public static void DeriveKey(byte[] password, ReadOnlySpan <byte> salt, Sha2 prf, uint iterationCount, Span <byte> destination)
{
Debug.Assert(password != null);
Debug.Assert(salt != null);
Debug.Assert(destination.Length > 0);
int numBytesWritten = 0;
int numBytesRemaining = destination.Length;
int saltWithBlockLength = salt.Length + sizeof(uint);
byte[]? saltArray = null;
byte[]? arrayToReturn = null;
try
{
Span <byte> saltWithBlockIndex = saltWithBlockLength > SaltSizeThreshold
? (saltArray = ArrayPool <byte> .Shared.Rent(saltWithBlockLength))
: stackalloc byte[SaltSizeThreshold];
saltWithBlockIndex = saltWithBlockIndex.Slice(0, saltWithBlockLength);
salt.CopyTo(saltWithBlockIndex);
Span <byte> hmacKey = stackalloc byte[Sha2.BlockSizeStackallocThreshold * 2];
var hashAlgorithm = new Hmac(prf, password, hmacKey.Slice(0, prf.BlockSize * 2));
int wSize = prf.GetWorkingSetSize(int.MaxValue);
int blockSize = hashAlgorithm.BlockSize;
Span <byte> W = wSize > Constants.MaxStackallocBytes
? (arrayToReturn = ArrayPool <byte> .Shared.Rent(wSize))
: stackalloc byte[Constants.MaxStackallocBytes];
W = W.Slice(0, wSize);
Span <byte> currentBlock = stackalloc byte[Sha2.HashSizeStackallocThreshold].Slice(0, hashAlgorithm.HashSize);
Span <byte> iterationBlock = stackalloc byte[Sha2.HashSizeStackallocThreshold].Slice(0, hashAlgorithm.HashSize);
Span <byte> blockIndexDestination = saltWithBlockIndex.Slice(saltWithBlockIndex.Length - sizeof(uint));
for (uint blockIndex = 1; numBytesRemaining > 0; blockIndex++)
{
BinaryPrimitives.WriteUInt32BigEndian(blockIndexDestination, blockIndex);
hashAlgorithm.ComputeHash(saltWithBlockIndex, currentBlock, W); // U_1
currentBlock.CopyTo(iterationBlock);
for (int iter = 1; iter < iterationCount; iter++)
{
hashAlgorithm.ComputeHash(currentBlock, currentBlock, W);
Xor(src: currentBlock, dest: iterationBlock);
}
int numBytesToCopy = Math.Min(numBytesRemaining, iterationBlock.Length);
iterationBlock.Slice(0, numBytesToCopy).CopyTo(destination.Slice(numBytesWritten));
numBytesWritten += numBytesToCopy;
numBytesRemaining -= numBytesToCopy;
}
}
finally
{
if (saltArray != null)
{
ArrayPool <byte> .Shared.Return(saltArray);
}
if (arrayToReturn != null)
{
ArrayPool <byte> .Shared.Return(arrayToReturn);
}
}
}