For decoded a portion of a PEM file, it seems possible although the requirement seems to be padding the file with characters so that it is divisible by 4.

This sounds like a pull request for someone that is smarter than me on their repository. Maybe I could do it but it sounds like a challenge.

https://stackoverflow.com/questions/42426346/is-it-possible-to-base64-decode-part-of-a-base64-encoded-message