static byte[] DeriveKey(IDictionary <string, object> header, int cekSizeBits, ECPublicKeyParameters externalPublicKey,
ECPrivateKeyParameters ephemeralPrvKey)
{
var z = EcdhKeyAgreementZ(externalPublicKey, ephemeralPrvKey);
var kdfGen = new ConcatenationKdfGenerator(new Sha256Digest());
byte[] algId = Encoding.ASCII.GetBytes(header["enc"].ToString());
byte[] apu = header.ContainsKey("apu") ? Base64Url.Decode((string)header["apu"]) : Arrays.Empty;
byte[] apv = header.ContainsKey("apv") ? Base64Url.Decode((string)header["apv"]) : Arrays.Empty;
byte[] kdl = CalcBeLengthArray(cekSizeBits);
var otherInfo = Arrays.Concat(PrependLength(algId), PrependLength(apu), PrependLength(apv), kdl);
//Console.Out.WriteLine($"otherInfo={VAU.ByteArrayToHexString(otherInfo)}");
kdfGen.Init(new KdfParameters(z, otherInfo));
byte[] secretKeyBytes = new byte[32];
kdfGen.GenerateBytes(secretKeyBytes, 0, secretKeyBytes.Length);
return(secretKeyBytes);
}
static byte[] PrependLength(byte[] data)
{
return(Arrays.Concat(CalcBeLengthArray(data.Length), data));
}