Sign merges, CRUD, Wiki and Repository initialisation with gpg key (#7631)

This PR fixes #7598 by providing a configurable way of signing commits across the Gitea instance. Per repository configurability and import/generation of trusted secure keys is not provided by this PR - from a security PoV that's probably impossible to do properly. Similarly web-signing, that is asking the user to sign something, is not implemented - this could be done at a later stage however.

## Features
- [x] If commit.gpgsign is set in .gitconfig sign commits and files created through repofiles. (merges should already have been signed.)
- [x] Verify commits signed with the default gpg as valid
- [x] Signer, Committer and Author can all be different
    - [x] Allow signer to be arbitrarily different - We still require the key to have an activated email on Gitea. A more complete implementation would be to use a keyserver and mark external-or-unactivated with an "unknown" trust level icon.
- [x] Add a signing-key.gpg endpoint to get the default gpg pub key if available
    - Rather than add a fake web-flow user I've added this as an endpoint on /api/v1/signing-key.gpg
    - [x] Try to match the default key with a user on gitea - this is done at verification time
- [x] Make things configurable?
    - app.ini configuration done
    - [x] when checking commits are signed need to check if they're actually verifiable too
- [x] Add documentation

I have decided that adjusting the docker to create a default gpg key is not the correct thing to do and therefore have not implemented this.

1 files changed, 19 insertions, 0 deletions

diff --git a/docs/content/doc/advanced/config-cheat-sheet.en-us.md b/docs/content/doc/advanced/config-cheat-sheet.en-us.md
index 959607dd11..0df88c23e8 100644
--- a/docs/content/doc/advanced/config-cheat-sheet.en-us.md
+++ b/docs/content/doc/advanced/config-cheat-sheet.en-us.md
@@ -76,6 +76,25 @@ Values containing `#` or `;` must be quoted using `` ` `` or `"""`.
 
 - `LOCK_REASONS`: **Too heated,Off-topic,Resolved,Spam**: A list of reasons why a Pull Request or Issue can be locked
 
+### Repository - Signing (`repository.signing`)
+
+- `SIGNING_KEY`: **default**: \[none, KEYID, default \]: Key to sign with.
+- `SIGNING_NAME` & `SIGNING_EMAIL`: if a KEYID is provided as the `SIGNING_KEY`, use these as the Name and Email address of the signer. These should match publicized name and email address for the key.
+- `INITIAL_COMMIT`: **always**: \[never, pubkey, twofa, always\]: Sign initial commit.
+  - `never`: Never sign
+  - `pubkey`: Only sign if the user has a public key
+  - `twofa`: Only sign if the user is logged in with twofa
+  - `always`: Always sign
+  - Options other than `never` and `always` can be combined as a comma separated list.
+- `WIKI`: **never**: \[never, pubkey, twofa, always, parentsigned\]: Sign commits to wiki.
+- `CRUD_ACTIONS`: **pubkey, twofa, parentsigned**: \[never, pubkey, twofa, parentsigned, always\]: Sign CRUD actions.
+  - Options as above, with the addition of:
+  - `parentsigned`: Only sign if the parent commit is signed.
+- `MERGES`: **pubkey, twofa, basesigned, commitssigned**: \[never, pubkey, twofa, basesigned, commitssigned, always\]: Sign merges.
+  - `basesigned`: Only sign if the parent commit in the base repo is signed.
+  - `headsigned`: Only sign if the head commit in the head branch is signed.
+  - `commitssigned`: Only sign if all the commits in the head branch to the merge point are signed.
+
 ## CORS (`cors`)
 
 - `ENABLED`: **false**: enable cors headers (disabled by default)