73 lines
2.2 KiB
Dart
73 lines
2.2 KiB
Dart
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/// Whether 32 bytes are an X25519 public key a device could have made: a
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/// point of Curve25519 (not of its twist) in its prime-order subgroup, as
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/// every key made from a scalar is. Random bytes are such a point one time
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/// in sixteen, so an answer for an address nobody has that isn't one would
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/// tell it from an account's (the second reading's M1).
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library;
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final _p = (BigInt.one << 255) - BigInt.from(19);
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final _a24 = BigInt.from(121665);
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/// The prime order of the subgroup the base point generates.
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final _order =
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(BigInt.one << 252) +
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BigInt.parse('27742317777372353535851937790883648493');
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/// Whether [publicKey] (X25519's u-coordinate, little-endian) is a point
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/// of the prime-order subgroup.
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bool isX25519PublicKey(List<int> publicKey) {
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final u = _littleEndian(publicKey) % _p;
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return _onCurve(u) && _ladderIsInfinity(u, _order);
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}
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BigInt _littleEndian(List<int> bytes) {
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var value = BigInt.zero;
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for (var i = bytes.length - 1; i >= 0; i--) {
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value = (value << 8) | BigInt.from(bytes[i]);
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}
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return value;
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}
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/// Whether u³ + Au² + u is a square: the point is on the curve, not its
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/// twist.
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bool _onCurve(BigInt u) {
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final rhs = (u * u * u + BigInt.from(486662) * u * u + u) % _p;
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return rhs == BigInt.zero ||
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rhs.modPow((_p - BigInt.one) >> 1, _p) == BigInt.one;
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}
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/// Whether [k] times the point at [u] is the point at infinity (an x-only
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/// Montgomery ladder, RFC 7748's).
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bool _ladderIsInfinity(BigInt u, BigInt k) {
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var x2 = BigInt.one;
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var z2 = BigInt.zero;
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var x3 = u;
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var z3 = BigInt.one;
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var swap = 0;
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for (var t = k.bitLength - 1; t >= 0; t--) {
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final bit = ((k >> t) & BigInt.one).toInt();
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swap ^= bit;
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if (swap == 1) {
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(x2, x3) = (x3, x2);
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(z2, z3) = (z3, z2);
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}
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swap = bit;
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final a = (x2 + z2) % _p;
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final aa = a * a % _p;
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final b = (x2 - z2) % _p;
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final bb = b * b % _p;
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final e = (aa - bb) % _p;
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final c = (x3 + z3) % _p;
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final d = (x3 - z3) % _p;
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final da = d * a % _p;
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final cb = c * b % _p;
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x3 = (da + cb) * (da + cb) % _p;
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z3 = u * ((da - cb) * (da - cb) % _p) % _p;
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x2 = aa * bb % _p;
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z2 = e * ((aa + _a24 * e) % _p) % _p;
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}
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if (swap == 1) {
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z2 = z3;
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}
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return z2 % _p == BigInt.zero;
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}
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