cmd/age: confirm encryption passphrase

Fixes #39

cmd/age/age.go

@@ -148,7 +148,7 @@ func main() {
 	case decryptFlag:
 		decrypt(identityFlags, in, out)
 	case passFlag:
-		pass, err := passphrasePromptWithDefault()
+		pass, err := passphrasePromptForEncryption()
 		if err != nil {
 			logFatalf("Error: %v", err)
 		}
@@ -158,7 +158,7 @@ func main() {
 	}
 }
 
-func passphrasePromptWithDefault() (string, error) {
+func passphrasePromptForEncryption() (string, error) {
 	fmt.Fprintf(os.Stderr, "Enter passphrase (leave empty to autogenerate a secure one): ")
 	pass, err := readPassphrase()
 	if err != nil {
@@ -172,6 +172,15 @@ func passphrasePromptWithDefault() (string, error) {
 		}
 		p = strings.Join(words, "-")
 		fmt.Fprintf(os.Stderr, "Using the autogenerated passphrase %q.\n", p)
+	} else {
+		fmt.Fprintf(os.Stderr, "Confirm passphrase: ")
+		confirm, err := readPassphrase()
+		if err != nil {
+			return "", fmt.Errorf("could not read passphrase: %v", err)
+		}
+		if string(confirm) != p {
+			return "", fmt.Errorf("passphrases didn't match")
+		}
 	}
 	return p, nil
 }