The evil maid and the brother-in-law

In 2009 Joanna Rutkowska published a blog post about hotel rooms. Leave an encrypted laptop in one, she wrote, and a maid with a USB stick and two minutes can replace the boot loader. Next time you type your password, the laptop keeps a copy. The disk encryption was never broken. The attacker just had a second visit. She called it the evil maid attack, and the name stuck because the shape of it is everywhere.

A seed phrase has the same problem, and the maid is usually someone you know.

Why repeated access is different

Most advice about hiding a seed assumes one search by a stranger. A burglar has minutes, doesn't know what a seed phrase is, and wants things that sell. Against that attacker, a list of numbers on the back of a receipt in a drawer of receipts is close to perfect.

The insider has none of those limits:

• Time. They come back. Every visit is another look, and a hiding place only has to be found once.
• Knowledge. They know you hold bitcoin, because you told them, or they watched you.
• Context. They know which drawer you keep going back to, which book never leaves the shelf, which receipt you'd never throw out.
• Cover. Nothing has to look like a break-in. A seed copied by photograph leaves the original exactly where it was.

That last point is the one that makes this attacker worse than a thief. A stolen plate tells you to move the coins tonight. A photographed plate tells you nothing, for months.

Who the insider is

Rutkowska's maid is a stand-in. In practice:

• The brother-in-law who stays for a week at Christmas.
• The cleaner, the contractor, the dog-sitter, the tenant.
• The ex, who still has a key, or still knows where the spare is.
• Your own child, at the age where they know what the words are worth.
• The co-worker who sees the plate in your desk drawer.
• You, at a different age: the version of you who writes the passphrase on the same page as the words "because I'll forget."

What repeated access defeats

Be honest about the hiding places that only work once.

Anything in the house that you visit. If you check on the seed, you lead them to it. People who hide things look at the hiding place; that is how the places get found.

Anything that looks different from its surroundings. A single receipt in a shoebox of receipts is fine. A laminated card in a shoebox of receipts is a flag.

Steganography the insider has seen you set up. A seed hidden in a photo, a word search, a book's margin, is hidden from someone who doesn't know the trick. The person who saw the book on your desk the day you did it knows the trick.

A photographed hardware wallet PIN. A PIN entered in a room with the same person in it, a dozen times, is not secret.

The passphrase on the same page. This is the common one. Words and passphrase together, so the backup is complete. The insider now has everything, and you still have both. The whole point of the passphrase is that it is not where the words are.

What holds up

Somewhere they don't go. The numbered list from your cipher belongs away from the house: a bank box, a lawyer, a relative in another town who doesn't know what the envelope is. The insider's advantage is access to your home. Take the secret out of it.

Split so one visit isn't enough. A 2-of-3 multisig, or a Shamir-style split, means the brother-in-law needs to find two things in two places, and one of them should be a place he has never been.

Tamper evidence. Seal the envelope and sign across the flap. Photograph the plate in its spot, with the photo stored elsewhere. Put the seed in a numbered bag of the kind cash-handling companies use. None of this stops a copy; all of it tells you that a copy was made, which is what turns a slow disaster into a quick move of funds.

A passphrase that lives in your head and nowhere in the house. With the passphrase elsewhere, the insider who photographs the words gets the decoy, if you keep one, or nothing. Choose a passphrase you can hold for years, and test it on a wiped device before funding the wallet. A forgotten passphrase is the same loss as a stolen seed.

Count the people who know. The practical defence against the brother-in-law is that he doesn't know there's anything to look for. The sticker on the laptop, the toast at dinner, the conversation in the car: each one adds a name to the list.

The device version

The original evil maid attack is still real for the computer you use with your wallet. A laptop left in a hotel room, an office, or a shared flat can come back with a keylogger in the boot chain. Against that:

• Sign on a device that never leaves the house and never touches the internet, or on a hardware wallet that shows the address on its own screen.
• Treat any computer that has been out of your sight as a different computer, especially before a large transaction.
• Verify the receiving address on the signer's screen, not on the laptop's.

We will never ask for a seed word or a passphrase. Never type one into this site or any tool it links to.

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