Section: New Results

Building blocks

Participants : Marine Minier, Vincent Roca.

• Symmetric cryptography

  In [7], we introduce Constraint Programming (CP) models to solve a cryptanalytic problem: the chosen key differential attack against the standard block cipher AES. The problem is solved in two steps: In Step 1, bytes are abstracted by binary values; In Step 2, byte values are searched. We introduce two CP models for Step 1: Model 1 is derived from AES rules in a straightforward way; Model 2 contains new constraints that remove invalid solutions filtered out in Step 2. We also introduce a CP model for Step 2. We evaluate scale-up properties of two classical CP solvers (Gecode and Choco) and a hybrid SAT/CP solver (Chuffed). We show that Model 2 is much more efficient than Model 1, and that Chuffed is faster than Choco which is faster than Gecode on the hardest instances of this problem. Furthermore, we prove that a solution claimed to be optimal in two recent cryptanalysis papers is not optimal by providing a better solution.

  Using dedicated hardware is common practice in order to accelerate cryptographic operations: complex operations are managed by a dedicated co-processor and RAM/crypto-engine data transfers are fully managed by DMA operations. The CPU is therefore free for other tasks, which is vital in embedded environments with limited CPU power. In this work we discuss and benchmark XTS-AES, using either software or mixed approaches, using Linux and dm-crypt, and a low-power At-mel(tm) board. This board featurs an AES crypto-engine that supports ECB-AES but not the XTS-AES mode. We show that the dm-crypt module used in Linux for full disk encryption has limitations that can be relaxed when considering larger block sizes. In particular we demonstrate in [14] that performance gains almost by a factor two are possible, which opens new opportunities for future use-cases.