A seed phrase is 12 or 24 ordinary English words with the entire key to a crypto wallet baked into them.

Whoever has the phrase has the money: no password needed, no phone, not even the wallet app itself. Here's how it's built, why you can't pick the last word yourself, why a phrase generated on a web page must never hold real money, and how to store it so you neither lose it nor hand it over.

  WHAT A SEED PHRASE IS

  12 or 24 words from a list of 2048
       └─ inside: a random key
       └─ last word: a check
  [!] the phrase = the whole wallet
      no password, no phone needed

How it's built

The standard is called BIP-39. The wallet takes a random number, 128 bits for twelve words or 256 for twenty-four, and translates it into words from a fixed list of 2048. Each word encodes 11 bits, so the words map back to the number unambiguously, and from the number the wallet derives all its keys and addresses.

The last word isn't arbitrary. A checksum (a few bits of the SHA-256 of that same number, a typo check) gets appended to the random number, and the last word carries exactly that. Change any word and the sum stops adding up; that's how the wallet knows you mistyped. You can check your own phrase for typos here: seed phrase check. The sum is computed in the browser, the words don't go anywhere.

Twelve words or twenty-four: the difference is headroom, not practical security. 128 bits of randomness is already beyond any brute force. Twenty-four is what you pick when compliance rules or future-proofing matter. The real danger isn't the phrase's length, it's where it's written down.

Why you never generate one in a browser

The tool on our page can generate a phrase, and the first thing it tells you is: don't use it for real money. Not because we send anything, the page sends nothing anywhere. It's because right next to it in the browser there are extensions, clipboard sync, autofill, and someone's eyes over your shoulder. Any of those can see the words on the screen. A phrase somebody saw once is compromised forever; you can't rotate it like a password.

For a real wallet, the phrase is generated by a hardware wallet (a separate device the key never leaves) or by the wallet app on a device with no internet. Our generator is there so you can see how the thing works, and to check the checksum of a phrase you already have.

How to store it: two opposite dangers

Storing a seed phrase has a built-in contradiction. One copy gets lost: flooded, burned, tossed during a move. Many copies, and each one is extra risk: any of them can land in the wrong hands, and the whole phrase is in every one.

Simple rules that cover most cases:

  • On paper or metal, not in a file. A file sits in the cloud, in a backup, in the clipboard, and leaks when the device leaks. A screenshot of the phrase in your photo gallery is the most common way to lose it.
  • Not in the same place as the device. If the phone and the slip of paper are in the same bag, that bag only has to get stolen once.
  • Not in a "message to yourself." That's the same file in the cloud, just with an illusion of privacy.
  • Never retyped into services that ask you to "confirm your phrase." A wallet asks for the phrase once, on recovery. Anything else asking you to type it in is theft.
  WHERE THE PHRASE DOESN'T BELONG

  file on a computer     leaks with the backup
  screenshot in gallery  leaks with the cloud
  "message to myself"    same cloud
  next to the phone      one bag gets stolen
  "confirm your phrase"  that's theft
     └─ paper, away from the device,
        in two places

How to split it so no single copy is dangerous

There's a way around "one copy gets lost, many copies are dangerous": cut the secret so that any K pieces put it back together, and fewer than K say nothing about it at all. That's Shamir's scheme, it's over forty years old, and it's a mathematical property, not a promise.

Here's how it goes. Split the phrase into parts, say three with a threshold of two. Three parts with three people you trust, or in three places. Any two rebuild the whole phrase. One part, stolen or found, gives away nothing: you can't even be sure of the original's length from it. Lose one, and you can still recover from the remaining two.

Why that beats just cutting the phrase into three chunks of four words: a chunk of a normal phrase is already part of the answer. With eight words out of twelve, the other four get brute-forced. Here a part on its own carries no information about the secret.

One trap: a threshold equal to the number of parts is dangerous. Three of three means losing any part is losing the secret forever. For home use it's usually three parts, threshold two. Everything is computed in the browser, neither the secret nor the parts go anywhere, and the same warning as above applies: splitting a real phrase on a web page is only for a device you trust, with no stray extensions.

What I'd actually do

Straight up. For a real wallet I'd generate the phrase on a hardware device, write it on paper twice, and put the two copies in two different places, neither of them where the phone lives. If the amount is big enough that losing it is scary and trusting one person is scary too, I'd split it into three parts with a threshold of two and hand the parts to people who don't talk to each other. And never, in any form, keep it in a file.

  STORAGE, SHORT VERSION

  [ ] not generated in a browser
  [ ] on paper, not in a file
  [ ] two copies in two places
  [ ] away from the device
  [ ] big amount: three parts, threshold two
      └─ you can't rotate a phrase
         like a password

What the phrase doesn't solve

The phrase doesn't protect you from typing it into a fake site yourself. No storage scheme helps if someone asked you to "confirm your phrase" and you did. That's the most common way to lose everything, and it isn't technical.

And the phrase has nothing to do with whether anyone can see your transfers. Addresses and transactions on a blockchain are public; the phrase only guards the right to spend. How to check whether a transfer arrived, and why coins "vanish" on the wrong network, is in why is my crypto transaction pending, and how to safely hand the phrase or a part of it to another person is in how to send a password or file securely.

If you found an old phrase and don't know what to do with it, ask an AI, but don't type the phrase itself.

Prompt for an AI
Help me figure out a seed phrase. I won't show
the phrase itself or type it anywhere.

How many words: (12 / 24 / other).
Where it's from: (my old wallet / found it /
was given it).
Where it's stored now: (paper / file /
screenshot / messenger).

Tell me how to check the phrase is intact
without revealing it, how to store it from
here on, and what I must never do with it.

In short

A seed phrase is the whole wallet in 12 or 24 words, with the last word checking the rest. Generating it on a web page for real money is out: not because of sending, but because of everything that can see the screen. Store it on paper, in two places, away from the device. Split a big amount with Shamir: three parts, threshold two, and no single part means anything on its own.