TDH2 implementation

[ramtinms]

This PR adds TDH2 implementation. TDH2 is a threshold encryption scheme, where decryption is secreted shared among n parties. Decryption requires a threshold of t+1 parties to cooperate. The scheme is based on ElGamal encryption and uses non-interactive zero-knowledge proofs (NIZK) to ensure the correctness of encryption.
See the original paper for more theoretical details:
https://www.shoup.net/papers/thresh1.pdf by Victor Shoup and Rosario Gennaro

This scheme is proven to be secure against chosen ciphertext attack
(CCA secure) under the decisional Diffie-Hellman (DDH) assumption

For this implementation, this specification is followed,
https://github.com/coinbase/cb-mpc/blob/master/docs/spec/tdh2-spec.pdf
Similar to the specification, this implementation also provides an
alternative to the original one time-pad encryption of the message.
If useAESGCM is set to true, AES-GCM (256-bit) is used instead of the one-time-pad.
We also support labels that can be used as associated data in the AEAD scheme.

[CLAassistant]

All committers have signed the CLA.

[github-actions]

🔒 Could not start CI tests due to missing safe PR label. Please contact a DEDIS maintainer.

[ramtinms]

Sorry for cold pings, but I saw you are also working on some go implementation of tdh2, thought might wanna contribute to or review this as well @pszal , @stchrysa , @kaleofduty.

[kaleofduty]

Sorry for cold pings, but I saw you are also working on some go implementation of tdh2, thought might wanna review this as well @pszal , @stchrysa , @kaleofduty.

I am afraid we likely won't have time for that, but it's always great to see more open-source cryptography implementations. Hope everything goes well for your project!

[ramtinms]

Any feedback from the maintainers?

[pierluca]

Maintainer here - thank you for the contribution, it's much appreciated.
We'll be looking into it in the coming weeks, please bear with us - we're a bit understaffed at the moment.

@AnomalRoil do you have time to look into this?

[ramtinms]

Hi there, any update or feedback on this one?