Seed phrases: how to back up properly

Everything about self-custody security reduces to one question: is your seed phrase safe, and will you still have it in ten years?

What the phrase actually is

A seed phrase is a human-readable encoding of a large random number. From that number, your wallet deterministically derives every private key and address it will ever use.

“Deterministically” is the key word. The same phrase always produces the same keys, in any compatible wallet, on any device, forever. That is why a restore works — and why the phrase is functionally identical to the funds themselves.

The words come from a standard list of 2,048, which is why your wallet can spot a typo: an invalid word is not on the list. It also means the phrase is not arbitrary text; it must be exact and in order.

The two ways people lose crypto

Someone else gets the phrase. Theft is immediate and irreversible. There is nothing to freeze and no one to appeal to.

You lose the phrase. Equally permanent. An enormous quantity of Bitcoin is believed to be permanently inaccessible for exactly this reason.

Good backup design protects against both at once, and the two goals pull against each other: copies that are easy for you to reach are easier for others to find. The right balance depends on how much you hold.

What not to do

• No photographs. Phone photos sync to cloud accounts, which are breached routinely and are often protected by a password you have reused.
• No cloud notes, email or documents. Same problem, plus anyone who gains access to that account has everything.
• No password managers. They are good tools, but a compromise of the manager becomes a total loss rather than an inconvenience.
• No typing it anywhere except your own wallet during a deliberate restore.
• No single copy. One fire, flood or house move and it is gone.
• No telling anyone. Not support, not a “recovery service”, not a stranger who has been very helpful.

What to do instead

Paper, done carefully

Adequate for modest amounts. Write clearly in pen, number each word, and store it somewhere private and dry — a sealed envelope in a locked drawer or a home safe. Make a second copy for a different location.

Metal, for anything meaningful

Metal backup plates let you stamp or engrave the words into steel. They survive house fires and flooding, which paper does not. If your holding is large enough that losing it would genuinely hurt, this is a small and sensible expense.

Two locations

Keep copies in at least two places you control — for instance home and a trusted relative’s house, or a safe deposit box. This protects against a single-site disaster. Both locations must be secure, since either copy is sufficient to spend your funds.

A note on splitting

People often split a phrase, keeping half in each of two places. Done naively this is worse, not better: you now have two ways to lose access (either half missing) and each half meaningfully narrows the search space for an attacker who finds one. Proper schemes for splitting a secret exist, but they are an advanced topic and easy to get wrong. For most people, two complete copies in two secure places is both simpler and safer.

The passphrase option

Many wallets support an optional extra passphrase — sometimes called a 25th word — which produces a completely different set of keys from the same seed. It defends against someone finding your written phrase, since the phrase alone reaches an empty wallet.

The trade-off is severe: forget the passphrase and the funds are unrecoverable, even with the seed phrase in hand. It also needs its own backup, stored separately. Useful for experienced users with a clear plan; a way to lose money for everyone else.

Plan for not being around

If you hold a meaningful amount, consider how someone you trust would access it if you could not. Crypto has no probate process and no next-of-kin recovery. Sealed instructions with a solicitor, or a documented plan someone knows exists, prevents your holdings from simply vanishing.

What to read next

Next: hot vs cold wallets — which do you actually need?