What is a brain wallet in crypto?

A brain wallet is a crypto wallet where the secret that controls the funds lives only in your head. Instead of writing down a key or recovery phrase, you memorize something and rebuild the wallet from it when needed. The idea sounds appealing, but the most common early version proved easy for thieves to crack.

What is a bitcoin brain wallet?

The Bitcoin Wiki describes two kinds. The obsolete kind starts with a passphrase the user makes up, such as a sentence or quote. Software runs it through a single SHA-256 hash, and the result becomes the private key for one bitcoin address. Anyone who types the same phrase gets the same key.

The modern kind means memorizing a normal seed phrase that wallet software generated at random. The wallet itself is ordinary. Only the backup is kept in memory instead of on paper or metal.

Are brain wallets safe?

The passphrase kind is not, according to the evidence. People are poor at producing randomness, so phrases often come from quotes, song lyrics, common sayings or dates. Attackers can test billions of these with dictionary and rule-based guessing, because turning a guess into a key takes only one fast hash.

Researchers measured this directly. A 2016 study by Vasek, Bonneau, Castellucci, Keith and Moore tested about 300 billion candidate passwords and found 884 brain wallets used between 2011 and 2015. All but 21 had been drained, usually within 24 hours and often within minutes of being funded, by about a dozen competing thieves. The researchers also found that wallets holding more bitcoin did not use stronger passwords. A later follow-up by Castellucci, testing 3.9 trillion guesses, identified about 2,005 brain wallets that together had held about 3,219 BTC.

Even a memorized random seed carries risk. The Bitcoin Wiki notes that if the owner forgets it, dies or becomes incapacitated, the coins are lost for good. It does not recommend brain wallets for general use and advises moving funds out of old passphrase wallets.

Brain wallet vs seed phrase, what is the difference?

The difference is where the randomness comes from. A seed phrase under the BIP-39 standard starts with 128 to 256 bits of random data created by the wallet. That data is turned into 12 to 24 words from a fixed list of 2,048, with a checksum to catch typing errors. The BIP-39 word list guide explains the list.

BIP-39 also slows down guessing. It runs the phrase through 2,048 rounds of a key derivation function before producing the wallet seed. A random seed has so many possible combinations that guessing is not realistic, as can someone guess your seed phrase explains. A human-chosen sentence has no such guarantee.

BIP-39 does allow an optional extra passphrase on top of the seed. That passphrase adds protection, but it is not a brain wallet, because the random seed still has to exist and be backed up.