[bdonlan]

Both sides have to generate secure random numbers in order to perform an ephemeral diffie-helmann exchange securely; if the client's random number is insecure, a man in the middle attack becomes possible.

Non-DH-based exchanges are even worse - the entire exchange hinges on a random number generated by the _client_. The server effectively doesn't even have a chance to generate a random number at all.

[mdda]

But surely the client could use human input as it's source of randomness too : After all, in this case (browser purchases) the user is going to be hitting keys and/or moving the mouse.

[ars]

It could, and today it does. But back then it didn't.