TAPSIGNER FAQ

What is TAPSIGNER^®?

A reusable NFC Bitcoin hardware signer. A compatible wallet provides the interface, constructs transactions, and shows the details; TAPSIGNER holds the BIP-32 signing key and signs after a tap and PIN. The card has no display of its own.

What app can I use with my TAPSIGNER?

Nunchuk

Android iOS Desktop

Cove

Android iOS Website

Bitcoin Keeper

Android iOS Website

Sparrow

Desktop

Your app could be here

Learn more

What's the motivation for TAPSIGNER? You already make COLDCARD^®.

TAPSIGNER's lower cost makes hardware wallet features and security available to a wider market around the world.
An NFC card provides fast and easy user experiences.
TAPSIGNER is a great way to keep your keys separate from your wallet(s).
Card form factor makes it easy to carry and easy to conceal.

What are some possible uses for TAPSIGNER?

Phone wallet cosigning
Opening and closing Lightning Network channels
Using an NFC module with a Lightning Network node
Message signing
Collaborative multisig service applications
Raw transactions
Partially signed Bitcoin transactions (PSBTs) through supported wallet workflows
App Authentication

Do I need this paper (plastic) envelope it came in?

Yes! That's a special radio frequency (RF) blocking sleeve. It prevents unwanted access by RF readers with bad intentions. Be sure to insert TAPSIGNER fully into the sleeve: even 5 mm sticking out can allow a sneaky reader to get a signal.

I lost the RF-blocking sleeve! What can I do?

Thousands of RF-blocking (you can search for RFID) sleeves, wallets, and other card protectors are available for purchase at Amazon or elsewhere online. Any of these should work.

We have replacements too.

Can my organization order custom-branded TAPSIGNER cards?

Yes. Coinkite offers custom-branded and white-label TAPSIGNER production runs for organizations, events, wallets, and communities. Contact sales@coinkite.com to discuss a run.

Can I use TAPSIGNER for multisig co-signing?

Yes. A compatible wallet can use TAPSIGNER as one BIP-32 key in a multisignature policy. Preserve the complete wallet configuration and recovery plan.

Can I use TAPSIGNER with a computer (desktop, laptop)?

Yes. Sparrow supports TAPSIGNER with compatible USB NFC readers, and the cktap command-line software implements the open protocol. Check the wallet and reader documentation for current operating-system support.

Is the private key unique and secret?

Yes. TAPSIGNER comes without a private key. The setup process combines your entropy (random bits) with secret entropy picked by the card.

How do I see the deposit address?

Use your mobile wallet to view the deposit address.

How do I know a TAPSIGNER is genuine?

There are a number of ways to verify the card:

Check it with any TAPSIGNER-ready mobile wallet.
Use our cktap command-line software.
Tap the card on your phone. TAPSIGNER opens a webpage verifying the card logic (but not the certificate chain).

How do I know I'm depositing to the correct address?

You must use a mobile wallet that supports TAPSIGNER. Ideally, you tap the card as part of verifying the deposit address. However, once a wallet is set up, it can generate deposit addresses using the usual BIP-32 derived address paths.

Can I store data on my TAPSIGNER?

No. The TAPSIGNER cannot hold arbitrary data.

How long will a TAPSIGNER last?

The presumed operational lifespan is 10 years when the card is stored within its documented temperature limits.

For long-term storage and large amounts, consider a COLDCARD^® Hardware Wallet, a display-equipped Bitcoin wallet also made by Coinkite.

Can't the phone just grab the private key and use it later?

Normal signing keeps the unencrypted private key (XPRV) on the card.

The backup feature provides an encrypted (AES-128-CTR) copy of the key. Using it requires the decryption key printed on the card. Rogue software cannot read the back of the card without your help.

What if someone takes my TAPSIGNER?

If you changed the PIN, someone holding only the card still faces PIN authentication and rate limiting. They can record the printed AES backup key, and joint access to that key plus your encrypted backup file would reveal the master XPRV.

Can I use it on an untrusted computer?

Normal signing keeps the unencrypted XPRV on TAPSIGNER, which helps isolate it from malware that reads computer storage.

However, a paired wallet can ask TAPSIGNER to sign an unintended transaction. The card has no screen and cannot display the amount, destination, fee, or change. Use a trusted workflow and consider COLDCARD when independent on-device review is required.

What's the seed phrase (BIP-39)?

TAPSIGNER does not use a BIP-39 seed phrase. Recovery uses an encrypted master-XPRV backup file, the separate decryption key printed on the card, and the wallet configuration.

The TAPSIGNER encrypts the XPRV with AES using a 16-byte key printed on the back of the card. The backup file plus the key lets you recover the XPRV.

How do I back up a TAPSIGNER?

Run the backup command to capture the data on your phone.
Record the 16-byte decryption key on the back of the card. We recommend writing it down on paper.

