Your wallet, qui encode le secret maître dont dérivent toutes ses clés privées. Elle permet de restaurer le wallet sur n'importe quel appareil compatible.">seed phrase is the master key to a self-custody wallet. Anyone who reads those 12 or 24 words can move your funds from any device in the world, and if you lose them, nobody can restore your access: there is no “forgot password” button on a blockchain. This guide explains seed phrase security step by step: what a recovery phrase actually is, how to write it down and store it, when a metal backup or a passphrase makes sense, and how to test your backup before you need it.
It assumes you already know the difference between custodial and self-custody wallets. If not, start with our guide to crypto wallets explained.
What a seed phrase is (BIP-39)
A seed phrase, also called a recovery phrase or mnemonic, is a human-readable version of a large random number. Almost every modern wallet follows the BIP-39 standard, published in 2013 for Bitcoin and now used across Ethereum, Solana and most other networks.
Here is how it works in practice:
- The wallet generates 128 to 256 bits of randomness (entropy).
- It adds a short checksum, then splits the result into groups of 11 bits. Each group points to one word in a fixed list of 2,048 words.
- 128 bits give 12 words, 256 bits give 24 words. 15, 18 and 21 words also exist but are rarer.
- The words are then run through a key-stretching function (PBKDF2, 2,048 rounds) to produce the seed from which every private key and address of the wallet is derived.
Two practical consequences follow. First, the order of the words matters as much as the words themselves. Second, the checksum lets a wallet detect most typos, but it cannot tell you which word is wrong.
In the English list, the first four letters of each word are enough to identify it. That is why some metal backup plates only ask you to stamp four letters per word.
Seed phrase, private key, PIN: what is the difference?
Your seed phrase generates all your private keys. The PIN of a hardware wallet only unlocks that particular device. If the device is lost or broken, the PIN is useless and the seed phrase is the only way back. If the seed phrase is stolen, the PIN does not protect you at all.
How to back up your seed phrase: step by step
Follow these steps when you create a new wallet, ideally a hardware wallet (see how to set up a hardware wallet).
- Prepare a private moment. Be alone, away from cameras, smart speakers and video calls. Never set up a wallet while screen sharing.
- Generate the phrase on the device itself. A hardware wallet shows the words on its own screen. Never use a phrase that came pre-printed in the box or that someone else gave you.
- Write the words by hand on the recovery card supplied, in pencil or permanent ink, numbered from 1 to 12 or 1 to 24. Check the spelling against the screen twice.
- Confirm the backup on the device when it asks you to re-enter some words.
- Store the card in a safe place at home immediately, then plan a durable copy (metal) and a second location.
- Test the recovery before sending significant amounts (see the last section of this guide).
- Note, separately from the phrase, which wallet and which standard you used (BIP-39 or Shamir, 12 or 24 words, passphrase or not). Your heirs will need this information.
Where to store it: paper, metal and locations
Paper is fine to start, but fragile
A paper card is enough on day one. It is offline, cheap and easy to verify. Its weaknesses are fire, water, ink fading and simply being thrown away by someone who does not know what it is. Keep it in a sealed envelope inside a drawer or safe that only you can access.
Metal backups
A metal backup is a steel or titanium plate, capsule or set of tiles on which you stamp or slot the words. Good models survive house fires and floods far better than paper. Before buying one, check three points:
- Material: stainless steel or titanium, not aluminium or brass, which melt at lower temperatures.
- Method: stamping with a punch set is more durable than sliding letter tiles that can fall out.
- Purchase channel: buy from the manufacturer or a trusted retailer, and never accept a “pre-filled” plate.
A metal plate does not replace discretion. Anyone who finds it can read it.
One copy or two?
A single copy protects against theft but not against a fire or a move gone wrong. Two copies in two places (your home and a trusted relative or a bank safe deposit box) protect against loss but double the theft risk. A common compromise is two copies plus a passphrase, or a Shamir backup, described below.
Passphrase: the optional 25th word
BIP-39 allows an optional passphrase, sometimes called the “25th word”. It is mixed into the seed calculation, so the same 24 words with a different passphrase open a completely different wallet. Every passphrase produces a valid wallet, which means a thief who finds your words but not your passphrase only sees an empty or decoy wallet.
The trade-off is serious: if you forget the passphrase, your funds are gone, exactly as if you had lost the seed. Use one only if you can store it reliably, in a different place from the words. Hardware wallets from Ledger and Trezor both support it.
Shamir backup (SLIP-39)
Some wallets offer an alternative standard. Trezor’s Shamir backup (SLIP-39) splits the recovery into several shares of 20 or 33 words, with a threshold such as 2 of 3. Any two shares restore the wallet; one share alone reveals nothing useful. It removes the single point of failure, but SLIP-39 shares are not compatible with BIP-39 wallets, so check your device before choosing it.
What never to do with your seed phrase
Almost every theft of a self-custody wallet starts with one of these mistakes:
- Never type your phrase into a website, a browser extension pop-up, a form, a chat or an email. No legitimate wallet, exchange or support agent will ever ask for it.
- Never take a photo or a screenshot, and never store it in a notes app, a password manager synced to the cloud, a cloud drive or your email.
- Never “validate”, “synchronise” or “unlock” your wallet on a site you reached through an ad, a direct message or an email. This is the classic phishing script.
- Never buy a second-hand hardware wallet or one that arrives with a recovery phrase already printed.
- Never split the phrase yourself into two halves of 12 words: each half makes brute force much easier. Use Shamir if you want shares.
- Never keep the seed and the hardware wallet in the same bag when you travel.
For the scams that try to obtain your phrase, read our guide on how to avoid crypto scams.
Test your recovery before you need it
A backup you have never tested is a hope, not a backup. The safest test uses your hardware wallet itself:
- Send a small amount to the wallet and note one receiving address.
- Reset the device (or use a second device of the same brand) and choose “restore from recovery phrase”.
- Enter the words, and the passphrase if you use one, directly on the device.
- Check that the same address and the same balance appear.
Some devices, such as recent Trezor and Ledger models, also offer a “check backup” function that verifies the words without wiping anything. Never run this test by typing the words into a phone app or a computer.
Repeat the check after any change: new passphrase, new metal plate, move to a new home.
Plan for inheritance
If something happens to you, your family needs to know that the crypto exists, where the backup is and how to use it, without being able to steal it today. A sealed letter with instructions kept by a notary or a lawyer, separate from the phrase itself, is a simple solution. Never write the words in a will, which can become a public document in some countries.
Key takeaways
| Situation | Good practice |
|---|---|
| Creating a wallet | Generate the words on the device, write them by hand, confirm |
| Long-term storage | Metal backup, sealed, in a private place |
| Theft risk | Optional passphrase stored separately, or Shamir shares |
| Loss risk | Second copy in another location, recovery tested |
| Anyone asks for the words | It is a scam, every time |
