If I only use Bitcoin Core, how can I pay someone in person?

Imagine you're a Bitcoin Core user on a Linux setup because you don't trust anything else. You want to meet someone in real life and do the transaction in person rather than from home. You're looking to swap BTC for cash right there or maybe for some product or service, but you can’t bring your laptop along. Is there any kind of portable device that wouldn’t attract too much attention? I mean, something like a hardware wallet could scream "rob me for my BTC". Is there a secure device that resembles a phone and runs Linux or something similar? Honestly, I’m stumped on how to manage this unless I'm okay using a typical Android phone. That brings up trust issues with the hardware and software though. I don’t need to make any big purchase, so maybe it’s fine, but I’m worried about privacy. I'd probably mix up my coins at home before dealing with this person using an Android wallet. So what would you do? You need to get the transaction out there, which means needing internet access, but if there’s no public Wi-Fi, you’ll definitely need a phone. What if I used my phone as a hotspot to connect wirelessly to this portable device for making the transaction? How can I do that without compromising my privacy? I’m really not sure how to handle this. Would love to hear how others are managing it.

There are a few options for that. An option for example would be the Coldcard Q1 from Coinkite which should be released soon. If you're considering a more phone-like device that runs Linux, there are several Linux-based smartphones that might meet your privacy and security needs while providing a conventional smartphone appearance. The Fairphone comes as a complete degoogled device and runs a Androind system by default. These smartphones are really designed for privacy and the protection of your own data. Certain Linux based OS, such as Ubuntu Touch or PostmarketOS, can be easily flashed onto these devices. There are other OS being advertised but I can't comment on them due to lack of experience. In addition to the Fairphone, there are also other providers: the Liberty Phone which comes from the American manufacturer Purism but costs more than three times as much as the Fairphone.

I don't get your point. laptop can be brought anywhere you want so why not bring it and only deal with someone in a safe place like a mall? Or if you don't want to bring your Laptop then the only solution for your problem is to make a signed transaction(offline transaction) first in your house(don't forget to ask him about the price and BTC address)and don't broadcast it yet get the raw/hex code and paste it to your phone or any device that you can bring. Now you can meet someone in person and only broadcast the transaction when you deal successfully with him but if not don't broadcast the transaction. Here's the list of broadcasting sites below. - https://en.bitcoin.it/wiki/Transaction_broadcasting If you want it private don't forget to mix your coins at home before you make a raw transaction with Bitcoin Core. Alternatively, you can make a portable Raspberry Pi with Linux installed you can bring it anywhere you want. Here's what it looks like https://youtu.be/MorJLzybb3I?si=Nw-1QH7RsqFdVp3y

You have various options to do this privately. You can run a hot wallet, which is pointed at your node running back home, such as Electrum. Transfer the coins you want to trade from Bitcoin Core to your mobile Electrum wallet before you go, and then make the transaction from Electrum to your trading partner while you are out. Everything goes via your own node. For extra security, you can do the above with a watch only wallet and a separate airgapped wallet. Run a watch only wallet on your phone pointed at your own node running back home. Also take with you either a hardware wallet containing the funds you want to trade, or a second phone in airplane mode with your coins stored on it. You could do this with Electrum on two phones, or with a hardware wallet such as Passport and their accompanying Envoy app. As BitMaxz has said, you can sign the transaction in advance if you know the address and amount, and take the signed transaction with you on your phone. Either broadcast via your own node, or via Tor using mempoolhqx4isw62xs7abwphsq7ldayuidyx2v2oethdhhj6mlo2r6ad.onion

According to the red text above, you are reluctant to use an android phone, because of privacy. As oeleo stated above, connecting to your own node will fix this issue. However, the best answer in my opinion is the one by BitMaxz. Sign the tx beforehand from your home. Take the tx id with you in any form you wish (android, paper, usb, sd card). There, you can broadcast the tx, or you can even give it to them to broadcast it. The broadcasting of a signed tx is a feature available in almost every wallet software.

Why not? Yes. It's called a laptop I can think of many potential problems. What if: Transaction fees went up between creating the transaction and meeting in person?You buy something at a shop and their automated payment system won't be able to give you an address and exact amount before making the payment?The seller doesn't want to email you a Bitcoin address for various reasons? @OP: I use different wallets for different purposes. The purpose of mobile wallets is convenience for small amounts. You simply can't have it all with Bitcoin Core. I often bring a laptop, not for making Bitcoin transactions but for other purposes. Nobody bats an eye if you pull out a laptop somewhere. Tether a hotspot on your phone and you have your own internet connection with you.

Sorry to step in but I would like to address your problems: This could be fixed by using the RBF-feature(Replace-by-fee) in Bitcoin Core. The requirement for this is that you activate the option when signing the transaction. By default, RBF is not activated. This is no longer possible afterwards! The Full-RBF feature should be implemented by version 24.0 Yeah.. not the best condition to sign the transaction offline. I think in this case you have to use a mobile wallet. In that case, I would still suggest that he perhaps encrypt the emails using PGP so that the seller feels more secure. Another alternative would be other encrypted messengers. Even if that wasn't an option for him, as in question/problem 2, I see no other option than to carry out the transaction face-to-face while using a mobile wallet. However, as a buyer I would be suspicious in that case aswell as PGP encryption has proven itself over the years. So it makes no sense to me to reject the suggestion.

