![]() Val startEpochMillis = timeBasedOneTimePasswordGenerator.timeslotStart(counter) Val totp = timeBasedOneTimePasswordGenerator.generate(timestamp) The constructor takes the shared secret and a configuration instance of the class HmacOneTimePasswordConfig as arguments: The HOTP generator is available through the class HmacOneTimePasswordGenerator. This maximum value means that a higher code digits value than ten adds more trailing zeros to the code (in the case of 10 digits, the first number is always 0, 1, or 2). But notice that through the design of the algorithm, the maximum code value is 2,147,483,647. However, this library does not set any requirements for this property. The RFC 4226 requires a code digits value between 6 and 8 to assure a good security trade-off. This filling is the reason why the code gets represented as a String. The computed code gets filled with zeros at the beginning to meet this requirement. The client then sends the usual login credentials header and the additional header Authorization: 2FA $code (or a more specific generator name instead of "2FA").Īll three one-time password generators create a code value with a fixed length given by the code digits property in the configuration instance.If the challenge generation is unknown in advance, this value must be transferred by appending to the header value, challenge="$challenge" (yes, with the comma). If a two-factor authentication activate for the user, the server answers with HTTP status code 401 Unauthorized and the header HOTP, TOTP or Google).The client sends an HTTP request with the header for the normal login credentials Authorization: Basic Base64($username:$password) to the server.If the one-time-password is used for two-factor authentication, a possible HTTP flow could look like this (even if it does not follow an official standardization): The client of the user and the server must use the same code generator with the same configuration (e.g., number of code digits, hash algorithm). Even if an attacker captures the code, he can't use it a second time to log in himself. This code also has the name one-time-password, as it only can be used once. The solution to the challenge is a numeric code.For example, the Google authenticator uses the current Unix timestamp as a challenge.) (This step is optional if the generation algorithm of the challenge is known to both sides. Instead, the server sends the user a challenge that he can only solve if he has the correct shared secret. For that, he could send the shared secret directly to the server (like a regular password), but a man-in-the-middle attack could capture this, and the attacker could log in with the password. The user now wants to authenticate to the server.The user and server need to agree on a shared secret, which must be negotiated in advance and remains constant over a longer time.This library is available at Maven Central: Gradle This library gets used by hundreds of active users every day to generate Google Authenticator codes for several years now, so I am very confident that the code correctly generates codes. Most problems arise from not following the two remarks correctly. ℹ️ If you want to use this library in conjunction with the Google Authenticator app (or similar apps), please carefully read the chapter Google Authenticator, especially the remarks regarding the Base32-encoded secret and the plain text secret length limitation. Since the code is relatively simple, follows the specifications of the two RFCs, and has good test coverage, there is hardly any need to change anything. However, this is not an abandoned project. ℹ️ In this repository, changes don't happen that often and the library gets updated very rarely. RFC 6238: "TOTP: Time-Based One-Time Password Algorithm".RFC 4226: "RFC 4226 HOTP: An HMAC-Based One-Time Password Algorithm".The implementations are based on the RFCs: This is a Kotlin library to generate one-time password codes for:
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