Paperkey (Was: Re: )

Ciprian Dorin, Craciun ciprian.craciun at
Sat Mar 13 07:52:27 CET 2010

    I've used the `dtmx` tool to export some GPG keys (exactly a 4096
bits one) and it worked.

    What I did was something like:
        paperkey --secret-key ./key.gpg --output ./key.paperkey
--output-type raw
        dmtxwrite --encoding 8 --format png --resolution 72
<./key.paperkey >./key.png

    And everything worked just perfectly.

    But after looking through my scripts (because I've automated the
entire process), I see a test where I check if the size of the file to
be encoded is less than 1555 bytes, because it seems that dtmx (in
fact the specification?) doesn't allow files larger than this.

    Another quirk that I've found is that decoding back that image
after being printed, photographed and put back in my computer worked
Ok for small sizes (~200-300 bytes), but for other files (closer to
~1000 bytes) I was not able to decode the image back even after some
image processing that enhances the contrast, sharpens the colors, etc.
(I haven't tried a scanner, but I guess that would have worked.)
    (But the original file spitted out by dtmx was decoded just fine.)


On Fri, Mar 12, 2010 at 10:42 PM, john espiro <john_espiro at> wrote:
> Hi David -
> Thanks for the quick reply.
> I should note that I am on a Windows XP box.
> When I run gpg --export-secret-key my at | paperkey  --output-type raw
> for both the 2048 and 4096, I note that the output for the 4096 key is about
> 10% logner than that of the 2048 key.
> I am not familiar with wc.exe, but will google it...
> John

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