Glossary
Seed phrase: the words that can rebuild your wallet
A seed phrase is the master backup of a self-custody wallet. It is also the one thing a scammer most wants you to type into their website.
A seed phrase is a random sequence of words that lets you restore a crypto wallet if you lose it or its private key. Anyone who has the words can rebuild the wallet and move the crypto in it.
Quick answer
A seed phrase is a random list of words, usually 12 to 24 [2], that can restore your crypto wallet if you lose it or your private key [1]. Whoever has the words can rebuild the wallet and take the crypto, so store them securely and share them with no one.
Key points
- A seed phrase restores a wallet and every key it generates [3]; the SEC says store it securely and share it with no one [1].
- Under BIP-39 a phrase has 12, 15, 18, 21 or 24 words chosen from a list of 2,048 [3].
- No genuine offer needs your seed phrase: the FBI warns that fake free-token sites ask for it to empty wallets [2].
On this page
What does seed phrase mean?#
The SEC defines a seed phrase as a random sequence of words that allows you to restore your crypto wallet if you lose it or your private key [1]. The FBI puts it more bluntly: a string of 12 to 24 words that serve as the key to a user's crypto [2].
BIP-39 is a published standard for seed phrases, and it calls them a mnemonic sentence. Its authors chose words because they are easier for people to handle than raw binary or hexadecimal numbers [3]. Behind the words is a random number. BIP-39 turns that number into words, and the words are then processed into the seed from which the wallet generates its keys [3]. That is why a single phrase can rebuild every private key in the wallet.
Longer phrases encode a bigger random number. The table shows the five lengths BIP-39 allows.
| Phrase length | Random bits | Possible phrases |
|---|---|---|
| 12 words | 128 | a 39-digit number |
| 15 words | 160 | a 49-digit number |
| 18 words | 192 | a 58-digit number |
| 21 words | 224 | a 68-digit number |
| 24 words | 256 | a 78-digit number |
Word counts and random bits from the BIP-39 standard [3]. Possible phrases = 2 to the power of the random bits, calculated; for 12 words that is about 3.4 × 10 to the power 38. Not every wallet uses BIP-39.
Why must you never share a seed phrase?#
Because the phrase is the wallet. Anyone who types your words into a wallet app gets the same keys you have, and with them the power to sign transfers. The SEC's advice is to store your seed phrase in a secure place and not share it with anyone [1]. A crypto transfer, once made, is designed to be computationally impractical to reverse [4].
The huge numbers in the table only protect you while the words stay secret. Nobody needs to guess a phrase that you photographed, saved in an email or typed into the wrong website. That is why scammers try to get you to hand it over.
Our guide to crypto scams covers other requests that end the same way.
What does an optional passphrase add?#
BIP-39 lets a wallet mix an extra passphrase into the seed alongside the words [3]. With a passphrase, the words alone no longer rebuild the same wallet; you need both. That can protect you if someone finds the written words, but it creates a second secret you must not lose. Restoring the wallet without the exact passphrase does not bring back your crypto.
Whether you need one depends on how you hold crypto. With an exchange account, the provider holds the keys [1] and you protect a login instead, ideally with 2fa for crypto accounts. For the full picture of how the words, keys and wallet fit together, read seed phrase vs private key.
Frequently asked questions#
Is a seed phrase the same as a private key?
No. A private key authorises transactions for the crypto it controls, while the seed phrase can restore the whole wallet and its keys if you lose the wallet or the private key [1]. Both must stay secret.
Will a wallet company or support agent ever need my seed phrase?
What happens if I lose my seed phrase?
If you still have the wallet, nothing changes yet, but you have lost your backup. If the wallet is also lost, stolen or damaged, the SEC warns you may permanently lose access to your crypto [1].
The bottom line#
A seed phrase is a short list of words with total control over a self-custody wallet. Keep it somewhere secure and private, never type it into a site or app you reached from a message, and never share it. Read the risk disclosure before buying any crypto.
Sources
- Crypto Asset Custody Basics for Retail Investors - Investor Bulletin.
- Cybercriminals Defraud Hedera Hashgraph Network Non-Custodial Wallet Users Through Nonfungible Token Airdrops Disguised as Free Rewards.
- BIP 39 - Mnemonic code for generating deterministic keys (bip-0039.mediawiki, bitcoin/bips repository).
- Bitcoin: A Peer-to-Peer Electronic Cash System.
Education only. This page is not investment, tax or legal advice. Trading and crypto can lose you money. See our risk disclosure.
