#!/bin/sh
# BlueShift: an experimental POSIX-compatible tool for
# high-density (1 byte per pixel) image-based steganography
# Depends on od, paste, dd, pr, awk, sed, ImageMagick and pngcrush
# Usage:
# - encoding: blueshift.sh e [cover image] [file to hide] [output PNG]
# - decoding: blueshift.sh d [container PNG] [output file]
# Created by Luxferre in 2026, released into public domain

# encode raw file as a netstring
to_netstring() {
  printf '%d:' "$(cat "$1" | wc -c)"
  cat "$1"
  printf ','
}

# decode a raw file from a netstring (inside another file)
from_netstring() {
  # get the container length
  flen=$(head -n 1 "$1" | cut -f 1 -d :)
  # skip the length itself and the : char
  dd if="$1" of="$2" skip=$((${#flen} + 1)) bs=1 count=$flen
}

# convert PPM to plain RGB list (pixel per line)
ppm2rgb() {
  tail -n +4 | sed 's/\s\+/\n/g' | pr -3ta
}

# for every input byte, print its 2-bit or 4-bit parts
# R: lsb 2 bits, G: msb 2 bits, B: hsb 4 bits
conv_input() {
  to_netstring "$1" | od -An -tu1 -v -w1 | awk '{n=int($0);printf("%u\t%u\t%u\n",n%4,int(n/4)%4,int(n/16))}'
}

# stego_encode cover_image hidden_text out.png
stego_encode() {
  coverfile="$(mktemp)"
  infile="$(mktemp)"
  tfile="$(mktemp)"
  temp_ppm="$(mktemp)"
  magick $1 -compress none ppm:$temp_ppm
  cat $temp_ppm | head -n 3 > $tfile
  cat $temp_ppm | ppm2rgb > $coverfile
  conv_input "$2" > $infile
  paste $coverfile $infile | awk '{if(NF>3)printf("%u %u %u\n", int(int($1)/4)*4+int($4), int(int($2)/4)*4+int($5), int(int($3)/16)*16+int($6));else print($0)}' >> $tfile
  magick ppm:$tfile png:$3
  pngcrush -ow $3
  rm -f $coverfile $infile $temp_ppm $tfile
}

# stego_decode in.png out_file
stego_decode() {
  temp_ppm="$(mktemp)"
  tfile="$(mktemp)"
  magick $1 -compress none ppm:- | ppm2rgb | awk '{printf("\\%03o", (int($3)%16)*16 + (int($2)%4)*4 + (int($1)%4))}' > $temp_ppm
  printf "$(cat $temp_ppm)" > $tfile
  from_netstring $tfile "$2"
  rm -f $temp_ppm $tfile
}

# entry point

if [ "$1" = "e" ]; then
  [ -z "$2" ] && echo 'No cover file specified! Exiting...' && exit 1
  [ -z "$3" ] && echo 'No input file specified! Exiting...' && exit 1
  [ -z "$4" ] && echo "No output file specified! Exiting..." && exit 1
  stego_encode "$2" "$3" "$4"
elif [ "$1" = "d" ]; then
  [ -z "$2" ] && echo 'No input file specified! Exiting...' && exit 1
  [ -z "$3" ] && echo "No output file specified! Exiting..." && exit 1
  stego_decode "$2" "$3"
else
  echo 'Invalid mode! Use e for encoding or d for decoding'
fi