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Document and test maximum deriveBits length for ECDH curves #289

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Document and test maximum deriveBits length for ECDH curves #289

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21 changes: 20 additions & 1 deletion lib/src/testing/webcrypto/ecdh.dart
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Original file line number Diff line number Diff line change
Expand Up @@ -59,13 +59,32 @@ final runner = TestRunner.asymmetric<EcdhPrivateKey, EcdhPublicKey>(
);

void main() async {
log('generate ECDH test case');
log('generate ECDH test cases');

// P-256: up to 256 bits (32 bytes)
await runner.generate(
generateKeyParams: {'curve': curveToJson(EllipticCurve.p256)},
importKeyParams: {'curve': curveToJson(EllipticCurve.p256)},
deriveParams: {},
maxDeriveLength: 32,
);

// P-384: up to 384 bits (48 bytes)
await runner.generate(
generateKeyParams: {'curve': curveToJson(EllipticCurve.p384)},
importKeyParams: {'curve': curveToJson(EllipticCurve.p384)},
deriveParams: {},
maxDeriveLength: 48,
);

// P-521: up to 528 bits (66 bytes)
await runner.generate(
generateKeyParams: {'curve': curveToJson(EllipticCurve.p521)},
importKeyParams: {'curve': curveToJson(EllipticCurve.p521)},
deriveParams: {},
maxDeriveLength: 66,
);

log('--------------------');

await runner.tests().runTests();
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9 changes: 5 additions & 4 deletions lib/src/webcrypto/webcrypto.ecdh.dart
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Expand Up @@ -206,15 +206,16 @@ final class EcdhPrivateKey {
/// two parties.
///
/// [length] specifies the length of the derived secret in bits.
/// The maximum length depends on the [EllipticCurve] used:
/// * [EllipticCurve.p256] — up to 256 bits.
/// * [EllipticCurve.p384] — up to 384 bits.
/// * [EllipticCurve.p521] — up to 528 bits.
///
/// [publicKey] is [EcdhPublicKey] from the other party's ECDH key pair.
///
/// Returns a [Uint8List] containing the derived shared secret.
///
/// {@macro EcdhPrivateKey:example}
// Note some webcrypto implementations (chrome, not firefox) supports passing
// null for length (in this primitive). However, you can always know the right
// length from the curve. Note p512 can provide up to: 528 bits!!!
//
// See: https://www.rfc-editor.org/rfc/rfc6090#section-4
// Notice that this is not uniformly distributed, see also:
// https://www.rfc-editor.org/rfc/rfc6090#appendix-B
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