yf_base32_decode.function

#! /bin/true
# vi: set tabstop=4 syntax=sh : # colorize it in 'vi' and use shorter tabstops
# SPDX-License-Identifier: GPL-2.0-or-later
#######################################################################################
#                                                                                     #
# base32_decode - convert STDIN from base32 format (AVMs character set) to the binary #
#                 content                                                             #
#                                                                                     #
# parameters:                                                                         #
#   none                                                                              #
#                                                                                     #
# stdin:                                                                              #
#   the content to decode, size has to be a multiple of 8 on each line and a newline  #
#   character is needed at the end (otherwise the 'read' command will fail), even if  #
#   the encoding function does not add one (it doesn't split its output into lines)   #
#                                                                                     #
# stdout:                                                                             #
#   the decoded data                                                                  #
#                                                                                     #
# return code:                                                                        #
#   0 - stdout value is valid                                                         #
#   1 - error occured, input data is invalid                                          #
#                                                                                     #
#######################################################################################
#                                                                                     #
# U: sed printf expr dd                                                               #
# W: -                                                                                #
# I: -                                                                                #
# F: yf_substring yf_index                                                            #
# K: avm encryption fritzbox                                                          #
#                                                                                     #
#######################################################################################
yf_base32_decode()
(
	[ -t 0 ] && return 1
	rc=1 # preset, if no newline is present on STDIN, our 'read' will signal instant EOF
	while read line; do
		rc=0
		! [ -z "$(expr \( "$line" : "\([^A-Z1-6\r]\)" \) )" ] && return 1
		line=$(printf "%s" $line | sed -e "s|\r||g")
		[ $(( ${#line} % 8 )) -gt 0 ] && return 1
		i=0
		f=0
		while [ $i -lt ${#line} ]; do
			part=$(printf "%s" $(yf_substring "$line" $i 8) | sed -e "s|.|& |g")
			i=$(( i + 8 ))
			v=0
			j=0
			for c in $part; do
				v=$(( ( v * 32 ) + $(yf_index "ABCDEFGHIJKLMNOPQRSTUVWXYZ123456" $c) - 1 ))
				j=$(( j + 1 ))
				[ $(( j % 4 )) -ne 0 ] && continue
				# process 16 and 24 bits in two separate steps, this avoids
				# problems with 32-bit-limited arithmetics
				r=$(( j % 8 ))
				o=$(( v >> r ))
				s=$(( 24 - ( r * 2 ) ))
				v=$(( v & ( 0x00FFFFFF >> ( r * 5 ) ) ))
				while [ $s -gt 0 ]; do
					s=$(( s - 8 ))
					d=$(( ( o >> s ) & 255 ))
					[ $d -eq 0 ] \
						&& dd if=/dev/zero bs=1 count=1 2>/dev/null \
						|| printf "%b" "\0$(( d >> 6 ))$(( ( d >> 3 ) & 7 ))$(( d & 7 ))"
				done
			done
		done
	done
	return $rc
)