Seed phrase backup

Durable media

1. Paper is fine for the first days. Use pencil or archival ink on the vendor's card and keep it dry.
2. Move to stamped or engraved stainless steel for fire and flood resistance. Number every word. The first four letters of each BIP-39 word are unique, so they are enough if space is tight.
3. Check the metal copy word by word against the paper copy, confirm with a restore test, then shred and burn drafts.

Storage

• Somewhere you can reach but a burglar, guest or contractor would not think to look. No labels like "Bitcoin seed".
• For larger amounts, separate the seed and the passphrase (if used) geographically.
• A bank safe-deposit box has access and seizure trade-offs; weigh them in your threat model.

Never: photos, screenshots, cloud notes, email drafts, cloud-synced password managers, printers, or voice assistants. Never type your seed into a website, phone or computer, including to "check" it.

Do not cut one seed into halves. That weakens it and makes recovery fragile. If you want no single point of failure, use multisig or a standard share scheme on a device that supports it.

Optional passphrase (the "25th word")

• A BIP-39 passphrase creates a completely different wallet from the same seed, so a thief with only the seed gets nothing.
• Make it strong and unique (for example 5 to 7 random words) and back it up separately from the seed. There is no recovery if it is lost.
• Record the wallet's master fingerprint (Sparrow shows it). On restore, a matching fingerprint proves you typed the passphrase correctly.
• Test seed + passphrase before depositing.

The 2026 weak-seed lesson

Coinkite disclosed that an integration error caused some COLDCARD firmware to generate seeds with far less randomness than intended (Coinkite advisory). Attackers recomputed affected seeds offline. Galaxy Research's August 25, 2026 tally put confirmed losses at about 1,789 BTC from 8,865 addresses (Cointelegraph).

Updating the firmware did not change seeds that already existed. Affected users had to create a new seed on fixed firmware and move their funds (COLDCARD migration guide). For every wallet:

• Update firmware before generating a seed, not after.
• Add your own entropy (dice or key presses) where the device supports it.
• Keep a private note of which device and firmware created each seed, so you can tell at once whether an advisory affects you.