Track 2 · Keys & identity · lesson 6

Seed phrases

7 min

Managing one private key per address would be unbearable. Twelve words solve it.

Seed lab

BIP-39 · 12 words · 128 bits of entropy

Generating…

What the words are

Not a password. Not a hint. Not a backup of the key.

The phrase is the entropy. Those twelve words encode 128 random bits, chosen from a fixed list of 2048 words, and every key in your wallet is derived from them deterministically.

Same phrase in, same addresses out — on any wallet, on any device, forever. That is why you can restore a MetaMask wallet into a Ledger, or a Ledger into a Rabby. The words are the standard, not the app.

It also means the phrase is worth exactly as much as everything it controls, combined and forever. There is no "change your seed phrase" — you would be creating a different wallet and moving funds into it.

How one phrase becomes many keys

The phrase seeds a hierarchical tree (BIP-32). Each address has a derivation path, and Ethereum's convention is m/44'/60'/0'/0/n, where n is the account index.

That is why the widget above showed three addresses from one phrase. It could have shown a billion.

Predict

Someone learns the address of your account 0. What can they work out about account 1?

Nothing — the addresses are unlinkable from public data aloneThey can derive it, since both come from one phraseThey can derive it only if both have transacted

Storing it

The rules are boring and they are the ones that matter:

Never type it into anything that did not generate it. No support agent, no "wallet validator", no airdrop checker. Ever.
Never photograph it or put it in a password manager that syncs to a cloud you do not control. A screenshot in your camera roll is one account breach away from being gone.
Write it on paper, or stamp it into metal. Store copies in more than one physical place.
A wallet will never ask for it after setup. Any prompt that does is an attack, without exception.

Check

A support agent for your wallet asks for your seed phrase to fix a stuck transaction. What is happening?

It is a scam, without exceptionIt is fine if it is official supportIt is fine if you only send the first six words

Worth remembering

A seed phrase is the entropy itself — 12 words encoding 128 bits, not a backup of a key.
The same phrase deterministically produces the same addresses in any wallet.
One phrase derives unlimited addresses via BIP-32 paths like m/44'/60'/0'/0/n.
One address reveals nothing about its siblings — but an xpub reveals all of them.
No legitimate party will ever ask for your phrase after setup.