Separate OTP generation into a library.

No functional changes are intended; the main package now imports the library
and uses it, but the implementation is unchanged.

Specific highlights:

- Change the names of the functions to avert stutter following the advice of
  Effective Go: https://golang.org/doc/effective_go.html#package-names

- Reorganize the helpers in main so control flow is easier to follow.

- Add documentation comments.

gauth.go

@@ -4,72 +4,20 @@ import (
 	"bytes"
 	"crypto/aes"
 	"crypto/cipher"
-	"crypto/hmac"
-	"crypto/sha1"
 	"crypto/sha256"
-	"encoding/base32"
 	"encoding/csv"
 	"fmt"
 	"io/ioutil"
 	"log"
-	"math/big"
 	"os/user"
 	"path"
 	"strings"
 	"syscall"
-	"time"
 
+	"github.com/pcarrier/gauth/gauth"
 	"golang.org/x/crypto/ssh/terminal"
 )
 
-func TimeStamp() (int64, int) {
-	time := time.Now().Unix()
-	return time / 30, int(time % 30)
-}
-
-func normalizeSecret(sec string) string {
-	noPadding := strings.ToUpper(strings.Replace(sec, " ", "", -1))
-	padLength := 8 - (len(noPadding) % 8)
-	if padLength < 8 {
-		return noPadding + strings.Repeat("=", padLength)
-	}
-	return noPadding
-}
-
-func AuthCode(sec string, ts int64) (string, error) {
-	key, err := base32.StdEncoding.DecodeString(sec)
-	if err != nil {
-		return "", err
-	}
-	enc := hmac.New(sha1.New, key)
-	msg := make([]byte, 8, 8)
-	msg[0] = (byte)(ts >> (7 * 8) & 0xff)
-	msg[1] = (byte)(ts >> (6 * 8) & 0xff)
-	msg[2] = (byte)(ts >> (5 * 8) & 0xff)
-	msg[3] = (byte)(ts >> (4 * 8) & 0xff)
-	msg[4] = (byte)(ts >> (3 * 8) & 0xff)
-	msg[5] = (byte)(ts >> (2 * 8) & 0xff)
-	msg[6] = (byte)(ts >> (1 * 8) & 0xff)
-	msg[7] = (byte)(ts >> (0 * 8) & 0xff)
-	if _, err := enc.Write(msg); err != nil {
-		return "", err
-	}
-	hash := enc.Sum(nil)
-	offset := hash[19] & 0x0f
-	trunc := hash[offset : offset+4]
-	trunc[0] &= 0x7F
-	res := new(big.Int).Mod(new(big.Int).SetBytes(trunc), big.NewInt(1000000))
-	return fmt.Sprintf("%06d", res), nil
-}
-
-func authCodeOrDie(sec string, ts int64) string {
-	str, e := AuthCode(sec, ts)
-	if e != nil {
-		log.Fatal(e)
-	}
-	return str
-}
-
 func main() {
 	user, e := user.Current()
 	if e != nil {
@@ -83,7 +31,7 @@ func main() {
 	}
 
 	// Support for 'openssl enc -aes-128-cbc -md sha256 -pass pass:'
-	if bytes.Compare(cfgContent[:8], []byte{0x53, 0x61, 0x6c, 0x74, 0x65, 0x64, 0x5f, 0x5f}) == 0 {
+	if bytes.HasPrefix(cfgContent, []byte("Salted__")) {
 		fmt.Printf("Encryption password: ")
 		passwd, e := terminal.ReadPassword(int(syscall.Stdin))
 		fmt.Printf("\n")
@@ -122,7 +70,7 @@ func main() {
 		log.Fatal(e)
 	}
 
-	currentTS, progress := TimeStamp()
+	currentTS, progress := gauth.IndexNow()
 	prevTS := currentTS - 1
 	nextTS := currentTS + 1
 
@@ -150,3 +98,24 @@ func main() {
 func leftPad(s string, padStr string, pLen int) string {
 	return strings.Repeat(padStr, pLen) + s
 }
+
+// normalizeSecret cleans up whitespace and adds any missing padding to sec to
+// use it as an OTP seed.
+func normalizeSecret(sec string) string {
+	noPadding := strings.ToUpper(strings.Replace(sec, " ", "", -1))
+	padLength := 8 - (len(noPadding) % 8)
+	if padLength < 8 {
+		return noPadding + strings.Repeat("=", padLength)
+	}
+	return noPadding
+}
+
+// authCodeOrDie returns a code for the specified parameters, or aborts if an
+// error occurred while generating the code.
+func authCodeOrDie(sec string, ts int64) string {
+	str, e := gauth.Code(sec, ts)
+	if e != nil {
+		log.Fatal(e)
+	}
+	return str
+}

gauth/gauth.go

@@ -0,0 +1,48 @@
+// Package gauth implements the time-based OTP generation scheme used by Google
+// Authenticator.
+package gauth
+
+import (
+	"crypto/hmac"
+	"crypto/sha1"
+	"encoding/base32"
+	"fmt"
+	"math/big"
+	"time"
+)
+
+// IndexNow returns the current 30-second time slice index, and the number of
+// seconds remaining until it ends.
+func IndexNow() (int64, int) {
+	time := time.Now().Unix()
+	return time / 30, int(time % 30)
+}
+
+// Code returns the OTP code for the given secret at the specified time slice
+// index. It will report an error if the secret is not valid Base32 or if HMAC
+// generation fails.
+func Code(sec string, ts int64) (string, error) {
+	key, err := base32.StdEncoding.DecodeString(sec)
+	if err != nil {
+		return "", err
+	}
+	enc := hmac.New(sha1.New, key)
+	msg := make([]byte, 8)
+	msg[0] = (byte)(ts >> (7 * 8) & 0xff)
+	msg[1] = (byte)(ts >> (6 * 8) & 0xff)
+	msg[2] = (byte)(ts >> (5 * 8) & 0xff)
+	msg[3] = (byte)(ts >> (4 * 8) & 0xff)
+	msg[4] = (byte)(ts >> (3 * 8) & 0xff)
+	msg[5] = (byte)(ts >> (2 * 8) & 0xff)
+	msg[6] = (byte)(ts >> (1 * 8) & 0xff)
+	msg[7] = (byte)(ts >> (0 * 8) & 0xff)
+	if _, err := enc.Write(msg); err != nil {
+		return "", err
+	}
+	hash := enc.Sum(nil)
+	offset := hash[19] & 0x0f
+	trunc := hash[offset : offset+4]
+	trunc[0] &= 0x7F
+	res := new(big.Int).Mod(new(big.Int).SetBytes(trunc), big.NewInt(1000000))
+	return fmt.Sprintf("%06d", res), nil
+}