Without internet access, these phones can't be hacked remotely, but think about what can actually happen to them:

• Phone gets lost/damaged
  Depending on whether you carry the phone with you or just leave it at home in a drawer, this is more or less likely.
  
• Phone gets stolen
  If the thief does not know what the seed means, this is the same as a lost phone. However, a thief who recognizes the data as a cryptowallet seed will probably find out pretty quickly how it can be used to access your wallet.
  

So what's the advantage of storing a seed in such a phone? Do you really need to carry it with you (for example, to enter it into a wallet on a desktop/laptop where it is not normally stored)?
You need a backup of the seed to protect against loss of the phone, and you need to store the seed on the phone in a way that is not recognizable as a wallet seed to protect against knowledgeable thieves.

Overall, looks like a lot of effort for dubious value.

Onkel Paul

You should also look at Andreas Schildbach's wallet (available from the app store) to compare features and characteristics.
On the spot I can't find docs on how it manages blockchain access (Mycelium uses dedicated servers for that) but you might have more time than me to look around...

Onkel Paul

"Onlineshop-Wallet"? Dann musst du dich mit dem Betreiber des Onlineshops auseinandersetzen.
Dass der Betrag auf der Adresse eingegangen ist, ist ja in der Blockchain eindeutig nachweisbar, z.B. über die Blockchain.info-Anzeige.

Aber ich denke, hier im Forum kann dir ohne konkretere Infos niemand wirklich weiterhelfen - wie soll irgend jemand wissen, warum diese Wallet deine Transaktion nicht als eingegangen anzeigt, vor allem, wenn man noch nicht einmal weiß, welche Wallet von welchem Shop das ist?

Onkel Paul

Wenn du verrätst, welche Wallet-Software du benutzt, ist es leichter...
Bei Bitcoin-QT steht es direkt in der Statusleiste, das ist eigentlich nicht zu übersehen.
Mit Elektrum und anderen Wallets kenne ich mich nicht aus, aber das sollte ähnlich aussehen.
In jedem Fall müsstet die Wallet eine Funktion haben, um eingehende Transaktionen anzusehen, dort sollte auch ein Hinweis darauf zu sehen, sein, wenn sie nicht synchronisiert ist.

Onkel Paul

I think it would be easier if you just calculated a fee structure and then let people decide whether your hosting offer is competitive.
The "trust based" approach is likely to cause disagreements about proper distribution of profits between you and your customers.

However, if you want to avoid running this as an actual business, your approach might work if you find people whose needs match your profile.
Good luck!

Onkel Paul

Ist die Empfänger-Wallet komplett mit der Blockchain synchronisiert? Wenn nicht, sieht sie möglicherweise die Transaktion noch gar nicht.
Dass eine neue Empfänger-Adresse angezeigt wird, ist normal. Es ist sinnvoll, für jeden Bitcoin-Empfang eine neue Adresse zu verwenden, um die Kombination von Transaktionen weniger leicht zu machen. Aber die Wallet speichert alle Empfangs-Adressen und zeigt (wenn sie synchronisiert ist) alle eingegangenen Beträge an, egal auf welche der verwendeten Adressen sie gingen.

Onkel Paul

Double spending means to create two transactions using the same output(s) from a previous transaction, and presenting one of these transactions to a seller as payment, while transferring the funds to another address using the other transaction. Only one of these transactions will eventually make it into the blockchain. If you want to scam the coffee shop, you'd need to control a substantial share of the mining pools so that you can get them to accept your second transaction after the first one has already propagatesd through the network.
There are basically two cases:
- you spend a small amount (for a coffee or a pretzel), and the seller accepts a zero-confirmation transaction (with sufficient fees) as "good enough", risking a small percentage of failed payments. In this case, the cost of doing a double spend far exceeds the possible gain in coffee or pretzels, even though determined crooks could probably pull this one off.
- you spend a large amount (for a house or a Lamborghini), and the seller will wait for 6 or more confirmations (which takes about one hour). Performing a double spend in such a situation requires you to control more than 50% of the mining power for a considerable time to effectively orphan the part of the chain that contains your first transaction and create an alternate blockchain that will be accepted by the rest of the bitcoin world. The cost of doing this far exceeds the value of a house or a Lamborghini, so this case, too, is economically infeasible. And in the case of the house, you can't simply drive away with the stolen goods...
One possible exception might be a trade that involves highly illegal stuff that both parties want to finalize very quickly. But if a drug dealer sells you drugs for several thousand bitcoins and accepts a zero-conf payment, I'm pretty convinced that he has means to find you after you've perfomed a double spend... For you as a buyer, this wouldn't be economically feasible either, and probably pretty painful.

Executive summary: Double spending does not work in reality (as has been discussed to death already)

Onkel Paul

Bitcoin core needs to store the whole blockchain on your hard disk. Downloading (and verifying) that amount of data takes quite some time, that's right.

Currently there's no easy way around this with bitcoin core, other wallets such as Electrum don't store the full blockchain but rely on external services for blockchain access. This is much faster for initial setup, but at the moment, this is not an option for you because you already sent funds from coinBase to an address within your bitcoin core wallet. So you'll definitely want to keep that running until you have full control over your funds.
It might be possible to export the private key for the address to which you sent your coinBase funds, and import this into another wallet which you can use sooner, but I don'd really have experience in this, so I won't advise you to do this. If you find someone who can explain this process and lead you through it, that might be an option, though.

