Seed phrases: what they are and how to store them

What the words are

When you set up a wallet it generates a large random number, the secret from which every key and address in the wallet is derived. The BIP 39 standard turns that number into words from a fixed list of 2,048, which are far easier to write down and read back correctly than a long string of digits. Twelve words encode 128 bits of randomness and 24 words encode 256. The last word includes a checksum, so a wallet can tell when a word has been copied wrongly.

Wallets derive keys from the seed along standard paths, which is why the same words restore the same addresses in any compatible wallet. It also means one phrase can control coins on several networks at once: a typical multi-coin wallet uses the same seed for its Bitcoin and Ethereum accounts.

Twelve words are already far beyond guessing. The danger is never that someone guesses your phrase. It is that someone reads it.

Rules that keep it safe

• No photos, screenshots or cloud storage. Phone photos sync to cloud accounts and screenshots end up in backups, both places you do not control.
• No password managers or notes apps for the seed of a wallet holding real money. They are online by design.
• Write it by hand, numbered and in order, exactly as the wallet shows it, and check the spelling of every word. On the English list each word is identified by its first four letters, which is why small metal plates often carry only those.
• Refuse pre-filled phrases. A hardware wallet or recovery card that arrives with words already on it is a trap, because the seller knows them.

Where to keep it

• Paper is cheap and fine for small amounts, but it burns, gets wet and can be thrown away by mistake.
• Metal plates or capsules with stamped or engraved words hold up to heat and water far better than paper. They cost more and take care to set up.
• Two copies in two places, such as your home and a safe-deposit box or a relative you trust, so that one fire or burglary cannot take both. Every copy is a full key to your funds, so each place must be one you would trust with the money itself.

Do not split the words into halves stored in different places. Half a phrase is a large head start for a thief, and losing either half loses everything. If you want no single place to hold the whole secret, use a method designed for it: a multisig wallet, or a wallet that supports Shamir backup (SLIP-39), which splits the secret into shares so that, for example, any 2 of 3 can restore it.

Tell someone you trust what exists and where the backups are kept, in a note that does not contain the words themselves.

The optional passphrase

Many wallets let you add a passphrase to the seed, sometimes called the 25th word. The same words with a different passphrase open a completely different wallet, so someone who finds your written words sees only the wallet without the passphrase, which you can leave empty or use for a small decoy balance.

The catch is absolute. A forgotten passphrase cannot be recovered or reset, and without it the funds are gone. Back it up as carefully as the seed, and store it separately from the words.

Test the backup before you trust it

1. Set up the wallet and write down the words.
2. Run the hardware wallet’s built-in backup check if it has one. Several models can confirm on the device itself that your written words match.
3. Receive a small amount and note the wallet’s first receive address.
4. If your device has no backup check, reset it and restore it from your written words, on the hardware wallet itself and never on a computer, phone or website. The same first address should appear.
5. Only then move larger amounts in.

If someone may have seen it

Treat a seed phrase that may have been exposed as compromised, even if nothing has moved yet; thieves sometimes wait. Set up a new wallet with a new seed phrase, then send everything from the old wallet to the new one, largest balances first. On Ethereum and other EVM chains, remember tokens and NFTs as well as ETH.

Wardcrest never asks for a seed phrase, and none of its tools accept one. To keep an eye on a wallet without exposing anything, set up a watch-only monitor with the public address, or with an xpub for a Bitcoin wallet.