The most important compatibility question is often not the file extension—it is the key used by the ransomware. STOP/Djvu cases can behave differently depending on whether the infection used an offline or online key.
What is an offline key?
An offline key is a fallback key that can be reused across multiple victims when the ransomware cannot obtain a victim-specific key from its server. If researchers have that offline key, compatible files may be decryptable.
What is an online key?
An online key is typically unique to a particular victim or infection. If that key is not available, a free decryptor cannot simply calculate it from the encrypted files.
Why an extension is not enough
Extensions such as .djvu or later STOP/Djvu suffixes can help identify the family, but they do not reveal whether the required key is known.
What to do after a no-key result
Do not delete encrypted files. Store them safely, record the decryptor log, and periodically check trusted ransomware-response resources for updated support.
Next, read how to interpret no-key and new-variant messages.