Modular Design of Secure yet Practical Cryptographic Protocols

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PoPETs Secure Messaging Protocols.

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Publications for: Secure Messaging Protocols. Research Program: Secure Messaging Protocols. A bit more precisely, such a protocol describes how: cryptographic keys are created, distributed, authenticated, used and deleted, messages are prepared encrypted, authenticated, etc for transport and how received packets are processed decrypted, verified, etc.

Roughly speaking, the security improvements over the original Signal protocol can be described as follows: As before, a main strengthening is in the type of PCS provided by the protocol. Moreover, the protocols also prevent certain forgery attacks. How well is the protocol implemented?

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How are default options in the platform set? What code review processes are being used? How are the capabilities, and especially the limitations of the platform communicated to the user? And much much more. Transcript Consistency means that the protocol ensures that all parties to the conversation have an identical view of the entire history of the conversation. P2P Transport refers to fully decentralized messaging platforms where packets are routed directly between end-points without any server infrastructure to help with things like contact discovery, key distribution or asynchronous communication.

In that spirit several recent research projects in the field can be traced back to the introduction and deployment of the Double Ratchet key agreement protocol and the various secure messaging protocols that are built onto of it. Thanks to its novel design and strong security claims it triggered a new line of cryptographic research into provable security for 2-party messaging protocols. With this result public cryptographers quickly expanded their focus beyond the original Double Ratchet protocol. While the specifics of these notions are quite complicated and well outside the scope of this page we observe that the flavor of the improvement over current protocols was in the precise type of Post Compromise Security PCS the new protocols provided.

Modular Design of Secure yet Practical Cryptographic Protocols

Unfortunately, this added security comes at a rather steep price at least from a practical perspective. Roughly speaking, the security improvements over the original Signal protocol can be described as follows:. While RECOVER-security is undoubtably a novel and interesting security property, at least in a theoretical sense, we at Wickr believe that it remains somewhat in the eye of the beholder as to wether it is truly desirable in practice.

In particular, the adversary may continue to impersonate Bob to Alice while Bob has no in-band means to notify Alice that this is happening. Thus, the value of the property may well depend on the particular use case of the SMP. In particular, when a message is dropped any and all future communication remains undecryptable until the dropped message is resent and delivered. Katriel Cohn-Gordon, Cas J. CSF PoPETs Secure Messaging Protocols. The reason for the verification toolbox only considering the verifier code is that by definition [2] the soundness of the protocol essentially concerns providing guarantees for the verifier, regardless of whether the prover is honestly executing the protocol or not.

The exception is the final algebraic verification that is performed on the last response from the prover, which deter- mines whether the proof should be accepted. The theoretical soundness proof that we construct essentially establishes that this algebraic check is correct with respect to the proof goal, i. The soundness proof is then generated in three steps: a An adequate proof template is selected from those built into the tool 3.

If no adequate template exists, the user is notified and the process terminates.

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If the proof assistant successfully finishes, then we have a formal proof of the theoretical sound- ness of the protocol. The process is fully automatic and achieving this was a major challenge to our design. As can be seen in Fig. In order to achieve automatic validation of the generated proofs, it was necessary to construct a library of general lemmata and theorems in HOL that capture, not only the properties of the algebraic constructions that are used in ZK-PoK protocols, but also the generic provable security stepping stones required to es- tablish the theoretical soundness property.

By relying on a set of existing libraries such as this, development time was greatly shortened, and we were able to create a proof environment in which we can express proof goals in a notation that is very close to the standard mathematical notation adopted in cryptography papers. No verification is carried out of the executable code generated from the PIL file.

This is a program correctness problem rather than a theoretical security problem, and must be addressed using different techniques not covered here. We next detail the most important aspects of our approach. Proof strategy. Proving the soundness property of the ZK-PoK protocols pro- duced by the compiler essentially means proving that the success probability of a malicious prover in cheating the verifier is bounded by the intended knowledge error. As all spe- cial homomorphisms used in cryptography fall into one of two easily recognizable classes, the verification toolbox is able to automatically find a pseudo-preimage for any concrete homomorphism that it encounters without human interaction.

A central stepping stone in formally proving the existence of an efficient knowl- edge extractor is the following lemma which actually proves Theorem 1 that we have formalized in HOL. Given a special homomorphism and two accepting protocol transcripts for a ZK- PoK of an atom, we prove the existence of a knowledge extractor by ensuring that we are able instantiate Lemma 2. If multiple predi- cates are combined by And, the verification tool defines as proof goal the exis- tence of a knowledge extractor for each and all of them separately: one needs to show that the witness for each predicate can be extracted independently from the other predicates.

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In case of Or proofs i. First, for each atom, an Isabelle theorem proves the existence of a knowledge extractor. In a second step, it is then shown that the assumptions of at least one of these theorems are satisfied i. The HOL theory file produced by the Protocol Verification Toolbox is typical, in the sense that it contains a set of auxiliary lemmata that are subsequently used as simplification rules, and a final lemma with the goal to be proved.

The purpose of the auxiliary lemmata is to decompose the final goal into simpler and easy to prove subgoals. They allow a systematic proof strategy that, because it is modularized, can handle proof goals of arbitrary complexity. Let G and H be commutative groups, where G rep- resents the group of integers.

A typical proof is then structured as follows. As we have embedded in our tool the domain specific knowledge to generate pseudo-preimages for the class of protocols that we formally verify, we can intro- duce another explicit pseudo-preimage as an hypothesis in our proof, e. At this point we can instantiate the formalization of Lemma 2, and complete the proof for the above theorem, which implies the existence of a knowledge extractor.

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In: Pfitzmann, B. Lipmaa, H. In: Laih, C. Paulson, L. Volume of LNCS. Springer MacKenzie, P. ACM, New York Malkhi, D. In: Jarecki, S. Springer, Heidel- berg Briner, T. In: EuroPKI to appear, Meiklejohn, S. Rivest, R. Communications of the ACM 21, — Backes, M. Baskar, A. In: Datta, A. ASIAN Blanchet, B. Barthe, G. Goubault-Larrecq, J.

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