Initial commit

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

common/googauth/htop.go

@@ -0,0 +1,64 @@
+package googauth
+
+import (
+	"crypto/hmac"
+	"crypto/sha1"
+	"encoding/binary"
+	"strconv"
+)
+
+// HOTP implementation
+type HOTP struct {
+	secret []byte
+	digits int
+}
+
+// At generate code
+func (h HOTP) At(counter uint64) string {
+	counterBytes := make([]byte, 8)
+	binary.BigEndian.PutUint64(counterBytes, counter)
+	hash := hmac.New(sha1.New, h.secret)
+	hash.Write(counterBytes)
+	hs := hash.Sum(nil)
+	offset := hs[19] & 0x0f
+	binCodeBytes := make([]byte, 4)
+	binCodeBytes[0] = hs[offset] & 0x7f
+	binCodeBytes[1] = hs[offset+1] & 0xff
+	binCodeBytes[2] = hs[offset+2] & 0xff
+	binCodeBytes[3] = hs[offset+3] & 0xff
+	binCode := binary.BigEndian.Uint32(binCodeBytes)
+	mod := uint32(1)
+	for i := 0; i < h.digits; i++ {
+		mod *= 10
+	}
+	code := binCode % mod
+	codeString := strconv.FormatUint(uint64(code), 10)
+	if len(codeString) < h.digits {
+		paddingByteLength := h.digits - len(codeString)
+		paddingBytes := make([]byte, paddingByteLength)
+		for i := 0; i < paddingByteLength; i++ {
+			paddingBytes[i] = '0'
+		}
+		codeString = string(paddingBytes) + codeString
+	}
+	return codeString
+}
+
+// Verify verify OTP code
+func (h HOTP) VerifyCode(code string, counter uint64) bool {
+	realCode := h.At(counter)
+	return realCode == code
+}
+
+// Verify verify OTP code
+func (h HOTP) GenerateCode(counter uint64) string {
+	return h.At(counter)
+}
+
+// NewHOTP generate new HOTP instance
+func NewHOTP(secret []byte, digits int) (h *HOTP) {
+	h = new(HOTP)
+	h.secret = secret
+	h.digits = digits
+	return
+}