A BC5B file is a file encrypted by ransomware, a virus utilized by cybercriminals. It contains a user's file, such as a .JPG, .WMV, or .DOCX file, which is encrypted by the malware.

The purpose of ransomware is to take your files hostage and force you to pay the perpetrator (commonly by way of bitcoin) to unlock your files. The virus is a trojan horse that was introduced through a personal document (most likely a fake invoice .DOC or DOCX file asking you to enable macros) attached to an email. Once the virus affects your computer it begins scrambling your files, renaming them, and encrypting them. The virus usually then generates a .TXT ransom note informing you of the encryption of your files and what you need to do to recover them. There is no program currently available to effectively restore your files. If you backup your files you can execute a full system restore. Otherwise, the only known way to decrypt your infected files is to pay the criminal for the private key and decrypt program.