Welcome to the CHES 2020 CTF

The CHES 2020 Capture the Flag (CTF) is a side-channel cryptanalysis challenge against masked implementations of the Clyde-128 Tweakable Block Cipher (TBC) which is used in the Spook AEAD encryption scheme (and has to be strongly protected against side-channel attacks if leveled implementations are considered).

Different targets are proposed in parallel, both in software and in hardware, corresponding to masked implementations with various number of shares. Challengers are provided with the source code of the implementations (C in software and Verilog in hardware/FPGA), a tool to predict intermediate values of the hardware implementation, profiling sets of traces including the nonces, (random) keys, (random) plaintexts and the randomness used for masking, test sets of traces corresponding to a few fixed keys (without the masking randomness), and finally prototype attacks against a single byte of the secret key for exemplary targets.

The goal of the challenge is to modify and improve the prototype attacks. The submitted attacks will be rated based on the number of measurements needed to reduce the rank of the master key below 2^32 using a rank estimation algorithm. All the attacks submitted will be made public to all challengers under an open-source license.

Prizes: Belgian chocolate and beers. Since Clyde and Shadow (respectivaly the tweakable block cipher and permutation primitives of Spook) are named after PacMan characters – Clyde for its (masked) random behavior, Shadow for its fast / to the point behavior – special prizes for the most impactful side-channel cryptanalyses will follow this theme.

Organizers: Davide Bellizia, Olivier Bronchain, Gaetan Cassiers, Charles Momin, François-Xavier Standaert, Balazs Udvarhelyi.