Yubikey KSM Decryption Protocol
-------------------------------

The protocol for asking the Yubikey Key Storage Module to decrypt an
OTP is to request a HTTP resource as follows:

 http://ykksm.example.com/wsapi/decrypt/?otp=dteffujehknhfjbrjnlnldnhcujvddbikngjrtgh

On success, the response will follow this format:

 ^OK .*

For example:

 OK counter=000f low=c541 high=a7 use=04

The content of the various fields are as follows:

counter::
 16-bit hex integer, counting upwards on each powerup&touch

low::
 16-bit hex integer, low part of time-stamp of OTP

high::
 8-bit hex integer, high part of time-stamp of OTP

use::
 8-bit hex integer, counting upwards on each touch

On soft errors, the response will follow this format:

 ^ERR .*

For example:

 ERR Invalid OTP format

The data matching .* will be a english error message in one line.

Any other kind of response means a hard error occured.