Add ssh certificate support (#12281)

* Add ssh certificate support

* Add ssh certificate support to builtin ssh

* Write trusted-user-ca-keys.pem based on configuration

* Update app.example.ini

* Update templates/user/settings/keys_principal.tmpl

Co-authored-by: silverwind <me@silverwind.io>

* Remove unused locale string

* Update options/locale/locale_en-US.ini

Co-authored-by: silverwind <me@silverwind.io>

* Update options/locale/locale_en-US.ini

Co-authored-by: silverwind <me@silverwind.io>

* Update models/ssh_key.go

Co-authored-by: silverwind <me@silverwind.io>

* Add missing creation of SSH.Rootpath

* Update cheatsheet, example and locale strings

* Update models/ssh_key.go

Co-authored-by: zeripath <art27@cantab.net>

* Update models/ssh_key.go

Co-authored-by: zeripath <art27@cantab.net>

* Update models/ssh_key.go

Co-authored-by: zeripath <art27@cantab.net>

* Update models/ssh_key.go

Co-authored-by: zeripath <art27@cantab.net>

* Update models/ssh_key.go

* Optimizations based on feedback

* Validate CA keys for external sshd

* Add filename option and change default filename

Add a SSH_TRUSTED_USER_CA_KEYS_FILENAME option which default is
RUN_USER/.ssh/gitea-trusted-user-ca-keys.pem

Do not write a file when SSH_TRUSTED_USER_CA_KEYS is empty.

Add some more documentation.

* Remove unneeded principalkey functions

* Add blank line

* Apply suggestions from code review

Co-authored-by: zeripath <art27@cantab.net>

* Add SSH_AUTHORIZED_PRINCIPALS_ALLOW option

This adds a SSH_AUTHORIZED_PRINCIPALS_ALLOW which is default
email,username this means that users only can add the principals
that match their email or username.

To allow anything the admin need to set the option anything.

This allows for a safe default in gitea which protects against malicious
users using other user's prinicipals. (before that user could set it).

This commit also has some small other fixes from the last code review.

* Rewrite principal keys file on user deletion

* Use correct rewrite method

* Set correct AuthorizedPrincipalsBackup default setting

* Rewrite principalsfile when adding principals

* Add update authorized_principals option to admin dashboard

* Handle non-primary emails

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

* Add the command actually to the dashboard template

* Update models/ssh_key.go

Co-authored-by: silverwind <me@silverwind.io>

* By default do not show principal options unless there are CA keys set or they are explicitly set

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

* allow settings when enabled

* Fix typos in TrustedUserCAKeys path

* Allow every CASignatureAlgorithms algorithm

As this depends on the content of TrustedUserCAKeys we should allow all
signature algorithms as admins can choose the specific algorithm on their
signing CA

* Update models/ssh_key.go

Co-authored-by: Lauris BH <lauris@nix.lv>

* Fix linting issue

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: techknowlogick <matti@mdranta.net>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

custom/conf/app.example.ini

@@ -297,6 +297,9 @@ SSH_ROOT_PATH =
 ; Gitea will create a authorized_keys file by default when it is not using the internal ssh server
 ; If you intend to use the AuthorizedKeysCommand functionality then you should turn this off.
 SSH_CREATE_AUTHORIZED_KEYS_FILE = true
+; Gitea will create a authorized_principals file by default when it is not using the internal ssh server
+; If you intend to use the AuthorizedPrincipalsCommand functionality then you should turn this off.
+SSH_CREATE_AUTHORIZED_PRINCIPALS_FILE = true
 ; For the built-in SSH server, choose the ciphers to support for SSH connections,
 ; for system SSH this setting has no effect
 SSH_SERVER_CIPHERS = aes128-ctr, aes192-ctr, aes256-ctr, aes128-gcm@openssh.com, arcfour256, arcfour128
@@ -312,7 +315,26 @@ SSH_KEY_TEST_PATH =
 ; Path to ssh-keygen, default is 'ssh-keygen' which means the shell is responsible for finding out which one to call.
 SSH_KEYGEN_PATH = ssh-keygen
 ; Enable SSH Authorized Key Backup when rewriting all keys, default is true
-SSH_BACKUP_AUTHORIZED_KEYS = true
+SSH_AUTHORIZED_KEYS_BACKUP = true
+; Determines which principals to allow
+; - empty: if SSH_TRUSTED_USER_CA_KEYS is empty this will default to off, otherwise will default to email, username.
+; - off: Do not allow authorized principals
+; - email: the principal must match the user's email
+; - username: the principal must match the user's username
+; - anything: there will be no checking on the content of the principal
+SSH_AUTHORIZED_PRINCIPALS_ALLOW = email, username
+; Enable SSH Authorized Principals Backup when rewriting all keys, default is true
+SSH_AUTHORIZED_PRINCIPALS_BACKUP = true
+; Specifies the public keys of certificate authorities that are trusted to sign user certificates for authentication.
+; Multiple keys should be comma separated.
+; E.g."ssh-<algorithm> <key>". or "ssh-<algorithm> <key1>, ssh-<algorithm> <key2>".
+; For more information see "TrustedUserCAKeys" in the sshd config manpages.
+SSH_TRUSTED_USER_CA_KEYS =
+; Absolute path of the `TrustedUserCaKeys` file gitea will manage.
+; Default this `RUN_USER`/.ssh/gitea-trusted-user-ca-keys.pem
+; If you're running your own ssh server and you want to use the gitea managed file you'll also need to modify your
+; sshd_config to point to this file. The official docker image will automatically work without further configuration.
+SSH_TRUSTED_USER_CA_KEYS_FILENAME =
 ; Enable exposure of SSH clone URL to anonymous visitors, default is false
 SSH_EXPOSE_ANONYMOUS = false
 ; Indicate whether to check minimum key size with corresponding type