Decrypt the output from the yubikey token

The hardware can be found at

http://www.yubico.com/home/index/

The decryption module does only one thing - decrypt the AES encrypted
OTP from the Yubikey. To this, it requires the OTP, and the AES
key.

Please note - this module does not perform authentication - it is
a required component to decrypt the token first before authentication
can be performed.