base64 decode while ignoring brackets - bash

Question

I'm trying to decode a file, which is mostly encoded with base64. What I want to do is to decode the following, while still maintaining the [_*_].
example.txt
wq9cXyjjg4QpXy/Crwo=
[_NOTBASE64ED_]
aGkgdGhlcmUK
[_CONSTANT_]
SGVsbG8gV29ybGQhCg==
Sometimes it'll be in this form
aGkgdGhlcmUK[_CONSTANT_]SGVsbG8gV29ybGQhCg==
Desired output
¯\_(ツ)_/¯
[_NOTBASE64ED_]
hi there
[_CONSTANT_]
Hello World!
hi there[_CONSTANT_]Hello World!
Error output
¯\_(ツ)_/¯
4��!:�#�H\�B�8ԓ��[��ܛBbase64: invalid input
What I've tried
base64 -di example.txt
base64 -d example.txt
base64 --wrap=0 -d -i example.txt
I tried to individually base64 the [_*_] using grep -o. Then find and
replacing them through a weird arrangement with arrays, but I couldn't
get it to work.
base64ing it all, then decoding. Results in double base64ed rows.
The file is significantly downsized!
Encoded using base64 --wrap=0, while loop, and if/else statement.
The [_*_] still need to be there after being decoded.

Answer 1

I am sure someone has a more clever solution than this. But try this
#! /bin/bash
MYTMP1=""
function printInlineB64()
{
local lines=($(echo $1 | sed -e 's/\[/\n[/g' -e 's/\]/]\n/g'))
OUTPUT=""
for line in "${lines[#]}"; do
MYTMP1=$(base64 -d <<< "$line" 2>/dev/null)
if [ "$?" != "0" ]; then
OUTPUT="${OUTPUT}${line}"
else
OUTPUT="${OUTPUT}${MYTMP1}"
fi;
done
echo "$OUTPUT"
}
MYTMP2=""
function printB64Line()
{
local line=$1
# not fully base64 line
if [[ ! "$line" =~ ^[A-Za-z0-9+/=]+$ ]]; then
printInlineB64 "$line"
return
fi;
# likely base64 line
MYTMP2=$(base64 -d <<< "$line" 2>/dev/null)
if [ "$?" != "0" ]; then
echo $line
else
echo $MYTMP2
fi;
}
FILE=$1
if [ -z "$FILE" ]; then
echo "Please give a file name in argument"
exit 1;
fi;
while read line; do
printB64Line "$line"
done < ${FILE}
and here is output
$ cat example.txt && echo "==========================" && ./base64.sh example.txt
wq9cXyjjg4QpXy/Crwo=
[_NOTBASE64ED_]
aGkgdGhlcmUK
[_CONSTANT_]
SGVsbG8gV29ybGQhCg==
==========================
¯\_(ツ)_/¯
[_NOTBASE64ED_]
hi there
[_CONSTANT_]
Hello World!
$ cat example2.txt && echo "==========================" && ./base64.sh example2.txt
aGkgdGhlcmUK[_CONSTANT_]SGVsbG8gV29ybGQhCg==
==========================
hi there[_CONSTANT_]Hello World!

Answer 2

You need a loop that reads each line and tests whether it's base64 or non-base64, and processes it appropriately.
while read -r line
do
case "$line" in
\[*\]) echo "$line" ;;
*) base64 -d <<< "$line" ;;
esac
done << example.txt

Answer 3

I would suggest using other languages other than sh but here is a solution using cut. This would handle the case where there are more than one [_constant_] in a line.
#!/bin/bash
function decode() {
local data=""
local line=$1
while [[ -n $line ]]; do
data=$data$(echo $line | cut -d[ -f1 | base64 -d)
const=$(echo $line | cut -d[ -sf2- | cut -d] -sf1)
[[ -n $const ]] && data=$data[$const]
line=$(echo $line | cut -d] -sf2-)
done
echo "$data"
}
while read -r line; do
decode $line
done < example.txt

Answer 4

If Perl is an option, you can say something like:
perl -MMIME::Base64 -lpe '$_ = join("", grep {/^\[/ || chomp($_ = decode_base64($_)), 1} split(/(?=\[)|(?<=\])/))' example.txt
The code below is equivalent to the above but is broken down into steps for the explanation purpose:
#!/bin/bash
perl -MMIME::Base64 -lpe '
#ary = split(/(?=\[)|(?<=\])/, $_);
foreach (#ary) {
if (! /^\[/) {
chomp($_ = decode_base64($_));
}
}
$_ = join("", #ary);
' example.txt
-MMIME::Base64 option loads the base64 codec module.
-lpe option makes Perl bahave like AWK to loop over input lines and implicitly handle newlines.
The regular expression (?=\[)|(?<=\]) matches the boundary between the base64 block and the maintaining block surrounded by [...].
The split function divides the line into blocks on the boundary and store them in an array.
Then loop over the array and decode the base64-encoded entry if found.
Finally merge the substring blocks into a line to print.