White-box AES Implementation Research Articles

Analysis and Improvement of Differential Computation Attacks against Internally-Encoded White-Box Implementations

White-box cryptography is the last security barrier for a cryptographic software implementation deployed in an untrusted environment. The principle of internal encodings is a commonly used white-box technique to protect block cipher implementations. It consists in representing an implementation as a network of look-up tables which are then encoded using randomly generated bijections (the internal encodings). When this approach is implemented based on nibble (i.e. 4-bit wide) encodings, the protected implementation has been shown to be vulnerable to differential computation analysis (DCA). The latter is essentially an adaptation of differential power analysis techniques to computation traces consisting of runtime information, e.g., memory accesses, of the target software. In order to thwart DCA, it has then been suggested to use wider encodings, and in particular byte encodings, at least to protect the outer rounds of the block cipher which are the prime targets of DCA.In this work, we provide an in-depth analysis of when and why DCA works. We pinpoint the properties of the target variables and the encodings that make the attack (in)feasible. In particular, we show that DCA can break encodings wider than 4-bit, such as byte encodings. Additionally, we propose new DCA-like attacks inspired from side-channel analysis techniques. Specifically, we describe a collision attack particularly effective against the internal encoding countermeasure. We also investigate mutual information analysis (MIA) which naturally applies in this context. Compared to the original DCA, these attacks are also passive and they require very limited knowledge of the attacked implementation, but they achieve significant improvements in terms of trace complexity. All the analyses of our work are experimentally backed up with various attack simulation results. We also verified the practicability of our analyses and attack techniques against a publicly available white-box AES implementation protected with byte encodings –which DCA has failed to break before– and against a “masked” white-box AES implementation –which intends to resist DCA.

Protect white‐box AES to resist table composition attacks

White-box cryptography protects cryptographic software in a white-box attack context (WBAC), where the dynamic execution of the cryptographic software is under full control of an adversary. Protecting AES in the white-box setting attracted many scientists and engineers, and several solutions emerged. However, almost all these solutions have been badly broken by various efficient white-box attacks, which target compositions of key-embedding lookup tables. In 2014, