Here are two attack scenarios on bitcoin private keys:
1) Assume that, BitCorp has just noticed that a Mallory has just compromised one of their servers holding their Bitcoin Private keys. Luckily, they are using a 2-out-3 multi-signature wallet, so Mallory has learnt only one of the 3 sets of keys. The other two sets are on different servers that Mallory cannot access. How do that re-secure their wallet and effectively revoke the information that Mallory has learned?
2) If BitCorp uses 2-out-,2 instead of 2-out-3 wallet, what steps can they take 'in advance' so that they can recover, even in the event of one of their servers getting broken into(and Mallory not just learning but also potentially 'deleting' the key material on that server)?