ssh: add support for server side multi-step authentication

[drakkan]

• add ErrPartialSuccess. Authentication callbacks must return this error for multi-step authentication when a specific authentication step succeed
• add PartialSuccessMethods to ConnMetadata interface, it returns the ordered list of authentication methods that returned ErrPartialSuccess. It can be used inside callbacks to find if a multi-step authentication is done using the correct sequence and to return the authentication methods that can continue
• add NextAuthMethodsCallback, this callback is called when an authentication callback returns ErrPartialSuccess or if, after an initial partial success, an authentication step fails. It must return the list of authentications methods that can continue.

This way an application can define per-user multi-step authentication.

Fixes #17889

[gopherbot]

This PR (HEAD: 2aafde1f2d6de5ede3d7d4b45a6fed378b77f9a9) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/crypto/+/227781 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off) See the Wiki page for more info

[gopherbot]

This PR (HEAD: aed7e1740ca2f1e66517c1368ee0acd9c06b4004) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/crypto/+/227781 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off) See the Wiki page for more info

[gopherbot]

This PR (HEAD: 83446a08cde28514db3f71ff25cb08d17b84bc7f) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/crypto/+/227781 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off) See the Wiki page for more info

[tt]

There's a pretty significant security issue in the current implementation.

If PublicKeyCallback returns ErrPartialSuccess, a client will be able to authenticate by presenting any public key without proving that it possess the corresponding private key.

This is because the client can first query if a key is acceptable before it signs anything.

https://github.com/golang/crypto/blob/2aafde1f2d6de5ede3d7d4b45a6fed378b77f9a9/ssh/server.go#L577-L586

candidate.result == nil needs to be candidate.result == nil || candidate.result == ErrPartialSuccess.

[gopherbot]

This PR (HEAD: 6d02a9b2fff0b8213b4ca6cec2496cdfe30365ea) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/crypto/+/227781 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off) See the Wiki page for more info

[drakkan]

There's a pretty significant security issue in the current implementation.

If PublicKeyCallback returns ErrPartialSuccess, a client will be able to authenticate by presenting any public key without proving that it possess the corresponding private key.

This is because the client can first query if a key is acceptable before it signs anything.

https://github.com/golang/crypto/blob/2aafde1f2d6de5ede3d7d4b45a6fed378b77f9a9/ssh/server.go#L577-L586

candidate.result == nil needs to be candidate.result == nil || candidate.result == ErrPartialSuccess.

this should be fixed now, thanks for reporting

[gopherbot]

Message from Dan Peterson:

Patch Set 4: Run-TryBot+1

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/227781. After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Gobot Gobot:

Patch Set 4:

TryBots beginning. Status page: https://farmer.golang.org/try?commit=9080eac4

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/227781. After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Gobot Gobot:

Patch Set 4: TryBot-Result+1

TryBots are happy.

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/227781. After addressing review feedback, remember to publish your drafts!

[samiponkanen]

Are there plans to accept this PR, or alternatively add server side multi auth via another PR?

I am very eagerly waiting for this feature, and can offer my help in reviewing this PR, if that will speed up the acceptance process!

[gopherbot]

Message from Sami Pönkänen:

Patch Set 4:

(1 comment)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/227781. After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Go Bot:

Patch Set 4:

TryBots beginning. Status page: https://farmer.golang.org/try?commit=9080eac4

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/227781. After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Go Bot:

Patch Set 4: TryBot-Result+1

TryBots are happy.

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/227781. After addressing review feedback, remember to publish your drafts!