# One Time Password (OTP) ## Overview This page contains recommendations for using One-Time Passwords (OTPs). ## General
* If you need to generate OTP on the server side and send it to a user via one of the communication channels (email, SMS, push, etc.), use a cryptographically strong random generator, see the [Cryptography: Random Generators](/application-security-handbook/web-application/cryptography/random-generators.md) page. * If you need to generate OTP on the client side, use time-based one-time passwords (TOTPs), see the [Time-Based One-Time Password Algorithm (TOTP)](#time-based-one-time-password-algorithm-totp) section. * Use an OTP once. Delete an OTP or transfer it to a final status that prohibits reusing. * Link an OTP to an individual user in a database. * If an application generates OTPs on the server side, limit the number of generation attempts in a certain period, see the [Vulnerability Mitigation: Brute-force](/application-security-handbook/web-application/vulnerability-mitigation/brute-force.md) page. * Limit the number of OTP verification attempts for a certain period, see the [Vulnerability Mitigation: Brute-force](/application-security-handbook/web-application/vulnerability-mitigation/brute-force.md) page. * Use OTPs of length 6+ symbols. * Set a short expiration time for an OTP (\~ 5 hours). * Make sure the generation of new OTP invalidates previous ones. * Store OTPs in a separate database, access to which must be limited. Do **not** store OTPs next to other application data. * Do **not** send OTPs in an URL (in the path, query, or fragment). * Comply with requirements from the [Sensitive Data Management](/application-security-handbook/web-application/sensitive-data-management.md) page. * Comply with requirements from the [Logging and Monitoring](/application-security-handbook/web-application/logging-and-monitoring.md) page. * Comply with requirements from the [Error and Exception Handling](/application-security-handbook/web-application/error-and-exception-handling.md) page.
* Store OTPs in encrypted form and decrypt only certain records when accessed. * Set a short expiration time for an OTP (\~ 1 minute). * Include information about the confirmed action in the text templates of messages with OTPs. For example, if a user uses OTP to cancel a subscription, the message can be like this: ``` Use this code: 133713 to confirm cancellation of subscription #13. For more information https://website.local/yJri7a ``` ## Time-Based One-Time Password Algorithm (TOTP) Time-Based One-Time Password Algorithm (TOTP) is described in [RFC6238](https://datatracker.ietf.org/doc/html/rfc6238). TOTP is an algorithm that generates OTPs based on a shared secret and the current time. To generate OTPs, a user needs an authenticator application (also known as a `prover`). In other words, a user and a backend generate an OTP separately, but thanks to the shared secret and the current time, a backend can reproduce the same OTP in a certain period.
* Generate a shared secret using a cryptographically strong random generator, see the [Cryptography: Random Generators](/application-security-handbook/web-application/cryptography/random-generators.md) page. * Use a unique shared secret for each authenticator application (prover). For example, if TOTP is used to generate OTPs for a second-factor authentication, generate a unique shared secret for each TOTP registration. * Use shared secrets of length 16+ bytes. * Set a short expiration time for an OTP (\~ 1 minute).
Clarification Since new OTPs are generated in steps of `n` seconds (generally `n` is `30` seconds), multiple OTPs are valid at the same time. Each OTP in the allowable window is a valid OTP. If OTPs are valid for an hour, many OTPs are valid at the same time within this window. Therefore, it is much easier to guess a valid OTP, because there is more than one valid OTP in the possible set of OTPs. For example, if OTPs are generated in steps of `30` seconds and a common validity range here is about `5` minutes in either direction from the time as set on a server, the allowable window is: ``` [current_time - 5 mins, current_time + 5 mins] ``` In such case, the number of valid codes at the same time is: ``` 10 mins * 60 sec / 30 sec = 20 codes ```
* The OTP generation step and OTP expiration time must be equal (\~ 1 minute). In other words, the generation of a new OTP must invalidate previous ones. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://0xn3va.gitbook.io/application-security-handbook/web-application/authentication/one-time-password-otp.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.