cmd/verifysig: tiny tool to verify commit signatures

Signed-off-by: Anirudh Oppiliappan <[e-mail]>

cmd/verifysig/main.go

···
+package main
+
+import (
+	"flag"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+
+	"tangled.sh/tangled.sh/core/crypto"
+)
+
+func parseCommitObject(commitData string) (string, string, error) {
+	lines := strings.Split(commitData, "\n")
+
+	var payloadLines []string
+	var signatureLines []string
+	var inSignature bool
+	var foundSignature bool
+
+	for _, line := range lines {
+		if strings.HasPrefix(line, "gpgsig ") {
+			foundSignature = true
+			inSignature = true
+			// remove 'gpgsig' prefix
+			sigLine := strings.TrimPrefix(line, "gpgsig ")
+			signatureLines = append(signatureLines, sigLine)
+			continue
+		}
+
+		if inSignature {
+			// check if this line is part of the signature (starts with space)
+			if strings.HasPrefix(line, " ") {
+				// remove the leading space and add to signature
+				sigLine := strings.TrimPrefix(line, " ")
+				signatureLines = append(signatureLines, sigLine)
+				continue
+			} else {
+				// end of signature block
+				inSignature = false
+				// this line is part of payload, so add it
+				payloadLines = append(payloadLines, line)
+			}
+		} else {
+			// regular payload line
+			payloadLines = append(payloadLines, line)
+		}
+	}
+
+	if !foundSignature {
+		return "", commitData, nil // no signature found, return empty signature and full data as payload
+	}
+
+	signature := strings.Join(signatureLines, "\n")
+	payload := strings.Join(payloadLines, "\n")
+
+	return signature, payload, nil
+}
+
+func main() {
+	var pubkeyPath string
+	flag.StringVar(&pubkeyPath, "pubkey", "", "Path to the public key file")
+	flag.Parse()
+
+	var pubKey []byte
+	var err error
+	if pubkeyPath != "" {
+		pubKey, err = os.ReadFile(pubkeyPath)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "error reading public key file: %v\n", err)
+			os.Exit(1)
+		}
+	}
+
+	input, err := io.ReadAll(os.Stdin)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "error reading from stdin: %v\n", err)
+		os.Exit(1)
+	}
+
+	commitData := string(input)
+
+	signature, payload, err := parseCommitObject(commitData)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "error parsing commit: %v\n", err)
+		os.Exit(1)
+	}
+
+	fmt.Println("signature")
+	fmt.Println(signature)
+	fmt.Println()
+	fmt.Println("payload:")
+	fmt.Println(payload)
+
+	err, ok := crypto.VerifySignature(pubKey, []byte(signature), []byte(payload))
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "%v\n", err)
+		os.Exit(1)
+	}
+	if ok {
+		fmt.Println("ok")
+	}
+}

crypto/verify.go

···
 	}
 
 	buf := bytes.NewBuffer(payload)
-	sshsig.Verify(buf, sig, pub, sshsig.HashSHA256, "git")
+	// we use sha-512 because ed25519 keys require it internally; rsa keys support
+	// multiple algorithms but sha-512 is most secure, and git's ssh signing defaults
+	// to sha-512 for all key types anyway.
+	err = sshsig.Verify(buf, sig, pub, sshsig.HashSHA512, "git")
 	return err, err == nil
 }