What is a seed phrase?

What a Seed Phrase Does

A seed phrase does one job: it turns the random number at the heart of a wallet into words a person can write down. The standard most wallets follow is BIP39, the mnemonic code proposal in the Bitcoin repository, which describes two steps: generate the words, then convert them into a binary seed.¹

That seed is where every key comes from. BIP32 describes how a wallet derives a whole tree of key pairs from a single seed, and BIP44 adds a path for each coin and account, so one phrase can restore Bitcoin, Ethereum and other accounts in the same wallet.²³ Each key pair gives a public key and, from it, the addresses people send coins to.

So the phrase is not a password that a company can reset. It is the wallet. Lose it and a broken phone takes the coins with it. Leak it and someone else holds the wallet too, from anywhere.

How Many Words Are in a Seed Phrase

A seed phrase has 12, 15, 18, 21 or 24 words under BIP39, and most wallets use 12 or 24. The count follows from the math: BIP39 starts from 128 to 256 bits of random data (entropy), adds a short checksum, and splits the result into 11-bit pieces, each pointing at one word in a list of 2,048.

The table shows the five sizes the spec allows.

12 Word Seed Phrase vs 24 Words

A 12 word seed phrase carries 128 bits of entropy, and a 24 word phrase carries 256 bits. Both are far beyond guessing when the words come from a proper random source. The difference is margin, plus four extra checksum bits in the longer phrase.

Which one you get usually depends on the wallet, not on a setting you pick. What matters more than length is where the words were made and who has seen them, which is the rest of this page.

How the Last Word Works as a Checksum

The last word of a seed phrase works as a partial checksum: part of it is computed from a SHA256 hash of the other bits instead of being random. BIP39 tells wallet software to check it and to warn when it does not match, which catches many typos and swapped words.

The check is weak by design, and the spec says so: a random error slips past it about 1 time in 16 on a 12-word phrase (4 checksum bits) and about 1 time in 256 on a 24-word phrase (8 bits), and it cannot point to which word is wrong. Write the words carefully, in order, and compare each one with the BIP39 word list. Each word is unique by its first four letters, so a smudged ending can still be read.

Why You Never Type a Seed Phrase Into a Website

You never type a seed phrase into a website because the site's owner, or anyone who has hacked it, can read what you type and take the wallet. Unlike a password, a seed phrase cannot be changed after it leaks. Whoever has a copy can move the coins at any hour without your phone, your email or your approval.

The rule covers every channel: browser forms, "wallet validation" pages, chat windows, support tickets, email replies, spreadsheets, notes apps and photos. ethereum.org puts it plainly in its security guide: no legitimate service, support agent, or website will ever ask for it.⁴ It also warns against screenshots, since cloud syncing can expose them.

The only place the words belong is the wallet itself, at setup or when restoring on a device you control, and on the paper or metal backup you keep offline.

Why a Website Should Never Generate a Seed Phrase

A website should never generate a seed phrase because whatever program makes the random number also knows the words. BIP39 describes the phrase as computer-generated randomness written in human-readable form, so the generator sees every word first. A web page runs code its server sends, and the server can change that code at any time, log the result or send it elsewhere.

Onchain Wiki does not offer a seed phrase generator for that reason, and you should treat any site that offers one as unsafe. Real wallets create the phrase on your own device, ideally a cold wallet that never touches the internet.

Seed Phrase Scams That Ask for the Words

Seed phrase scams all end the same way: a person types or reads out the words, and the wallet is emptied. The ethereum.org security guide describes the common setups:

• Fake support. Someone poses as wallet or exchange support in a chat or direct message and asks for the phrase to "fix" a problem.
• Imitation websites. Phishing links lead to copies of real wallet pages that ask for the seed phrase to "verify" or "sync" a wallet.
• Giveaways. ethereum.org says giveaways are always scams, whether they ask for coins or for keys.
• Claim pages. Airdrop and reward pages that ask for the phrase are the imitation-site trick in another costume. Pages that ask for a signature instead are covered under wallet drainers.

A request for the words is the scam, whatever reason comes with it.

Recovery Phrase, Seed Phrase and Passphrase

Recovery phrase, seed phrase, mnemonic and secret recovery phrase all name the same BIP39 word list backup; wallets just label it differently. A passphrase is something else: an optional extra word or sentence that BIP39 mixes into the seed along with the phrase.

With a passphrase, the same seed phrase opens a different wallet. BIP39 notes that every passphrase produces a valid wallet, but only the correct one shows the expected coins. That protects against someone who finds the paper alone. It also means a forgotten passphrase locks the funds as surely as a lost phrase.

How to Keep a Recovery Phrase Offline

Keeping a recovery phrase offline means the words exist only on paper or metal that you hold, never as a file, photo or cloud note. BIP39 was written so the sentence could go on paper, and that is still the safest form for most people.

• Write the words in order, numbered, and check the spelling against the word list.
• Store the backup where fire, water and curious visitors cannot reach it.
• Never photograph it, scan it or type it into a phone.
• Tell the people who would need it one day where it is, without showing it to them.

The self custody guide covers the wider setup, and the wallets hub links every key and address topic on this site.

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Sources

Every number on this page comes from these sources, checked November 9, 2026.

1. github.com/bitcoin/bips/blob/master/bip-0039.mediawiki
2. github.com/bitcoin/bips/blob/master/bip-0032.mediawiki
3. github.com/bitcoin/bips/blob/master/bip-0044.mediawiki
4. ethereum.org/en/security