This topic has already been discussed a couple of times. But this answer deserves a comment:
No, because PoS doesn't work as a decentralised consensus. Every single PoS coin is a private club, with trusted owners, much like Visa the company is. When you invest in a PoS coin you are being tricked into thinking you're investing in the future, when actually you're investing something that can never work as designed.
Read this for more details:
https://bitcointalk.org/index.php?topic=1382241.0The attack described in monsterer's thread is the well known "Long Range Attack". It has often been cited as "the final blow to PoS" or "the hard problem" or something similar by Proof of Stake critics.
However, if you look at the attack, it is so complicated and so expensive that it will probably be easier to attack a Proof of Work currency in 51% fashion. At this moment, you need about $400-800 million dollars to attack the $44 billion Bitcoin chain (and in PoW too, you can get some or even all of them back by short selling in the right moment).
To attack a PoS currency via the Long Range attack you would have to possess about 15% of the coin supply at some moment in the blockchain history (according to statistics from NXT, normally about 30% of the coin owners are "staking" or "forging", so 15% is enough for a "51% PoS attack") AND then trick the rest of the network into a long chain reorganization.
Imagine now a 44 billion $ PoSCoin. You have basically two options:
1) You buy old keys that at the same time in the blockchain history had 15% of all coins. Maybe some old exchange keys (BTC-E?
) will do the trick. (That is the "bribing attacker" Vitalik Buterin has described in his PoS analysis.)
2) You buy 15% of the supply and send them to your wallet, send them back to the exchange, privately mine a double-spend attack chain, and sell the coins again, and then you publish the chain for an attack.
Option 2 will cost you at least 5 billion $ (probably much more, above all if you try to buy them fast), although you will recover some by selling again. Maybe you will get most of them back, but it isn't at all sure that you, in the end, will "get away" cheaper than with the 400-800 million PoW 51% attack.
Option 1 looks cheaper, but first I don't think you will get the keys for free, and second, "buying a key" involves the risk that the old owner uses it in the same way. You would have to trust the old owners - and every single of them can tell the others your plan that you are attacking the coin.
And then you must trick the chain into a "reorg" - most PoS coins simply prohibit long reorganizations. So Option 1 is almost impossible, and option 2 will be very expensive - probably more than an 51% attack.
I know that PoS is,
in theory, a little bit weaker because no external resources are used, but in a practical sense I think PoW and PoS are equally secure.
