Script to decode JWT from paste buffer.

DecodeJWT.sh

#!/bin/bash

# Dependencies:
# JQ: https://stedolan.github.io/jq/download/
# if Linux, you'll need xclip: sudo apt install xclip.
# if OSX, then pbpaste is already there.

# To use this tool, just save it on your system and make
# it executable.  Then, if you have copied a base64 encoded
# JWT, then you can simply execute this from a command prompt
# and it will decode the token and dump the JSON to your 
# terminal window.

if [[ "$OSTYPE" == "linux-gnu" ]]; then
  PASTECMD="xclip -o"
elif [[ "$OSTYPE" == "darwin"* ]]; then
  PASTECMD="pbpaste"
else
  # following the above pattern, you should easily be able
  # to add an option for your OS
  echo "OS not supported."
  exit 0
fi

# Get the name of the paste executable, without any arguments
PASTERPATH=$(echo $PASTECMD | cut -d' ' -f1)

# Check to see if it is installed.  Otherwise exit.
if [[ -z "$(which $PASTERPATH)" ]]; then
  echo "$PASTERPATH is not installed on this system."
  exit 0
fi

# Check to see if JQ is installed.  Otherwise exit.
if [[ -z "$(which jq)" ]]; then
  echo "JQ is not installed on this system."
  exit 0
fi

# Execute the paste command to grab the token from the paste buffer
TOKEN=$($PASTECMD)

# Make a reasonable effort to determine if what is in the paste buffer actuall IS
# a token. This will check for something that starts with at least two unbroken
# strings of characters that are at least 30 characters long and separated by a
# '.' character.  It is not perfect, but not likely to fail often enough to warrant
# more effort.
if [[ ! $TOKEN =~ ^[A-Za-z0-9_-]{30,}\.[A-Za-z0-9_-]{30,}.*$ ]]; then
  echo "Your paste buffer does not appear to contain a valid token."
  exit 0
fi

# there are probably 2 parts of the token that we want to see, so we setup to 
# look at both of them.
for IDX in 1 2
do
  echo

  # split the contents of the paste buffer on the '.' and get the part
  # identified by the current value of $IDX.  Not sure why, but the final
  # "} is always removed from these token parts after docoding, so that
  # last bit just puts it back
  TKN="$(echo $TOKEN | cut -d'.' -f$IDX | base64 --decode 2>/dev/null)\"}"

  echo "Part $IDX"
  jq '.' <<< $TKN
done
echo