Link to decrypter download page. <- The decrypter will tell you if your files are decryptable, whether you're dealing with an "old" or "new" variant of STOP/Djvu, and whether your ID is online or offline.
Link to instructions for using the decrypter (PDF).
Link to "file pair" submission form.
Link to more information about the decrypter. <- Article at BleepingComputer.com
Link to more detailed information about STOP ransomware (covers more than just STOP/Djvu).