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I'm thinking of using a brain wallet for btc storage. Yes i know all the talk about it being insecure. But what if I used a bunch of information that only I would know making a >100 character brain wallet ? What is the max length of a brain wallet?
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Brainwallet algorithm uses hash function of entered data for creating privkey. So, there are no limits.
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A 256 bit hash of any string will result in a 256 bit (AKA 32 character) hash. For this reason, any string longer than 32 characters will collide with a hash of some other string, potentially one that is only 3 or 4 characters long. The risk is ridiculously low, of course, but it's still there.
Choosing a >100 character string to hash (which can certainly be done) is not necessarily more secure than choosing a 32 character string to hash.
Be super careful what you use as a password/passphrase, though. No full words or names, in any language. Do not use common phrases or quotes, or pieces of such. For example, using the first letters of all of the words of the Gettysburg address, although making for a long passphrase, is actually pretty likely to be stolen.
What you could do, however, is start with a root passphrase, hash it, and combine that hash with another passphrase, and repeat the process until you are satisfied with the result.
The reason you make it longer than 32 characters is that its really hard for a human being to make up a 32 character string with more entropy than 0x2aeccd2839a0ae1a6f186f9d1c009700c79720863a5cafe0a2bb759b4736b502. Remember those 32 characters would have to include byte values you probably couldn't type on your keyboard. – jeteon – 2016-02-13T07:12:33.900
I agree, a truly random or pseudorandom passphrase will have more entropy than a human-made passphrase. If you are saying that a 100 character human-made passphrase will have more entropy than a 32 character human-made passphrase, then you may be right. Still, it's going to collide with another passphrase's hash -- one that may or may not have a lot less entropy than your 100 character masterpiece. Do you want to chance it? – user32647 – 2016-03-01T14:04:16.437
Concur. Really limited by what your brain can remember... – skaht – 2015-07-07T03:24:38.003