use flags --output and --raw-output

Now logic for output looks like:
password from stdin (stays as was)
keys: private to stdout OR private to file (via -o flag) and public to stdout
stderr for errors and password prompt

README.md

@@ -7,3 +7,8 @@ Strong password highly recomended
 ## Password selection
 To keep entropy at at the same level of $\geq 2^{256}$ bits, as in private key of curve25519 (which is used in age encryption), you should use long password.
 Exact amount of required characters can be calculated by formula: $\lceil 256 / log_2(Nchars) \rceil$
+
+
+## TODO
+[ ] piped/terminal output as raw/verbose
+[ ] handle broken pipe signal since program will so much depend on pipes

cmd/age-gen-passphrase/main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"errors"
+	"flag"
 	"fmt"
 	"io"
 	"log"
@@ -21,30 +22,69 @@ type X25519Identity struct {
 	secretKey, ourPublicKey []byte
 }
 
+const usage = `Usage:
+    age-gen-passphrase [-o OUTPUT] [--raw-input]
+
+Options:
+    -o, --output OUTPUT       Write the result to the file at path OUTPUT.
+    --raw-output              Print stripped keys (without additional text or comments)
+
+Everything is similar to age-keygen`
+
 func main() {
+	log.SetFlags(0)
+	flag.Usage = func() { fmt.Fprintf(os.Stderr, "%s\n", usage) }
+
+	var (
+		rawOutput  bool
+		outputFile string
+	)
+
+	flag.BoolVar(&rawOutput, "raw-output", false, "Print stripped keys (without additional text or comments)")
+	flag.StringVar(&outputFile, "o", "", "Write the result to the file at path OUTPUT")
+	flag.StringVar(&outputFile, "output", "", "Write the result to the file at path OUTPUT")
+	flag.Parse()
+
 	passbytes, err := getPasswordBytes()
 	if err != nil {
 		errorf("Failed to get password, error: %s\n", err)
 	}
 
 	sum := sha256.Sum256(passbytes)
-	fmt.Printf("Password hash: %x\n", sum)
 
 	k, err := newX25519IdentityFromScalar(sum[:])
 	if err != nil {
 		errorf("internal error: %v", err)
 	}
 
-	fmt.Printf("Public key: %s\n", k.Recipient())
+	// if user is not seeing private keyfile, which also contains public key,
+	// also duplicate public key it to stderr,
+	// but if user sees public key via stdout, no need for duplication
+	if outputFile != "" {
+		if !rawOutput {
+			fmt.Printf("Public key: %s\n", k.Recipient())
+		} else {
+			fmt.Printf("%s", k.Recipient())
+		}
+	}
 
-	fmt.Printf("# created: %s\n", time.Now().Format(time.RFC3339))
-	fmt.Printf("# public key: %s\n", k.Recipient())
-	fmt.Printf("%s\n", k)
+	output := os.Stdout
+	if outputFile != "" {
+		output, err = os.Create(outputFile)
+		if err != nil {
+			errorf("failed to create output file, error: %s", err)
+		}
+	}
+
+	err = writeSecretKey(output, k, !rawOutput)
+	if err != nil {
+		fmt.Printf("Failed to write secret key to file, error: %s\n", err)
+	}
 }
 
 func getPasswordBytes() ([]byte, error) {
 	if term.IsTerminal(int(os.Stdin.Fd())) {
-		fmt.Print("Enter password: ")
+		fmt.Fprintf(os.Stderr, "Enter password: ")
 		passbytes, err := term.ReadPassword(int(os.Stdin.Fd()))
 		fmt.Println()
 		return passbytes, err
@@ -53,6 +93,28 @@ func getPasswordBytes() ([]byte, error) {
 	}
 }
 
+func writeSecretKey(f *os.File, key *age.X25519Identity, verbose bool) error {
+	var err error
+
+	if verbose {
+		_, err = fmt.Fprintf(f, "# created: %s\n", time.Now().Format(time.RFC3339))
+		if err != nil {
+			return err
+		}
+
+		_, err = fmt.Fprintf(f, "# public key: %s\n", key.Recipient())
+		if err != nil {
+			return err
+		}
+
+		_, err = fmt.Fprintf(f, "%s\n", key)
+	} else {
+		_, err = fmt.Fprintf(f, "%s", key)
+	}
+
+	return err
+}
+
 // almost a copy of private function in age/x25519.go
 func newX25519IdentityFromScalar(secretKey []byte) (*age.X25519Identity, error) {
 	if len(secretKey) != curve25519.ScalarSize {