Generating QR code of very big file?

Question

I want to generate a QR code of my 4096-bit armored GPG private key. The key is so big, the program qrencode seems to fail because of its size.

$ gpg --export-secret-keys --armor > ~/private.key
$ ./qrencode -o test.png < ~/private.key

Result:

Failed to encode the input data: Numerical result out of range

How can I make that happen? Are there alternative programs to qrencode which can handle a very big GPG key? I want to print it on paper as this security.SE question suggested.

The comments of @geruetzel and @ cuonglm are addressing this version of my question.

Remove `<`, the command is `qrencode -o test.png -t png "$( cat private.key)"` — cuonglm, Apr 30 '16 at 15:53
@cuonglm sry, didn't realize that mistake. I updated my answer. — toogley, Apr 30 '16 at 16:10
are you coming up against the designed size limit for QR codes? consider searching down to "Maximum character storage capacity" on the wikipedia page: https://en.wikipedia.org/wiki/QR_code — Theophrastus, Apr 30 '16 at 17:03

score 9 · Answer 1 · answered May 03 '16 at 12:13

9

The key could not be encoded as a single QR code. But two (or more) could work.

Export your key (as before):

gpg --export-secret-keys --armor > private.key

Generate files of a maximun size of 2500 byte:
```
split -C 2500 private.key splitkey-
```

Convert each to one QR file (same name with extension .qr)

for file in splitkey-??; do
    <"$file" qrencode -s 3 -d 150 -o "$file".qr
done

When it is needed to recover the key. You can scan each QR code which will each produce an string. Just concatenate each string in sequence of the filenames. Lets assume that the result key is stored in the file newkey.

And you may test that the internal CRC-24 of the key checks out.

 gpg --dearmor newkey >/dev/null

If there is no error message, the key file has been reconstructed correctly.

answered May 03 '16 at 12:13

This is what I was thinking also, why not split the key into three pieces, then create three qr codes that can be scanned and reassembled. – J363 May 03 '16 at 12:20
1

Why do you specify the dpi in the `qrencode` command? I'm curious what purpose that serves. Is it necessary? – mas Jan 01 '19 at 21:14
Just FYI for future readers, `2500` might be sufficiently small to generate the a QR code, but a QR code that dense is very hard to scan with a phone. It's probable that you can still read it by taking a picture and running it through a command line utility like [those found in the arch aur](https://aur.archlinux.org/packages/?O=0&C=0&SeB=nd&K=QR+Code&outdated=&SB=n&SO=a&PP=50&do_Search=Go) but most android apps will have trouble with it. It's not too much more work to set the byte limit to `1000` on the `split` operation and just generate a few more QR codes. Then it scans like a dream. – mas Jan 01 '19 at 22:38

score 8 · Accepted Answer · edited Apr 30 '16 at 20:39

Your error message already gives a hint as to what's wrong!

Your one-liner is providing the actual file content as filename to the qrencode program. Hence the error message.

Try qrencode -o test.png -t png < private.key.

You should take a look at shell input-output redirection. For example, I/O Redirection.

I see that you too have found your way to the developers GitHub repository of qrencode :) There is an explanation why a 4096-bit key cannot be encoded as a QR code:

qrencode is encoding your private GPG key as 8 bit (binary|utf-8), because the key is not pure alphanumeric. It contains special character. the alphanumeric mode only supports those special character .(%*+-./:). So the maximum GPG key can only be 2953 char long.

From https://github.com/fukuchi/libqrencode/issues/31

Sry, i didn't realize it that - but still It seems that my gpg key is too big for Qrencode. (I updated my answer to address that issue) — toogley, Apr 30 '16 at 16:07

score 4 · Answer 3 · edited Jan 12 '21 at 15:55

4

If you pass the options -Sv40 (40 is a version), qrencode will automatically create multiple images for input that's too large. For example:

qrencode -o private.png -t png -Sv 40 < private.key

will output private-01.png, private-02.png, and private-03.png.

edited Jan 12 '21 at 15:55

AdminBee

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answered Feb 26 '20 at 18:54

Pakman

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the problem with the generated qr codes is that they are too dense and phones don't seem to be able to parse them – ccpizza Apr 08 '22 at 20:18

score 3 · Answer 4 · answered Apr 30 '16 at 17:02

3

I just found out that this is not possible.

% wc -c ~/private.key
6709 /home/toogley/private.key

(-c counts characters.)

to cite from wikipedia:

max characters for alphanumerical characters: 4,296.

answered Apr 30 '16 at 17:02

toogley

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Yes and even this limit does not apply to your key - see my answer's update! – geruetzel Apr 30 '16 at 17:34

score 3 · Answer 5 · answered Jul 22 '16 at 10:01

3

I suggest that you minimize your key before encoding it.

gpg --export --export-options export-minimal

Depending on how you've used your key in the past, it may save you old binding signatures.

answered Jul 22 '16 at 10:01

Frederick Nord

265
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score 2 · Answer 6 · answered Apr 30 '16 at 22:00

You may be interested in paperkey, which is designed to take a GPG secret key, and transform it into a sequence of bytes that can be printed out on paper. The secret key can be subsequently recovered from the text after scanning it or keying it in.

There's also a discussion of various ways of archiving data on paper which you might find interesting.

Generating QR code of very big file?

6 Answers6