private static byte[] DeriveKeyImpl(HMAC hmac, byte[] label, byte[] context, int keyLengthInBits)
{
int num = (label != null) ? label.Length : 0;
int num2 = (context != null) ? context.Length : 0;
checked
{
byte[] array = new byte[4 + num + 1 + num2 + 4];
if (num != 0)
{
Buffer.BlockCopy(label, 0, array, 4, num);
}
if (num2 != 0)
{
Buffer.BlockCopy(context, 0, array, 5 + num, num2);
}
SP800_108.WriteUInt32ToByteArrayBigEndian((uint)keyLengthInBits, array, 5 + num + num2);
int num3 = 0;
int i = keyLengthInBits / 8;
byte[] array2 = new byte[i];
uint num4 = 1u;
while (i > 0)
{
SP800_108.WriteUInt32ToByteArrayBigEndian(num4, array, 0);
byte[] array3 = hmac.ComputeHash(array);
int num5 = Math.Min(i, array3.Length);
Buffer.BlockCopy(array3, 0, array2, num3, num5);
num3 += num5;
i -= num5;
num4 += 1u;
}
return(array2);
}
}
public static CryptographicKey DeriveKey(CryptographicKey keyDerivationKey, Purpose purpose)
{
CryptographicKey result;
using (HMACSHA512 hMACSHA = CryptoAlgorithms.CreateHMACSHA512(keyDerivationKey.GetKeyMaterial()))
{
byte[] label;
byte[] context;
purpose.GetKeyDerivationParameters(out label, out context);
result = new CryptographicKey(SP800_108.DeriveKeyImpl(hMACSHA, label, context, keyDerivationKey.KeyLength));
}
return(result);
}