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chore: remove internal precompile call #3

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Merged
kevaundray merged 1 commit into from
Mar 9, 2026
Merged

chore: remove internal precompile call #3

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2 changes: 1 addition & 1 deletion src/modexp/Modexp.sol
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Original file line number Diff line number Diff line change
Expand Up @@ -18,7 +18,7 @@ library Modexp {
bytes memory base,
bytes memory exponent,
bytes memory modulus
) internal view returns (bytes memory result) {
) internal pure returns (bytes memory result) {
if (modulus.length == 0) return new bytes(0);

// Check if modulus is odd (last byte has bit 0 set)
Expand Down
116 changes: 40 additions & 76 deletions src/modexp/ModexpBarrett.sol
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Expand Up @@ -12,7 +12,7 @@ library ModexpBarrett {
bytes memory base,
bytes memory exponent,
bytes memory modulus
) internal view returns (bytes memory result) {
) internal pure returns (bytes memory result) {
uint256 modLen = modulus.length;
if (modLen == 0) return new bytes(0);

Expand Down Expand Up @@ -46,10 +46,10 @@ library ModexpBarrett {
uint256 k = (modLen + 31) / 32;

uint256[] memory n = _bytesToLimbs(modulus, k);
uint256[] memory a = _reduceBase(base, modulus, k);
uint256[] memory a = _reduceBase(base, n, k);

// Barrett constant: mu = floor(2^(512k) / n), has k+1 limbs
uint256[] memory mu = _computeBarrettConstant(n, k, modulus);
uint256[] memory mu = _computeBarrettConstant(n, k);

// one = 1 as k-limb number (used as initial accumulator)
uint256[] memory r = new uint256[](k);
Expand Down Expand Up @@ -144,83 +144,31 @@ library ModexpBarrett {
}
}

// ── Precompile wrapper ────────────────────────────────────────────

function _callPrecompile(bytes memory b, bytes memory e, bytes memory m)
private view returns (bytes memory result)
{
uint256 modLen = m.length;
result = new bytes(modLen);
bytes memory input = abi.encodePacked(
uint256(b.length), uint256(e.length), uint256(modLen), b, e, m
);
assembly {
let ok := staticcall(gas(), 0x05, add(input, 0x20), mload(input), add(result, 0x20), modLen)
if iszero(ok) { revert(0, 0) }
}
}

function _reduceBase(bytes memory base, bytes memory modulus, uint256 k)
private view returns (uint256[] memory)
/// @dev Reduces base mod n via schoolbook division remainder.
function _reduceBase(bytes memory base, uint256[] memory n, uint256 k)
private pure returns (uint256[] memory)
{
return _bytesToLimbs(_callPrecompile(base, hex"01", modulus), k);
}

function _uint256ToMinBytes(uint256 val) private pure returns (bytes memory) {
if (val == 0) return hex"00";
uint256 byteLen = 0;
uint256 tmp = val;
while (tmp > 0) {
byteLen++;
tmp >>= 8;
}
bytes memory result = new bytes(byteLen);
unchecked {
for (uint256 i = 0; i < byteLen; i++) {
result[byteLen - 1 - i] = bytes1(uint8(val));
val >>= 8;
}
}
return result;
uint256 baseLen = base.length;
if (baseLen == 0) return new uint256[](k);
uint256 baseK = (baseLen + 31) / 32;
if (baseK < k) baseK = k;
uint256[] memory baseLimbs = _bytesToLimbs(base, baseK);
(, uint256[] memory rem) = _schoolbookDiv(baseLimbs, baseK, n, k);
return rem;
}

// ── Barrett constant computation ──────────────────────────────────

/// @dev Computes mu = floor(2^(512k) / n).
function _computeBarrettConstant(uint256[] memory n, uint256 k, bytes memory modulus)
private view returns (uint256[] memory)
function _computeBarrettConstant(uint256[] memory n, uint256 k)
private pure returns (uint256[] memory)
{
// r = 2^(512k) mod n via precompile
uint256 expVal = 512 * k;
bytes memory expBytes = _uint256ToMinBytes(expVal);
bytes memory base2 = hex"02";
uint256[] memory r = _bytesToLimbs(_callPrecompile(base2, expBytes, modulus), k);

// dividend = 2^(512k) - r, which is exactly divisible by n
// dividend has 2k+1 limbs: limbs 0..k-1 = two's complement of r, limb 2k = 1
// Construct 2^(512k) as a (2k+1)-limb number: all zeros except limb[2k] = 1
uint256 dLen = 2 * k + 1;
uint256[] memory dividend = new uint256[](dLen);

// Compute -r mod 2^(256k), i.e., two's complement
assembly {
let divP := add(dividend, 0x20)
let rP := add(r, 0x20)
let borrow := 0
for { let i := 0 } lt(i, k) { i := add(i, 1) } {
let ri := mload(add(rP, mul(i, 0x20)))
let d := sub(sub(0, ri), borrow)
borrow := or(gt(ri, 0), and(iszero(ri), borrow))
mstore(add(divP, mul(i, 0x20)), d)
}
for { let i := k } lt(i, mul(2, k)) { i := add(i, 1) } {
let d := sub(0, borrow)
mstore(add(divP, mul(i, 0x20)), d)
}
mstore(add(divP, mul(mul(2, k), 0x20)), sub(1, borrow))
}

// mu = dividend / n via schoolbook division
return _schoolbookDiv(dividend, dLen, n, k);
dividend[2 * k] = 1;
(uint256[] memory mu,) = _schoolbookDiv(dividend, dLen, n, k);
return mu;
}

// ── Division helpers ──────────────────────────────────────────────
Expand Down Expand Up @@ -277,15 +225,17 @@ library ModexpBarrett {
uint256 dLen,
uint256[] memory divisor,
uint256 k
) private pure returns (uint256[] memory quotient) {
) private pure returns (uint256[] memory quotient, uint256[] memory rem) {
rem = new uint256[](k);

// Find actual length of dividend (strip leading zero limbs)
uint256 m = dLen;
while (m > 0 && dividend[m - 1] == 0) {
m--;
}
if (m == 0) {
quotient = new uint256[](1);
return quotient;
return (quotient, rem);
}

// Find effective number of significant limbs in divisor
Expand All @@ -305,13 +255,15 @@ library ModexpBarrett {
(q, remainder) = _div512by256(remainder, dividend[i], d);
quotient[i] = q;
}
return quotient;
rem[0] = remainder;
return (quotient, rem);
}

// Multi-limb divisor: Knuth Algorithm D (using kEff significant limbs)
if (m < kEff) {
quotient = new uint256[](1);
return quotient;
for (uint256 i = 0; i < m; i++) rem[i] = dividend[i];
return (quotient, rem);
}
uint256 numQlimbs = m - kEff + 1;
quotient = new uint256[](numQlimbs);
Expand Down Expand Up @@ -454,7 +406,19 @@ library ModexpBarrett {
quotient[jj] = qHat;
}

return quotient;
// Extract remainder from u[0..kEff-1] and denormalize (shift right)
if (shift > 0) {
for (uint256 i = 0; i < kEff; i++) {
rem[i] = u[i] >> shift;
if (i + 1 < kEff) {
rem[i] |= u[i + 1] << (256 - shift);
}
}
} else {
for (uint256 i = 0; i < kEff; i++) {
rem[i] = u[i];
}
}
}

// ── Schoolbook multiplication ─────────────────────────────────────
Expand Down
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