BIP-39 Mnemonic
Generate or verify a BIP-39 seed phrase.
Overview
The BIP-39 mnemonic tool generates a fresh 12, 15, 18, 21, or 24-word seed phrase, and also verifies that an existing phrase has a valid checksum. Pick a strength, hit generate, and copy the wordlist into your wallet; or paste your backup phrase to confirm you wrote it down correctly.
Cryptocurrency users restoring or auditing a wallet, hardware wallet owners double-checking their backup, and developers writing wallet tooling all need a BIP-39 seed phrase generator that handles the checksum byte correctly. Treat the output like cash — anyone holding the phrase controls the wallet.
How it works
BIP-39 (Bitcoin Improvement Proposal 39, 2013) maps random entropy to memorable words. The tool draws 128–256 bits of cryptographically secure random bytes, appends a checksum equal to the first entropy_bits / 32 bits of SHA-256(entropy), splits the resulting bit string into 11-bit chunks, and looks each chunk up in the standard 2048-word BIP-39 English wordlist. Verification reverses the process: convert words back to indices, recompute SHA-256 over the entropy portion, and confirm the checksum bits match.
To derive a wallet seed from the phrase, BIP-39 specifies PBKDF2-HMAC-SHA-512 with 2048 iterations and a salt of "mnemonic" + optional passphrase. The output is a 512-bit seed used by BIP-32 to generate hierarchical deterministic keys.
Examples
Strength: 128 bits (12 words)
Output: legal winner thank year wave sausage worth useful legal winner thank yellow
Strength: 256 bits (24 words)
Output: void come effort suffer camp survey warrior heavy shoot primary clutch crush
open amazing screen patrol group space point ten exist slush involve unfold
Verify: "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about"
Output: Valid checksum
FAQ
How is this random?
The tool reads from the browser's crypto.getRandomValues, which is a cryptographically secure PRNG. For maximum paranoia, generate phrases on an air-gapped device.
Why do some 12-word combinations fail verification?
Because the final word encodes a checksum of the previous bits. Only 1 in 16 random 12-word combinations passes; only 1 in 256 random 24-word combinations does. This is what catches single-word transcription errors.
What about other languages?
BIP-39 defines wordlists for English, Japanese, Korean, Spanish, Chinese, French, Italian, Czech, and Portuguese. The first four letters of each word in the English list are unique, so abbreviated backups still verify. This tool uses the English list by default.
Should I add a passphrase (BIP-39 "25th word")?
Optional. It hardens the seed against an attacker who has only the wordlist, at the cost of a credential you must never forget. Lose the passphrase and the funds are unrecoverable.