cmd/age-keygen: add -y mode to convert identity file to recipients

Copied -y from ssh-keygen. Copied the INPUT as only optional argument
from cmd/age.

Fixes #122
Closes #146

cmd/age-keygen/keygen.go

@@ -9,6 +9,7 @@ package main
 import (
 	"flag"
 	"fmt"
+	"io"
 	"log"
 	"os"
 	"runtime/debug"
@@ -20,16 +21,22 @@ import (
 
 const usage = `Usage:
     age-keygen [-o OUTPUT]
+    age-keygen -y [-o OUTPUT] [INPUT]
 
 Options:
-    -o, --output OUTPUT       Write the key to the file at path OUTPUT.
+    -o, --output OUTPUT       Write the result to the file at path OUTPUT.
+    -y                        Convert an identity file to a recipients file.
 
 age-keygen generates a new standard X25519 key pair, and outputs it to
 standard output or to the OUTPUT file.
 
 If an OUTPUT file is specified, the public key is printed to standard error.
 If OUTPUT already exists, it is not overwritten.
 
+In -y mode, age-keygen reads an identity file from INPUT or from standard
+input and writes the corresponding recipient(s) to OUTPUT or to standard
+output, one per line, with no comments.
+
 Examples:
 
     $ age-keygen
@@ -38,7 +45,10 @@ Examples:
     AGE-SECRET-KEY-1N9JEPW6DWJ0ZQUDX63F5A03GX8QUW7PXDE39N8UYF82VZ9PC8UFS3M7XA9
 
     $ age-keygen -o key.txt
-    Public key: age1ql3z7hjy54pw3hyww5ayyfg7zqgvc7w3j2elw8zmrj2kg5sfn9aqmcac8p`
+    Public key: age1ql3z7hjy54pw3hyww5ayyfg7zqgvc7w3j2elw8zmrj2kg5sfn9aqmcac8p
+
+    $ age-keygen -y key.txt
+    age1ql3z7hjy54pw3hyww5ayyfg7zqgvc7w3j2elw8zmrj2kg5sfn9aqmcac8p`
 
 // Version can be set at link time to override debug.BuildInfo.Main.Version,
 // which is "(devel)" when building from within the module. See
@@ -50,17 +60,21 @@ func main() {
 	flag.Usage = func() { fmt.Fprintf(os.Stderr, "%s\n", usage) }
 
 	var (
-		versionFlag bool
-		outFlag     string
+		versionFlag, convertFlag bool
+		outFlag                  string
 	)
 
 	flag.BoolVar(&versionFlag, "version", false, "print the version")
+	flag.BoolVar(&convertFlag, "y", false, "convert identities to recipients")
 	flag.StringVar(&outFlag, "o", "", "output to `FILE` (default stdout)")
 	flag.StringVar(&outFlag, "output", "", "output to `FILE` (default stdout)")
 	flag.Parse()
-	if len(flag.Args()) != 0 {
+	if len(flag.Args()) != 0 && !convertFlag {
 		log.Fatalf("age-keygen takes no arguments")
 	}
+	if len(flag.Args()) > 1 && convertFlag {
+		log.Fatalf("Too many arguments")
+	}
 	if versionFlag {
 		if Version != "" {
 			fmt.Println(Version)
@@ -90,7 +104,21 @@ func main() {
 		}
 	}
 
-	generate(out)
+	in := os.Stdin
+	if inFile := flag.Arg(0); inFile != "" && inFile != "-" {
+		f, err := os.Open(inFile)
+		if err != nil {
+			log.Fatalf("Failed to open input file %q: %v", inFile, err)
+		}
+		defer f.Close()
+		in = f
+	}
+
+	if convertFlag {
+		convert(in, out)
+	} else {
+		generate(out)
+	}
 }
 
 func generate(out *os.File) {
@@ -107,3 +135,20 @@ func generate(out *os.File) {
 	fmt.Fprintf(out, "# public key: %s\n", k.Recipient())
 	fmt.Fprintf(out, "%s\n", k)
 }
+
+func convert(in io.Reader, out io.Writer) {
+	ids, err := age.ParseIdentities(in)
+	if err != nil {
+		log.Fatalf("Failed to parse input: %v", err)
+	}
+	if len(ids) == 0 {
+		log.Fatalf("No identities found in the input")
+	}
+	for _, id := range ids {
+		id, ok := id.(*age.X25519Identity)
+		if !ok {
+			log.Fatalf("Internal error: unexpected identity type: %T", id)
+		}
+		fmt.Fprintf(out, "%s\n", id.Recipient())
+	}
+}