tools to write (block) data to png files and vice versa

blocktoimg.py

#!/usr/bin/env python3
# Distributed under the MIT software license
import binascii, struct, sys, io, argparse
from PIL import Image

IMG_WIDTH = 512 # could be made adaptive...
MIN_HEIGHT = 4 # minimum height of image; twitter won't let us upload anything smaller
BYTES_PER_PIXEL = 4 # RGBA, 8 bit

def div_roundup(x,y):
    return (x+y-1)//y

def block_to_png(blockdata):
    '''
    Embed data in PNG image.
    Pass in raw block data, returns raw PNG image.
    '''
    metaheader = struct.pack('>II', len(blockdata), binascii.crc32(blockdata))

    data = metaheader + blockdata

    # determine size;
    # add an extra pixel to make sure that there's always padding, twitter will
    # convert to JPG in the unlikely case that the entire alpha channel is 255
    pixels = div_roundup(len(data), BYTES_PER_PIXEL) + 1
    width = IMG_WIDTH
    height = max(div_roundup(pixels, width), MIN_HEIGHT)

    # add zero-padding
    padding_len = width*height*BYTES_PER_PIXEL - len(data)
    data += b'\x00' * padding_len

    # compress as PNG
    img = Image.frombytes("RGBA", (width, height), data)
    outf = io.BytesIO()
    img.save(outf, "PNG", optimize=True) # optimize sets compress_level to 9(max) as well
    return outf.getvalue()

def parse_arguments():
    parser = argparse.ArgumentParser(description='Convert bitcoin block data to PNG image')
    parser.add_argument('infilename', metavar='INFILE.hex', type=str, nargs='?',
            help='Block data (hex output from RPC "getblock <hash> false"). If not specified, read from standard input.')
    parser.add_argument('outfilename', metavar='OUTFILE.png', type=str, nargs='?',
            help='PNG image name. If not specified, write to standard output.')
    return parser.parse_args()

def main():
    args = parse_arguments()

    if args.infilename is not None:
        with open(args.infilename, 'r') as f:
            blockdata = binascii.a2b_hex(f.read().strip())
    else:
        blockdata = binascii.a2b_hex(sys.stdin.read().strip())

    imgdata = block_to_png(blockdata)

    if args.outfilename is not None:
        with open(args.outfilename, 'wb') as f:
            f.write(imgdata)
    else:
        sys.stdout.buffer.write(imgdata)

if __name__ == '__main__':
    main()

imgtoblock.py

#!/usr/bin/env python3
# Distributed under the MIT software license
import binascii, struct, sys, io, argparse
from PIL import Image

def png_to_block(blockdata):
    '''
    Extract embedded data in PNG image.
    Pass in raw PNG image, return raw block data.
    '''
    inf = io.BytesIO(blockdata)
    img = Image.open(inf)
    if img.format != 'PNG' or img.mode != 'RGBA':
        raise ValueError('Image has invalid format-mode {}-{}'.format(img.format, img.mode))
    imgdata = img.tobytes()

    metaheader = imgdata[0:8]
    (blocksize,crc) = struct.unpack('>II', metaheader)
    print('size {:d}×{:d}'.format(img.width, img.height))
    print('metaheader:')
    print('  size  : {:d}'.format(blocksize))
    print('  CRC32 : {:x}'.format(crc))

    if 8+blocksize > len(imgdata):
        raise ValueError('Block size does not fit in image, image was cropped or header corrupted')
    blockdata = imgdata[8:8+blocksize]

    if binascii.crc32(blockdata) != crc:
        raise ValueError('Block CRC mismatch')

    return blockdata

def parse_arguments():
    parser = argparse.ArgumentParser(description='Convert PNG image back to bitcoin block data')
    parser.add_argument('infilename', metavar='INFILE.png', type=str, nargs='?',
            help='PNG image name. If not specified, read from standard input.')
    parser.add_argument('outfilename', metavar='OUTFILE.txt', type=str, nargs='?',
            help='Block data (hex output like RPC "getblock <hash> false"). If not specified, write to standard output.')
    return parser.parse_args()

def main():
    args = parse_arguments()

    if args.infilename is not None:
        with open(args.infilename, 'rb') as f:
            imgdata = f.read()
    else:
        imgdata = sys.stdin.buffer.read()

    blockdata = png_to_block(imgdata)

    if args.outfilename is not None:
        with open(args.outfilename, 'w') as f:
            f.write(binascii.b2a_hex(blockdata).decode() + '\n')
    else:
        sys.stdout.write(binascii.b2a_hex(blockdata).decode() + '\n')

if __name__ == '__main__':
    main()

Here's some additional help for anyone trying to use these scripts:

1. Get the file: https://pbs.twimg.com/media/DjNNVO1XcAMzSAx.png:orig
2. $ pip3 install Pillow
3. Run the example in the tweet:
   $ python3 imgtoblock.py DjNNVO1XcAMzSAx.png output.txt
4. This process will create a file called "output.txt" in addition to printing the size (8 byte length) and CRC32 (header) in the CLI:
   size 512×571
metaheader:
size : 1168239
CRC32 : 5e8dabe1
5. Open output.txt and notice that the HEX matches what's shown here: https://blockchain.info/block/0000000000000000000972454a8f0553eaae550dccfc0abd7c9be41e5b0f3078?format=hex

So far, so good. Now let's go the other direction and encode this block back to an image:

1. Using the file we just created, output.txt (the HEX representation of block 534106) we run blocktoimg.py:
   $ python3 blocktoimg.py output.txt output.png
2. This process will create a file called "output.png" in addition to printing the dimensions of the exported image:
   exporting 512×571 to output.png
3. This image is identical to our original reference image in the tweet.
   You can verify this by running the images through a diff viewer.
   The comparison results in this output, showing no difference between the two images (no red coloring): https://i.imgur.com/4cHAldg.jpg

@christianboyle Thank you for the write-up. My snippet is really under-documented, hadn't expected it to catch on so much!

Edit: improved the code a bit, so that it can be used as module and in shell pipe constructs, and arguments are documented.

hello, Trying to runt the script but I get "SyntaxError: Non-ASCII character '\xc3' in file(imgtoblock.py) on line 19, but no encoding declared;"
What encoding should I declare in the file?

@radWorx are you trying to run it with python 2? This is python 3, and for python 3 all source files are UTF-8 and there is no need to specify it.