Seed Phrase

The words are not the key itself. They encode the entropy that a wallet uses to derive every private key it will ever hold, under the BIP-39 standard. That is why the same 12 or 24 words restore the same addresses on any compatible wallet, and why the phrase is worth exactly as much as everything it controls.

Word count is a proxy for entropy: 12 words carry 128 bits, 24 carry 256. Both are far beyond brute force. The attacks that succeed do not attack the cryptography, they attack where the words were written down. Cloud backups, password managers synced to a compromised laptop, a photograph in a camera roll, a support agent asking to verify the phrase.

An optional passphrase, sometimes called the 25th word, produces an entirely separate wallet from the same phrase and gives a plausible-deniability layer, but adds a second secret that can be lost.

The failure mode institutions underweight is not theft. It is a phrase that nobody can produce when the holder is no longer able to produce it.

A bank account has a power of attorney for that situation. A seed phrase has nothing. See Self-Custody Has No Answer for Incapacity for why that gap matters more than the theft statistics suggest.