Those two items recover the master XPRV. Preserve the derivation path and complete wallet configuration as well so recovery software can reconstruct the same addresses and policy.

What if I lose my TAPSIGNER?

Use the encrypted backup file and your separate copy of the decryption key to recover the XPRV in a trusted offline environment. You also need the derivation path and wallet configuration. There is no command to restore the key onto another TAPSIGNER.

How do I know the verification link is genuine?

A different random nonce (short for "number once," a single-use numerical value used in cryptography) is signed each time you tap the card on your phone to receive the URL over NFC. Our server verifies the signature and uniqueness of the nonce.

You can also tap again to get a new nonce and corresponding signature.

cktap can do additional verification over the NFC interface that is not possible via the single NFC tap to webpage method. All verification code is open source Python.

Is this a centralized service?

Direct TAPSIGNER protocol use does not require a Coinkite service. The protocol is open and the card stores the signing key; a chosen companion wallet or assisted-custody plan may introduce its own service dependencies.

TAPSIGNER works with any Bitcoin wallet that uses our NFC protocol.

How do I know the manufacturer doesn't know the private key?

When setting up your TAPSIGNER for the first time, you provide a 32-byte chain code for entropy. That chain code plus a private key picked by the TAPSIGNER are combined using the BIP-32 standard to derive the payment address.

Because you provided the chain code, and the TAPSIGNER shares the public part of its key, you can derive the payment address and confirm that it matches the address given by the card.

Effectively this means you know the XPUB, the card knows the XPRV, and it's easy to prove the two correspond.

How can I verify the wallet's chain code was used?

The wallet provides a 32-byte chain code during setup and can verify that the master XPUB returned by TAPSIGNER contains that same value. The card supplies the private-key contribution with its internal TRNG. This check still depends on correct card hardware and companion software.

Do I give the TAPSIGNER to other people as payment?

No. Keep TAPSIGNER under your control. SATSCARD^™ is the Coinkite card designed for physical bearer transfer.

Can I use a TAPSIGNER to sign a message?

Yes, a TAPSIGNER can sign arbitrary messages. It's the same as signing a transaction.

What if I make a malicious TAPSIGNER?

Each TAPSIGNER made by Coinkite carries a certificate signed through the factory chain. Unlike the X.509 format used for OPENDIME^®, the Tap Protocol uses Bitcoin signatures over the documented root, batch, and card values.

The Python code in cktap will always verify the certificate chain when speaking to TAPSIGNER, and mobile wallets should do the same.

What about an active MiTM attack or relay attack over NFC?

Protected commands use an ephemeral ECDH (Elliptic-curve Diffie-Hellman) session so values such as the CVC and returned sensitive data are encrypted over NFC. This does not protect against a compromised companion wallet requesting an unintended signature.

Still have questions? Contact support: support@coinkite.com

Does "tapping" reveal my public key?

When you tap the TAPSIGNER on a phone it provides a URL for verification purposes, which contains a signature. The URL will typically take you to tapsigner.com/start which decodes it and shows a verification message if it all checks out. This lets you know that a real TAPSIGNER card (not a generic NFC card) is being used, and helps to on-board new users by providing useful links to suitable wallet apps.

That URL does not contain any part of your public key (or XPUB) and cannot be linked to any of your on-chain activity. It is unique for each card.

What about an Evil Maid accessing my TAPSIGNER card when I am absent?

(We will assume you have changed the CVC (PIN code) from the value printed on the card.)

An evil maid could try to guess your PIN; however, after 3 wrong PINs, there is a 15-second delay before she can try again. The delay is enforced by requiring the 'wait' command to be executed 15 times. If you try any command which requires a PIN, you'll get an error unless you do the delay steps first.

Chain code cannot be changed once set (nor can private key).

An evil maid without CVC (PIN code) can take pictures of the outside; this gives them the AES key for the backup decryption. This is a problem going forward if they ever gain access to a copy of the encrypted backup file.

Without the CVC, the maid cannot make a new backup. If they can also access an existing encrypted backup file, the printed AES key is enough to decrypt the master XPRV without the card or current PIN.

The maid can confirm the card's identity (printed on back, but also available by NFC without authentication). But that doesn't reveal XPUB or anything related to blockchain. It could be a privacy issue if they are tracking your wallet by long-range NFC, but we assume the RF-blocking sleeve is used.

Last used derivation path (hardened components) will be revealed with a tap. Small info leak there, but will be constant value for most users.

Do you have any Tutorial Videos?

Where is the NFC sweet-spot on my phone?

Watch this video to find the NFC sensor on your phone.

Card Specifications

Card size: ID-1: 85.6 × 53.98mm
Temperature limits: -15℃ ~ 50℃
There are no magnetic or gravity constraints.
The presumed operational lifespan is 10 years.
Communications by NFC via ISO-7816, specifically ISO/IEC 7816-4:2020 and/or ISO/IEC 14443-A.