NFC KeyCard

Passport Prime
1

Can you please explain a bit more about the NFC KeyCards? Are they in a standard format? Can they be read by any phone? If you tap them on a phone, can their shard be viewed and written down as seed words?

2

Hey Mason,

  1. Standard format? They’re NFC cards but designed with an antenna optimized for Prime. Some off-the-shelf NFC cards may technically work but aren’t tested or supported.

  2. Readable by any phone? Yes, a phone with an NFC reader app can detect the card and read what’s stored on it.

  3. Could the shard be written down as seed words? No. The share on the card is a Shamir Secret Share serialized as CBOR (a compact binary encoding). Some Shamir-based formats like SLIP-39 do define a human-readable wordlist representation for shares, but our implementation writes binary only. Nothing on the card maps to BIP39 seed words you could copy down.

3

Thanks! So if this card was found and tapped on a phone, would it give any indication what was on it or what it’s for or does it only work when read by a Passport Prime? Thinking about the possibility of keeping it in a (metal) wallet and risks associated with that.

4

Used my agent to check our source on this for you:

Tapping a KeyCard with a generic NFC reader app (NFC Tools, NFC TagInfo, etc.) shows:

1. One NDEF record with type cbor.io:cbor. That string is a generic identifier meaning “the payload is CBOR-encoded data”. It is not branded. There is no “Foundation”, “Passport”, “KeyCard”, “Shamir”, or “Magic Backup” string anywhere in what’s written to the card.

2. A binary CBOR payload. Inside that payload, the structure uses integer keys (0, 1, 2…) rather than named fields, so even someone who decodes the CBOR sees a map of integers pointing to byte strings and small numbers. Nothing in the data self-identifies as a Bitcoin backup or as Foundation-related.

So a finder tapping the card would see “an NFC card with some CBOR data on it” and have no way to recognise what it’s for without prior knowledge of our format.

A couple of practical notes for the metal wallet idea:

  • Keep the card in the Faraday sleeve we ship with it, inside the wallet. That blocks the read entirely so the question of what shows up never comes into play. Metal wallets vary a lot in how well they shield NFC. The sleeve is purpose-built.
  • The card’s chip serial number (UID) is always readable, like any NFC tag. It’s a random factory-assigned number with no link back to you or your wallet.