This is my handout for paranoid people who want a way to store bitcoin safely. It requires a little work, but this is the method I use because it should be resistant to risks associated with:
- Bad random number generators
- Malicious or flawed software
- Hacked computers
If you want a method that is less secure but easier, skip to the bottom of this post. The Secure Method
- Download bitaddress.org. (Try going to the website and pressing "ctrl+s")
- Put the bitaddress.org file on a computer with an operating system that has not interacted with the internet much or at all. The computer should not be hooked up to the internet when you do this. You could put the bitaddress file on a USB stick, and then turn off your computer, unplug the internet, and boot it up using a boot-from-CD copy of linux (Ubuntu or Mint for example). This prevents any mal-ware you may have accumulated from running and capturing your keystrokes. I use an old android smart phone that I have done a factory reset on. It has no sim-card and does not have the password to my home wifi. Also the phone wifi is turned off. If you are using a fresh operating system, and do not have a connection to the internet, then your private key will probably not escape the computer.
- Roll a die 62 times and write down the sequence of numbers. This gives you 2160 possible outcomes, which is the maximum that Bitcoin supports.
- Run bitaddress.org from your offline computer. Input the sequence of numbers from the die rolls into the "Brain Wallet" tab. By providing your own source of randomness, you do not have to worry that the random number generator used by your computer is too weak. I'm looking at you, NSA ಠ_ಠ
- Brain Wallet tab creates a private key and address.
- Write down the address and private key by hand or print them on a dumb printer. (Dumb printer means not the one at your office with the hard drive. Maybe not the 4 in 1 printer that scans and faxes and makes waffles.) If you hand copy them you may want to hand copy more than one format. (WIF and HEX). If you are crazy and are storing your life savings in Bitcoin, and you hand copy the private key, do a double-check by typing the private key back into the tool on the "Wallet Details" tab and confirm that it recreates the same public address.
- Load your paper wallet by sending your bitcoin to the public address. You can do this as many times as you like.
- You can view the current balance of your paper wallet by typing the public address into the search box at blockchain.info
- If you are using an old cell phone or tablet do a factory reset when you are finished so that the memory of the private keys is destroyed. If you are using a computer with a boot-from-CD copy of linux, I think you can just power down the computer and the private keys will be gone. (Maybe someone can confirm for me that the private keys would not be able to be cached by bitaddress?)
- To spend your paper wallet, you will need to either create an offline transaction, or import the private key into a hot wallet. Creating an offline transaction is dangerous if you don't know what you are doing. Importing to a client side wallet like Bitcoin-Qt, Electrum, MultiBit or Armory is a good idea. You can also import to an online wallet such as Blockchain.info or Coinbase.
The only thing you need bitaddress.org to do is to honestly convert the brainwallet passphrase into the corresponding private key and address. You can verify that it is doing this honestly by running several test passphrases through the copy of bitaddress that you plan on using, and several other brainwallet generators. For example, you could use the online version of bitaddress, and brainwallet
. If you are fancy with the linux command line, you can also try "echo -n my_die_rolls | sha256sum". The linux operating system should reply with the same private key that bitaddress makes. This protects you from a malicious paper wallet generator. Trusting your copy of bitaddress.org
Bitaddress publishes the sha1 hash of the bitaddress.org website at this location: https://www.bitaddress.org/pgpsignedmsg.txt
The message is signed by the creator, pointbiz. I found his PGP fingerprint here: https://github.com/pointbiz/bitaddress.org/issues/18
"527B 5C82 B1F6 B2DB 72A0 ECBF 8749 7B91 6397 4F5A"
With this fingerprint, you can authenticate the signed message, which gives you the hash of the current bitaddress.org file. Then you can hash your copy of the file and authenticate the file.
I do not have a way to authenticate the fingerprint itself, sorry. According to the website I linked to, git has cryptographic traceability that would enable a person to do some research and authenticate the fingerprint. If you want to go that far, knock yourself out. I think that the techniques described in this document do not really rely on bitaddress being un-corrupt. Anyway, how do we know pointbiz is a good guy? ;-)
There are a lot of skilled eyes watching bitaddress.org and the signed sha1 hash. To gain the most benefit from all of those eyes, it's probably worthwhile to check your copy by hashing it and comparing to the published hash. "But we aren't supposed to use brainwallets"
You are not supposed to use brainwallets that have predictable passphrases. People think they are pretty clever about how they pick their passphrases, but a lot of bitcoins have been stolen because people tend to come up with similar ideas. If you let dice generate the passphrase, then it is totally random, and you just need to make sure to roll enough times. How to avoid spending your life rolling dice
When I first started doing this, I rolled a die 62 times for each private key. This is not necessary. You can simply roll the die 62 times and keep the sequence of 62 numbers as a "seed". The first paper address you create would use "my die rolls-1" as the passphrase, the second would be "my die rolls-2" and so on. This is safe because SHA256 prevents any computable relationship between the resulting private key family.
Of course this has a certain bad security scenario -- if anyone obtains the seed they can reconstruct all of your paper wallets. So this is not for everyone! On the other hand, it also means that if you happen to lose one of your paper wallets, you could reconstruct it so long as you still had the seed.
One way to reduce this risk is to add an easy to remember password like this: "my die rolls-password-1".
If you prefer, you can use a technique called diceware
to convert your die rolls to words that still contain the same quantity of entropy, but which could be easier to work with. I don't use diceware because it's another piece of software that I have to trust, and I'm just copy/pasting my high entropy seed, so I don't care about how ugly it is. Why not input the dice as a Base 6 private key on the Wallet Details tab?
