If the paper notes contain a bitcoin address where the denoted amount is held, they could be checked occasionally while a merchant holds them who is online. That still leaves the problem that separate paper notes could contain the same bitcoin address, either because the bank cheats and prints several notes with the same address, or because a counterfeiter creates a fake note with the same address.

Onkel Paul

That's a big IF


That's an even bigger IF

Let's consider this scenario. Assume that there is no fiat money around, and people use bitcoin everywhere. Additionally assume that because of the inconvenience of being online all the time (hey, how do those poor folks live without facebook?), people resort to paper bitcoins. As banks see that printing paper bitcoins is a lot easier than mining real bitcoins, they just print some more when they need more money. As long as all the paper bitcoins stay in circulation and are used to pay for goods or services, nobody will complain. Only when someone does the math and realizes that there are many more than 21 million paper bitcoins in circulation, they will realize that one paper bitcoin is not worth a true bitcoin anymore.

This has happened a number of times in history, there's no reason to assume that it would not happen again. People are greedy.

Onkel Paul

It does not make a lot of sense.
Fiat paper money works pretty well since it is accepted everywhere within its region and it is issued by the country's central bank. By keeping the inflation at some moderate level, the central bank can devalue its money without everybody losing trust in them, which is actually a pretty clever scheme (look up "boiling frog" sometime, it's been found to be a myth, and probably the belief by central banks that nobody will revolt if they just devalue their money slowly enough will be found to be a myth, too.)
Certificates issued by private banks are not as fungible as state-issued money, you'd need to redeem them at the issuing bank, which incurs quite some additional cost. If you can't trust the issuing bank, certificates can lose value incredibly fast (think Lehman Brothers...)
As you already noticed, handing over a medium containing a private key (whether it's paper or a physical coin) does not really work either. Without being online, the receiver cannot check whether the corresponding address has sufficient funding, and in any case they can't be sure that the key hasn't been copied in advance.

Of course, you can accept any kind of certificate statement (a.k.a. promise) as a payment anyway, whether the issuer promises to pay the bearer one bitcoin, an ounce of gold, or three bags of potatoes. It's just a matter of trusting the issuer, and trusting that the certificate itself is not counterfeit.

But that's not how bitcoin works, if you receive bitcoins, you don't have to trust anyone (except your own ability to keep your private keys secret) and there is no risk of counterfeit. "Paper bitcoins" are not bitcoins.

Onkel Paul

You receive bitcoins in the form of transactions. Once a transaction has been confirmed (put into the bitcoin blockchain) it is valid, and you did in fact receive the amount of bitcoin transferred in the transaction.
As a security precaution, for higher amounts you should wait for a number of confirmations (additional blocks built upon the block in which the original transaction was recorded.) A typical confirmation count is 6, although that is pretty arbitrary, 3 would be pretty safe in the current blockchain environment.
If you want to check whether someone actually has the bitcoins they offer you for a trade, you can let them sign a message using the bitcoin address with the balance.
As an example of such a signed message, here is one that I signed using my signature address:

Address:
1HvaoWCtvG2pvd8B9nvr9DjiErnSmjCqAY
Message:
I don't want to buy gold.
Signature:
HBxJ6x/QwG4W8a+uCwtMt56f9AaXzMOP/pJdyUNxBskKN0rG4WqjzpIH2992nZl/C4BImeZ/t2AcQgJWMPobTxM=

(you can verify the message at https://coining.com, and you can check the current balance at https://blockchain.info/address/1HvaoWCtvG2pvd8B9nvr9DjiErnSmjCqAY?sort=0 so if I proposed to buy gold from you you'd know that I don't have enough bitcoins at this address for an ounce of gold)

Directly exchanging bitcoin for gold and vice versa is probably not very efficient, as there are few companies offering this service, so they can charge a premium for it due to missing competition. Trading with private (possibly anonymous) persons is pretty risky, especially when buying gold with bitcoins, as verifying the gold is much harder technically than verifying a bitcoin transaction. You also need to consider that you risk getting mugged, or if you want to do the trade due to legal reasons (India comes to mind) that you might get caught in a law enforcement sting operation.

Onkel Paul

Kann sein, dass es nicht funktioniert, weil es zu wenig definierte Simulationsmechanismen gibt.
Risto Pietila hat etwas ähnliches schon mal angesetzt, da steckte erheblich mehr Vorarbeit und laufende "Spielstandsaktualisierung" drin, das habe ich mir eine Zeitlang angesehen, bis mir zum passiven Zuschauen einfach die Zeit fehlte.

