cmd/age: offer to autogenerate a secure passphrase

As suggested by @tqbf: https://twitter.com/tqbf/status/1210715916381511680

cmd/age/age.go

@@ -13,6 +13,7 @@ import (
 	"io"
 	_log "log"
 	"os"
+	"strings"
 
 	"filippo.io/age/internal/age"
 	"golang.org/x/crypto/ssh/terminal"
@@ -147,7 +148,7 @@ func main() {
 	case decryptFlag:
 		decrypt(identityFlags, in, out)
 	case passFlag:
-		pass, err := passphrasePrompt()
+		pass, err := passphrasePromptWithDefault()
 		if err != nil {
 			logFatalf("Error: %v", err)
 		}
@@ -157,13 +158,22 @@ func main() {
 	}
 }
 
-func passphrasePrompt() (string, error) {
-	fmt.Fprintf(os.Stderr, "Enter passphrase: ")
+func passphrasePromptWithDefault() (string, error) {
+	fmt.Fprintf(os.Stderr, "Enter passphrase (leave empty to autogenerate a secure one): ")
 	pass, err := readPassphrase()
 	if err != nil {
 		return "", fmt.Errorf("could not read passphrase: %v", err)
 	}
-	return string(pass), nil
+	p := string(pass)
+	if p == "" {
+		var words []string
+		for i := 0; i < 10; i++ {
+			words = append(words, randomWord())
+		}
+		p = strings.Join(words, "-")
+		fmt.Fprintf(os.Stderr, "Using the autogenerated passphrase %q.\n", p)
+	}
+	return p, nil
 }
 
 func encryptKeys(keys []string, in io.Reader, out io.Writer, armor bool) {
@@ -229,6 +239,15 @@ func decrypt(keys []string, in io.Reader, out io.Writer) {
 	}
 }
 
+func passphrasePrompt() (string, error) {
+	fmt.Fprintf(os.Stderr, "Enter passphrase: ")
+	pass, err := readPassphrase()
+	if err != nil {
+		return "", fmt.Errorf("could not read passphrase: %v", err)
+	}
+	return string(pass), nil
+}
+
 func logFatalf(format string, v ...interface{}) {
 	_log.Printf(format, v...)
 	_log.Fatalf("[ Did age not do what you expected? Could an error be more useful?" +