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?