LSBBA: ECIES Encryption Version #27
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Alternate encryption scheme that I'm considering to replace the RSA-based one.
The main potential issue I see with RSA is that it may be possible to come up with a combination of (amountOut, seed) that matches an encrypted bid, but that is different than what the user submitted. This should be very difficult, but we have no way to detect it.
In this version, we use a simplified version of the Elliptic Curve Integrated Encryption Scheme (ECIES) where the auction creator provides a public key on the AltBN128 curve. Bidders create a shared key off-chain and conceal it using the public key of the auction. To settle, the private key for the auction can be provided and the encrypted amounts out can be decrypted directly using the AltBN128 ecMul precompile for ~6,000 gas. We use a simple, hash-based key derivation function and XOR encryption, which are weak by themselves, but are likely sufficient behind the EC public key cryptography.