Generate BIP-39 mnemonic seed phrases (12/24 words) with entropy validation. Verify existing mnemonics and compute seeds.
This tool generates BIP-39 mnemonic seed phrases of 12 or 24 words, using a cryptographically secure random number generator in your browser. It also verifies that an existing phrase is valid according to the BIP-39 checksum, and computes the 512-bit seed from the phrase and an optional passphrase. The most common surprise: the passphrase is not the same as the 25th word; changing it changes the derived seed entirely, so forgetting it locks you out of funds derived from that seed.
A concrete input and expected output from the current implementation.
Input
abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about
Expected output
Seed: 0x5eb00bbddcf069084889a8ab9155568165f5c453ccb85e70811aaed6f6da5fc19a5ac40b389cd370d086206dec8aa6c43daea6690f20ad3d8d48b2d2ce9e38e4
This is the phrase from the BIP-39 test vectors for a 12-word mnemonic with entropy of all zero bits. The seed shown here is for an empty passphrase. The tool computes the seed using PBKDF2-HMAC-SHA512 with 2048 iterations and the passphrase treated as an empty string.
The tool generates 128 or 256 bits of entropy using the browser's secure random API. It then computes a SHA-256 hash, takes the first 4 or 8 bits as a checksum, and appends them to the entropy. The resulting bit string is split into 11-bit groups, each mapped to a word from the 2048-word BIP-39 English list. The last word is derived from both the entropy and the checksum bits together; it is not independent of the entropy.
When you paste a phrase, the tool splits it into words, looks up each in the list, and reconstructs the bit string. It checks that the last word's bits, when the checksum portion is extracted and compared to the computed hash of the entropy, match. If valid, it derives the seed with PBKDF2 using the phrase (with spaces) as the password, the passphrase prefixed with 'mnemonic' as the salt, 2048 iterations, and SHA-512. The output is 64 bytes displayed in hex.
Yes, the passphrase adds an extra layer of security. It is not stored anywhere; you must remember it. Without it, you cannot derive the same seed from the phrase. It is not a password that can be reset.
No, but the probability of collision is astronomically small. For 128 bits of entropy, there are 2^128 possible phrases. Generating a phrase twice is practically impossible, so you can treat each generated phrase as unique.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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