Braavos Account Contract v1.0.0

Class Hashes

Braavos Account - 0x00816dd0297efc55dc1e7559020a3a825e81ef734b558f03c83325d4da7e6253

Braavos Base Account - 0x013bfe114fb1cf405bfc3a7f8dbe2d91db146c17521d40dcf57e16d6b59fa8e6

Architecture

Deployment

Braavos account deployment enables account addresses to be dependent only on the Stark public key while still supporting additional initialization parameters and using the most up-to-date account implementation class hash. This is achieved by:

1. Having a base implementation class hash that rarely changes (src/presets/braavos_base_account.cairo). Account deployments should always use this class hash.
2. Send additional initialization parameters via the deployment signature instead of constructor params.
3. Additional initialization parameters are signed and verified in the base implementation (see assert_valid_deploy_base).
4. After validation, the base implementation replaces the underlying implementation to the latest one via a replace_class_syscall (src/presets/braavos_account.cairo) and the actual implementation initializer is called.

NOTE: The actual account implementation is not expected to be directly deployed and therefore its constructor panics.

Signers (src/signers/)

The Braavos account supports 3 different types of signers:

1. Stark Signer - the native signer based on the Stark friendly curve.
2. Hardware Signer - Uses the physical device secure enclave to sign - based on the secp256r1 curve.
3. Webauthn Signer - Uses the physical device Webauthn protocol (aka Passkeys) implementation to sign - also based on the secp256r1 curve with a webauthn compatible signature format.

The native Stark signer is added by default on account deployment and is usually derived from the seed phrase.

The Hardware Signer and Webauthn Signer are considered to be Strong Signers as they facilitate 2FA. When a Strong Signer is added to the account the default Stark Signer is disabled and not allowed to issue transactions. Only a Strong Signer can.

Note that an account can have both Hardware Signer and Webauthn Signer defined, in which case, any of them can sign as a 'Strong Signer' in the account.

Multisig

In addition, a multisig threshold can be enabled and support m-of-n signer threshold in order to execute a transaction. When an account have both Hardware signer and Webauthn signer defined, then both of them MUST be present in the multisig.

Deferred Removal of Strong Signers and Multisig

When there is a Strong Signer defined in the account, the Stark Signer role is reduced to removing all Strong Signers and Multisig with a time lock. The purpose of this mechanism is to allow the user to recover access to his account even if he lost access to the device containing his Strong Signer. The time-lock guarantees that the user have ample time to respond in case his seed phrase was stolen and an attacker tries to remove Strong Signer protection and take over the account.

The default time lock is defined to be 4 days.

Signature Format

The signature format is as follows: [ signer #1 sig #1, signer #2 sig, signer #3 sig, ... ]

Where each instance of a signer sig is: [ signer type, signer pub key, ...actual signature fields]

Some important details to note here:

1. We send the signer pub key since each signer is saved in the account as a single felt252 - guid. When verifying a signature, the guid is calculated from the public key and we verify that it matches one of the guids that are saved in the account. For the Hardware Signer and Webauthn Signer the guid is poseidon(secp256r1 pub key)

2. Signer type indicates in which signer list we should look for the computed guid. The applicable types are: Stark Signer: 1, Hardware Signer: 2, Webauthn Signer: 5

3. Since currently only the default Stark Signer is supported, when sending a Stark signature there is no need to send the pub key together with the signature. In this scenario there are 2 possible formats:

   • The native (r, s) format for compatibility with tooling and SDKs
   • [ signer type (==1), r, s]

4. More than one signer can be sent in the signature array to allow for m-of-n signers in Multisig mode.

Daily Withdrawal Limit (src/dwl/)

The Daily Withdrawal Limit is a feature that allows relaxtion of Strong Signer and Multisig requirements for certain types of transactions. All applicable transactions are analyzed for their value (actual token value + gas) and are accumulated per calendar day. While the accumulated amount is under a certain threshold, a weaker signer can be used to validate enabling lower fees and simpler User Experience for lower value transactions.

The Withdrawal Limit thresholds are set in USDC. For example, to set a low withdrawal limit of 100$, withdrawal limit should be set to 100 * 10**6. We use MySwap-CL's TWAP pricing to determine the value of an applicable transaction in USDC terms.

Applicable transactions are either transfer or approve transactions on the most commonly used tokens (ETH, USDC, USDT, DAI, WBTC) or a whitelisted set of protocol entrypoints. This whitelist for both tokens and contract entrypoints can be configured manually using the update_rate_config entrypoint

Two thresholds can be set as part of the daily withdrawal limit:

1. Low Limit - controls under which accumulated daily transaction value the Stark Signer can sign even if there are Strong Signers or Multisig defined in the account.
2. High Limit - Defines under which accumulated daily transaction value a Strong Signer can be used even if Multisig is defined

When a transaction is validated without utilizing the Daily Withdrawal Limit's use of a weaker signer, i.e. using Strong Signer even though Daily Withdrawal Limit allows usage of Stark Signer for that transaction then the transaction value is not accumulated towards the Withdraw Limit.

Outside Execution (src/outside_execution/)

This feature is intended to allow different protocols to submit transactions on behalf of the user, given the user had signed the appropriate transactions beforhand. This contract implements SNIP-9 version 2.

Building and Testing

Build

This repo is built using Scarb v2.5.1. After installation, run:

scarb build

Tests

Prerequisites:

1. Install Python requirements

pip install -r requirements.txt

2. Setup starknet-devnet-rs
3. Define STARKNET_DEVNET env variable to point to starknet-devnet-rs executable

To run tests:

pytest

Acknowledgements

• This project utilzes 2 separate implementations of SHA256 for pre/post regenesis support:
  • Cairo 0 implementation (as a library call) by the Cartridge team: cairo-sha256
  • Cairo implementation from the Alexandria project