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.

3 files changed, 77 insertions, 8 deletions

diff --git a/routers/api/v1/api.go b/routers/api/v1/api.go
index 04ff91fbbf..f8ab9025b7 100644
--- a/routers/api/v1/api.go
+++ b/routers/api/v1/api.go
@@ -507,6 +507,7 @@ func RegisterRoutes(m *macaron.Macaron) {
 			m.Get("/swagger", misc.Swagger)
 		}
 		m.Get("/version", misc.Version)
+		m.Get("/signing-key.gpg", misc.SigningKey)
 		m.Post("/markdown", bind(api.MarkdownOption{}), misc.Markdown)
 		m.Post("/markdown/raw", misc.MarkdownRaw)
 
@@ -771,6 +772,7 @@ func RegisterRoutes(m *macaron.Macaron) {
 						m.Delete("", bind(api.DeleteFileOptions{}), repo.DeleteFile)
 					}, reqRepoWriter(models.UnitTypeCode), reqToken())
 				}, reqRepoReader(models.UnitTypeCode))
+				m.Get("/signing-key.gpg", misc.SigningKey)
 				m.Group("/topics", func() {
 					m.Combo("").Get(repo.ListTopics).
 						Put(reqToken(), reqAdmin(), bind(api.RepoTopicOptions{}), repo.UpdateTopics)
diff --git a/routers/api/v1/convert/convert.go b/routers/api/v1/convert/convert.go
index e0e7f609c7..0262051390 100644
--- a/routers/api/v1/convert/convert.go
+++ b/routers/api/v1/convert/convert.go
@@ -12,6 +12,7 @@ import (
 	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/markup"
+	"code.gitea.io/gitea/modules/structs"
 	api "code.gitea.io/gitea/modules/structs"
 	"code.gitea.io/gitea/modules/util"
 
@@ -84,17 +85,21 @@ func ToCommit(repo *models.Repository, c *git.Commit) *api.PayloadCommit {
 // ToVerification convert a git.Commit.Signature to an api.PayloadCommitVerification
 func ToVerification(c *git.Commit) *api.PayloadCommitVerification {
 	verif := models.ParseCommitWithSignature(c)
-	var signature, payload string
+	commitVerification := &api.PayloadCommitVerification{
+		Verified: verif.Verified,
+		Reason:   verif.Reason,
+	}
 	if c.Signature != nil {
-		signature = c.Signature.Signature
-		payload = c.Signature.Payload
+		commitVerification.Signature = c.Signature.Signature
+		commitVerification.Payload = c.Signature.Payload
 	}
-	return &api.PayloadCommitVerification{
-		Verified:  verif.Verified,
-		Reason:    verif.Reason,
-		Signature: signature,
-		Payload:   payload,
+	if verif.SigningUser != nil {
+		commitVerification.Signer = &structs.PayloadUser{
+			Name:  verif.SigningUser.Name,
+			Email: verif.SigningUser.Email,
+		}
 	}
+	return commitVerification
 }
 
 // ToPublicKey convert models.PublicKey to api.PublicKey
diff --git a/routers/api/v1/misc/signing.go b/routers/api/v1/misc/signing.go
new file mode 100644
index 0000000000..f5428670af
--- /dev/null
+++ b/routers/api/v1/misc/signing.go
@@ -0,0 +1,62 @@
+package misc
+
+import (
+	"fmt"
+	"net/http"
+
+	"code.gitea.io/gitea/models"
+	"code.gitea.io/gitea/modules/context"
+	"code.gitea.io/gitea/modules/log"
+)
+
+// SigningKey returns the public key of the default signing key if it exists
+func SigningKey(ctx *context.Context) {
+	// swagger:operation GET /signing-key.gpg miscellaneous getSigningKey
+	// ---
+	// summary: Get default signing-key.gpg
+	// produces:
+	//     - text/plain
+	// responses:
+	//   "200":
+	//     description: "GPG armored public key"
+	//     schema:
+	//       type: string
+
+	// swagger:operation GET /repos/{owner}/{repo}/signing-key.gpg repository repoSigningKey
+	// ---
+	// summary: Get signing-key.gpg for given repository
+	// produces:
+	//     - text/plain
+	// parameters:
+	// - name: owner
+	//   in: path
+	//   description: owner of the repo
+	//   type: string
+	//   required: true
+	// - name: repo
+	//   in: path
+	//   description: name of the repo
+	//   type: string
+	//   required: true
+	// responses:
+	//   "200":
+	//     description: "GPG armored public key"
+	//     schema:
+	//       type: string
+
+	path := ""
+	if ctx.Repo != nil && ctx.Repo.Repository != nil {
+		path = ctx.Repo.Repository.RepoPath()
+	}
+
+	content, err := models.PublicSigningKey(path)
+	if err != nil {
+		ctx.ServerError("gpg export", err)
+		return
+	}
+	_, err = ctx.Write([]byte(content))
+	if err != nil {
+		log.Error("Error writing key content %v", err)
+		ctx.Error(http.StatusInternalServerError, fmt.Sprintf("%v", err))
+	}
+}