Spiele in einer virtuellen Welt funktionieren normalerweise nur, wenn es ein ausbalanciertes Regelsystem gibt, dessen Einhaltung entweder vom Spielleiter oder einer Simulationsumgebung erzwungen wird oder an das sich die Spieler aus eigener Disziplin halten. Die Erfahrung in der realen Welt ist, dass die einzigen Regeln, an die sich jeder ziemlich konsequent hält, die Gesetze der Physik sind, also eine strenge Simulationsumgebung :-)

Onkel Paul

Danke, Mario, dass du den Kurs noch stabilisierst 
Ich hätte aber auch sonst jetzt keine großen Sorgen mehr, dass er in der nächsten Zeit unter 700 gehen würde.

Onkel Paul

Well the transaction posted by shorena matches the description in the original post, and for someone whose native language is not english it might be possible to mistake a "donation address" appearing somewhere within the electrum client as the address to which receiving payments should be sent.

Contacting the server operator is most likely the only possible way of getting the payment back. Don't know whether he would be willing to send you that amount without some reasonable proof that the money was indeed intended for you and not for him. A good proof would be a signed message from your friend using one of the addresses from which the transaction inputs came. However, apparently your friend sent from an online wallet or other service, at least there are many outputs to different addresses in the transaction, which is atypical for a single payment from a wallet, so he will most likely not be able to provide such a signed message.

Onkel Paul

Do you only need to generate payment addresses on your web server, or do you need to check whether payments have been made, too?

To generate payment addresses, HD key derivation (BIP32, see https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki) would be the most appropriate - your webserver would only need to know the public parent key from which the payment addresses are generated. There are Javascript implementations available, so integrating that into your Nodejs software should be fairly easy.

To check whether payments have been made, your server could either run a full node and use its API (but you wrote that you want to avoid that), or use an external service for checking address balances. If you just need to check payments manually, any old HD wallet should do.

Onkel Paul

The safest way is to give each prospective funder a fresh address (and make a record of that bitcoin address and the info about the funder that you need to give them their "little something").
The addresses could be generated by a HD wallet, you will have to search a little to find something that fits into your crowdfunding infrastructure. Using a HD wallet has the advantage that you can generate the public keys (and addresses) from the public parent key, for example on the server with which your users interact, and keep the private parent key (and all the generated private child keys) safely out of reach of any hackers. Of course, if you use a HD wallet, you only store the key derivation paths for each funder, not the generated addresses, so you can compute the matching private key for each funder's address.

Do you think you can get this to work?

Onkel Paul

That site is well-known, a kind of inside joke...

So far, that sounds reasonable. Of course you can use that page to enumerate addresses (there are other, more efficient ways, though)

Extremely unlikely. Enumerating private keys to find some containing bitcoin balances does not work. If it would, bitcoin would be worthless (nobody would use money that can spontaneously disappear because someone happens to guess a private key).

Ah, you're selling the secret script. I'd bet that you'll make some money from that (probably not 4 BTC/month) because there are enough gullible people who hope to make a quick BTC using some "trick" they don't understand...

Onkel Paul

Did you read the postings from 2 years ago? Only the people who gave you negative trust can take it back, and it does not looks like they are inclined to do so.
Even though you probably like that account name a lot you should really consider using a different name if you want to avoid the negative trust. There just have been too many impostors on this forum, so the preventive negative trust is quite understandable.
Since you're still a newbie with very few postings, you would not even lose significant activity points or membership rank by giving up this account.

Onkel Paul

Na ja. Bei ca. 660€ ist Schluss. Sag ich mal so aus dem Bauch raus.

Onkel Paul

As others have noted, questions about legality of bitcoin are mostly useless if you don't state what country you're living in.
What I'm writing here only applies if you intend not to break the law. If you don't care about the law anyway, why ask? 

If there is explicit law about bitcoin, you should look at that law, of course.
If there is no explicit ruling about bitcoin in your country, you should first consider the general attitude towards "unspecified" behavior:

• In more liberal countries, one can assume that if something is not explicitly forbidden then it is allowed.
• In restrictive/totalitarian countries, you should assume that if something is not explicitly allowed you could get into trouble when you do it (especially when officials want to get you into trouble).
  

Then it might be helpful to look at how foreign currencies would be handled. For example, if there are rules restricting how much value in foreign currencies you can legally hold, you can safely assume that holding more value in bitcoin would not be allowed. In addition, if businesses need a special license if they want to receive and spend foreign currencies, you probably need such a license as well. Note that even if you are allowed to use bitcoin for payments, this probably does not mean that you may operate a money exchange - for that you most likely need a specific license.

