The design of software countermeasures against active and passive adversaries is a challenging problem that has been addressed by many authors in recent years. The proposed solutions adopt a theoretical foundation (such as a leakage model) but often do not offer concrete reference implementations to validate the foundation. Contributing to the experimental dimension of this body of work, we propose a customized processor called SKIVA that supports experiments with the design of countermeasures against a broad range of implementation attacks. Based on bitslice programming and recent advances in the literature, SKIVA offers a flexible and modular combination of countermeasures against power-based and timing-based side-channel leakage and fault injection. Multiple configurations of side-channel protection and fault protection enable the programmer to select the desired number of shares and the desired redundancy level for each slice. Recurring and security-sensitive operations are supported in hardware through custom instruction-set extensions. The new instructions support bitslicing, secret-share generation, redundant logic computation, and fault detection. We demonstrate and analyze multiple versions of AES from a side-channel analysis and a fault-injection perspective, in addition to providing a detailed performance evaluation of the protected designs. To our knowledge, this is the first validated end-to-end implementation of a modular bitslice-oriented countermeasure.

Cryptographic implementations deployed in real world devices often aim at (provable) security against the powerful class of side-channel attacks while keeping reasonable performances. Last year at Asiacrypt, a new formal verification tool named tightPROVE was put forward to exactly determine whether a masked implementation is secure in the well-deployed probing security model for any given security order t. Also recently, a compiler named Usuba was proposed to automatically generate bitsliced implementations of cryptographic primitives. This paper goes one step further in the security and performances achievements with a new automatic tool named Tornado. In a nutshell, from the high-level description of a cryptographic primitive, Tornado produces a functionally equivalent bitsliced masked implementation at any desired order proven secure in the probing model, but additionally in the so-called register probing model which much better fits the reality of software implementations. This framework is obtained by the integration of Usuba with tightPROVE + , which extends tightPROVE with the ability to verify the security of implementations in the register probing model and to fix them with inserting refresh gadgets at carefully chosen locations accordingly. We demonstrate Tornado on the lightweight cryptographic primitives selected to the second round of the NIST competition and which somehow claimed to be masking friendly. It advantageously displays performances of the resulting masked implementations for several masking orders and proves their security in the register probing model.

Cryptographic primitives are subject to diverging imperatives. Functional correctness and auditability pushes for the use of a high-level programming language. Performance and the threat of timing attacks push for using no more abstract than an assembler to exploit (or avoid!) the microarchitectural features of a given machine. We believe that a suitable programming language can reconcile both views and actually improve on the state of the art of both. Usuba is an opinionated dataflow programming language in which block ciphers become so simple as to be "obviously correct" and whose types document and enforce valid parallelization strategies at the granularity of individual bits. Its optimizing compiler, Usubac, produces high-throughput, constant-time implementations performing on par with hand-tuned reference implementations. The cornerstone of our approach is a systematization and generalization of bitslicing, an implementation trick frequently used by cryptographers. CCS Concepts• Software and its engineering → Domain specific languages; Data flow languages; • Security and privacy → Block and stream ciphers.

Bitslicing is a programming technique commonly used in cryptography that consists in implementing a combinational circuit in software. It results in a massively parallel program immune to cache-timing attacks by design. However, writing a program in bitsliced form requires extreme minutia. This paper introduces Usuba, a synchronous dataflow language producing bitsliced C code. Usuba is both a domain-specific language-providing syntactic support for the implementation of cryptographic algorithms-as well as a domain-specific compiler-taking advantage of welldefined semantics invariants to perform various optimizations before handing the generated code to an (optimizing) C compiler. On the Data Encryption Standard (DES) algorithm, we show that Usuba outperforms a reference, hand-tuned implementation by 15% (using Intel's 64 bits general-purpose registers and depending on the underlying C compiler) whilst our implementation also transparently supports modern SIMD extensions (SSE, AVX, AVX-512), other architectures (ARM Neon, IBM Altivec) as well as multicore processors through an OpenMP backend.

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