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We need a system to compare cipher-texts for equality, without (the checking system) having access to the plaintext.

We found a library that uses AES in CTR mode, with a single key, and an IV based on the SHA1 has of the plaintext.

This question is closely related to another question here on crypto.SE, but in our case the IV is derived by calculating a SHA1 has of the plaintext.

The plaintext in question are key value pairs, where the key is a short textual identifier and the value is a key. The total plaintext size is around 1k in bytes, and, additionally, but separately, SSH key files.

Would there by be any implications for the secrecy of the encrypted plaintext?

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    $\begingroup$ Note what you want is: Miss-use resistant (authenticated) encryption. Modern examples being AES-SIV and AES-GCM-SIV, they essentially do what you want, but also add authentication on-top and formal analysis that at most equality of plaintexts is leaked (only if nonces are re-used). $\endgroup$ – SEJPM♦ Feb 1 '18 at 13:55
  • $\begingroup$ I think that the hashing approach also works. But personally I don't see any drawback against using SIV or GCM-SIV compared to that approach. $\endgroup$ – Maarten Bodewes♦ Feb 1 '18 at 14:26
  • $\begingroup$ If the checking system should only have access to the IV and not to the key, how does it make sure that it's got the correct IV; how does it protect against tampering with this equality? $\endgroup$ – Ruben De Smet Feb 1 '18 at 14:49
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    $\begingroup$ @RubenDeSmet, the checking system would only have access to the ciphertext. $\endgroup$ – Jacco Feb 1 '18 at 14:50
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In the context, the IV is part of the ciphertext (as usual with AES in CTR mode). That part of the ciphertext leaks information about the plaintext including for one who does not know the AES key, which is bad in theory at least. This is because the (SHA-1) hash of the plaintext (or part of it) is the IV. In particular, an adversary can use a ciphertext to comfort a guess of the plaintext, by hashing the guess and checking if (the appropriate part of) that matches the IV.

One academic solution to the problem is a MAC of the plaintext, such as HMAC-SHA-256, with a secret key. This gives a short deterministic digest of the plaintext that does not leak any information about plaintext to adversaries not holding the key (beyond equality of two plaintexts, of course). That digest could be added as a prefix or suffix of the ciphertext obtained by AES-CTR or some other mode (authenticated encryption is a nice-to-have).

Using the same key for the encryption (authenticated or not) and the MAC is poor in theory, and could be disastrous with an AES-based MAC. However, with HMAC-SHA-256, the only foreseeable drawback would be that a key-recovery attack on HMAC (e.g. by some side-channel) would leak the encryption key; or vice versa.

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    $\begingroup$ Yeah, the hashing idea is bad, I don't know why I didn't see that immediately. That said, you might want to consider a SIV mode as well, in that case the authentication tag is also dependent on the key, and the issue of the hash leaking information should ot be present. $\endgroup$ – Maarten Bodewes♦ Feb 5 '18 at 10:47

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