If you use a decimal number with leading zeros instead of base64 you can have a solution that is completely human compatible. It would be 15 digits for 2^48 or

If you use a decimal number with leading zeros instead of base64 you can have a solution that is completely human compatible. It would be 15 digits for 2^48 or 20 digits for 2^64 if you want to be future-proof. Since the goal is to store really long numbers, it seems like a false economy to worry about a few bytes at the front.