How do I spend my Bitcoins without waiting for Bitcoin-Qt to sync?

I purchased $50 from ExpressCoins and now it’s transferred to the address, but I hadn’t realized that to use your program, I have to download 20 gigs of information. Is there a way to access my coins without doing that and transfer them to the market in order to spend them? I can’t use the address at another wallet that doesn’t do that?

A clear step-by-step would be a godsend at this point, and looking for threads and faqs to read has just confused me with more vague answers and piling on new programs and terms.

Monitor non-wallet bitcoin addresses for new transactions

This is usual checkout functionality. Show address to the user and needs to monitor it for new transaction during n minutes. Addresses are not unique for each payment.

Optional:it would be good to have listener for transaction with certain incoming amount, but this is optional.

blockchain.info have something similar, but I want to it using local bitcoin-qt from .Net application.

*Update: this should be done for non-wallet addresses/transactions.

using coinbase with uc_bitcoin drupal module

I need a simple hack to change the Drupal uc_bitcoin modules code so it gets the exchange rate from Coinbase, it is designed to use MtGox. I want to avoid using the entire coinbase API because I just want the exchange rate, not the kitchen sink. There is a lot of documentation there.

I have found the code in the uc_bitcoin drupal module which gets the information from MtGox. It needs some simple changes so it gets this data from Coinbase. Since both are so different, and I’m not fluent with PHP I thought someone might be able to help me with this.

Here is the code which gets the exchange rate from MtGox:

function uc_bitcoin_exchange_rate($order) {

  static $rate;

  if(isset($rate)) {
    return $rate;
  }

  if ($order->currency == 'BTC') {
    // BTC 1:1
    return 1;
  }

  // Supported MtGox currencies, Dec 15 2012
  $mtGox_currencies = array(
    'USD',
    'EUR',
    'JPY',
    'CAD',
    'GBP',
    'CHF',
    'RUB',
    'AUD',
    'SEK',
    'DKK',
    'HKD',
    'PLN',
    'CNY',
    'SGD',
    'THB',
    'NZD',
    'NOK'
  );

  // @todo add cache
  if (in_array($order->currency, $mtGox_currencies)) {
    if ($response = drupal_http_request('https://data.mtgox.com/api/1/BTC' . $order->currency . '/ticker')) { 
      if ($response->code == 200) {
        $data = json_decode($response->data);
        if(!empty($data->return->avg->value)) {
          $rate = $data->return->avg->value;
        }
      }
    }
  }

// @todo add cache
  if (in_array($order->currency, $mtGox_currencies)) {
    if ($response = drupal_http_request('https://data.mtgox.com/api/1/BTC' . $order->currency . '/ticker')) { 
      if ($response->code == 200) {
        $data = json_decode($response->data);
        if(!empty($data->return->avg->value)) {
          $rate = $data->return->avg->value;
        }
      }
    }
  }

  if (!$rate || !is_numeric($rate)) {
    watchdog('uc_bitcoin', 'MtGox lookup - Unable to get current exchange rate', WATCHDOG_ERROR);
    $rate = FALSE; // just to be super sure.
  }

  return $rate;
}

ECDSA x,y coordinate validity verification doesn’t seem to work

The ECDSA algorithm, secp256k1 for Bitcoin allegedly uses the equation

y ^ 2 = x ^ 3 + 7 mod P

to determine the validity of an alleged point upon the elliptical curve. Utilizing
http://web2.0calc.com/

When verifying public key 1 which has the following qualities:

x = 55066263022277343669578718895168534326250603453777594175500187360389116729240

y = 32670510020758816978083085130507043184471273380659243275938904335757337482424

I applied x ^ 3, then added 7, then mod P on that webpage. Then I square rooted it and got

180964706334543048141953325634608696715.426683454595872

Which is not y. How am I doing this wrong? My results suggests that this point is not a valid point on the curve, but everyone knows it is. Obviously I’m the one who’s wrong.

EDIT: I’d like to further clarify the question. My question is, how does one actually determine (as an example) the Y value while only having the X value. Bitcoin does it, the “Bitcoin Address Utility” also does it. When someone has a compressed key (which only contains the x coordinate) they are able to get the y coordinate as well. Utilizing the aforementioned webpage calculator does not work and others are saying its “modular root”. Anyone have Python 2.7.7 code which would do this or have a relatively simple way of explaining how to accomplish this whole thing? Thanks.

Negative Dogecoin Balances?

Through dogecoin-cli, I sent half the dogecoins I own to an external address. Now, when I call the listaccounts function, I get this:

[root@web ~]# ./dogecoin-cli listaccounts
{
    "" : -25001.00000000,
    "doge" : 48136.85072100
}

Notice how the “” has -25001.00000000. I am using a query from php to get the balance of “doge” and display it to users. That returns 48136.85072100, which is a problem, because I only have 23135.85072100 doge. How can I fix this problem? Thanks!

Do Miners get a reward for re-confirming blocks?

As far as I know, to ensure double-spending didn’t happened and also for other security reasons, there is minimum number of confirmations that a transaction should have to be accepted. That value is around 4-6 blocks I think, but merchants can put their own threshold.

My question is if with the sub-sequent confirmations after the first one, the miners get the mining reward, or a share of it.

Or maybe a transaction is included in different blocks per each confirmation? But how the mining fees are paid then?

If there is no reward for re-confirmations, I’m wondering what would happen after the Bitcoin limit is reached and the only reward for the miners will be the transaction fees. Will transaction get more than 1 confirmation then?

What is a thin client?

I’ve heard that if you don’t want do download gigabytes of data just to use bitcoin you gotta use a thin client. But what is a thin client?
What are the main advantages of a thin bitcoin client and what defines it as such?

Bitcoin transaction with dust output stuck

I made this transaction more than 24 hours ago, and it still has no confirmations at all.

What should I do? This transaction was created by the mobile Blockchain.info wallet. At the time, it didn’t warn me that transaction was problematic. However, I later noticed that one of the outputs of transaction is dust, using Blockchain.info’s block explorer.

Any suggestions how to solve this issue?

Is there a fallback plan if bitcoin’s underlying cryptography is cracked?

Lets say someone started making real viable quantum computers that were affordable enough that large companies could buy one. And lets say a quantum algorithm was devised to crack the private keys of bitcoin public keys in days rather than years.

Is there a plan for that possibility? What would happen? Would there be a smooth migration to another system? Would bitcoin be able to update somehow to protect itself from such a breach?

Can I use my Bitcoins to buy household commodities just like ordinary money?

Can I use my Bitcoins to buy household commodities just like ordinary money? You see, I’m a newbie and have yet to learn the basics including all the facts about Bitcoins. However, I want to be ascertained as to whether my time and efforts in learning the trade will be rewarding in such a way that, I’ll be able to use whatever amount of Bitcoins I may earn someday to actually purchase things from the nearby store.