Anon Proxies
New crypto for private transaction delegation.
About
The problem it solves Our project has two contributions: 1. New crypto primitive implementation: **anonymous proxy signatures** [[AN23]](https://eprint.iacr.org/2023/833). In arkworks and with WASM bindings for in-browser execution. 2. Toy applications: on-chain **company cards** and on-chain **pocket money**. In both cases, an account holder (company or parent) can delegate spending authority to a depending party (employee or child). No external observers, including the verifier, can know that delegation took place. We can also add a *private* spending policy. > In general, anonymity enables safe proxy delegation in case of targeted attacks. For instance, an Ethereum validator can designate a few other parties as backups to sign on her behalf: for instance, if Alice's machine is down, Bob can sign on Alice’s behalf after a preset timeout. Here, even if Alice is a victim of a targeted attack, Bob can stay anonymous and avoid the attack, and continue to sign for Alice until she recovers.  We wrote the **signature verifier in Noir** and, if compiled to the Aztec Virtual Machine, can be used to authorise transactions using an [account contract](https://docs.aztec.network/developers/tutorials/codealong/contract_tutorials/write_accounts_contract). Unfortunately, as explained in our Challenges, Blake2s does not work in the AVM. Therefore, we couldn't deploy the account contract we were hoping for. Instead we deployed the following proof of concept: a regular smart contract that verifies a proxy signature (but sadly using a not-so-collision-resistant hash function since Blake2 was unavailable). See the README on GitHub for more info. Things you can run: - we have runnable arkworks examples in the repo (explained in the [README](https://github.com/therealyingtong/schnorr-tokens/tree/main?tab=readme-ov-file#how-to-run-the-examples)). - a proof-of-concept Noir smart contract that you can compile and upload to the Aztec Playground. The contract verifies a proxy signature. 
How it's made
Challenges we ran into Most of our issues have been related to the Noir standard library and deploying to the Aztec testnet. - No way to interact with contracts on Aztec testnet; deployment is subsidised but we needed "fee juice" to call methods. - The Noir standard lib function for Blake2s hashing cannot be compiled to the AVM (Aztec Virtual Machine). - [Hash to field](https://github.com/noir-lang/noir/blob/d7030e8c4e05dcba0eb30adac7f0f18458b306f0/noir_stdlib/src/hash/mod.nr#L128-L137) in the Noir standard library is broken; it is easy to find collisions for it 😬.