HOTP Generator: Compute RFC 4226 One-Time Passwords

How to use the hotp generator

1. Enter your base32 secret in the first field. Spaces are ignored automatically.
2. Set the counter value. This is the moving factor that increments with each authentication attempt.
3. Choose the number of digits for the output code (6, 7 or 8).
4. The HOTP code appears immediately as you type.

How HOTP works under RFC 4226

The HMAC-based one-time password algorithm combines a shared secret key with a counter using HMAC-SHA-1. The steps are:

1. Convert the counter to an 8-byte big-endian integer.
2. Compute HMAC-SHA-1(key, counter_bytes), producing a 20-byte hash.
3. Extract 4 bytes from the hash using dynamic truncation: the offset is the lower nibble of the last byte, then read 4 bytes starting at that offset.
4. Mask the most significant bit to avoid signed/unsigned issues, yielding a 31-bit integer.
5. Reduce modulo 10^d where d is the desired digit count, and pad with leading zeros.

For example, with the standard test secret 12345678901234567890 (base32: GEZDGNBVGY3TQOJQGEZDGNBVGY3TQOJQ) and counter 0, the HMAC-SHA-1 result is cc93cf18508d94934c64b65d8ba7667fb7cde4b0. Dynamic truncation yields the decimal value 1284755224, and taking modulo 1000000 gives the 6-digit code 755224.

RFC 4226 test vectors

The following table lists the official test vectors from Appendix D of RFC 4226. The shared secret is the ASCII string 12345678901234567890, which encodes to GEZDGNBVGY3TQOJQGEZDGNBVGY3TQOJQ in base32. You can paste this secret into the HOTP generator above to verify each code.

Base32 encoding for secrets

HOTP secrets are typically exchanged as base32 strings because the alphabet avoids visually ambiguous characters. Standard base32 uses A through Z and 2 through 7, with optional padding using the equals sign. When entering a secret into this HOTP generator, spaces and lowercase letters are handled automatically.

A common mistake is confusing base32 with base64. Base64 includes lowercase letters, digits, plus and slash, while base32 does not. If your authenticator app rejects a secret, verify that it is actually base32 encoded. For tools that handle other encoding formats, see the base64 encoder.

HOTP compared with TOTP

HOTP uses a counter that increments with each use, while TOTP (Time-based One-Time Password) replaces the counter with a timestamp divided by a time step, usually 30 seconds. Both rely on HMAC-SHA-1 and dynamic truncation, but their synchronization models differ.

Because HOTP depends on a counter, both the client and server must track it precisely. If they drift out of sync, the server typically checks a window of upcoming counter values to resynchronize. TOTP avoids this problem by deriving the moving factor from the current time, though it requires accurate clocks on both sides. If you need time-based codes instead, look for a TOTP tool rather than this HOTP generator.

Security considerations

The security of an HOTP system depends on keeping the shared secret confidential. If the secret is exposed, any party can generate valid codes for any counter value. The secret should be generated using a cryptographically secure random number generator and stored securely on both the client and server.

RFC 4226 recommends a minimum of 6 digits for the output code. Shorter codes reduce the search space for brute-force attacks. Servers should also implement throttling or lockout policies after a small number of failed attempts to prevent online guessing. Because this tool computes codes locally in the browser, it is useful for development and testing, but production systems should use audited cryptographic libraries on the server side.

Worked examples