ECDSA.sol

// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

// A copy from https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2237/files

/**
 * @dev Elliptic Curve Digital Signature Algorithm (ECDSA) operations.
 *
 * These functions can be used to verify that a message was signed by the holder
 * of the private keys of a given address.
 */
library ECDSA {
  /**
   * @dev Returns the address that signed a hashed message (`hash`) with
   * `signature`. This address can then be used for verification purposes.
   *
   * The `ecrecover` EVM opcode allows for malleable (non-unique) signatures:
   * this function rejects them by requiring the `s` value to be in the lower
   * half order, and the `v` value to be either 27 or 28.
   *
   * IMPORTANT: `hash` _must_ be the result of a hash operation for the
   * verification to be secure: it is possible to craft signatures that
   * recover to arbitrary addresses for non-hashed data. A safe way to ensure
   * this is by receiving a hash of the original message (which may otherwise
   * be too long), and then calling {toEthSignedMessageHash} on it.
   */
  function recover(bytes32 hash, bytes memory signature) internal pure returns (address) {
    // Check the signature length
    if (signature.length != 65) {
      revert("ECDSA: invalid signature length");
    }

    // Divide the signature in r, s and v variables
    bytes32 r;
    bytes32 s;
    uint8 v;

    // ecrecover takes the signature parameters, and the only way to get them
    // currently is to use assembly.
    // solhint-disable-next-line no-inline-assembly
    assembly {
      r := mload(add(signature, 0x20))
      s := mload(add(signature, 0x40))
      v := byte(0, mload(add(signature, 0x60)))
    }

    return recover(hash, v, r, s);
  }

  /**
   * @dev Overload of {ECDSA-recover-bytes32-bytes-} that receives the `v`,
   * `r` and `s` signature fields separately.
   */
  function recover(
    bytes32 hash,
    uint8 v,
    bytes32 r,
    bytes32 s
  ) internal pure returns (address) {
    // EIP-2 still allows signature malleability for ecrecover(). Remove this possibility and make the signature
    // unique. Appendix F in the Ethereum Yellow paper (https://ethereum.github.io/yellowpaper/paper.pdf), defines
    // the valid range for s in (281): 0 < s < secp256k1n ÷ 2 + 1, and for v in (282): v ∈ {27, 28}. Most
    // signatures from current libraries generate a unique signature with an s-value in the lower half order.
    //
    // If your library generates malleable signatures, such as s-values in the upper range, calculate a new s-value
    // with 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141 - s1 and flip v from 27 to 28 or
    // vice versa. If your library also generates signatures with 0/1 for v instead 27/28, add 27 to v to accept
    // these malleable signatures as well.
    if (uint256(s) > 0x7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF5D576E7357A4501DDFE92F46681B20A0) {
      revert("ECDSA: invalid signature 's' value");
    }

    if (v != 27 && v != 28) {
      revert("ECDSA: invalid signature 'v' value");
    }

    // If the signature is valid (and not malleable), return the signer address
    address signer = ecrecover(hash, v, r, s);
    require(signer != address(0), "ECDSA: invalid signature");

    return signer;
  }

  /**
   * @dev Returns an Ethereum Signed Message, created from a `hash`. This
   * replicates the behavior of the
   * https://github.com/ethereum/wiki/wiki/JSON-RPC#eth_sign[`eth_sign`]
   * JSON-RPC method.
   *
   * See {recover}.
   */
  function toEthSignedMessageHash(bytes32 hash) internal pure returns (bytes32) {
    // 32 is the length in bytes of hash,
    // enforced by the type signature above
    return keccak256(abi.encodePacked("\x19Ethereum Signed Message:\n32", hash));
  }
}