EasyCrypt Security Proof of Private Count Retrieval (PCR) Protocol
This repository contains the specification and proof of security for a three party cryptographic protocol called PCR (Private Count Retrieval) using the EasyCrypt proof assistant.
PCR involves three parties: a Server, a Client and a Third Party (TP). The protocol works with one-dimensional databases: a database consists of a list of elements (which can be anything). Database queries are single elements: a query is a request for the number of occurrences of the query in the database. The database is held by the Server, and queries are made by the Client. The Client is only allowed to learn the counts for the queries it makes, whereas the Server must not learn what queries the Client makes. The TP is an intermediary between the Server and Client, but isn't trusted - it is only allowed to learn certain element patterns. The parties are assumed to be non-colluding.
In the PCR protocol, the Server starts by sharing a secret, sec, with the Client (but not the TP). It then randomly shuffles its database, and turns the result of the shuffling into a hashed database, which it sends to the TP. Each element, elem, of the shuffled database is transformed into the hash - computed using a random oracle - of (elem, sec). The Server's work is then complete. For each query, qry, that the Client wants to make, the Client hashes (qry, sec), and sends the resulting hash tag, tag, to the TP. The TP counts the number of occurrences of tag in the hashed database, and sends the resulting count back to the Client.
Honest but curious security against each of the three protocol parties is specified using the real/ideal paradigm. The definition of the PCR protocol maintains protocol views for the three protocol parties. The protocol is then specialized to the "real" games for the Server, TP and Client. Each party also has an "ideal" game in which it is obvious the protocol party doesn't learn more than it should, but where the party's view is still constructed by a Simulator.
Both real and ideal games are parameterized by an Adversary, which has access to the random oracle, and adaptively chooses protocol inputs, as a function of the protocol views that are supplied to it and its interactions with the random oracle. At the end of a game's execution, the Adversary is asked to make a boolean judgment, which is returned as the game's result. The security theorems upper-bound the absolute values of the differences between the probabilities that the real and ideal games return true. For these upper bounds to be small, we must limit the Adversary in different ways, depending upon the protocol party we are proving security against. For instance, we can limit the Adversary's use of the random oracle, or limit the number of distinct elements in the database it proposes. Because our proofs are information-theoretic - as opposed to relying on hardness assumptions - in each of our security theorems, the Simulator is existentially quantified, so that the Simulator is part of the proof, instead of part of the specification, of security.
The EasyCrypt proofs are structured using the sequence of games approach, in which a protocol party's real and ideal games are connected using some number of intermediate games. This horizontal structure is supplemented with a vertical one, in which cryptographic reductions are carried out, using previously proved lemmas that may themselves have been proven using game sequences. To increase the efficiency and stability of proof checking, all uses of SMT solvers explicitly specify the EasyCrypt lemmas that the solvers may use.
See the CSF 2017 paper Mechanizing the Proof of Adaptive, Information-theoretic Security of Cryptographic Protocols in the Random Oracle Model for more information.
The repository contains the EasyCrypt proof scripts (files with
.ec (ordinary theories) and
.eca (abstract theories))
proving the security of PCR.
There is also a shell script
check-all-scripts for checking all
theories using two SMT provers: Alt-Ergo and Z3.
Proof of Security Against Server:
Proof of Security Against Third Party:
Proof of Security Against Client:
We intend to maintain the PCR security proof as the EasyCrypt development continues.
The reported work was partially completed while the authors were employed at MIT Lincoln Laboratory, funded by the Intelligence Advanced Research Projects Activity under Air Force Contract FA8721-05-C-0002. Varia's contributions were subsequently supported by the National Science Foundation under Grant No. 1414119.
It is a pleasure to acknowledge helpful discussions with Gilles Barthe, Ran Canetti, Robert Cunningham, François Dupressoir, Benjamin Grégoire, Jonathan Herzog, Aaron D. Jaggard, Jonathan Katz, Catherine Meadows, Adam Petcher, Emily Shen, Pierre-Yves Strub, Arkady Yerukhimovich and Santiago Zanella Béguelin. Special thanks to Zanella Béguelin for suggesting that security against the Third Party could be strengthened were the Server to begin by randomly shuffling its database. Our theory for removing redundant hashing is based on a technique invented by Grégoire, but our implementation of this technique doesn't build on Grégoire's code and uses a slightly different approach.