Encryption proof

[zwilling]

Encryption Proof circuit including:

• private public keypair derived from ethereum key
• MiMC encryption circuit (uses symmetric encryption)
• ECDH symmetric key derivation from (sender's priv key and receiver's pub key == the other way around)
• put together in encryptionProof.circom to show that the sender encrypted a msg correctly so that the receiver can read it

The tests work fine and should be enough for a prototype.
The security should be reviewed because I took the MiMC circuits and ECDH code from other github projects that are not audited and battle tested. (links in the code)

[zwilling]

To be more specific about the things that should be reviewed in my opinion:

• MiMC encryption. I took it from Adds MiMC encryption and decryption modes iden3/circomlib#16 because it was the only circuit I found for encryption that was also easy to integrate. It seems to work, but I am not sure about how secure it is. An alternative is Poseidon encryption and decryption iden3/circomlib#60 based on this spec. But it was much harder to integrate in the current version of circom.
• Implementation of ECDH. The method seems to be solid (wiki page). But I do not know if the practical implementation is secure as well. It is inspired from this implementation in TypeScript and circuit.
• MiMC uses a single signal as encryption key, but ECDH outputs a 2D point on the curve. Is it secure, that I just use the first coordinate of the point for the MiMC encryption?