You will need to do all of this via the command line. Given you're already dealing with Base64 data I am going to assume you know how to bring that up on your operating system. Scroll down to the relevant section based on your OS below, also substitute your file names as appropriate.
I am going to use .txt for the decoded data file extension and .b64 for the Base64 encoded file extension.
Note: encoding with the above command will leave a temporary file, tmp.b64, on your file system. If you do not wish to have that file present simply add this to the end of the command: && del tmp.b64