How to require two keyholders to decrypt a document? I want to create a system to encrypt a document and store it with a 3rd party, but not have the 3rd party be able to decrypt it until some unspecified later date. It seems like the solution would be to split the key into two parts, given to two different trusted 3rd parties. Then when the date arrives those 3rd parties can come together to decrypt the document.

I realize that splitting a key effectively reduces the strength of the key in half for anyone who has half of the key.


Is splitting an AES key into two parts is an effective method of preventing either party from decrypting a document?
Is there a preferred method of splitting a key? Down the middle? Every other byte, etc.?
When splitting a key will one party have an advantage over the other party in a brute force attack? In other words, are certain parts of the key more important than others?
Splitting a key does not reduce the key strength at all. Simply generate two random 128-bit strings and give one to each party. Encrypt the data with the exclusive OR of the two random strings.

Each string alone gives no information whatsoever about the final key, assuming your random number generator is sound. No party has any advantage.