Add HashAlgorithm Span-based APIs

[stephentoub]

Separated out of https://github.com/dotnet/corefx/issues/21281 for tracking purposes.

• Implement in System.Security.Cryptography.Primitives in corefx
• Expose from System.Security.Cryptography.Primitives contract in corefx
• Add tests to System.Security.Cryptography.Primitives tests in corefx

namespace System.Security.Cryptography
{
    public abstract class HashAlgorithm
    {
        public bool TryComputeHash(ReadOnlySpan<byte> source, Span<byte> destination, out int bytesWritten);
        protected virtual void HashCore(ReadOnlySpan<byte> source);
        protected virtual bool TryHashFinal(Span<byte> destination, out int bytesWritten);
        …
    }
}

These methods should then be overridden as-is appropriate on the implementations for:

• MD5
• SHA1
• SHA256
• SHA384
• SHA512

[KrzysztofCwalina]

Often the source bytes will not be in a single span. Maybe we should also expose these two protected members publicly?

[stephentoub]

Often the source bytes will not be in a single span. Maybe we should also expose these two protected members publicly?

I believe the intent for such a case is to use IncrementalHash. @bartonjs?

[bartonjs]

Yup. HashAlgorithm is for one-shot, IncrementalHash is for cumulative/iterative.

[KrzysztofCwalina]

OK, makes sense.

[Tornhoof]

Is Rfc2898DeriveBytes getting the Span Treatment too or is this rather dotnet/corefx#22615?