Missing clamping when generating x25519 KeyPair on web platform

[a-morand-tecost]

The key pair created using brower version X25519.newKeyPair() is not clamped where X25519.newKeyPairFromSeed(seed) is. This has the consequence that:

final generated = await X25519.newKeyPair();
final skBytes = await baseKeyPair.extractPrivateKeyBytes();
final recreated =  await algorithm.newKeyPairFromSeed(skBytes);

-> generated != recreated

It is possible to achieve stability by modifying browser /x25519.dart code like this:

  @override
  Future<SimpleKeyPair> newKeyPair() async {
    late web_crypto.Jwk jwk;
    try {
      final jsCryptoKey = await web_crypto.generateKeyWhenKeyPair(
          _jsAlgorithm, true.toJS, ['deriveBits'.toJS].toJS);
      jwk = await web_crypto.exportKeyWhenJwk(jsCryptoKey.privateKey);
    } catch (e) {
      final fallback = _fallback;
      if (fallback != null) {
        return fallback.newKeyPair();
      }
      throw StateError('$runtimeType.newKeyPair(...) failed: $e');
    }
    final keyPair = SimpleKeyPairData(
      Uint8List.fromList(web_crypto.base64UrlDecode(jwk.d!.toDart)),
      publicKey: SimplePublicKey(
        Uint8List.fromList(web_crypto.base64UrlDecode(jwk.x!.toDart)),
        type: KeyPairType.x25519,
      ),
      type: KeyPairType.x25519,
    );

    final seed = await keyPair.extractPrivateKeyBytes();

    return newKeyPairFromSeed(seed);
  }