support service principal with slash

[ah-]

Hi,

we're using gokrb in vault-plugin-auth-kerberos to authenticate users, and noticed that we were getting unexpected "Matching key not found in keytab" errors when service principals contained slashes/multiple components, e.g. "HTTP/full-hostname.keytab@EXAMPLE.COM".

There is more detail in wintoncode/vault-plugin-auth-kerberos#1

It turns out that this is because the the sa is represented differently, in the keytab it's a list of components (["HTTP", "full-hostname.keytab@EXAMPLE.COM"]), in ValidateAPREQ it's a single string "HTTP/full-hostname.keytab@EXAMPLE.COM".
DecryptEncPart in https://github.com/jcmturner/gokrb5/blob/master/messages/Ticket.go#L191 currently simply converts the single string into a list containing that single string: ["HTTP/full-hostname.keytab@EXAMPLE.COM"].

Therefore when GetEncryptionKey is called, the lengths of both lists don't match and it can't find the key in the keytab.

The reason that the version in the keytab is split up is the keytab format and parsing here: https://github.com/jcmturner/gokrb5/blob/master/keytab/keytab.go#L246

The proposed solution is to simply split up the string into it's components.

All the work for this was done by @kristian-lesko in https://github.com/wintoncode/vault-plugin-auth-kerberos/pull/2/files, thanks a lot Kristian!

[ah-]

The CI failure seems to be a formatting issue affecting only go master: https://travis-ci.org/jcmturner/gokrb5/jobs/369170616

$ test -z $(gofmt -s -l $GO_FILES)
The command "test -z $(gofmt -s -l $GO_FILES)" exited with 1.

[jcmturner]

relates to #108

[jcmturner]

Thanks very much for your contribution!

I have fixed the gofmt testing. Looks like later versions of Go have a fix for some formatting.

The commit looks good. I'm going to review the code for other areas that this should be done. Once I've done this I'll create a release version.

I hope you will contribute further in the future. If so, can I ask that you raise an issue for any contributions (I created on for this) as it helps with the history and for people to find issues they may be facing. For other details on contribution guidelines please see: https://github.com/jcmturner/gokrb5/blob/master/CONTRIBUTING.md

Thanks again.

[ah-]

Fantastic, thanks for merging so quickly! gokrb5 has been really great, without it we probably wouldn't have built our vault plugin at all.

I'll create an issue first next time, agree that it helps people find these changes more easily.

[jcmturner]

It's good to hear gokrb5 is useful to people.
Release v4.1.2 now has the fix for this.