How does the BIP-39 standard work?

BIP-39 is the technical standard that turns a gigantic random number into a sequence of words — the Seed Phrase. It defines an official list of 2,048 words and the rule for converting random data into those words and back again. Thanks to BIP-39, a Seed created in one wallet can be restored in virtually any other.

BIP stands for Bitcoin Improvement Proposal. BIP-39 was proposed in 2013 and is now used by Bitcoin and Ethereum wallets, as well as those for most other networks.

Why use words instead of numbers

A wallet's security depends on a random number of 128 or 256 bits — something like a string of 39 or 78 decimal digits. Writing that down by hand without mistakes is nearly impossible. Common words are much easier to record, double-check and read out loud.

How the Seed Phrase is generated, step by step

• Entropy: the wallet generates a random number of 128 bits (for 12 words) or 256 bits (for 24 words).
• Checksum: a few verification bits, calculated from that number, are added to the end (4 bits for 12 words, 8 for 24).
• Splitting: the result is split into 11-bit chunks. Each chunk is a number from 0 to 2,047.
• Words: each number becomes the matching word on the 2,048-word list.

The math checks out: 12 words × 11 bits = 132 bits (128 of entropy + 4 of checksum). 24 words × 11 bits = 264 bits (256 + 8).

Seed Phrase lengths in BIP-39

• 12 words: 128 bits of entropy — about 3.4 × 10³⁸ combinations
• 15, 18 and 21 words: intermediate, less common lengths
• 24 words: 256 bits of entropy

In practice, 12 words are already impossible to guess by brute force. The real risk lies in how you store it, not in its length.

The role of the checksum

Because of the verification bits, not every combination of words is a valid Seed. If you get a word wrong when restoring, the wallet will usually warn you that the Seed is invalid. But the checksum doesn't catch every error — and it can't tell if you've mixed up the Seed of one wallet with another.

Why word order matters

The words represent chunks of a number, so the order is part of the information. The same 12 words in a different order produce a different wallet. Your Seed must be stored exactly as it was created: same words, same order, same spelling.

The word list

The English list is the most widely used. It was designed to avoid confusion: the first four letters of each word are enough to identify it uniquely, which helps correct typos and lets you engrave just those four letters on metal plates. The standard also includes official lists in other languages.

Passphrase: the "25th word"

BIP-39 lets you add an optional password, called a passphrase. The same Seed with different passphrases produces completely different wallets. It adds protection in case someone finds your words — but if you forget the passphrase, the Seed alone won't recover that wallet.

From Seed Phrase to private keys

The words (and the passphrase, if any) go through a derivation function that produces the 512-bit binary "seed." From it, other standards — such as BIP-32 and BIP-44 — derive all of the wallet's private keys and addresses for each network. That's why a single Seed recovers every address in a wallet.

BIP-39 compatibility across wallets and backups

Because it's a standard, a BIP-39 Seed works with any compatible tool — wallets, hardware wallets and backup solutions such as the SafeVault Card, which generates or imports 12-word Seeds and keeps an encrypted backup.

Frequently asked questions

Can I choose my own words?
It's not recommended. Words chosen by people aren't random and are much easier to guess. Let the wallet generate the Seed.

Does every wallet use BIP-39?
The vast majority do, but some use their own formats. Check compatibility before restoring a Seed in another wallet.

Do I need to store the passphrase with the Seed?
Store it, but in a separate place. If the two are kept together, the passphrase loses its purpose; if you lose the passphrase, you lose the wallet.

Conclusion

BIP-39 made self-custody practical for everyday people: it turned numbers that are impossible to write down into words anyone can record and double-check. That simplicity, however, doesn't lessen the responsibility.

A BIP-39 Seed Phrase is the master key to your wallet: it must be stored exactly as it was created, offline and away from prying eyes.