Smeared private key on bitcoin bill.

[PrintCoins]

Someone wrote me about a bill they printed from http://print.printcoins.com, and couldn't recover their private key from it. I figured I would post it here since so many people are printing these bills now, and it might become a common problem.

Hi,
Hate to be a bother,
Have downloaded source and gotten help getting it setup.
now i've done something stupid,
loaded a bill, then smeared the #@$#$%ing private key QR (on a ronpaul note with no manual priv key in human chars)

Is there a way i can retrieve it, or a location where the pdf may be saved in the files somewhere?

Thank you much for any help you can give,
Byron

I am sorry to say that the code does not store any data, so there is no way to recover the private key from the server. If you downloaded a PDF and printed your bills from that, then you can always go back to the pdf, and just scan the QR from that. Maybe check browser caches, trash, etc. (These all should be cleared after print bills, but if you didn't, they are good places to look)

If all that fails, you can photocopy or scan your bill and maybe reconstruct the qr code by either using a pen to color in the qr code squares or use the scanned image in photoshop to manually try and fix it by eye. QR codes have error checking built in, so if you can get about 75% of the pattern reconstructed, you might be able to recover the private key.

Finally if all else fails, at least you can take solace in the fact that you increased the value of everyone else's bitcoins.

In the future, cotton paper does a pretty good job of holding printer ink, even in the wash. You can make this even tougher by putting scotch tape or clear packaging tape on the private key of the bill (both sides of the paper).

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If anyone wants to order printcoin.com bills, note that both sides have a protective tamper evident hologram who's glue seems to hold up very well in the wash.  They are printed on 32lb 100% cotton paper for durability.

[franky1]

lessons to be learned if using a inkjet.. USE FIXATIVE SPRAY!! its a kind of varnish.. google it.

or use colour laser printers

[MPOE-PR]

Putting clear tape over the qr is a good idea, laser printed stuff erases easily (especially if you're using cheap refills).

[casascius]

I would suggest trying to regenerate the code from your own sight. Open up a paint program, make a new file. Zoom really big, click the squares black where you see black.

The human eye can do far better at reading bar codes than any scanner, the human brain just doesn't understand the meaning. Use your eyes to read what you can into a new file and then scan it with the scanner.

QR can tolerate between 7-25% error rate so it is ok if you can't read some of the squares. Read what you can and guess on the rest. Just don't mess up the grid or skip rows or columns, it can't correct that.

Disclosure: I can read quite a few bar code symbologies by sight because I am geeky like that, but QR is not one of them, mostly due to complexity added by the XOR "masking" stage.  Still though, I can read many low quality things that scanners can't scan, so I can attest to the fact that scanners have poor sight.

[Peter Todd]

If you give up, at least post a scan of the bill here on the forums. Maybe someone can figure out how to decode it.

[casascius]

If you give up, at least post a scan of the bill here on the forums. Maybe someone can figure out how to decode it.

I would recommend against this, if he does this then he will certainly lose his funds.

[SgtSpike]

I would suggest trying to regenerate the code from your own sight. Open up a paint program, make a new file. Zoom really big, click the squares black where you see black.

The human eye can do far better at reading bar codes than any scanner, the human brain just doesn't understand the meaning. Use your eyes to read what you can into a new file and then scan it with the scanner.

QR can tolerate between 7-25% error rate so it is ok if you can't read some of the squares. Read what you can and guess on the rest. Just don't mess up the grid or skip rows or columns, it can't correct that.

Disclosure: I can read quite a few bar code symbologies by sight because I am geeky like that, but QR is not one of them, mostly due to complexity added by the XOR "masking" stage.  Still though, I can read many low quality things that scanners can't scan, so I can attest to the fact that scanners have poor sight.

To add to this, draw gridlines through the existing QR code so you can easily tell where each row/column starts and ends.

[DannyHamilton]

If you give up, at least post a scan of the bill here on the forums. Maybe someone can figure out how to decode it.

Just send me a scan/picture of the bill.  I'm trustworthy. You know this to be true, because I just told you so.

[Peter Todd]

If you give up, at least post a scan of the bill here on the forums. Maybe someone can figure out how to decode it.

I would recommend against this, if he does this then he will certainly lose his funds.

As I said if you give up.

[BkkCoins]

Photo the QR and use a graphics program like Photoshop or Gimp.
1. Crop it tightly.
2. use "threshold" function with various settings to try and recover most of it.
3. Scale it to a pixel count matching the grid size.
4. Visually fix pixels as best you can.
5. Feed the resulting image into a QR reader and check the result with bitaddress.org for a valid key.
6. Goto step 4 and tweak again until you get a valid key.