I'm using "uint64" or "unsigned 64 bit integer" as a shorthand for "natural number between 0 and 2^64 - 1 inclusive".

@keks

May I suggest: [...]

What are you suggesting this for (this question is genuine, not rethoric)? It isn't the legacy signing encoding (we should just use a simple bijection (e.g. base32 as you suggested)). It isn't the transport encoding either (that one will use a compact binary representation based on some variable length integer encoding). But those are the only encodings in the spec.

Are you designing some sort of human-producable input format?