This paper presents Riposte, a new system for anonymous broadcast messaging. Riposte is the first such system, to our knowledge, that simultaneously protects against traffic-analysis attacks, prevents anonymous denial-of-service by malicious clients, and scales to million-user anonymity sets. To achieve these properties, Riposte makes novel use of techniques used in private information retrieval and secure multi-party computation. In a configuration allowing 10,000 users to upload messages to the system, our prototype Riposte cluster handles 48 posts per second. For latency-tolerant workloads with many more readers than writers (e.g. Twitter, Wikileaks), we demonstrate that, in 32 hours, Riposte can build an anonymity set of 2,895,216 users.