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45 lines
1.6 KiB
Solidity
45 lines
1.6 KiB
Solidity
// SPDX-License-Identifier: AGPL-3.0-only
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pragma solidity ^0.8.26;
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// See https://github.com/noot/schnorr-verify for implementation details
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library Schnorr {
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// secp256k1 group order
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uint256 private constant Q = 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141;
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// We fix the key to have:
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// 1) An even y-coordinate
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// 2) An x-coordinate < Q
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uint8 private constant KEY_PARITY = 27;
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// px := public key x-coordinate, where the public key has an even y-coordinate
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// message := the message signed
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// c := Schnorr signature challenge
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// s := Schnorr signature solution
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function verify(bytes32 px, bytes32 message, bytes32 c, bytes32 s)
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internal
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pure
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returns (bool)
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{
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// ecrecover = (m, v, r, s) -> key
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// We instead pass the following to obtain the nonce (not the key)
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// Then we hash it and verify it matches the challenge
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bytes32 sa = bytes32(Q - mulmod(uint256(s), uint256(px), Q));
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bytes32 ca = bytes32(Q - mulmod(uint256(c), uint256(px), Q));
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/*
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The ecrecover precompile checks `r` and `s` (`px` and `ca`) are non-zero,
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banning the two keys with zero for their x-coordinate and zero challenge.
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Each has negligible probability of occuring (assuming zero x-coordinates
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are even on-curve in the first place).
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`sa` is not checked to be non-zero yet it does not need to be. The inverse
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of it is never taken.
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*/
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address R = ecrecover(sa, KEY_PARITY, px, ca);
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// The ecrecover failed
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if (R == address(0)) return false;
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// Check the signature is correct by rebuilding the challenge
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return c == keccak256(abi.encodePacked(R, px, message));
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}
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}
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