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What you're describing is pretty similar to the SIV block cipher mode. It also uses a deterministic function of the message to derive the nonce for CTR encryption. Under some pretty widely accepted assumptions about HMAC-SHA256 this is a perfectly fine way of achieving deterministic authenticated encryption. It doesn't meet IND-CPA (as you pointed out) but ...


[D]oes adding HMAC to each encrypted field and validating it before decryption makes sense? No. Since you authenticate the data once, you can trust that it is the same as was originally encrypted and authenticated. That is, authenticating the ciphertext also authenticates the plaintext. The goal is to make make the app as secure as possible and ...

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