Some open-source or custom-built applications implement a simple AES-256 or ChaCha20 encryption for .xcbg files. If you have the key, you can attempt decryption via OpenSSL.
This only works if you know exactly the encryption algorithm used. Guessing can corrupt the file irreversibly. xcbg decrypt
Finding your files appended with the extension is a clear indicator that your system has been compromised by a variant of the STOP/Djvu ransomware family . This specific malware encrypts personal data and demands payment for a decryption key. xcbg decrypt
: If the ransomware couldn't reach its server during the attack, it used a built-in "offline key." These are often decryptable. xcbg decrypt