models/asymkey: Implement Tag verification

This is, in large part, a refactoring: we rename `CommitVerification` to
`ObjectVerification`, and adjust `ParseObjectWithSignature` (previously
`ParseCommitWithSignature`) to work on an object, rather than a commit.

This in turn, lets us implement `ParseTagWithSignature` on top of it, so
commit & tag signature verification will share most of the code.

Work sponsored by @glts.

Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>

models/asymkey/ssh_key_object_verification.go

@@ -0,0 +1,85 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package asymkey
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"strings"
+
+	"code.gitea.io/gitea/models/db"
+	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/log"
+
+	"github.com/42wim/sshsig"
+)
+
+// ParseObjectWithSSHSignature check if signature is good against keystore.
+func ParseObjectWithSSHSignature(ctx context.Context, c *GitObject, committer *user_model.User) *ObjectVerification {
+	// Now try to associate the signature with the committer, if present
+	if committer.ID != 0 {
+		keys, err := db.Find[PublicKey](ctx, FindPublicKeyOptions{
+			OwnerID:    committer.ID,
+			NotKeytype: KeyTypePrincipal,
+		})
+		if err != nil { // Skipping failed to get ssh keys of user
+			log.Error("ListPublicKeys: %v", err)
+			return &ObjectVerification{
+				CommittingUser: committer,
+				Verified:       false,
+				Reason:         "gpg.error.failed_retrieval_gpg_keys",
+			}
+		}
+
+		committerEmailAddresses, err := user_model.GetEmailAddresses(ctx, committer.ID)
+		if err != nil {
+			log.Error("GetEmailAddresses: %v", err)
+		}
+
+		// Add the noreply email address as verified address.
+		committerEmailAddresses = append(committerEmailAddresses, &user_model.EmailAddress{
+			IsActivated: true,
+			Email:       committer.GetPlaceholderEmail(),
+		})
+
+		activated := false
+		for _, e := range committerEmailAddresses {
+			if e.IsActivated && strings.EqualFold(e.Email, c.Committer.Email) {
+				activated = true
+				break
+			}
+		}
+
+		for _, k := range keys {
+			if k.Verified && activated {
+				commitVerification := verifySSHObjectVerification(c.Signature.Signature, c.Signature.Payload, k, committer, committer, c.Committer.Email)
+				if commitVerification != nil {
+					return commitVerification
+				}
+			}
+		}
+	}
+
+	return &ObjectVerification{
+		CommittingUser: committer,
+		Verified:       false,
+		Reason:         NoKeyFound,
+	}
+}
+
+func verifySSHObjectVerification(sig, payload string, k *PublicKey, committer, signer *user_model.User, email string) *ObjectVerification {
+	if err := sshsig.Verify(bytes.NewBuffer([]byte(payload)), []byte(sig), []byte(k.Content), "git"); err != nil {
+		return nil
+	}
+
+	return &ObjectVerification{ // Everything is ok
+		CommittingUser: committer,
+		Verified:       true,
+		Reason:         fmt.Sprintf("%s / %s", signer.Name, k.Fingerprint),
+		SigningUser:    signer,
+		SigningSSHKey:  k,
+		SigningEmail:   email,
+	}
+}