Having helped Sergey Brin and Larry Page edit their initial business plan, which turned into some company called Google, Guido Appenzeller knows a fair amount about start-ups. Although he passed on an opportunity to join his Stanford buddies in taking their venture public--what's a couple of billion dollars between friends?--Appenzeller, 34, has followed their lead, co-founding Voltage Security, a fast-growing firm that has more than 150 clients.
Voltage provides encryption software to banks and health-care providers to safeguard financial and medical records. The company also works with government agencies and ran trials with the Department of Homeland Security and Department of...