P256 algorithm support

Abstract

This EIP adds a new EIP-7932 algorithm of type 0x0 for supporting P256 signatures.

Motivation

P256 (a.k.a secp256r1) is a widely-used NIST standardized algorithm that already has a presence within the Ethereum codebase. This makes it a great algorithm to write test cases against implementations of EIP-7932.

Specification

This EIP defines a new EIP-7932 algorithmic type with the following parameters:

N = 0xffffffff00000000ffffffffffffffffbce6faada7179e84f3b9cac2fc632551

def gas_cost(signing_data: Bytes) -> Uint64:
    # This is the precompile cost from [EIP-7951](./eip-7951.md) with 3000 gas
    # subtracted (the cost of the secp256k1 precompile)
    BASE_GAS = Uint64(3900)

    # Calcuate extra overhead for keccak256 hashing
    if len(signing_data) == 32:
        return BASE_GAS
    else:
        minimum_word_size = (len(signing_data) + 31) // 32
        return BASE_GAS + Uint64(30 + (6 * minimum_word_size))

def validate(signature: Bytes) -> None | Error:
    # This function is a noop as there is no
    # exposed function defined in [EIP-7951](./eip-7951.md)

def verify(signature: Bytes, signing_data: Bytes) -> Bytes | Error:
    if len(signing_data) != 32:
        # Hash if non-standard size
        signing_data = keccak256(signing_data)

    # Ignore initial alg_type byte
    signature = signature[1:]

    (r, s, x, y) = (signature[0:32], signature[32:64], signature[64:96], signature[96:128])

    # This is similar to [EIP-2](./eip-2.md)'s malleability verification.
    assert(s <= N/2)

    # This is defined in [P256Verify Function](#p256verify-function)
    assert(P256Verify(signing_data, r, s, x, y) == Bytes("0x0000000000000000000000000000000000000000000000000000000000000001"))
        
    return x.to_bytes(32, "big") + y.to_bytes(32, "big")

P256Verify Function

The P256Verify function is the logic of the precompile defined in EIP-7951, the only exception is that this function MUST NOT charge any gas.

Rationale

Why P256?

P256 or secp256r1, is used globally but (more importantly) has an existing implementation in all execution clients. This allows easy implementation of a known-safe algorithm, which is perfect for a test algorithm.

Backwards Compatibility

No backward compatibility issues found.

Security Considerations

Needs discussion.

Copyright

Copyright and related rights waived via CC0.