Offline bitcoin transactions

Saw this today: Payments Firm Backs 'Crazy' Offline Bitcoin Transactions Experiment

One of the commentators asked how they deal with double-spending - that’s my question too. If they have solved the offline problems in a mostly trustworthy way (understanding that security comes in degrees and is not absolute) then this looks really interesting.

Wondering also if SafeCoin would be able to do something like this. Thus far, SafeCoin seems as close to anonymous digital cash as we’re going to get, but taking it to the offline world would be another major leap.

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See referenced proposal for “minting safecoin” –


I still don’t see that the double-spend problem is resolved in that paper.

Alice takes the coins offline, copies them and spends them in two or more places. Only the first to verify the coin actually gets the value.

Or did I miss something? This seems likely the case with the netopia technology as well.

Per the paper, it generates a special transaction type, an open transaction.

Alice could hand the usb (etc.), to more than one person, I guess, so whoever validates the transaction with the network first gets the loot. No possibility of double spend. She’d just be committing fraud, like writing a bad check.

The minting bit is a proposal rather than something baked in, as yet. Don’t know how the removal of Transaction Managers might affect all this.


I think we may have a semantic difference here - “writing a check twice” is a double spend by my understanding. So, yeah, fraud, but also a temporary form of counterfeiting.


She can’t spend the same coin twice, what you’re saying is actually double promising.


That’s really my point. It’s the same as if you gave her something on the promise that she’d put cash in the mail, or whatever. It’s not double-spent on the network. You’re trusting her word that the safecoin is there. If it’s not, it’s just personal error or dishonesty. Can’t get around that with bitcoin either.


This offline bitcoin could work. I talked about it before.

Like with SAFEcoin you can also mint bitcoins, you simply send them to a new address and extract the privatekey. The problem is once you have the privatekey, you could just simply copy the privatekey and spend the coins. This problem can be solved, because with bitcoins you can also have a password. So entering a password would prevent the previous user to spend the coin. But this still means that you need an app, to communicate a new password to the bitcoin network. Changing the password is for free, so there would be no costs only a message to the bitcoin network to change the password. With every new owner of the bitcoin you would only have to change the password.

When you mint coins, it’s like coinage.

If some one could print out the privatekey, it could be scanned and a new password could be send to the Bitcoin network, then you have a trustworthy offline bitcoin.

It seems like they depend on the previous owner to immediately delete the privatekey.Well this doesn’t help. The only solution is to change the passphrase of the privatekey, that way only the new user knows the passphrase and the previous can’t spend the bitcoin, because a privatekey alone won’t grant you access to the bitcoins.

I really wonder if SAFEcoins will have a passphrase or even multisigs from the get go? Will it even be possible to mint SAFEcoins now that the Transaction manager is removed?

Let’s not forget - you can double spend fiat money by writing cheques for more than your available balance.

Does this stop cheques being useful? No. You just accept that there is an element of risk.

If you want the least risk exposure, you complete the transaction online.

Edit: someone beat me to my point already!


Yeah I’d say offline safecoin or bitcoin would be like writing cheques with fiat currency with much the same risk. If you receive an offline minted safecoin “cheque” be it a usb stick or printed qr code or however they want to do it then it’s only good if it’s the original. If it’s copied that’s the same as receiving a bad or forged cheque and reflects badly on whomever you received the copy from. But an offline safecon cheque could still be useful for those that have no, or limited, access to the internet. Say you can only access the net once or twice a week at an internet cafe or something but still want to use safecoin for your transactions. Safecoin cheques would be ideal for that, you just have to make sure their reliable. Or say you want to exchange safecoin for goods and services with someone that doesn’t use internet at all. Consider this: Person A uses internet trades with person B who does not who in turn trades with person C who also uses internet. Person B does not personally have a use for safecoin but does know people who do use it. Therefore because it is of value as a currency to others (A and C) it is of value to B, even tho they can’t personally get online and use it as a cryptocurrency.

Having an offline SAFEcoin doesn’t have to be risky. If we had something like this:

An app that can scan a offline SAFEcoins qrcode/digits. The app would communicate (wifi/sms/txt) with the SAFE Network authenticate and give the offline SAFEcoin another password, so that the previous owner can’t spend the offline SAFEcoin anymore. In this scheme at least if sms with the SAFE Network would be possible, you could even use a feature phone. To your sms to the SAFE Network you would get a message like (Yes the offline SAFEcoin is authentic and your new password is UseY0ur1MaGIn4tION).


What problem does offline Safecoin solve, when you have a SAFE client on the mobile device…and remembering coins dont exist outside the network.

  • It would enable you to have a brainwallet
  • Obviously offline SAFEcoin tx’s
  • Some people like something tangible so it would solve that hurdle (just receiving a qrcode or digits, just give different formats to an offline SAFEcoin (so no fancy pancy paper needed))
  • It would enable community currencies (chiemgauer) with the SAFE Network as ledger

True indeed sir,
But if they are minted, they could exist outside of the SAFE network. It would be fun if the SAFE Network had an sms equivalent because that would take care of a closed loop transaction. Who knows the return sms of the SAFE Network could also include the fiat price of a SAFEcoin.

input: 26423874het3673_EUR

btw if you didn’t have a calculator you could

input: 26423874het3673_2,5 SAFEcoin=EUR

Hmmmm now that I think about it, maybe the brainwallet part is not true yet, because the SAFE Network create a code when you mint a SAFEcoin, instead of you typing in a phrase to remember.

What do you mean by ‘mint a SAFEcoin’

Just a reminder: Not everywhere has internet and cell service.


I’m sorry for not explaining further in the SAFEcoin pdf

6 Minting safecoin

It’s explains that you can mint an SAFEcoin, in other words create an offline SAFEcoin. I hope this is still possible, due to the fact that the transaction manager will dissapear from the SAFE Network, I wonder if this is still possible. :confused:


Ah ha…very relevant and indeed we’ll need an update to the situation and a revised Safecoin Whitepaper I expect.

Nice work :wink:

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Yes, I think this was discussed previously in the situation of ad-hoc mesh with no current path to the SAFEnetwork. Much the same situation as a whole continent going down, coming back up and re-syncing.