Assume that Client and Server are using SSL 2.0 which supports DES ciphersuite. Also, assume that the server's RSA key is unfactorable yet. In SSL 2.0, the client chooses the master secret and encrypts it with the server's public-key. If the client and server agreed on a broken ciphersuite that uses DES. How a MITM can take advantage of the weak symmetric-key (DES)? It seems to me that MITM can't do anything unless the server's public-key gets factored. Can an expert elaborate or clarify to me?
The main weakness of DES is its short key length. This makes DES quite easy to brute force. An attacker would record the whole HTTPS session and then try each key until he has found the right one that can decode the conversation.
There are other attacks on DES which allow breaking it faster than brute force, but these need quite a lot of known or chosen plaintexts. This would not be feasible in your scenario.