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.

5 files changed, 111 insertions, 5 deletions

diff --git a/modules/git/commit.go b/modules/git/commit.go
index eb442f988d..45b943e79e 100644
--- a/modules/git/commit.go
+++ b/modules/git/commit.go
@@ -498,3 +498,11 @@ func GetFullCommitID(repoPath, shortID string) (string, error) {
 	}
 	return strings.TrimSpace(commitID), nil
 }
+
+// GetRepositoryDefaultPublicGPGKey returns the default public key for this commit
+func (c *Commit) GetRepositoryDefaultPublicGPGKey(forceUpdate bool) (*GPGSettings, error) {
+	if c.repo == nil {
+		return nil, nil
+	}
+	return c.repo.GetDefaultPublicGPGKey(forceUpdate)
+}
diff --git a/modules/git/repo.go b/modules/git/repo.go
index 1a9112132f..dd886f3a2e 100644
--- a/modules/git/repo.go
+++ b/modules/git/repo.go
@@ -32,6 +32,16 @@ type Repository struct {
 
 	gogitRepo    *gogit.Repository
 	gogitStorage *filesystem.Storage
+	gpgSettings  *GPGSettings
+}
+
+// GPGSettings represents the default GPG settings for this repository
+type GPGSettings struct {
+	Sign             bool
+	KeyID            string
+	Email            string
+	Name             string
+	PublicKeyContent string
 }
 
 const prettyLogFormat = `--pretty=format:%H`
diff --git a/modules/git/repo_gpg.go b/modules/git/repo_gpg.go
new file mode 100644
index 0000000000..b4c3f3b431
--- /dev/null
+++ b/modules/git/repo_gpg.go
@@ -0,0 +1,59 @@
+// Copyright 2015 The Gogs Authors. All rights reserved.
+// Copyright 2017 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package git
+
+import (
+	"fmt"
+	"strings"
+
+	"code.gitea.io/gitea/modules/process"
+)
+
+// LoadPublicKeyContent will load the key from gpg
+func (gpgSettings *GPGSettings) LoadPublicKeyContent() error {
+	content, stderr, err := process.GetManager().Exec(
+		"gpg -a --export",
+		"gpg", "-a", "--export", gpgSettings.KeyID)
+	if err != nil {
+		return fmt.Errorf("Unable to get default signing key: %s, %s, %v", gpgSettings.KeyID, stderr, err)
+	}
+	gpgSettings.PublicKeyContent = content
+	return nil
+}
+
+// GetDefaultPublicGPGKey will return and cache the default public GPG settings for this repository
+func (repo *Repository) GetDefaultPublicGPGKey(forceUpdate bool) (*GPGSettings, error) {
+	if repo.gpgSettings != nil && !forceUpdate {
+		return repo.gpgSettings, nil
+	}
+
+	gpgSettings := &GPGSettings{
+		Sign: true,
+	}
+
+	value, _ := NewCommand("config", "--get", "commit.gpgsign").RunInDir(repo.Path)
+	sign, valid := ParseBool(strings.TrimSpace(value))
+	if !sign || !valid {
+		gpgSettings.Sign = false
+		repo.gpgSettings = gpgSettings
+		return gpgSettings, nil
+	}
+
+	signingKey, _ := NewCommand("config", "--get", "user.signingkey").RunInDir(repo.Path)
+	gpgSettings.KeyID = strings.TrimSpace(signingKey)
+
+	defaultEmail, _ := NewCommand("config", "--get", "user.email").RunInDir(repo.Path)
+	gpgSettings.Email = strings.TrimSpace(defaultEmail)
+
+	defaultName, _ := NewCommand("config", "--get", "user.name").RunInDir(repo.Path)
+	gpgSettings.Name = strings.TrimSpace(defaultName)
+
+	if err := gpgSettings.LoadPublicKeyContent(); err != nil {
+		return nil, err
+	}
+	repo.gpgSettings = gpgSettings
+	return repo.gpgSettings, nil
+}
diff --git a/modules/git/repo_tree.go b/modules/git/repo_tree.go
index f5262ba81c..8f91f4efac 100644
--- a/modules/git/repo_tree.go
+++ b/modules/git/repo_tree.go
@@ -56,10 +56,11 @@ func (repo *Repository) GetTree(idStr string) (*Tree, error) {
 
 // CommitTreeOpts represents the possible options to CommitTree
 type CommitTreeOpts struct {
-	Parents   []string
-	Message   string
-	KeyID     string
-	NoGPGSign bool
+	Parents    []string
+	Message    string
+	KeyID      string
+	NoGPGSign  bool
+	AlwaysSign bool
 }
 
 // CommitTree creates a commit from a given tree id for the user with provided message
@@ -90,7 +91,7 @@ func (repo *Repository) CommitTree(sig *Signature, tree *Tree, opts CommitTreeOp
 	_, _ = messageBytes.WriteString(opts.Message)
 	_, _ = messageBytes.WriteString("\n")
 
-	if opts.KeyID != "" {
+	if version.Compare(binVersion, "1.7.9", ">=") && (opts.KeyID != "" || opts.AlwaysSign) {
 		cmd.AddArguments(fmt.Sprintf("-S%s", opts.KeyID))
 	}
 
diff --git a/modules/git/utils.go b/modules/git/utils.go
index 83cd21f34e..e791f16041 100644
--- a/modules/git/utils.go
+++ b/modules/git/utils.go
@@ -7,6 +7,7 @@ package git
 import (
 	"fmt"
 	"os"
+	"strconv"
 	"strings"
 	"sync"
 )
@@ -86,3 +87,30 @@ func RefEndName(refStr string) string {
 
 	return refStr
 }
+
+// ParseBool returns the boolean value represented by the string as per git's git_config_bool
+// true will be returned for the result if the string is empty, but valid will be false.
+// "true", "yes", "on" are all true, true
+// "false", "no", "off" are all false, true
+// 0 is false, true
+// Any other integer is true, true
+// Anything else will return false, false
+func ParseBool(value string) (result bool, valid bool) {
+	// Empty strings are true but invalid
+	if len(value) == 0 {
+		return true, false
+	}
+	// These are the git expected true and false values
+	if strings.EqualFold(value, "true") || strings.EqualFold(value, "yes") || strings.EqualFold(value, "on") {
+		return true, true
+	}
+	if strings.EqualFold(value, "false") || strings.EqualFold(value, "no") || strings.EqualFold(value, "off") {
+		return false, true
+	}
+	// Try a number
+	intValue, err := strconv.ParseInt(value, 10, 32)
+	if err != nil {
+		return false, false
+	}
+	return intValue != 0, true
+}