How do I securely store a Bitcoin private key on an NFC tag?

[Abiky]

I've never used NFC tags for writing sensitive information on them, since they're easily readable by anyone using an NFC reader. It's this "openness" which makes NFC tags a terrible solution for storing Bitcoin private keys. I know that it's possible to encrypt the private key and store it that way on the NFC tag. But then, I'd have to decrypt it after reading it which is not very convenient if you ask me.

I'd like to know of a way to store a Bitcoin private on an NFC tag that can be easily read/interpreted by any wallet on a computer or mobile device (able to read encrypted private keys prompting for the passphrase upon scanning the tag). I'm not sure how long NFC tags last, but I've bought a couple of ones a long time ago and they're still working these days. A keypair of public and private keys on an NFC tag for safekeeping would prove to be extremely convenient for mobile users. If somehow there was a way to "hide" the private key part, while making the public key visible on the NFC tag, it would make my life much easier.

Any help, suggestions, or recommendations will be greatly appreciated. Thanks in advance.

[20kevin20]

I'm not sure you can store a hidden information on the NFC tag. If that's possible, programming the tag is probably not as easy as it sounds and a bad guy might someday want to inspect it and find the hidden container. I'd personally not do that.

One thing you can do is write an encrypted seed on it. As long as you have a strong password, you can pretty much safely store it around the house. According to an earlier reply from o_e_l_e_o, you'd need 39 random characters from the full ASCII set in order to have an encryption key as secure as the seed is.

[Bradoets]

Why do not you encrypt the key and print the solution in the form of a question that you can only answer, encrypting the private key and making the answer is a password you can hide the other part that contains a question you know an answer and that answer is the password if you fail to remember it?
You need three of them with all of them linked to the key relationship or any tool.
you can add some cool colors.

[franky1]

wallets by standard dont just program themselves to suddenly accept a NFC imput and recognise it and prompt for questions
the wallet would need to be programmed by someone to interpret/react to a certain imput and activate a function in the wallet software they program

so with this said you can encrypt a private key. and put it on a nfc
and then do all the complicated stuff on the wallet software.

there are many options
like have the wallet software require multiple NFC inputs and a passphrase

EG
1 nfc input can be for a 'hotwallet' that only contains daily spend amount. and requires 2 nfc inputs for your main hoard.
that way when you are at home you can move a small amount of hoard to a small value address. because at home you have a NFC you keep at home and an NFC on your keyring you carry with you.
then when out and about only having the keyring with you. you can only access the small address

EG
wallet requires 2 passphrases
mothers maiden name and first girlfriends name for instance. along with the NFC

heck you can even make it so the passphrase encrypts to a certain latitude/longitude range. meaning people would need to know a certain location you ping on a map to get the decryption to be successful

..
in short the data on the NFC chip is not that important.
but how the device interpreted it and encodes/decodes it

so care less about the NFC and think about the ways in which you can add as much random/multiple input 'salt' into the encryption. to reduce the risk of someone getting to your stash unless they are in your house and know your secret holiday spot you fantasise about visiting as well as your mothers birth surname and your ex girlfriend

[ChiBitCTy]

I bought a few NFC "collectibles" in the past.  I guess the most prominent one is the Tangem NFC wallet cards ( recommend checking them out to those who've not seen them).  I'm however embarrassed   to say I've never used NFC and haven't the slightest clue how to get started. Some good info in this thread but does anyone know of a good tutorial on how to use NFC by chance?

[peterurb]

There are many types of NFC tags and they can save on average 200 B of text. BTC private key has around 70 B and there is enough space for encryption. But never use only 4-digit code. It is too easy to guess.

It is a really good idea, but never save private key without encryption.

[Abiky]

I'm not sure you can store a hidden information on the NFC tag. If that's possible, programming the tag is probably not as easy as it sounds and a bad guy might someday want to inspect it and find the hidden container. I'd personally not do that.

One thing you can do is write an encrypted seed on it. As long as you have a strong password, you can pretty much safely store it around the house. According to an earlier reply from o_e_l_e_o, you'd need 39 random characters from the full ASCII set in order to have an encryption key as secure as the seed is.

Interesting. I'd say that an encrypted seed may be all I need to secure my Bitcoin on an NFC tag. The real issue here would be "legacy" BTC addresses with public and private keypairs. But I think going the BIP38 route, would achieve nearly the same thing as BIP39 on a mnemonic/seed (if I'm not mistaken). All of this is great, but I'd be nice if I could somehow hide the private key on an NFC tag. It's possible to lock and even set a password on the tag for further security. Something similar to steganography on an NFC tag, would work like a charm. For instance, I could place some "dummy" text that would contain the private key "hidden" underneath.

As far as mobile wallets go, I don't think they'll be able to "read" the encrypted seed or private key from an NFC tag "on the fly". I'm talking about any mobile wallet with the capability of asking the user for the passphrase or password to "unlock" the encrypted private key or seed. Right now, NFC tags have very limited data capacity which greatly limits our options to store lots of crypto keys on a single tag. But if technology continues to evolve and people continue to use NFC, it might be possible to do this someday.