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@postmodern
Created December 8, 2023 07:37
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WIP code to convert a file into a BMP image, which can be screenshotted, then decoded in order to exfiltrate data from TryHackMe's network-less VMs
#!/usr/bin/env bash
## bmplib.sh v1.0
#
# Make your own BMP from scratch, no external dependencies.
# Useful in BitBar plugins.
#
# pixels=() # set pixels to empty or with background of same size
# # as will be declared in init_bmp
# curcol=(bb 66 44 aa) # set current color, BGRA, hex (%02x)
# init_bmp 1 25 16 # initialize BMP, version 1 width 25 height 16
# # if pixels not valid, initialize to $curcol
# curcol=(00 00 00 ff) # set current color to fully opaque black
# point $x $y # set ($x, $y) to $curcol. (0, 0) is bottom left
# line $x1 $y1 $x2 $y2 # draw horizontal or vertical line
# rect $x $y $w $h # draw rectangle. ($x, $y) is bottom left
# fill $x $y $w $h # draw filled rectangle. ($x, $y) is bottom left
# output_bmp # output BMP to stdout
bmp_ver=5 # set to 1 if you want most compatible BMP. no alpha.
width=25 # width of image
height=16 # height of image
curcol=(00 00 00 00) # current color
# No user-servicable parts below
# We avoid subshells for performance reasons
bpp=4
rowbytes=$((width * bpp))
pixbytes=$((width * height * bpp))
OLDIFS=$IFS
bmp_header=()
pixels=()
# Takes number, prints hex bytes in little endian
# e.g. hexle32 3142 will output 46 0c 00 00
hexle32() {
local num
printf -v num "%08x" "$1"
retval="${num:6:2} ${num:4:2} ${num:2:2} ${num:0:2}"
}
errmsg() {
>&2 echo "$@"
}
# make_bmp_header
# version can be 1 or 5
# v1 is the most compatible, but the graph will be opaque - no alpha support.
# v5 supports alpha channel.
make_bmp_header() {
local headerbytes comp pixoffset filebytes _filebytes _pixoffset
local _headerbytes _width _height _pixbytes
bmp_header=()
headerbytes=40
comp="00"
if [ "$bmp_ver" -eq 5 ]; then
headerbytes=124
comp="03"
fi
pixoffset=$((headerbytes + 14))
filebytes=$((pixbytes + pixoffset))
hexle32 $filebytes
_filebytes=$retval
hexle32 $pixoffset
_pixoffset=$retval
hexle32 $headerbytes
_headerbytes=$retval
hexle32 $width
_width=$retval
hexle32 $height
_height=$retval
hexle32 $pixbytes
_pixbytes=$retval
# Common bits for version 1 and 5
bmp_header+=(
42 4d # "BM" magic
$_filebytes # size of file
00 00 # reserved
00 00 # reserved
$_pixoffset # offset of pixel data
$_headerbytes # remaining bytes in header
$_width # width
$_height # height
01 00 # 1 color plane
20 00 # 32 bits per pixel
$comp 00 00 00 # compression
$_pixbytes # size of pixel data
13 0b 00 00 # ~72 dpi horizontal
13 0b 00 00 # ~72 dpi vertical
00 00 00 00 # colors in palette
00 00 00 00 # all colors are important
)
if [ "$bmp_ver" -eq 5 ]; then
bmp_header+=(
00 00 ff 00 # red channel mask (BGRA)
00 ff 00 00 # green channel mask
ff 00 00 00 # blue channel mask
00 00 00 ff # alpha channel mask
42 47 52 73 # sRGB
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 # red gamma
00 00 00 00 # green gamma
00 00 00 00 # blue gamma
00 00 00 00 # intent
00 00 00 00 # profile data
00 00 00 00 # profile size
00 00 00 00 # reserved
)
fi
}
