I can provide you two files: source file and encrypted output file.
You have to find out how the encryption is done.
After you found the decryption algorithm I will test your work and send you a second encrypted output file and you have to send me the original source file.
To get access to the two files, please send me a PM and tell me about your experience with file encryption.
If there are any questions please do not hesitate to ask me.
With best regards