Onkel Paul

Thanks, but I'm not inclined to be a Mod. Too much hassle, and I'd be frustrated over the cases that are not so clear cut. The posts I reported have been simple spam posts without any relation to bitcoin at all, clearly done by bots. There are much fewer posts of that kind nowadays...

Onkel Paul

As soon as such an agreement (or just a reasonably plausible suspicion of one) would surface somehow, the value of bitcoin would most likely go down a lot, making the pools lose really big.
The would not be able to make up for that through any double spending tricks.
Of course, that would not make it impossible for the pools to try, but according to game theory they are very unlikely to try it because loss would be inevitable.

Onkel Paul

The string that is being hashed consists of the block's header data.
Since the header contains the hash of the previous block, nobody can start building a block (and hashing it) before the previous block is known.
Other parts of the header contain a hash of the block's contents (which is normally different for each miner, because it includes not only the transactions from mempool, but also the coinbase transaction which goes to an address created from a random private key, and the "nonce" which is incremented sequentially to find a block header whose hash is small enough.)

So there is no single entity who creates and distributes a new challenge, every miner has his own challenge built from the previous block and the new block's data.

Onkel Paul

Definitely not.
Random data does not compress. Any compressibility is a sure sign of non-randomness.

Onkel Paul

See https://bitcointalk.org/index.php?topic=1033997.0 for a discussion.

Oops, maybe the problem is with trailing newline. The signature was created with bitcoin-qt, which does not create the formatted "signed message" but just the signature.
When I created the signature the text did not have a newline at the end. With a newline, the signature was different.
Does anyone know what the canonical procedure for creating a "-----BEGIN BITCOIN SIGNED MESSAGE-----" formatted multi-line signature with bitcoin-qt is?

Onkel Paul

Ah, ok, 375€ sind ja nicht besonders viel - das kann man wirklich noch direkt in Mycelium mit sich rumtragen (man sollte aber ein Backup machen, wäre sonst schade drum, wenn man das Handy verliert).

Ein Angebot in Local Trader hast du ja schon eingestellt. Wenn da keiner anbeißt, musst du wohl sehen, ob es mit einem der Käufer in der Umgebung klappt, bringt halt einen niedrigeren Preis...

Onkel Paul

Hängt wirklich von den ganzen Umständen ab - einfach so empfehlen würde ich nichts.

Wenn ich einen großen Betrag unter die Leute bringen wollte, würde ich wahrscheinlich mehrere kleinere Transaktionen machen - zum einen dürfte man wenige Handelspartner finden, die gleich auf einen Rutsch 20 oder 30 Bitcoins kaufen wollen, zum anderen wäre mir das Risiko, beschissen oder ausgeraubt zu werden, doch etwas zu hoch. Da ich weder einen größere Bitcoin-Summe besitze noch mich von meinen Bitcoins trennen will, ist das für mich sowieso hypothetisch.
Dann ist es vermutlich sinnvoll, den Betrag entweder auf der blockchain.info Wallet zu lassen oder von dort auf eine PC-Wallet zu verschieden, von der man ihn dann schluckweise je nach Bedarf auf das Phone transportiert.

Ein bisschen wundert es mich allerdings, dass du eine Blockchain.info Wallet mit ein bisschen (viel?) BTC hast, ohne dich vorher über Exchange-Möglichkeiten schlau gemacht zu haben. Die Bitcoins sind dir ja sicher nicht einfach zugelaufen...

Onkel Paul

Mycelium hat nur eine PIN (6 Digits, ein bisschen besser als eine Bankkarte).
Man sollte dort nur so viel BTC drauf halten, wie man auch Bargeld im Portemonnaie mit sich rumtragen würde - also je nach Reichtum und Risikofreudigkeit irgendwas zwischen max. 500 EUR / 2 BTC und 5000 EUR / 20 BTC...
Alles andere am besten auf einer Wallet auf dem PC (bevorzugt *nicht* Windows) bzw. größere Beträge, die man eine Zeitlang nicht anfassen will, in einer Cold Wallet.

Onkel Paul

Edit: Das mit "nur noch einmalig" hatte ich überlesen. Dann ist Cold Wallet natürlich nicht unbedingt angesagt...

The recommended transaction fee depends on a number of factors, basically number of inputs (which make up the bulk of a transaction's size) and age of the source transactions (when they are sufficiently old, then zero-fee works well).
In general, transactions without fee might eventually get processed but it can take a longer time, as miners would prefer to mine transactions with fees, but they normally include some no-fee transactions in blocks, too.
If you can accept a long transaction time (for example, it you move funds between different wallets of your own, or send a bit to a friend who does not mind if he gets it this week) then you might try to send without a fee. The worst that can happen is that after a long time (on the order of days) your wallet decides that re-broadcasting the transaction is not going to work, and shows its inputs as unspent again.

Onkel Paul

You probably had your wallet set to show mBTC instead of BTC, and sent 0.216 mBTC (which is incidentically equal to 0.000216 BTC) instead of 0.216 BTC (216 mBTC) to the destination address.
The other amount is so-called "change" and goes to a new address within your wallet (so it's not lost).
Depending on what the recipient expects, you might be able to send the missing amount (215.784 mBTC, which is equal to 0.215784 BTC) to the recipient to complete the payment.
Your best bet is to talk to them first to see whether they will be able to recognize those two payments as one.
Of course, due to this mistake in using the application you will have to pay miner's fee for two transactions instead of just one, but that's a minor loss compared to the one you've initially assumed.

Onkel Paul