On August 7th I will be giving a talk at DEF CON about cracking brainwallets. As part of that talk, I will be releasing a fast brainwallet cracker. I’m writing this post to provide a little insight as to why I’m giving away a tool that could be used to steal. I also hope that people who are currently using brainwallets will take notice and move to a more secure storage method.
By my estimates, a single day should be more than enough time for a botnet to check every possible eight character ASCII password and XKCD-style passphrase against every Bitcoin address that has ever received funds. There are already people cracking brainwallets, but it’s unclear what exactly their capabilities are. I will be presenting some research on that at DEFCON (particularly weak brainwallets have been robbed within seconds), but I can only divine so much information indirectly. Releasing a cracker will give concrete, indisputable evidence of what’s actually possible, and mine probably isn’t faster than what bad guys are already using. Hopefully this will convince people not to use (or stop using) brainwallets.
In computer security, there’s a concept known as responsible disclosure. The idea is that if someone like me discovers a bug, they make a good faith effort to get the bug fixed before sharing it with the world. I’ve done this in the past, and I think it’s generally the right approach. Sometimes, as in the current situation, there’s just no getting the bug fixed, or it’s already being exploited. In such a case, the best thing is to let everyone know so they can take appropriate steps to protect themselves.
If you’re using a brainwallet, move your coins - NOW! Your passphrase is not as strong as you think it is. Don’t think you’re safe because you use some other cryptocurrency - the same tools and techniques work just as well against them.
I recommend a BIP38 paper wallet with a passphrase generated using diceware with at least eight words. If you must use something that is “purely in your brain”, look into WarpWallet, but use it with a salt and a diceware password (again - at least eight words). Humans brains are too predictable. If you’re using a password or passphrase that has been used for a brainwallet anywhere else, change it. I’ll be posting again soon about my work on passphrase schemes designed for human memory.