This is a server that validates Yubikey OTPs. It is written in PHP, for use with web servers such as Apache.

The wiki and source code repository is now at https://github.com/Yubico/yubikey-val-server-php.

The server implements the Yubico API protocol as defined in:

http://yubico.com/developers/api/

This server talks to another service for decrypting the OTPs, to avoid storing any AES keys within the validation server. One implementation of this service is YKKSM:

http://code.google.com/p/yubikey-ksm/

Another implementation of a KSM using the YubiHSM hardware is Python-PyHSM:

https://github.com/Yubico/python-pyhsm

Note that version 1.x is a minimal centralized server. Version 2.x is a replicated system that uses multiple machines.