There are various reasons why such laws exist. Most often the argument revolves around money laundering prevention. This argument has quite some merit, since tracking down organized crime often amounts to "follow the money", and money laundering makes that difficult or impossible. The anarchist or conspiracy theorist would probably argue that the evil government likes to watch and control its subjects closely.
As a somewhat critical citizen I see some truth in both arguments. Organized crime and corruption are parasites and negatively affect a society's well-being, both in terms of simple economic health as well as regarding personal safety. However, political and economic power almost always tends to stabilize itself and take steps to impede necessary change, which can lead to a different kind of corruption and parasitic accumulation of wealth.

Onkel Paul

1. Lauda
2. Mitchell
3. Dabs

I won't go into personal attacks as I don't think that is helpful, but perhaps you can see that the wording of this topic can cause aggressive reactions easily.
The topic "Chinese miners rejecting transactions originating in the US" is worded as a factual statement when your post actually just raises a concern that *if* chinese miners started rejecting such transactions *then* american bitcoin users might be experiencing difficulties.
The reactions might have been much milder had you written the topic as a question, something along the lines of "Could chinese miners cause trouble by rejecting transactions originating in the US?".

Apart from the topic, it is actually not really clear what percentage of the global hashing power is controlled by chinese miners, and which of those could possibly be coerced by the chinese government to refuse to mine such transactions (if they can reliably be identified as originating from the U.S.A. at all). Pools might not get away with such a change - as a pool miner, you can switch pools relatively quickly if you don't like their behavior, making the pool irrelevant in a short time. Operators of actual mining hardware within China would be more susceptible to such a coercion, but even though they control quite some mining power, they would most likely only cause some delays in transaction processing.

So unless we have hard evidence that a significant percentage of miners implement policies that discriminate against some group of bitcoin users, I would file this concern under "What If?". There are probably a dozen "What If" posts each week on BCT, and it just does not make sense to act on them. One should probably look at the more realistic scenarios and think about possible workarounds, but worrying about the rather unlikely cases won't help much.

Onkel Paul

Hängt davon ab, was man als Testen ansieht. Ich habe mir Wallets auf Smartphone und Desktop-Rechner eingerichtet.
Das Desktop-Wallet braucht ewig, um sich zu synchronisieren. Ob das an begrenzter Testnet-Performance liegt oder an der Software, kann ich nicht sagen.
Android-Wallet geht so.

Zum Programmieren irgendwelcher Schnittstellen oder Bots bin ich nicht gekommen (wie gesagt, das Desktop-Wallet hat ewig zum Sync gebraucht, und auf dem Phone programmiert man nicht).
Im Ganzen ist mir die Sache mit den Witnesses noch nicht so ganz geheuer - im Testnet gibt es nur eine Liste von natürlich anonymen Witnesses, wie das dann im Livenet aussehen wird, muss man sehen. Wenn die Witnesses quasi zentral vorgegeben (vorgeschlagen) sind und kein Node die Liste prüft und ggf. anpasst, hat man kein wirklich dezentrales System.

Onkel Paul

My desktop wallet takes ages to sync, and sometimes there is no change in the database for half an hour or more. Is this normal? Maybe this is related to the relatively small number of testnet nodes, is it possible to connect to a different one for faster sync?
The mobile wallet also had some performance issues, but synced a lot faster since it does not store the database locally.

Onkel Paul

Mal ein, zwei technische Fragen:
- wie groß ist die aktuelle Datenbank? Mein Client synct inzwischen etliche Stunden und hat bis jetzt eine 100 MByte große Datenbank aufgebaut, ich hatte ein bisschen damit gerechnet, dass es zu diesem Zeitpunkt mit dem Testnet etwas schneller geht... (die Frage wird sich wahrscheinlich von selbst beantworten, bevor ich hier eine Antwort sehe )
- kann jemand die technische Robustheit der Implementierung einschätzen? So wie es aussieht, ist die Datenbank eine sqlite-Datei, skaliert das ordentlich, wenn richtig was los ist?

Onkel Paul

How much would you pay for it? If the price is right, someone ought to be able to prepare it and send it to you as a stack of DVDs
And do you really want CSV data for every single transaction? With or without pay-to-script and other "weird" transactions?
Currently the (binary) data is about 80 GB in binary form, as CSV it will probably be a bit more. Is Gephi able to handle that amout of data?

Onkel Paul

Did you try to start the application again?
Sometimes it might just have gotten into some inconsistent state, and the next time you run it, everything works fine.
You should at least try that. If it does not work (that is, if the error is reproducible) then posting the error log would certainly help people here to analyze the problem with you.
You should also check whether the hard drive has enough space to store the blockchain. I would assume that you'd get a reasonable error message when it runs out of space, but it would make sense to check anyway.

Onkel Paul