That doesn't help you if you brought a signed transaction to a meeting, and left your Bitcoin Core at home.

When you increase the fee, the transaction data changes, the signature becomes invalid and it's like that you are making a new transaction. If OP wants to sign the transaction again, he/she would have to get back home as he wouldn't have access to his/her laptop out of the home.

Yes, of course. I know what you're getting at. But the OP will have to use something to be able to transfer his transaction, won't he? Of course there are other wallets that support RBF, but he explicitly said that he only trusts Bitcoin Core on Linux. This is correct if you did not allow the RBF feature when signing the transaction. Just as it is set by default. However, if it is activated and the transaction is signed, there is a possibility that the transaction can later be replaced with a newer version with a higher fee. At least that was my understanding, but I haven't had any practical experience with it yet. Please correct me if I misunderstood the feature and how it works irl. EDIT: Ok, I must have had a long brain lag.  Of course you're right @hosseinimr93. Because it acts like a new transaction with a different hash, it must be re-signed.

There are many online tools that can be used for that. All you need to do is copy the raw (signed) transaction to your mobile and broadcast it using any of the avaliable tools. In this way, you don't need to have your private keys in your mobile phone and that's what OP is trying to achieve.

Install a privacy filter on your screen. Or does that make you stand out too?

Yes, that's really crazy, especially when you look at the specs of the phone. To compare this with the Fairphone 5 just from a memory perspective: The Fairphone has twice as much RAM (8GB vs 4GB) and also twice the memory (256GB vs 128GB). They probably justify the price with their advertising where they proudly claim that a smartphone is "built outside China".

There are a lot of devices like this out there: https://www.amazon.com/Industry-Portable-Computer-Notebook-Graphics/dp/B07QYZHM8F I know running core on that would not work it's just the 1st one I found with a quick search. Small enough to fit in a jacket pocket and you just look like a nerd / geek with an oddball computer. There are a ton of other 'pocket computers' out there. Just how much do you want to spend. If you are worried about your phone now being secure but you only do a couple of $200 transactions a year the payback time is long. If you are doing a couple of $1000 transactions a week it's a different story. -Dave

It has a 6" screen and weighs 1.03 pounds. This is designed for specific industrial applications. For me, 10" is the minimum to be able to type "normally". That brings me into the netbook category, but most of them lack RAM capacity. How about a good old paper wallet? Bitaddress.org is the easiest, but doesn't support Segwit. You can encrypt it with BIP38, and fund it with approximately the amount of Bitcoin you're going to need. When completing the trade, your trade partner can just sweep your private key from his own mobile device.

<cough> CLI </cough> If it's not for regular use for a few minutes of dealing with a very difficult screen setup is probably not the worst. And if you know where you are sending the BTC and the amount you can pre-populate it at home and just hit send once you get a wi-fi signal wherever you are doing the exchange. -Dave

Over a dozen billion, called smartphones.  No, seriously. Look. If you sell small amounts of bitcoin, then getting yourself a Pixel with GrapheneOS installed wouldn't be a bad idea. It comes a lot cheaper than Fairphone. The TL;DR of Graphene is that it focuses on security and privacy. That can work, and it doesn't make you look like a socially awkward fella with Arch Linux, having an NSFW anime-girl wallpaper. (No offense to those who do!  ) But seriously, why not a laptop? Having an Electrum wallet, running Ubuntu doesn't tell me anything. We are not living in the 00s anymore.

For those that ask why I don't want a laptop, beside it just not being convenient to carry around, I don't want to look like an high level nerd. You see, the problem is, you never know who you are dealing with, a laptop is typically a place you would store more BTC than a mobile device, where one would expect to carry pocket change, so you want to minimize this risk. Let alone those Coldcard type devices. Those make stand out like someone that is invested into this way too much. If you are a regular fella with a fashionable android phone, it's a much better look than showing up with some fancy, strange device that looks like some sort of Bitcoin-only device. Im going to look at the Fairphone thing. Im familiar with Purism, their prices are a scam, they get massive cuts for what it is. Im going to also need to know what OS to install in there. Someone mentioned Graphene but this is only for the Google Pixel for some reason, im not buying a Google phone. I have a dated Samsung Galaxy for a phone, does the trick. I don't even know what Android version this uses. Im not familiar with phones. Btw, what I meant with this thread, wasn't to say, I want to use Bitcoin Core on a phone, I can reasonably compromise, I just want to do it properly. I will mix at home, get the amount to use ready, and put it into mobile Electrum or whatever. Im not going to know the address until I arrive, so I cannot sign it at home. Then I also need to sort out how to transfer this through Tor from the phone. What I don't like is the traces that this would leave in terms of, imagine you lose your phone somewhere or something. Since im not familiar with phones, I don't know what to expect if that happens. Could some experienced phone users comment on this whole process?

It is quite difficult to lug around everywhere (unless it's a macbook air or something else that's light), requires a WiFi access point, is inconvenient for scanning QR codes and is overall quite cumbersome to use for making payments when you're on the street. That's why your other suggestion about getting a GrapheneOS phone is a much better idea.

Average people don't use CLI. And i just remember most OS have option to increase display scaling, which usually problem about small screen with big resolution. Install Orbot on your Android phone. The default setting use "full device VPN" mode which means all application use Tor network.