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Document and test maximum deriveBits length for ECDH curves #249

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4 changes: 4 additions & 0 deletions lib/src/webcrypto/webcrypto.ecdh.dart
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Original file line number Diff line number Diff line change
Expand Up @@ -206,6 +206,10 @@ final class EcdhPrivateKey {
/// two parties.
///
/// [length] specifies the length of the derived secret in bits.
/// The maximum length that can be derived depends on the elliptic curve:
/// * [EllipticCurve.p256] can derive up to 256 bits.
/// * [EllipticCurve.p384] can derive up to 384 bits.
/// * [EllipticCurve.p521] can derive up to 528 bits.
/// [publicKey] is [EcdhPublicKey] from the other party's ECDH key pair.
///
/// Returns a [Uint8List] containing the derived shared secret.
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67 changes: 67 additions & 0 deletions test/crypto_subtle_test.dart
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Expand Up @@ -310,6 +310,73 @@ void main() {
);
});
});
group('ECDH deriveBits', () {
test('P-256 allows maximum deriveBits length', () async {
final aliceKeyPair = await EcdhPrivateKey.generateKey(EllipticCurve.p256);
final bobKeyPair = await EcdhPrivateKey.generateKey(EllipticCurve.p256);

final secret = await aliceKeyPair.privateKey.deriveBits(
256,
bobKeyPair.publicKey,
);

expect(secret.length, equals(32));
});

test('P-256 rejects deriveBits larger than maximum', () async {
final aliceKeyPair = await EcdhPrivateKey.generateKey(EllipticCurve.p256);
final bobKeyPair = await EcdhPrivateKey.generateKey(EllipticCurve.p256);

expect(
aliceKeyPair.privateKey.deriveBits(257, bobKeyPair.publicKey),
throwsA(anyOf(isA<subtle.JSDomException>(), isA<Error>())),
);
});

test('P-384 allows maximum deriveBits length', () async {
final aliceKeyPair = await EcdhPrivateKey.generateKey(EllipticCurve.p384);
final bobKeyPair = await EcdhPrivateKey.generateKey(EllipticCurve.p384);

final secret = await aliceKeyPair.privateKey.deriveBits(
384,
bobKeyPair.publicKey,
);

expect(secret.length, equals(48));
});

test('P-384 rejects deriveBits larger than maximum', () async {
final aliceKeyPair = await EcdhPrivateKey.generateKey(EllipticCurve.p384);
final bobKeyPair = await EcdhPrivateKey.generateKey(EllipticCurve.p384);

expect(
aliceKeyPair.privateKey.deriveBits(385, bobKeyPair.publicKey),
throwsA(anyOf(isA<subtle.JSDomException>(), isA<Error>())),
);
});

test('P-521 allows maximum deriveBits length', () async {
final aliceKeyPair = await EcdhPrivateKey.generateKey(EllipticCurve.p521);
final bobKeyPair = await EcdhPrivateKey.generateKey(EllipticCurve.p521);

final secret = await aliceKeyPair.privateKey.deriveBits(
528,
bobKeyPair.publicKey,
);

expect(secret.length, equals(66));
});

test('P-521 rejects deriveBits larger than maximum', () async {
final aliceKeyPair = await EcdhPrivateKey.generateKey(EllipticCurve.p521);
final bobKeyPair = await EcdhPrivateKey.generateKey(EllipticCurve.p521);

expect(
aliceKeyPair.privateKey.deriveBits(529, bobKeyPair.publicKey),
throwsA(anyOf(isA<subtle.JSDomException>(), isA<Error>())),
);
});
});
}

extension on JSArray<JSString> {
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