Fix SAML/DSig Signature Verification

[ahacker1-securesaml]

• Use authenticated signedInfoBytes, when processing digestValue.
  This approach is more secure, than using the entire unsigned Signature node to obtain the digestValue.
  Furthermore, it makes sure that the signedInfoBytes is first authenticated, before making decisions

• Compare digests at byte level rather than at string level

[ucarion]

Thanks! I know we've talked in the past about this ordering making more sense from first principles, and I have come around to agree with your perspective. I'm grateful you've even taken the time to do the legwork here.

I'm gonna merge this, but there are a few stylistic nits I have, mostly with wording and whitespace. Is it cool if I add onto your PR, and then if you're good with my additions, we merge it? Alternatively, I can pepper this PR with nits, but I don't want to add more work for you.

[ahacker1-securesaml]

Yes, I am fine if you add onto my PR.