Reset server MAC after TransformFinalBlock on .NET FW

[scott-xu]

Call _serverMac.Initialize() after TransformFinalBlock when running on .NET Framework. This ensures the MAC is properly reset, addressing differences in cryptographic API behavior across different versions of mscorlib.dll.

Close #1829

[Rob-Hague]

I'm just guessing without having looked at the old mscorlib source, but perhaps it ought to be after the get on .Hash i.e. after the FixedTimeEquals check?

[scott-xu]

I reviewed the mscorlib source and found that .Hash is not cleared by .Initialize(). It gets cleared in Dispose().
I understand your concern and there's no harm to move .Initialize() after FixedTimeEquals check.

As a reference, the ComputeHash implementation calls .Initialize() at the very end, just before it returns.
The Initialize method name is kind of misleading.

public byte[] ComputeHash(byte[] buffer, int offset, int count)
{
	if (buffer == null)
	{
		throw new ArgumentNullException("buffer");
	}
	if (offset < 0)
	{
		throw new ArgumentOutOfRangeException("offset", Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
	}
	if (count < 0 || count > buffer.Length)
	{
		throw new ArgumentException(Environment.GetResourceString("Argument_InvalidValue"));
	}
	if (buffer.Length - count < offset)
	{
		throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
	}
	if (m_bDisposed)
	{
		throw new ObjectDisposedException(null);
	}
	HashCore(buffer, offset, count);
	HashValue = HashFinal();
	byte[] result = (byte[])HashValue.Clone();
	Initialize();
	return result;
}

[scott-xu]

there's no harm to move .Initialize() after FixedTimeEquals check.

The only minor difference is that .Initialize() will no longer execute if the FixedTimeEquals check fails. This is not a significant concern because an SshConnectionException will be thrown in that scenario.

An alternative approach is to capture the hash into a local variable, invoke .Initialize(), and then perform the FixedTimeEquals check.

Please let me know which option you prefer:

1. Move .Initialize() after the FixedTimeEquals check
2. Capture the hash → call .Initialize() → run the FixedTimeEquals check
3. Keep the existing behavior in this PR: call .Initialize() before the FixedTimeEquals check