[Cryptography] Elgamal Variant

[Nathaniel McCallum]

Hi everyone!

I'd like some review on a small variant of Elgamal (attached).
Encryption should just be standard Elgamal (no modifications). During
the decryption step, the only modification to Elgamal is using an
ephemeral key (X) to provide PFS (even from the server).

On a practical level, should a and b be accompanied by ZKPs? Besides
confirming group membership of all transferred items (and possibly
validating ZKPs), what other defensive checks should I perform?

Thanks!

Nathaniel

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