Add new output format: "signify"

This is BSD tool used to sign release, with simpler interface and
without support of old key types. It does not do encryption.

This is how signify can be used:

  $ signify -G -n -p new.pub -s new.key
  $ signify -S -s new.key -m message.txt # creates message.txt.sig
  $ signify -V -p new.pub -m message.txt # checks signature

Now first step can be replaced with:

  $ passphrase2pgp -f signify >new.combined

First two lines are the public key, and the next two are the secret key.
Note: signify(1) insists each begins with "untrusted comment: ".

Co-Authored: Christopher Wellons <[email protected]>

README.md

@@ -63,7 +63,7 @@ Use `--help` (`-h`) for a full option listing:
 ```
 Usage:
    passphrase2pgp <-u id|-l key> [-hv] [-c id] [-i pwfile] [--pinentry[=cmd]]
-       -K [-anps] [-e[n]] [-f pgp|ssh|x509] [-r n] [-t secs] [-x[spec]]
+       -K [-anps] [-e[n]] [-f pgp|ssh|x509|signify] [-r n] [-t secs] [-x[spec]]
        -S [-a] [-r n] [files...]
        -T [-r n] >doc-signed.txt <doc.txt
 Commands:
@@ -74,7 +74,7 @@ Options:
    -a, --armor               encode output in ASCII armor
    -c, --check KEYID         require last Key ID bytes to match
    -e, --protect[=ASKS]      protect private key with S2K
-   -f, --format pgp|ssh|x509 select key format [pgp]
+   -f, --format FORMAT       select key format [pgp]
    -h, --help                print this help message
    -i, --input FILE          read passphrase from file
    -l, --load FILE           load key from file instead of generating

passphrase2pgp.go

@@ -7,8 +7,10 @@ import (
 	"bytes"
 	"crypto/ed25519"
 	"crypto/sha256"
+	"crypto/sha512"
 	"crypto/x509"
 	"crypto/x509/pkix"
+	"encoding/base64"
 	"encoding/hex"
 	stdpem "encoding/pem"
 	"fmt"
@@ -40,6 +42,7 @@ const (
 	formatPGP = iota
 	formatSSH
 	formatX509
+	formatSignify
 )
 
 var version = "1.2.0"
@@ -137,7 +140,7 @@ func usage(w io.Writer) {
 	}
 	f("Usage:")
 	f(i, p, "<-u id|-l key> [-hv] [-c id] [-i pwfile] [--pinentry[=cmd]]")
-	f(b, "-K [-anps] [-e[n]] [-f pgp|ssh|x509] [-r n] [-t secs] [-x[spec]]")
+	f(b, "-K [-anps] [-e[n]] [-f pgp|ssh|x509|signify] [-r n] [-t secs] [-x[spec]]")
 	f(b, "-S [-a] [-r n] [files...]")
 	f(b, "-T [-r n] >doc-signed.txt <doc.txt")
 	f("Commands:")
@@ -148,7 +151,7 @@ func usage(w io.Writer) {
 	f(i, "-a, --armor               encode output in ASCII armor")
 	f(i, "-c, --check KEYID         require last Key ID bytes to match")
 	f(i, "-e, --protect[=ASKS]      protect private key with S2K")
-	f(i, "-f, --format pgp|ssh|x509 select key format [pgp]")
+	f(i, "-f, --format FORMAT       select key format [pgp]")
 	f(i, "-h, --help                print this help message")
 	f(i, "-i, --input FILE          read passphrase from file")
 	f(i, "-l, --load FILE           load key from file instead of generating")
@@ -270,6 +273,8 @@ func parse() *config {
 				conf.format = formatSSH
 			case "x509":
 				conf.format = formatX509
+			case "signify":
+				conf.format = formatSignify
 			default:
 				fatal("invalid format: %s", result.Optarg)
 			}
@@ -537,6 +542,8 @@ func main() {
 			ck.outputSSH(config)
 		case formatX509:
 			ck.outputX509(config)
+		case formatSignify:
+			ck.outputSignify(config)
 		}
 
 	case cmdSign:
@@ -718,6 +725,50 @@ func (k *completeKey) outputSSH(config *config) {
 	}
 }
 
+func (k *completeKey) outputSignify(config *config) {
+	key := k.key
+
+	pubkeyHash := sha512.Sum512([]byte(key.Pubkey()))
+	keynum := pubkeyHash[0:8]
+
+	salt := pubkeyHash[8:24]
+	output := bufio.NewWriter(os.Stdout)
+
+	// https://github.com/aperezdc/signify/blob/7960f78/signify.c#L62
+	output.WriteString("untrusted comment: signify public key for ")
+	output.Write(k.userid.ID)
+	output.WriteRune('\n')
+	pubkey := base64.NewEncoder(base64.StdEncoding, output)
+	pubkey.Write([]byte("Ed"))
+	pubkey.Write(keynum)
+	pubkey.Write(key.Pubkey())
+	pubkey.Close()
+	output.WriteRune('\n')
+
+	if !config.public {
+		hash := sha512.Sum512(key.Key)
+
+		// https://github.com/aperezdc/signify/blob/7960f78/signify.c#L52
+		output.WriteString("untrusted comment: signify private key for ")
+		output.Write(k.userid.ID)
+		output.WriteRune('\n')
+		privkey := base64.NewEncoder(base64.StdEncoding, output)
+		privkey.Write([]byte("Ed"))       // pkalg
+		privkey.Write([]byte("BK"))       // kdfalg
+		privkey.Write([]byte{0, 0, 0, 0}) // kdfrounds
+		privkey.Write(salt)               // salt
+		privkey.Write(hash[0:8])          // checksum
+		privkey.Write(keynum)             // keynum
+		privkey.Write(key.Key)            // seckey (64 bytes)
+		privkey.Close()
+		output.WriteRune('\n')
+	}
+
+	if err := output.Flush(); err != nil {
+		fatal("%s", err)
+	}
+}
+
 func (k *completeKey) outputX509(config *config) {
 	key := k.key
 	uid := string(k.userid.ID)