totp: add method to generate n number of passwords

The GenerateN method will return list of password from
(current time - N*timeStep) until the current time.

2 files changed, 32 insertions, 7 deletions

diff --git a/cmd/totp/main.go b/cmd/totp/main.go
index f175827c..75aebc0b 100644
--- a/cmd/totp/main.go
+++ b/cmd/totp/main.go
@@ -7,6 +7,7 @@ import (
 	"crypto/sha1"
 	"crypto/sha256"
 	"crypto/sha512"
+	"encoding/base32"
 	"flag"
 	"fmt"
 	"hash"
@@ -22,8 +23,8 @@ func main() {
 	flag.Usage = usage
 
 	paramDigits := flag.Int("digits", 6, "number of digits to generated")
-	paramHash := flag.String("hash", "sha1", "hash names, valid values is sha1, sha256, sha512")
-	paramTimestep := flag.Int("timestep", 30, "time step in seconds")
+	paramHash := flag.String("alg", "sha1", "hash name, valid values is sha1, sha256, sha512")
+	paramTimestep := flag.Int("period", 30, "time step in seconds")
 	paramHelp := flag.Bool("help", false, "show command usage")
 	flag.Parse()
 
@@ -45,19 +46,25 @@ func main() {
 	}
 
 	totproto := totp.New(hashFn, *paramDigits, *paramTimestep)
-	secret := []byte(os.Args[1])
-	p0, err := totproto.Generate(secret)
+	secret, err := base32.StdEncoding.DecodeString(os.Args[1])
 	if err != nil {
 		log.Fatal(err)
 	}
 
-	fmt.Printf("%s\n", p0)
+	listOTP, err := totproto.GenerateN(secret, 3)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	for _, otp := range listOTP {
+		fmt.Printf("%s\n", otp)
+	}
 }
 
 func usage() {
 	log.Printf(`%s is command line interface to generate time-based one-time password.
 Usage:
-	%s [OPTIONS] <SECRET_KEY>
+	%s [OPTIONS] <BASE32-SECRET>
 
 Available OPTIONS:
 `, os.Args[0], os.Args[0])
diff --git a/lib/totp/totp.go b/lib/totp/totp.go
index 98b7142a..fa5fe070 100644
--- a/lib/totp/totp.go
+++ b/lib/totp/totp.go
@@ -64,6 +64,24 @@ func (p *Protocol) Generate(secret []byte) (otp string, err error) {
 }
 
 //
+// GenerateN generate n number of passwords from (current time - N*timeStep)
+// until the curent time.
+//
+func (p *Protocol) GenerateN(secret []byte, n int) (listOTP []string, err error) {
+	mac := hmac.New(p.fnHash, secret)
+	ts := time.Now().Unix()
+	for x := 0; x < n; x++ {
+		t := ts - int64(x*p.timeStep)
+		otp, err := p.generateWithTimestamp(mac, t)
+		if err != nil {
+			return nil, fmt.Errorf("GenerateN: %w", err)
+		}
+		listOTP = append(listOTP, otp)
+	}
+	return listOTP, nil
+}
+
+//
 // Verify the token based on the prover secret key.
 // It will return true if the token matched, otherwise it will return false.
 //
@@ -107,7 +125,7 @@ func (p *Protocol) generateWithTimestamp(mac hash.Hash, time int64) (
 ) {
 	steps := int64((float64(time) / float64(p.timeStep)))
 
-	msg := fmt.Sprintf("%016x", steps)
+	msg := fmt.Sprintf("%016X", steps)
 	msgb, err := hex.DecodeString(msg)
 	if err != nil {
 		return "", err