Bitcoin Forum

Be it a computer that is offline (I understand this a bit more, the transfer of files and certificates through USB makes more sense) or a paper wallet, I am really having a hard time understanding how that works.

It would be awesome if someone could do a good job of explaining how they work (especially paper wallets) or could point me in the direction of or make an explanation video on the topic.

https://i.imgur.com/s4kv4.jpg

Here's the wiki article on them:

 - http://en.bitcoin.it/wiki/Paper_wallet

Simply, the private key (the "Spend" QR Code on the wallet above) is created using computer code .. javascript, in the case of BitAddress.org:
 - http://BitAddress.org

That private key is needed to spend any coins sent to that Bitcoin address (the Load & Verify QR Code on the wallet above) that is derived from that private key.

The computer code that will use that private key to spend needs to do a few things like determine the unspent transactions and choose one or more of them to include in a spend transaction.  Blockchain.info will do this with an API call.  

Or you can import the private key into the Bitcoin.org client (or into your Blockchain.info/wallet, or any other client like Multibit, Electrum, etc.) and it will be treated just like any other private key in the wallet.   Also you can import it into Mt. Gox and they will sweep any unspent funds for that Bitcoin address to your Mt. Gox account.

Isn't risky sending money to an address that I can easily mistype because I have to write it on the client manually from the paper?  ???

No, since Bitcoin addresses include a checksum for exactly this reason. The chance of a mistyped address being accepted as valid is 4,294,967,296 to 1.

How does it work? I mean, if I change a letter of your address my client doesn't send the btcs?

Correct. Also (in the standard client at least), when you try (and fail) to send, the address in question will be highlighted in red to indicate that is invalid.

You can also print out QR-Codes that can be scanned with a webcam, smartphone, or document scanner to make address entry easier and less error prone.

I have done it. With a simple camera the online readers can't read my qr code. Is the quality wrong or I need a smartphone? I knew that even not perfect qr code can be read.

http://img824.imageshack.us/img824/4695/dscn8646b.jpg (http://imageshack.us/photo/my-images/824/dscn8646b.jpg/)

My phone was able to read that.

Adress beginning with 1Geb3w......


Looks like an issue with your phone.  Does it scan other QR codes without issue?  Does it have problem with some barcodes?  The library may be incomplete it may have problems with certain QR code versions.

Not sure what your issue is.  I've just read your image directly from the my computer monitor with my smartphone and it worked fine.  I'll try downloading it and sending it to my favorite online reader...

He's not using a phone to read the QR-Code.  He is using a (dumb)phone to take the picture and then sending the picture to an online QR-Decoder.

Wow that is really nice.

I tried sending your image to http://www.webqr.com/ and it was unable to read it.

I opened your image with image editing software, cropped out everything that wasn't part of the code, boosted the contrast, and reduced the size by 50%.

I ended up with an image which http://www.webqr.com/ was able to decode without a problem.

Perhaps try for better lighting, and a lower resolution image (if your phone allows you to adjust the image capture resolution).

Thanks, I'll do it. The photo was made by a photocamera, not a phone, it's funny that I have to lower the resolution in order to read it better  

It seems that smartphone reads qr better than the online readers.

Ok, now I have the privatekey. I followed these instructions (https://en.bitcoin.it/wiki/How_to_import_private_keys_v7%2B (https://en.bitcoin.it/wiki/How_to_import_private_keys_v7%2B)) to import it but the client says: "Error adding key to wallet (code -4)".

My wallet was unlocked, what's the problem? I didn't find the code -4 on the internet.  :-\

The code that you posted an image of in this discussion was the Bitcoin Address, not the Private Key.  If that is what you are trying to import it won't work.

NOTE: DO NOT POST AN IMAGE OF THE PRIVATE KEY OR ITS QR-CODE ANYWHERE ON THE INTERNET INCLUDING HERE.  IF ANYONE ELSE HAS ACCESS TO THAT CODE THEY CAN TAKE THE COINS ASSOCIATED WITH THAT ADDRESS.

Are you trying to load up a paper bitcoin with some amount of bitcoin or verify the amount that it is loaded with (in either case you need the Bitcoin Address only), or are you trying to redeem the value from a paper bitcoin into your wallet so you can spend it electronically (in which case you need the Private Key which is labeled "Spend" on the the printed paper bitcoin)?

I do know that I didn't posted the private key  ;)


Yes. And I used the instructions on the wiki. But I had the code 4 error.

Odd.  I suspect there is something wrong in the importprivkey command parameters you used, but I'm not sure what.

Generally when in wallet import format the private key will be start with a 5 and will be 51 characters long. Perhaps you had a copy/paste error when trying to get the private key to the command from the QR-Reader?  You might want to double check the private key you used in the importprivkey command and compare it to the one from the QR-Code (or printed on the paper).

Assuming that you were trying to import the following private key:

5Kb8kLf9zgWQnogidDA76MzPL6TsZZY36hWXMssSzNydYXYB9KF

Your command should look like this:

importprivkey 5Kb8kLf9zgWQnogidDA76MzPL6TsZZY36hWXMssSzNydYXYB9KF  "Testing import"

Nothing else extra, nothing missing.

If your wallet is encrypted with a password, you'll need to provide the password before you'll be able to run that command.  That is the purpose of the command:

walletpassphrase "YourLongPassphrase" 600

The 600 indicates that the wallet private keys are to remain unlocked for the next 10 minutes (600 seconds).

If none of this helps, then you'll need to wait for someone more knowledgeable than me to discover this discussion.  You may want to try posting a question about this over in one of the "Technical" fourms ("Technical Support" or "Technical Discussion").  You might have a better chance of finding someone who can help.

My guess is lowering the resolution removes the white streaks that cut through all the blocks due to the print quality.

BlockChain.info's private key import works really well and you can spend the funds instantly upon import.  They also have a webcam-based QR code scanner right in their website.

The bitcoins appeared in my wallet today, probably the client is slower than I tought.

Thank you all for your tips, now I can use some paper wallets  :)

(I think a guide would be useful for the no-smartphone-users)

Is it possible to recreate an online wallet from the hard copy alone in case the online wallet has been lost/destroyed/damaged/corrupted/forgot password/stolen/ect?  I have an online wallet at blockchain.info and will be making a paper wallet "savings account".  I just want to make sure I can redeem my funds somehow in case I cant access the online part.

Yes Blockchain makes it super easy, just go into the advanced tools and "import private key".

However, you're only recreating the wallet to the extent it involves that one private key.  Wallets tend to contain lots of private keys.  So if you want your funds on a single private key, it's far better to use the wallet to send your funds to a paper wallet, rather than to try to export keys.