One premise underlies every argument about usability and security that has ever raged: "Secure software is doomed to be unusable, and usable software is doomed to be insecure." This talk will examine the faulty assumptions behind that belief, using the dual lenses of linguistics and formal language theory. We'll explore what makes software -- particularly software that developers use, e.g., APIs -- easy or difficult to use, how mismatches between what developers expect and what users expect lead to vulnerabilities, and how architects and developers can design and code for improved security and improved usability at the same time. By day a mild-mannered build engineer, by night the ringleader of the Langsec Conspiracy, Meredith L. Patterson lives in Brussels, Belgium. She wrote and maintains the Hammer parser generator library, and is currently working on Tongs, a "standard library" of reference implementations of protocol, file format, and message format parsers. When not traveling to far too many infosec conferences, she enjoys bicycling, cooking, and target shooting.