Determining which Law Firm Worked on a Particular Patent Application

I have invented a secure computing platform that has a large cryptographic component to it and am preparing to apply for patents on my invention. I am looking for a firm with an IP practice that has experience in the area of cryptography and secure computing-related patents. One approach I had in mind for finding such a firm would be to search for closely related existing patents and approach the firms who worked on those patents. However, it appears that only the inventors and assignee's are listed on the applications. Is there a way to determine which law firm (when applicable) worked on a particular patent? Alternatively, if someone can recommend a firm with expertise in this area I would be appreciative.