user2349: Can anyone explain the vulnerability of PPCoin and possibly other Proof of Stake coins that might allow an attacker to double spend? I've seen one explanation here: 
https://bitcointalk.org/index.php?topic=169204.msg1761154#msg1761154
but it wasn't clear. 

user1507: PPCoin and other coins use Proof of Stake in addition to Proof of Work.  Stake is how long a coin has been idle multiplied by its value.  If you've been holding many coins for a long time, you can spend their coin age in lieu of poof of work.  The fear is that someone will hoard coins, gather a bunch of age, and then double-spend and use his accrued coin age to give each spend 6 confirmations immediately, without needing CPU work.
It's a valid concern, in my opinion.

2349: <p>Can anyone explain the vulnerability of PPCoin and possibly other Proof of Stake coins that might allow an attacker to double spend? I've seen one explanation here: </p>

<p><a href="https://bitcointalk.org/index.php?topic=169204.msg1761154#msg1761154" rel="nofollow">https://bitcointalk.org/index.php?topic=169204.msg1761154#msg1761154</a></p>

<p>but it wasn't clear. </p>
 1507: <p>PPCoin and other coins use Proof of Stake in addition to Proof of Work.  Stake is how long a coin has been idle multiplied by its value.  If you've been holding many coins for a long time, you can spend their coin age in lieu of poof of work.  The fear is that someone will hoard coins, gather a bunch of age, and then double-spend and use his accrued coin age to give each spend 6 confirmations immediately, without needing CPU work.</p>

<p>It's a valid concern, in my opinion.</p>
