pnginator: pack Javascript into a self-extracting PNG

pnginator.rb

#!/usr/bin/env ruby -w

# pnginator.rb: pack a .js file into a PNG image with an HTML payload;
# when saved with an .html extension and opened in a browser, the HTML extracts and executes
# the javascript.

# Usage: ruby pnginator.rb input.js output.png.html

# By Gasman <http://matt.west.co.tt/>
# from an original idea by Daeken: http://daeken.com/superpacking-js-demos


MAX_WIDTH = 4096
USE_PNGOUT = true

require 'zlib'
require 'tempfile'

input_filename, output_filename = ARGV

f = File.open(input_filename, 'rb')
js = f.read
f.close

if js.length < MAX_WIDTH
	# js fits onto one pixel line
	js += "\x00"
	scanlines = [js]
	width = js.length
	height = 1

	# Daeken's single-pixel-row bootstrap (requires js string to be reversed)
	# (edit by Gasman: change eval to (1,eval) to force global evaluation and avoid massive slowdown)
	# html = "<canvas id=q><img onload=for(p=q.width=#{width},(c=q.getContext('2d')).drawImage(this,0,e='');p;)e+=String.fromCharCode(c.getImageData(--p,0,1,1).data[0]);(1,eval)(e) src=#>"

	# p01's single-pixel-row bootstrap (requires an 0x00 end marker on the js string)
	# (edit by Gasman: move drawImage out of getImageData params (it returns undef, which is invalid) and change eval to (1,eval) to force global evaluation)
	html = "<canvas id=c><img onload=with(c.getContext('2d'))for(p=e='';drawImage(this,p--,0),t=getImageData(0,0,1,1).data[0];)e+=String.fromCharCode(t);(1,eval)(e) src=#>"
else
	js = "\x00" + js
	width = MAX_WIDTH
	# split js into scanlines of 'width' pixels; pad the last one with whitespace
	scanlines = js.scan(/.{1,#{width}}/m).collect{|line| line.ljust(width, "\x00")}
	height = scanlines.length

	# p01's multiple-pixel-row bootstrap (requires a dummy first byte on the js string)
	# (edit by Gasman: set explicit canvas width to support widths above 300; move drawImage out of getImageData params; change eval to (1,eval) to force global evaluation)
	html = "<canvas id=c><img onload=for(w=c.width=#{width},a=c.getContext('2d'),a.drawImage(this,p=0,0),e='',d=a.getImageData(0,0,w,#{height}).data;t=d[p+=4];)e+=String.fromCharCode(t);(1,eval)(e) src=#>"
end

# prepend each scanline with 0x00 to indicate 'no filtering', then concat into one string
image_data = scanlines.collect{|line| "\x00" + line}.join
idat_chunk = Zlib::Deflate.deflate(image_data, 9) # 9 = maximum compression

def png_chunk(signature, data)
	[data.length, signature, data, Zlib::crc32(signature + data)].pack("NA4A*N")
end

if USE_PNGOUT
	# Create a valid (no format hacks) .png file to pass to pngout
	f = Tempfile.open(['pnginator', '.png'])

	begin
		f.write("\x89PNG\x0d\x0a\x1a\x0a") # PNG file header
		f.write(png_chunk("IHDR", [width, height, 8, 0, 0, 0, 0].pack("NNccccc")))
		f.write(png_chunk("IDAT", idat_chunk))
		f.write(png_chunk("IEND", ''))
		f.close

		system("pngout", f.path, "-c0", "-y")

		# read file back and extract the IDAT chunk
		f.open
		f.read(8)
		while !f.eof?
			length, signature = f.read(8).unpack("NA4")
			data = f.read(length)
			crc = f.read(4)

			if signature == "IDAT"
				idat_chunk = data
				break
			end
		end
	ensure
		f.close
		f.unlink
	end
end

File.open(output_filename, 'wb') do |f|
	f.write("\x89PNG\x0d\x0a\x1a\x0a") # PNG file header

	f.write(png_chunk("IHDR", [width, height, 8, 0, 0, 0, 0].pack("NNccccc")))

	# a custom chunk containing the HTML payload; stated chunk length is 4 less than the actual length,
	# leaving the final 4 bytes to take the place of the checksum
	f.write([html.length - 4, "jawh", html].pack("NA4A*"))

	# can safely omit the checksum of the IDAT chunk  
	# f.write([idat_chunk.length, "IDAT", idat_chunk, Zlib::crc32("IDAT" + idat_chunk)].pack("NA4A*N"))
	f.write([idat_chunk.length, "IDAT", idat_chunk].pack("NA4A*"))

	# can safely omit the IEND chunk
	# f.write([0, "IEND", "", Zlib::crc32("IEND")].pack("NA4A*N"))
end

why is canvas not created from within onload?

@NyonCat It would take more bytes.

This works on Chrome and Firefox for me, and it's great! But on Safari or Edge, it just shows the png header text on the page, so I assumed it doesn't work on those browsers.

However, a friend said it works on Edge on his PC, and showed me a screenshot. Does anyone know if it's supposed to work or not on Edge, and Safari?

It didn't work with alert('hello world') in the input.js. Is there a specific way I have to write the js file?

@LinuxRocks2000 What browser did you try it in? It works for me in Firefox, but not Chrome - it seems that Chrome has tightened up its security restrictions since I created this, and now it will only work if you serve it over HTTP rather than opening the file locally.

@gasman Thanks for sharing your code,Is it still working? Tried with an alert message but not working.

@AmitSivaLevi See my comment above... have just retested, and the same thing applies on current versions. Firefox works completely, Chrome only works if you serve the file over HTTP (or, possibly, if you launch it with the command-line option --allow-file-access-from-files).

I've now put up a test copy, so you can see what's supposed to happen: https://gasman.github.io/pnginator/test.png.html . Hopefully this should work on both Chrome and Firefox; if you save the page and launch it locally, you'll probably find it only works on Firefox. If this copy works but the one you built doesn't, then something has gone wrong in the build process...

Fun fact, you can use the same trick with .webp for lower header overhead and better compression ratios