EC Recover is slow

[recmo]

The recover function is unnecessarily slow. On my machine it takes about 7 ms instead of 60 μs.

The cause is this line:

        let public_key = Secp256k1::verification_only()
            .recover(&message, &signature)

This calls Secp256k1::verification_only() for each recover, creating a new context and recomputing all the lookup tables each time. This is documented in secp256k1:

[...] the library uses context objects that contain precomputation tables which are created on object construction. Since this is a slow operation (10+ milliseconds, vs ~50 microseconds for typical crypto operations, on a 2.70 Ghz i7-6820HQ) the tables are optional, giving a performance boost for users who only care about signing, only care about verification, or only care about parsing [...]

So we should statically allocate the context and initialize it once:

use once_cell::sync::Lazy;
use secp256k1::{
    recovery::{RecoverableSignature, RecoveryId},
    Error, Message, Secp256k1, VerifyOnly,
};
use sha3::{Digest, Keccak256};
use web3::types::Address;

static CONTEXT: Lazy<Secp256k1<VerifyOnly>> = Lazy::new(Secp256k1::verification_only);

pub fn recover(message: &[u8], signature: &[u8], recovery_id: i32) -> Result<Address, Error> {
    // Recover public key
    let message = Message::from_slice(message)?;
    let recovery_id = RecoveryId::from_i32(recovery_id)?;
    let signature = RecoverableSignature::from_compact(signature, recovery_id)?;
    let public_key = CONTEXT.recover(&message, &signature)?;

    // Hash public key into address
    let public_key = public_key.serialize_uncompressed();
    debug_assert_eq!(public_key[0], 0x04);
    let hash = {
        let mut hasher = Keccak256::new();
        hasher.update(&public_key[1..]);
        hasher.finalize()
    };
    let address = Address::from_slice(&hash[12..]);
    Ok(address)
}

Benchmarking the above shows a 100x improvement:

web3_recover            time:   [7.0468 ms 7.1243 ms 7.2109 ms]                         
my_recover              time:   [58.773 us 63.094 us 67.377 us]                          
                        change: [-99.225% -99.186% -99.147%] (p = 0.00 < 0.05)

[recmo]

(Note that the same argument likely applies for the two usages of Secp256k1::signing_only())