Quote from: enuma on February 11, 2014, 04:23:57 PM

Well newbies thats why bitcoin consider a near 100% transaction confirmed only once it hits the god damm 6 confirmations @ minimum. Exchanges and other services rely on txid to simply cut the long time you have to wait for the 6 confirmations.

Yes but there isn't anything magical about 6 confirmations. I would say the network considers a transaction confirmed once it is confirmed. How many confirmations you should wait depends on your risk threshold. For most tx 2 confirmations provides a high level of security. For some transactions you probably want to wait more than 6.

Satoshi never indicated 6 was some magical barrier it simply was the output of his example on the risk of reversal. If an attacker has 10% of the network, and you want a less than 0.1% chance of the attacker being able to reverse your transaction you need to wait for 6 confirmations. However if the attacker has say 20% of the network you would need to wait for 10 confirmations to reduce the attackers chances to 0.3%.

Meni wrote a good paper on the economics of transaction reversing because the attacker has a cost (in terms of potentially lost block rewards) trying to re-org the blockchain. So the more important factor is what is the VALUE of your transaction.

For example if you assume the attacker has 48% of the network (if they have 51% no amount of confirmations will keep you safe) then you need at least this many confirmations to make the attack non-economical

48%
<= 4 BTC = 1 confirmation
6 BTC = 2 confirmations
8 BTC = 3 confirmation
9 BTC = 4 confirmation
10 BTC = 5 confirmations
12 BTC = 6 confirmations
13 BTC = 7 confirmations
14 BTC = 8 confirmations
15 BTC = 9 confirmations
16 BTC = 10 confirmations

Hopefully we can eventually kill the "6 is good for everyone" way of thinking.
https://bitcoil.co.il/Doublespend.pdf

Many merchants hinder the user experience by demanding an excessive amount of confirmations relative to the transaction risk. For example someone accepting 0.01 BTC for some game credits shouldn't be demanding 6 confirmations. The user wants to play right away and making them wait 30-120 minutes just creates the (false) perception that Bitcoin is slow. Even 1 confirmation provides a higher level of security than other payment methods.