BIP39

BIP39 is a Bitcoin Improvement Proposal that defines a mnemonic code for generating deterministic keys. It specifies a standardized way to encode entropy into a human-readable list of words and to convert that mnemonic back into usable binary data, enabling portable backups and recovery of wallets. The proposal also provides multilingual wordlists and defines how mnemonics are used in the key derivation process.

BIP39 details a process to convert random entropy into a mnemonic sentence. Entropy of 128 to 256 bits (in increments of 32) is generated, a checksum of ENT/32 bits is computed, and the two bitstreams are concatenated and split into 11-bit segments. Each 11-bit segment maps to a word in a 2048-word preapproved vocabulary, producing a mnemonic of 12, 15, 18, 21, or 24 words. The availability of multiple language wordlists allows mnemonic phrases to be created in several languages while preserving the same structure and checksum properties.

From mnemonic to seed, BIP39 specifies the mnemonic-to-seed derivation using PBKDF2 with HMAC-SHA512, 2048 iterations, a 64-byte output, and a salt formed from the string "mnemonic" plus an optional user-provided passphrase (normalized). The resulting seed can be fed into BIP32/BIP44 style hierarchical deterministic wallets to derive a full set of private keys and addresses.

Security and usage considerations include keeping the mnemonic secret, encrypting or backing it up securely, and using an optional passphrase for added protection. A mnemonic is not itself a private key, but a representation of entropy used to derive one; loss or exposure can compromise access to funds. BIP39 has been widely adopted by cryptocurrency wallets for interoperability.