Without this patch, the setting SSH.StartBuiltinServer decides whether
the native (Go) implementation is used rather than calling 'ssh-keygen'.
It's possible for 'using ssh-keygen' and 'using the built-in server' to
be independent.
In fact, the gitea rootless container doesn't ship ssh-keygen and can be
configured to use the host's SSH server - which will cause the public
key parsing mechanism to break.

This commit changes the decision to be based on SSH.KeygenPath instead.
Any existing configurations with a custom KeygenPath set will continue
to function. The new default value of '' selects the native version. The
downside of this approach is that anyone who has relying on plain
'ssh-keygen' to have special properties will now be using the native
version instead.
I assume the exec-variant is only there because /x/crypto/ssh didn't
support ssh-ed25519 until 2016. I don't see any other reason for using
it so it might be an acceptable risk.

Fixes #23363

EDIT: this message was garbled when I tried to get the commit
description back in.. Trying to reconstruct it:

## :warning: BREAKING :warning: Users who don't have SSH.KeygenPath
explicitly set and rely on the ssh-keygen binary need to set
SSH.KeygenPath to 'ssh-keygen' in order to be able to continue using it
for public key parsing.

There was something else but I can't remember at the moment.

EDIT2: It was about `make test` and `make lint`. Can't get them to run.
To reproduce the issue, I installed `golang` in `docker.io/node:16` and
got:
```
...
go: mvdan.cc/xurls/v2@v2.4.0: unknown revision mvdan.cc/xurls/v2.4.0
go: gotest.tools/v3@v3.4.0: unknown revision gotest.tools/v3.4.0
...
go: gotest.tools/v3@v3.0.3: unknown revision gotest.tools/v3.0.3
...
go: error loading module requirements
```

Signed-off-by: Leon M. Busch-George <leon@georgemail.eu>

This is useful in case you want to use them for signature verification.

A notice is added to avoid confusion.

The error reported when a user passes a private ssh key as their ssh
public key is not very nice.

This PR improves this slightly.

Ref #22693

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: delvh <dev.lh@web.de>

Related to
https://github.com/go-gitea/gitea/pull/22262#discussion_r1059010774

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lauris BH <lauris@nix.lv>

Change all license headers to comply with REUSE specification.

Fix #16132

Co-authored-by: flynnnnnnnnnn <flynnnnnnnnnn@github>
Co-authored-by: John Olheiser <john.olheiser@gmail.com>

Found using
`find . -type f -name '*.go' -print -exec vim {} -c
':%s/fmt\.Errorf(\(.*\)%v\(.*\)err/fmt.Errorf(\1%w\2err/g' -c ':wq' \;`

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

Found via `codespell -q 3 -S ./options/locale,./vendor -L ba,pullrequest,pullrequests,readby,te,unknwon`

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

* Move keys to models/keys

* Rename models/keys -> models/asymkey

* change the missed package name

* Fix package alias

* Fix test

* Fix docs

* Fix test

* Fix test

* merge

* go build / format tools
* re-format imports

The io/ioutil package has been deprecated as of Go 1.16, see
https://golang.org/doc/go1.16#ioutil. This commit replaces the existing
io/ioutil functions with their new definitions in io and os packages.

Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

`models` does far too much. In particular it handles all `UserSignin`.

It shouldn't be responsible for calling LDAP, SMTP or PAM for signing in.

Therefore we should move this code out of `models`.

This code has to depend on `models` - therefore it belongs in `services`.

There is a package in `services` called `auth` and clearly this functionality belongs in there.

Plan:

- [x] Change `auth.Auth` to `auth.Method` - as they represent methods of authentication.
- [x] Move `models.UserSignIn` into `auth`
- [x] Move `models.ExternalUserLogin`
- [x] Move most of the `LoginVia*` methods to `auth` or subpackages
- [x] Move Resynchronize functionality to `auth`
  - Involved some restructuring of `models/ssh_key.go` to reduce the size of this massive file and simplify its files.
- [x] Move the rest of the LDAP functionality in to the ldap subpackage
- [x] Re-factor the login sources to express an interfaces `auth.Source`?
  - I've done this through some smaller interfaces Authenticator and Synchronizable - which would allow us to extend things in future
- [x] Now LDAP is out of models - need to think about modules/auth/ldap and I think all of that functionality might just be moveable
- [x] Similarly a lot Oauth2 functionality need not be in models too and should be moved to services/auth/source/oauth2
  - [x] modules/auth/oauth2/oauth2.go uses xorm... This is naughty - probably need to move this into models.
  - [x] models/oauth2.go - mostly should be in modules/auth/oauth2 or services/auth/source/oauth2 
- [x] More simplifications of login_source.go may need to be done
- Allow wiring in of notify registration -  *this can now easily be done - but I think we should do it in another PR*  - see #16178 
- More refactors...?
  - OpenID should probably become an auth Method but I think that can be left for another PR
  - Methods should also probably be cleaned up  - again another PR I think.
  - SSPI still needs more refactors.* Rename auth.Auth auth.Method
* Restructure ssh_key.go

- move functions from models/user.go that relate to ssh_key to ssh_key
- split ssh_key.go to try create clearer function domains for allow for
future refactors here.

Signed-off-by: Andrew Thornton <art27@cantab.net>