Unfortunately your variant of STOP ransomware encryption cannot be decrypted without a key.
Your options
without a backup:
1) Recovery: If you got ZIP archives that were encrypted by STOP, you can still retrieve parts of their contents. If you use file cloud services like Dropbox or OneDrive, those provide a file version history and might be able to restore previous versions of your files.
In rare cases ransomware fails to delete shadow volume copies or fails to delete the original files properly. You can try to
recover files via shadow volume copies and file recovery software.
2) Repair: Certain file types, mainly video and audio files, can possibly be repaired with tools like
MediaRepair. But these files will loose some data.
3) Wait: Backup encrypted files and a ransom note and wait in case a solution comes up later. Maybe law enforcement gets hands on the keys or the criminals publish the keys as it happened with, e.g., GandCrab. I suggest reading the news on this. Emsisoft will update their decrypter if that happens.
4) Pay: There is the option of paying the criminals, but we highly recommend against this step. You will just fund later attacks. You may also pay without getting your files back. These are criminals and as such not trustworthy.
Please let me know you want assistance in any of the mentioned options above.
Furthermore, I recommend that you apply
STOP vaccine to prevent future STOP infections from encrypting your files.