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| func getHOTPToken(secret string, interval int64) string { | |
| //Converts secret to base32 Encoding. Base32 encoding desires a 32-character | |
| //subset of the twenty-six letters A–Z and ten digits 0–9 | |
| key, err := base32.StdEncoding.DecodeString(strings.ToUpper(secret)) | |
| check(err) | |
| bs := make([]byte, 8) | |
| binary.BigEndian.PutUint64(bs, uint64(interval)) | |
| //Signing the value using HMAC-SHA1 Algorithm | |
| hash := hmac.New(sha1.New, key) | |
| hash.Write(bs) | |
| h := hash.Sum(nil) | |
| // We're going to use a subset of the generated hash. | |
| // Using the last nibble (half-byte) to choose the index to start from. | |
| // This number is always appropriate as it's maximum decimal 15, the hash will | |
| // have the maximum index 19 (20 bytes of SHA1) and we need 4 bytes. | |
| o := (h[19] & 15) | |
| var header uint32 | |
| //Get 32 bit chunk from hash starting at the o | |
| r := bytes.NewReader(h[o : o+4]) | |
| err = binary.Read(r, binary.BigEndian, &header) | |
| check(err) | |
| //Ignore most significant bits as per RFC 4226. | |
| //Takes division from one million to generate a remainder less than < 7 digits | |
| h12 := (int(header) & 0x7fffffff) % 1000000 | |
| //Converts number as a string | |
| otp := strconv.Itoa(int(h12)) | |
| return prefix0(otp) | |
| } |
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