Key-schedule Security for the TLS 1.3 Standard
Transport Layer Security (TLS) is the cryptographic backbone of secure communication on the Internet. In its latest version 1.3, the standardization process has taken formal analysis into account both due to the importance of the protocol and the experience with conceptual attacks against previous versions. To manage the complexity of TLS (the specification exceeds 100 pages), prior reduction-based analyses have focused on some protocol features and omitted others, e.g., included session resumption and omitted agile algorithms or vice versa. This article is a major step towards analysing the TLS 1.3 key establishment protocol as specified at the end of its rigorous standardization process. Namely, we provide a full proof of the TLS key schedule, a core protocol component which produces output keys and internal keys of the key exchange protocol. In particular, our model supports all key derivations featured in the standard, including its negotiated modes and algorithms that combine an optional Diffie-Hellman exchange for forward secrecy with optional pre-shared keys supplied by the application or recursively established in prior sessions. Technically, we rely on state-separating proofs (Asiacrypt '18) and introduce techniques to model large and complex derivation graphs. Our key schedule analysis techniques have been used subsequently %by Brzuska, Cornelissen and Kohbrok to analyse the key schedule of Draft 11 of the MLS protocol (S&P'22) and to propose improvements.
State Separation for Code-Based Game-Playing Proofs
The security analysis of real-world protocols involves reduction steps that are conceptually simple but still have to account for many protocol complications found in standards and implementations. Taking inspiration from universal composability, abstract cryptography, process algebras, and type-based verification frameworks, we propose a method to simplify large reductions, avoid mistakes in carrying them out, and obtain concise security statements.Our method decomposes monolithic games into collections of stateful packages representing collections of oracles that call one another using well-defined interfaces. Every component scheme yields a pair of a real and an ideal package. In security proofs, we then successively replace each real package with its ideal counterpart, treating the other packages as the reduction. We build this reduction by applying a number of algebraic operations on packages justified by their state separation. Our method handles reductions that emulate the game perfectly, and leaves more complex arguments to existing game-based proof techniques such as the code-based analysis suggested by Bellare and Rogaway. It also facilitates computer-aided proofs, inasmuch as the perfect reductions steps can be automatically discharged by proof assistants.We illustrate our method on two generic composition proofs: a proof of self-composition using a hybrid argument; and the composition of keying and keyed components. For concreteness, we apply them to the KEM-DEM proof of hybrid-encryption by Cramer and Shoup and to the composition of forward-secure game-based key exchange protocols with symmetric-key protocols.