Two reasons. First of all, this option requires that you roll the die 99 times, but you do not get meaningful additional protection by rolling more than 62 times. Why roll more times if you don't have to? Second, I use the "high entropy seed" method to generate multiple private keys from the same die rolls. Using the Base 6 option would require rolling 99 times for every private key. I'm a big nerd with exotic dice. How many times to roll?
Put this formula in Excel to get the number of times to roll: "=160*LOG(2,f)" where f = number of faces on the die. For example, you would roll a d16 40 times. By the way, somewhat unbelievably casino dice are more fair than ordinary dice The "Change address" problem:
You should understand change addresses because some people have accidentally lost money by not understanding it.
Imagine your paper wallet is a 10 dollar bill. You use it to buy a candy bar. To do this you give the cashier the entire 10 dollar bill. They keep 1 dollar and give you 9 dollars back as change.
With Bitcoin, you have to explicitly say that you want 9 dollars back, and you have to provide an address where it should go to. If you just hand over the 10 dollar bill, and don't say you want 9 dollars back, then the miner who processes the transaction gives 1 dollar to the store and keeps the remainder themselves.
Wallet software like Bitcoin-Qt handles this automatically for you. They automatically make "change addresses" and they automatically construct transactions that make the change go to the change address.
There are three ways I know of that the change problem can bite you:
- You generate a raw transaction by hand, and screw up. If you are generating a transaction "by hand" with a raw transaction editor, you need to be extra careful that your outputs add up to the same number as your inputs. Otherwise, the very lucky miner who puts your transaction in a block will keep the difference.
- You import a paper wallet into a wallet software and spend part of it, and then think that the change is in the paper wallet. The change is not in the paper wallet. It is in a change address that the wallet software generated. That means that if you lose your wallet.dat file you will lose all the change. The paper wallet is empty.
- You import a paper wallet into a wallet software and spend part of it, and then think that the change is in the change address that the wallet software generated. If the transaction did not need to consume all of the "outputs" used to fund the paper wallet, then there could be some unspent outputs still located at the address of the paper wallet. If you destroyed the paper wallet, and destroyed the copy of the private key imported to the wallet software, then you could not access this money. (E.g. if you restored the software wallet from its seed, thinking all of the money was moved to the wallet-generated change addresses.)
For more on this, see here The hot paper wallet problem
Your bitcoin in your paper wallet are secure, so long as the piece of paper is secure, until you go to spend it. When you spend it, you put the private key onto a computer that is connected to the internet. At this point you must regard your paper wallet address as hot
because the computer you used may have been compromised. It now provides much less protection against theft of your coins. If you need the level of protection that a cold paper wallet provides, you need to create a new one and send your coins to it. Destroying your paper wallet address
Do not destroy the only copy of a private key without verifying that there is no money at that address. Your client may have sent change to your paper wallet address without you realizing it. Your client may have not consumed all of the unspent outputs available at the paper wallet address. You can go to blockchain.info
and type the public address into the search window to see the current balance. I don't bother destroying my used/empty paper wallet addresses. I just file them away. Encrypting your private key BIP 0038
describes a standardized way to encrypt your paper wallet private key. A normal paper wallet is vulnerable because if anyone sees the private key they can take the coins. The BIP38 protocol is even resistant to brute force attacks because it uses a memory intensive encryption algorithm called scrypt. If you want to encrypt your wallets using BIP38, I recommend that you use bitcoinpaperwallet
because they will let you type in your own private key and will encrypt it for you. As with bitaddress, for high security you should only use a local copy of this website on a computer that will never get connected to the internet. Splitting your private key
Another option for protecting the private key is to convert it into multiple fragments that must be brought together. This method allows you to store pieces of your key with separate people in separate locations. It can be set up so that you can reconstitute the private key when you have any 2 out of the 3 fragments. This technique is called Shamir's Secret Sharing
. I have not tried this technique, but you may find it valuable. You could try using this website http://passguardian.com/
which will help you split up a key. As before, you should do this on an offline computer. Keep in mind if you use this service that you are trusting it to work properly. It would be good to find other independently created tools that could be used to validate the operation of passguardian. Personally, I would be nervous destroying the only copy of a private key and relying entirely on the fragments generated by the website.
Looks like Bitaddress has an implementation of Shamir's Secret Sharing now under the "Split Wallet" tab. However it would appear that you cannot provide your own key for this, so you would have to trust bitaddress. Durable Media
Pay attention to the media you use to record your paper wallet. Some kinds of ink fade, some kinds of paper disintegrate. Moisture and heat are your enemies.
In addition to keeping copies of my paper wallet addresses I did the following:
- Order a set of numeric metal stamps. ($10)
- Buy a square galvanized steel outlet cover from the hardware store ($1)
- Buy a sledgehammer from the hardware store
- Write the die rolls on the steel plate using a sharpie
- Use the hammer to stamp the metal. Do all the 1's, then all the 2's etc. Please use eye protection, as metal stamp may emit sparks or fly unexpectedly across the garage. :-)
- Use nail polish remover to erase the sharpie
If you trust electrum
you might try running it on an offline computer, and having it generate a series of private keys from a seed. I don't have experience with this software, but it sounds like there are some slick possibilities there that could save you time if you are working with a lot of addresses. Message to the downvoters
I would appreciate it if you would comment, so that I can learn from your opinion. Thanks! The Easy Method
This method is probably suitable for small quantities of bitcoin. I would not trust it for life-altering sums of money.
- Download the bitaddress.org website to your hard drive.
- Close your browser
- Disconnect from the internet
- Open the bitaddress.org website from your hard drive.
- Print a paper wallet on your printer
- Close your browser
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