[layer8]

The original specification is in RFC 989 [0] from 1987, called “Printable Encoding”, where it explains “The bits resulting from the encryption operation are encoded into characters which are universally representable at all sites, though not necessarily with the same bit patterns […] each group of 6 bits is used as an index into an array of 64 printable characters; the character referenced by the index is placed in the output string. These characters, identified in Table 1, are selected so as to be universally representable, and the set excludes characters with particular significance to SMTP (e.g., ".", "<CR>", "<LF>").”

Using the array-indexing method, the noncontiguity of the characters doesn’t matter, and the processing is also independent of the character encoding (e.g. works exactly the same way in EBCDIC).

[0] https://www.rfc-editor.org/rfc/rfc989.html#page-9