Gordon Welchman
Welchman's main work at Bletchley was "traffic analysis" of encrypted German communications. This is, first, looking for an increase in signals. Major events in warfare are signalled in advance by a huge increase in messages. Second, there are standardized parts of a message, such as message origination, message destination, time/date information, and so on. These are much easier to decipher than the actual message content. Welchman developed this approach. Today we describe this as "metadata" analysis.