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Add HKDF (SHA256)-Expand #128
Description
HKDF is a key derivation function
that has two parts: first "extract" and second "expand". For some encryption tasks, we need only the expand part.
I made a code for that for my project and, I guess, it would be great to add to the lib.
Future<SecretKey> expandKey({
required SecretKey secretKey,
List<int> info = const <int>[],
}) async {
// T(0)
var bytes = const <int>[];
// T(1), T(2), ...
final hashLength = hmac.hashAlgorithm.hashLengthInBytes;
final n = outputLength ~/ hashLength;
final result = Uint8List(outputLength);
for (var i = 0; i <= n; i++) {
final sink = await hmac.newMacSink(secretKey: secretKey);
sink.add(bytes);
if (info.isNotEmpty) {
sink.add(info);
}
final added = <int>[0xFF & (1 + i)];
sink.add(added);
sink.close();
final mac = await sink.mac();
bytes = mac.bytes;
final offset = i * hashLength;
if (offset + bytes.length <= result.length) {
result.setAll(offset, bytes);
} else {
result.setAll(offset, bytes.take(result.length - offset));
}
}
return SecretKey(result);
}
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