Add Cross-Platform Cryptography document

[qmuntal]

This document is a port of Cross-Platform Cryptography in .NET.

This guide aims to replace our FIPS User Guide by being more user-friendly and easier to maintain. It is also a nice visual summary of what each backend supports and what needs more work.

Preview: https://github.com/microsoft/go/blob/dev/qmuntal/cpdoc/eng/doc/CrossPlatformCryptography.md

For #1377.

[Copilot]

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

Tip: If you use Visual Studio Code, you can request a review from Copilot before you push from the "Source Control" tab. Learn more

[dagood]

Is it worth being more exact? Are there more potential situations than this one example?

Suggested change

	The RSA key size is subject to the limitations of the underlying cryptographic library. For example, on Windows and when using SCOSSL, the key size should be multiple of 8.
	The RSA key size is subject to the limitations of the underlying cryptographic library:
	* On Windows or when using SCOSSL, the key size must be a multiple of 8 (in bits).

But now I wonder if this was intended to be saying something about algorithms implemented by SCOSSL vs. by the built-in OpenSSL provider...

[qmuntal]

I intentionally didn't want to enumerate a possible limitations, as can vary in time and are normally not part of the public documentation of the crypto libraries.

[dagood]

Ah, makes sense. I think the doc probably should state that variability pretty clearly, because otherwise it might be a bit confusing why more detail isn't provided. Also, it sounds like the user is meant to get a hint that they should avoid depending on exact details here.

[qmuntal]

I've rephrased the paragraph to be less confusing.

[dagood]

Nice!

[Copilot]

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

Tip: Turn on automatic Copilot reviews for this repository to get quick feedback on every pull request. Learn more