OTP routines

crypto.go

@@ -0,0 +1,57 @@
+package main
+
+import (
+	"crypto/hmac"
+	"crypto/sha1"
+	"crypto/sha256"
+	"encoding/base32"
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"hash"
+	"math"
+	"strings"
+	"time"
+)
+
+func ComputeHmac256(message string, secret string) []byte {
+	h := hmac.New(sha256.New, []byte(secret))
+	h.Write([]byte(message))
+	return h.Sum(nil)
+}
+
+func encodeSecret(secret []byte) string {
+	return strings.TrimRight(base32.StdEncoding.EncodeToString(secret), "=")
+}
+
+func GenericTotpCode(secret string, t time.Time, algo string, digits int, period int) (string, error) {
+	var mac hash.Hash
+	var format string
+
+	now := [8]byte{}
+
+	binary.BigEndian.PutUint64(now[:], uint64(math.Floor(float64(t.Unix())/float64(period))))
+	binsecret, _ := base32.StdEncoding.DecodeString(secret)
+
+	switch algo {
+	case "sha1":
+		mac = hmac.New(sha1.New, binsecret)
+	case "sha256":
+		mac = hmac.New(sha256.New, binsecret)
+	default:
+		return "", errors.New("unsupported algorithm")
+	}
+
+	switch digits {
+	case 6:
+		format = "%06d"
+	case 8:
+		format = "%08d"
+	default:
+		return "", errors.New("unsupported code size")
+	}
+
+	mac.Write(now[:])
+	sum := mac.Sum(nil)
+	return fmt.Sprintf(format, (binary.BigEndian.Uint32(sum[sum[len(sum)-1]&0xf:])&0x7fffffff)%1000000), nil
+}