ssh: fix server to accept rsa-sha2-256 and rsa-sha2-512 signatures

[samiponkanen]

This pull request fixes serverAuthenticate() to accept "rsa-sha2-256" and "rsa-sha2-512" public key algorithm names and signature format in user auth messages. Additionally this pr adds unit tests for RSA publickey authentication using rsa-sha2-256 and rsa-sha2-512 signatures.

Fixes golang/go#46569

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[samiponkanen]

@googlebot I fixed it.

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We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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[samiponkanen]

@googlebot I fixed it.

[gopherbot]

This PR (HEAD: ab4f6d8) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/crypto/+/325089 to see it.

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[gopherbot]

This PR (HEAD: ded2006) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/crypto/+/325089 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off)
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[samiponkanen]

I noticed the unit test had a bug: incorrect public key algorithm name was used woth rsa-sha2-* signatures. Fixed it.

[Ereski]

@samiponkanen as I understand, the server would still not advertise support for SHA2 during KEX with your PR. Is that the case?

crypto/ssh/keys.go

Lines 338 to 340 in 0c34fe9

	func (r *rsaPublicKey) Type() string {
	return "ssh-rsa"
	}

[samiponkanen]

@samiponkanen as I understand, the server would still not advertise support for SHA2 during KEX with your PR. Is that the case?

You are correct, this PR does not add support for sending the server-sig-algs extension.