[taway2012]

Base64 can maybe result in strings unsuitable for pasting as-is into bad HTTP servers' URLs (the default character set include '/' and '+').

Using only the uppercase letters lets them worry less about whether the string got mangled because some systems interpreted it as Unicode and some didn't etc.

Something like Bitcoin and Flickr's base58 would have also worked, but the encoder and decoder would have been more complicated. They got away with only division by 16.

They get a little bit of integrity checking too.

Basically, this is base16 with a poor man's integrity checking baked in. Pretty interesting.

[jarrett]

Ah, so the design of this encoding is specifically targeting URLs? That makes a lot more sense. Thanks!

Knowing that, it does in fact seem like a sensible encoding for certain applications. Encoding, not encryption, mind you. I assume (or at least hope) the designers intended it as the former only, and never as the latter.