BIP 39 (Mnemonic Phrases)

BIP 39 is the Bitcoin improvement proposal that defines mnemonic phrases, the human readable backup format used by most cryptocurrency wallets. Proposed in 2013, it converts between 128 and 256 bits of random entropy into a sequence of 12 to 24 words drawn from a fixed list of 2,048 English words, giving users a way to write down and restore an entire wallet.

Why it matters

Before mnemonic phrases, backing up a wallet meant exporting individual private keys or copying wallet files, and a single missed key could mean permanent loss. A BIP 39 phrase is a complete backup of every key a wallet will ever generate, and because the standard is shared across vendors, a phrase written down from one wallet can usually be restored in another. An optional passphrase, often called the 25th word, adds a second factor that is never stored with the words.

The same property creates the format's main risk. Anyone who obtains the words controls the funds, which is why seed phrase phishing remains one of the most common attacks against bitcoin holders.

How it works

The wallet generates random entropy, appends a checksum, and splits the result into 11 bit segments that each index one word in the 2,048 word list. To recover keys, the phrase is stretched through 2,048 rounds of PBKDF2 with HMAC-SHA512 into a 512 bit seed, which then feeds the BIP 32 hierarchical deterministic derivation tree that produces every address in the wallet.