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Teaching Old Crypto New Tricks

Periodic Reporting for period 3 - TOCNeT (Teaching Old Crypto New Tricks)

Reporting period: 2019-04-01 to 2020-09-30

TThe bulk of the research in modern cryptography goes into constructing new schemes for which stronger security guarantees can be proven. However, often it is not clear whether simple existing schemes already provide the required security, and it’s just that we don’t know how to prove it. As these new schemes are usually less efficient, there are not being applied resulting in a large discrepancy between what security the schemes applied in practice are supposed to provide, and what is actually proven. This project aims at closing this gap in different contexts: We will revisit simple schemes (including widely deployed ones) using new tools, developed by us and others in the last years, towards proving much stronger security properties than what is currently known.
The schemes developed and proven secure in this project will allow for more efficient and/or more secure solution to various challenges of information security.
A notable early result of this project is a proof that SCRYPT is memory-hard, this function was already widely deployed -- in blockchains and for password hashing -- without any formal security guarantees (this result won the Eurocrypt’17 best paper award).
We made progress on several of the work packages, but we also started some new projects which fall into the general theme of the project which were not foreseen in the original proposal. Let us first mention the main results for each of the three work packages.
WP1 (adaptive security). At CRYPTO’17 we published a paper entitled “Be Adaptive, Avoid Overcommitting” which provides a general methodology to prove adaptive security of schemes. This methodology unifies and simplifies several previous results, and has also been useful in proving new results by us and others. For example, we used the framework to give the first “meaningful” security proof for group messaging schemes (concretely, for TreeKEM, which is the the proposal of an IETF working group) and for proxy re-encryption schemes.
WP2 (symmetric cryptography). In the paper “The Exact Security of PMAC” which appeared at FSE’17 (and was invited to the Journal of Cryptology) we determine the exact security of PMAC, which is a popular message authentication code. This result is not only important as PMAC is a popular scheme, and thus we should know it’s exact security, but also because many of the candidates of the currently running CEASAR competition (which aims at standardizing a authenticated encryption scheme) are based on the design principles of PMAC. We also developed a new symmetric primitive, called “delayed message authentication codes” to address some of the security issues of the contact tracing apps used to help with the COVID19 pandemic.
WP3 (pseudoentropy). After making progress on proving many positive results on various computational entropy notions during my previous ERC grant, we now started investigating from the other direction. That is, proving lower bounds to understand how far we can possibly push this line of research. The two main publications in this direction were “Non-Uniform Attacks Against Pseudoentropy” at ICALP’17 and “Pseudoentropy: Lower-Bounds for Chain Rules and Transformations” at TCC’17.
Some topics we worked on which fall into the aim of the project but were not already suggested as work packages in the proposal include “memory hard functions” (MHF), “proofs of space” (PoS) and “verifiable delay functions (VDF)”.
We published several papers on MHFs, and our proof that scrypt is memory hard won the best paper award at Eurocrypt’17.
The PoS and VDFs we developed will be used in an upcoming cryptocurrency https://chia.network/ where I’m involved in (as scientific advisor). Our VDF is also used in numerous other blockchain projects.