[rainsford]

SIV mode is misuse resistant with regard to repeated nonce values, but that property doesn't seem to apply to the SID use-case. And if you assume the user isn't validating the decrypted value, SID has a far more catastrophic malleability problem than the zero padded AES approach.

Because SID encrypts the 64-bit ID using counter mode, not validating the SIV value allows an attacker to make specific changes to the decrypted ID (e.g. flipping individual bits) or inserting chosen ID values if the attacker ever learns the mapping between an 128-bit encrypted ID and the decrypted 64-bit value.

Even if you don't validate the zero padding in the other approach, an attacker is only able to get the system to accept random 64-bit ID values as far as I can tell. Still not great, but less catastrophic and it's not a malleability problem as I understand the term because the attacker is unable to make specific changes in the decrypted output.