Time-based One-Time Password

Short description: Cryptographic algorithm

Time-based One-time Password (TOTP) is a computer algorithm that generates a one-time password (OTP) that uses the current time as a source of uniqueness. As an extension of the HMAC-based One-time Password algorithm (HOTP), it has been adopted as Internet Engineering Task Force (IETF) standard RFC 6238.

TOTP is the cornerstone of Initiative for Open Authentication (OATH), and is used in a number of two-factor authentication (2FA) systems.

History

Through the collaboration of several OATH members, a TOTP draft was developed in order to create an industry-backed standard. It complements the event-based one-time standard HOTP, and it offers end user organizations and enterprises more choice in selecting technologies that best fit their application requirements and security guidelines. In 2008, OATH submitted a draft version of the specification to the IETF. This version incorporates all the feedback and commentary that the authors received from the technical community based on the prior versions submitted to the IETF. In May 2011, TOTP officially became RFC 6238.

Algorithm

To establish TOTP authentication, the authenticatee and authenticator must pre-establish both the HOTP parameters and the following TOTP parameters:

• T₀, the Unix time from which to start counting time steps (default is 0),
• T_X, an interval which will be used to calculate the value of the counter C_T (default is 30 seconds).

Both the authenticator and the authenticatee compute the TOTP value, then the authenticator checks whether the TOTP value supplied by the authenticatee matches the locally generated TOTP value. Some authenticators allow values that should have been generated before or after the current time in order to account for slight clock skews, network latency and user delays.

TOTP uses the HOTP algorithm, substituting the counter with a non-decreasing value based on the current time:

TOTP value(K) = HOTP value(K, C_T),

calculating counter value

[math]\displaystyle{ C_T = \left\lfloor\frac{T - T_0}{T_X}\right\rfloor, }[/math]

where

• C_T is the count of the number of durations T_X between T₀ and T,
• T is the current time in seconds since a particular epoch,
• T₀ is the epoch as specified in seconds since the Unix epoch (e.g. if using Unix time, then T₀ is 0),
• T_X is the length of one time duration (e.g. 30 seconds).

Unix time is not strictly increasing. When a leap second is inserted into UTC, Unix time repeats one second. But a single leap second does not cause the integer part of Unix time to decrease, and C_T is non-decreasing as well so long as T_X is a multiple of one second.

Security

TOTP values can be phished like passwords, though this requires attackers to proxy the credentials in real time.

An attacker who steals the shared secret can generate new, valid TOTP values at will. This can be a particular problem if the attacker breaches a large authentication database.

Because of latency, both network and human, and unsynchronised clocks, the one-time password must validate over a range of times between the authenticator and the authenticatee. Here, time is downsampled into larger durations (e.g., 30 seconds) to allow for validity between the parties. For subsequent authentications to work, the clocks of the authenticatee and the authenticator need to be roughly synchronized (the authenticator will typically accept one-time passwords generated from timestamps that differ by ±1 time interval from the authenticatee's timestamp). TOTP values are typically valid for longer than 30 seconds so that client and server time delays are accounted for.

See also

• Botan (programming library)
• FreeOTP
• Google Authenticator
• multiOTP
• Aegis Authenticator

External links

• RFC 6238
• Step by step Python implementation in a Jupyter Notebook
• Designing Docker Hub Two-Factor Authentication, (section "Using Time-Based One-Time Password (TOTP) Authentication").