It can also be done using a few small books, mapping strings of 4 base 64 characters into 3 ASCII characters. With 100 printable characters (according to Python), and assuming you can get 96 per page (4 columns of 24; possible with clever layout), that would fill about 50 books.

Okay, maybe that's not very efficient.

Each pair of Base 64 characters contains a single ASCII character, plus some change. That would take three lookup tables of about 1000 entries each; given that the “change” letters could use character ranges, that would take 3 or 4 sheets of A4 (or A3, if you wanted large print).

I've always imagined that somewhere out there is a dude who has spent time to teach himself how to decode/encode base64 in his head.

Best I've done so far is to recognize that "eyJ" means I'm probably looking at JSON, since `{"` followed by any letter will always begin with that.

  > It's hard to call this encryption.

It's a lie to call this encryption. It is encoding, not encryption.