HOTP Definition

What is HOTP?

HOTP stands for HMAC-based one-time password (OTP). It is an algorithm for generating OTPs that is based on the HMAC algorithm. When setting up HOTP, the client and server share a secret key. Using this value and a counter, both sides can generate a sequence of OTPs that can be used as part of a multi-factor authentication (MFA) scheme.

How Does HOTP Work?

HOTP uses a deterministic algorithm to generate a sequence of values. The client and the server use a shared secret key as the starting point and combine this key with an incrementing counter to create a series of OTPs. Since both sides use the same algorithm and starting point, they can generate the same sequence with no interaction after the initial setup.

HOTP works via the following process:

• Initialization: The client and server set the secret key and counter value.
• OTP Generation: The shared secret key and counter value are hashed using the HMAC algorithm.
• Truncation: The hash value is truncated to the 6-8 digit code displayed in an authenticator app.
• Counter Update: The counter is updated to prepare to generate the next OTP.
• Entry: The user enters the 6-8 digit code into the webpage, which sends it to the server.
• Validation: The server generates its own version of the OTP by hashing the shared secret and counter value with HMAC and truncating the result. It then compares the two values to see if they match and the user should be granted access.

Since the counter value increments each time the client or server generates an OTP, they may become misaligned. The protocol also allows for synchronization if the client and server’s counters become misaligned.

Pros and Cons of HOTP

HOTP is a useful protocol for implementing OTPs and MFA. Some of the main benefits of it include:

• Simplicity: HOTP is a simple protocol to implement. Each side of the protocol simply needs to maintain a copy of the secret key and counter and periodically generate and truncate a hash value.
• Security: The security of HOTP depends on the security of the secret key. If this remains confidential, then the protocol is secure.
• No Time Synchronization: Time-based OTP (TOTP) is an alternative to HOTP that relies on the client and server having the same clock time. HOTP doesn’t require synchronized clocks.

In addition to these benefits, HOTP does have its limitations as well. These include:

• Counter Synchronization: HOTP requires synchronization of counters between the client and server. This can be problematic if counters often become desynchronized and require frequent synchronization.
• Replay Attacks: HOTP is vulnerable to replay attacks if the attacker can intercept an OTP and send it to the server to authenticate themselves.

Conclusion

HOTP is an algorithm for generating OTPs for multi-factor authentication. The client and server both maintain a shared secret and an incrementing counter that enable them to generate a synchronized series of OTPs. These OTPs can then be used for authentication but — like all OTPs — are potentially vulnerable to replay attacks.