Signature Verification

Source: Verifying Signature, solidity-by-example.org [7].

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.0;

contract Yul {
    function splitSignature(bytes memory signature) public pure returns (
        bytes32 r,
        bytes32 s,
        uint8 v
    ) {
        // Signatures are 65 bytes in length. r => 32, s => 32, v => 1.
        // Although some signatures can be packed into 64 bytes.
        if (signature.length != 65) revert();

        assembly {
            r := mload(add(signature, 0x20))
            s := mload(add(signature, 0x40))
            v := byte(0, mload(add(signature, 0x60)))
        }
    }

    function verifySignature(
        address signer,
        bytes32 hash,
        bytes memory sig
    ) public pure returns (bool) {
        (bytes32 r, bytes32 s, uint8 v) = splitSignature(sig);
        address recovered = ecrecover(hash, v, r, s);

        return (recovered != address(0) && recovered == signer);
    }
}

byte(a, b) retrieves a single byte at position a in a 32-byte memory word, b. In our case, we are simply saying "Return the first byte in the 32-byte location at add(signature, 0x60)."

ecrecover(a, b, c, d) returns the address of a signer when given four arguments, a, the hash that was signed and b, c, d being the v, r and s components of the signature, respectively.