passes and can't start the second encryption/MAC phase until after the plaintext MAC is done. SIV (and similar modes that use a MAC of the plaintext as the IV for the encryption) have this issue, but are otherwise better than MtEtM. $\endgroup$ Nov 30 '20 at 14:24

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