Dear all, We are writing to ask about the possible security impact of variable-length secrets on the "Hybrid key exchange in TLS 1.3" RFC [1].
As you probably know, when using key material of variable length and processing this material using hash functions, a timing side channel may arise. In broad terms, when the secret is longer, the hash function may need to process more blocks internally. In some unfortunate circumstances, this has led to timing attacks, e.g. Lucky Thirteen [2] and the newly-disclosed Raccoon Attack [3]. To be clear, we are not aware of any attack on the proposed standard. Rather, we view this as an opportunity to defend-in-depth against such attacks, while work on the standard is in progress. Our proposal is to add language to the RFC explaining that variable-length secrets may enable such attacks, and should therefore be avoided when possible. Currently, the language seems to allow for variable-length secrets, should the need arise: "Variable-length shared secrets ... if it is envisioned that this specification be used with algorithms which do not have fixed-length shared secrets ..." We also note that a related RFC exists, "Hybrid Post-Quantum Key Encapsulation Methods (PQ KEM) for Transport Layer Security 1.2" [4]. However, that RFC apparently only uses BIKE, Kyber or SIKE as the PQ KEM. To our knowledge, all three KEMs have fixed-length secrets. It may be prudent to add cautionary language to that document as well, in case other KEMs are used in the future. Kind regards, The Raccoon Team [1] https://www.ietf.org/id/draft-ietf-tls-hybrid-design-00.html#name-open-questions [2] https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6547131 [3] https://raccoon-attack.com/ [4] https://tools.ietf.org/id/draft-campagna-tls-bike-sike-hybrid-05.html#name-key-exchange-algorithms
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