SPEAKER: Tom Ristenpart TITLE: From Credit Cards to Censorship Circumvention: Building Encryption Schemes with Specialized Ciphertext Formats Abstract: Traditional symmetric encryption schemes map plaintexts to ciphertexts that are unformatted bit strings. In this talk we will explore encryption that takes into account formatting requirements for ciphertexts. We start with format-preserving encryption (FPE), for which we gave the first cryptographic definitions in 2009 and which is now used widely in industry. FPE ensures that ciphertexts preserve the format of the underlying plaintext, for example enabling the encryption of credit card numbers to yield ciphertexts that are themselves valid credit cards. We will present definitions of FPE and a generic construction in the form of Rank-Encipher-Unrank, which combines classic results regarding the ranking of languages in the sense of Goldberg-Sipser with small-domain ciphers to produce FPE schemes that work for formats described by arbitrary regular languages. We will then overview our recent results on building fully secure small-domain block-ciphers, giving a new paradigm for building PRPs from pseudorandom separators, a primitive we introduce. We will finish with a recent generalization of FPE called format-transforming encryption (FTE). Here we use a variant of an Encrypt-Unrank approach to implement encryption tunnels whose ciphertexts on the wire are guaranteed to match against arbitrary regular expressions. This enables, for example, confuse Internet censors that use regular-expression based deep-packet inspection to identify and block circumvention tools such as Tor. This talk will cover joint work with Mihir Bellare, Scott Coull, Kevin Dyer, Phil Rogaway, Thomas Shrimpton, Till Stegers, and Scott Yilek