Getting Started Writing Clients


While the canonical description of the validation protocol is documented in Validation Protocol V2.0, it may be difficult to grasp the high level of how a client typically works. The intention with this page is to illustrate one simple case.

This does not cover the replicated protocol.

If you want to make your own client implementation, here is the quickest way to get started:

API key

API key is used to optionally sign the OTP validation request and to verify the OTP validation response. We recommend all production deployments use either API key or HTTPS.

If you use HTTPS to access Yubico’s validation web service and you validate the Yubico server SSL certificate, you don’t need to use the shared key to further authenticate response signatures from Yubico.

If you for some reason do not want to rely on HTTPS. You may also sign requests using this API key.

To get an API key, first use our online API key generator. It will assign you an ID and create a shared key. You can use the shared key to authenticate that the API responses do come from Yubico.

Capture an OTP

Capture an OTP output from your YubiKey. You can do that by opening a text editor and simply pressing the button on the YubiKey. The OTP is a simple string of characters, like this:


Validate OTP format

Prudent clients should validate the data entered by the user so that it is what the software expects. YubiKey OTPs consists of 32-48 characters in the ModHex alphabet cbdefghijklnrtuv. Unfortunately, this has turned out to be over-aggresive because if the keyboard layout is Dvorak-based, it will look differently. For example, the modhex alphabet in the US Dvorak layout is jxe.uidchtnbpygk. There are other Dvorak layouts as well. For this reason, our recommendation is that clients only check that the input consists of 32-48 printable characters.

Send OTP to our server

Send the authentication request to our servers following the protocol specified below. Your request should include the verifier ID (a number that identifies the signature key), the OTP you captured in the previous instruction, and a random looking string of characters to make the request unique. The request is part of the HTTP GET URL, encoded using normal parameter/value pairs. For example (broken into two lines for legibility):\

The servers that Yubico provides are:

These servers are hosted in different places and by different organizations.

Clients should send authentication requests to all of them in parallel, and utilize the first response received for added reliability should one or more of the api servers be offline, see Yubico C client for technical details.

Parse response

You will recieve a response back from the server, a string of text that resembles the following:


You can check the authenticity of this response by checking that the OTP and nonce was the same as the one you requested validation for and verify the HMAC-SHA-1 signature.

Make a decision

Use the status= codes to make a decision whether to authenticate your user or not. All the status codes and their meanings are described by the protocol specification, see below.

Binding an OTP to an Identity

Since a valid OTP on its own is not terribly useful, you need to connect it to a user in some way. This is normally not something the client needs to do, it belongs elsewhere in the application, however for completeness we mention how it is done here.

In some situations, the user is typing her username directly into your application, which solves the issue.

More generally, the YubiKey OTP contains as the initial part an identity of the YubiKey, and it can be used to identify the user. The identity part is the same for every OTP, and it is the initial 2-16 modhex characters of the OTP. (The identity can be programmed to 0 characters, but then obviously this scheme does not apply.) Since the rest of the OTP is always 32 characters, the method to extract the identity is to remove 32 characters from the end and then use the remaining string, which should be 2-16 characters, as the YubiKey identity.

Note that the validation server is case sensitive, so you need to lower-case the string in order to avoid caps lock resulting in different identities with the only differ being case of the prefix.

Typically you would store the association between the user and the YubiKey prefix in a database.

The End

There are some more subtle matters that can be important, so please read the entire protocol specification for all the details.