The literature on Base64 is terrible and that includes the RFCs.
Roughly is to encode binary data as ASCII.
There is a silent assumption that all binary data comes in 8 bit hunks.
I think that encoding assumes that the reader of the coded data already knows how bytes are expected.
The coded values themselves need demarcation.
Neither is assumed to be self delimiting.
The documentation is comprehensible only with the idea that data starts at the left and goes on to the right.
Perhaps we should optionally write keys in this format.
‘mpint’ sort of explained here.