# point x y
point() {
local off
off=$(($2 * rowbytes + $1 * bpp))
pixels[$off]=${curcol[0]}
pixels[$((off + 1))]=${curcol[1]}
pixels[$((off + 2))]=${curcol[2]}
pixels[$((off + 3))]=${curcol[3]}
}
# line x1 y1 x2 y2
line() {
local x1 y1 x2 y2 x y
if [ "$1" -eq "$3" ]; then
if [ "$2" -gt "$4" ]; then y1=$4; y2=$2; else y1=$2; y2=$4; fi
for ((y = y1; y <= y2; y++)); do
point "$1" $y
done
elif [ "$2" -eq "$4" ]; then
if [ "$1" -gt "$3" ]; then x1=$3; x2=$1; else x1=$1; x2=$3; fi
for ((x = x1; x <= x2; x++)); do
point $x "$2"
done
else
errmsg "Only vertical and horizontal lines supported" "$@"
fi
}
# fill x y w h
function fill() {
local x2 y2 y
x2=$(($1 + $3 - 1))
y2=$(($2 + $4 - 1))
for ((y = $2; y <= y2; y++)); do
line "$1" $y $x2 $y
done
}
# rect x y w h
function rect() {
local x2 y2
x2=$(($1 + $3 - 1))
y2=$(($2 + $4 - 1))
line "$1" "$2" $x2 "$2"
line "$1" $y2 $x2 $y2
line "$1" "$2" "$1" $y2
line $x2 "$2" $x2 $y2
}
output_bmp() {
local _bmp=(${bmp_header[@]/#/'\x'})
_bmp+=(${pixels[@]/#/'\x'})
local IFS=''
#echo -ne "${_bmp[*]}" >/tmp/mtop.bmp
echo -ne "${_bmp[*]}"
IFS=$OLDIFS
}
# init_bmp bmp_ver width height
init_bmp() {
local i
bmp_ver=${1:-$bmp_ver}
width=${2:-$width}
height=${3:-$height}
rowbytes=$((width * bpp))
pixbytes=$((width * height * bpp))
make_bmp_header
if [ ${#pixels[@]} -ne $pixbytes ]; then
pixels=()
for ((i = 0; i < width * height; i++)); do
pixels+=(${curcol[@]});
done
fi
}
## End of bmplib.sh
file="$1"
file_size=$(stat --printf="%s" "$file")
columns=1024
block_length=1024
rows=$(( file_size / columns ))
# round up for the last partial row
if (( rows > 1 )) && (( (file_size % columns) > 0 )); then
rows=$(( rows + 1 ))
fi
blocks=$(( rows / block_length ))
if (( blocks > 1 )) && (( (rows % block_length) > 0 )); then
blocks=$(( blocks + 1 ))
fi
image_width=$columns
image_height=$(( 2 + rows + (10 * blocks) ))
pixels=()
curcol=(ff 00 00 ff) # set the color to blue
init_bmp 5 "$image_width" "$image_height"
current_x=0
current_y="$(( image_height - 1 ))"
function next_line()
{
current_x=0
current_y=$(( current_y - 1 ))
}
function next_pixel()
{
current_x=$(( current_x + 1 ))
}
function write_start_header()
{
# every image starts with a black column
curcol=(00 ff 00 ff)
line 0 "$current_y" "$(( image_width - 1 ))" "$current_y"
next_line
}
function write_byte()
{
curcol=("$1" "$1" "$1" ff)
point "$current_x" "$current_y"
next_pixel
}
function write_file_size()
{
write_byte "$(printf '%.2x' "$(( (file_size & 0xff000000) >> 24 ))")"
write_byte "$(printf '%.2x' "$(( (file_size & 0xff0000) >> 16 ))")"
write_byte "$(printf '%.2x' "$(( (file_size & 0xff00) >> 8 ))")"
write_byte "$(printf '%.2x' "$(( file_size & 0xff ))")"
next_line
}
function write_block_spacer()
{
rect 0 "$current_y" "$(( image_width - 1 ))" 10
current_y=$(( current_y - 10 ))
}
write_start_header
write_file_size
byte_count=0
row_count=0
for byte in $(od -An -t xC "$file"); do
write_byte "$byte"
byte_count=$(( byte_count + 1 ))
if (( byte_count >= columns )); then
byte_count=0
row_count=$(( row_count + 1 ))
next_line
fi
if (( row_count >= block_length )); then
write_block_spacer
row_count=0
fi
done
output_bmp
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