This is very cool, regardless of how serious it was intended to be taken. Before base-64 encoders/decoders became more common as preinstalled commands in the environments I found myself on, I wrote a base64 utility in mostly pure POSIX shell:

  https://25thandClement.com/~william/2023/base64.sh

If this project had existed I might have opted to compile my C-based base-64 encoder and decoder routines, suitably tweaked for pnut's limitations.

I say base64.sh is mostly pure not because it relies on shell extensions, but because the only non-builtins it depends on are od(1) or, alternatively, dd(1) to assist with binary I/O. And preferably od(1), as reading certain control characters, like NUL, into a shell variable is especially dubious. The encoder is designed to operate on a stream of decimal encoded bytes. (See decimals_fast for using od to encode stdin to decimals, and decimals_slow for using dd for the same.)

It looks like pnut uses `read -r` for reading input. In addition to NULs and related raw byte issues, I was worried about chunking issues (e.g. truncation or errors) on binary data, e.g. no newlines within LINE_BUF bytes. Have you tested binary I/O much? Relatedly, how many different shell implementations have you tested your core scheme with? In addition to bash, dash, and various incarnations of /bin/sh on the BSDs, I also tested base64.sh with Solaris' system shells (ksh88 and ksh93 derivatives), as well as AIX's (ksh88 derivative). AIX had some odd quirks with pipelines even with plain text I/O. (Unfortunately Polar Home is gone, now, so I have no easy way to play with AIX; maybe that's for the better.)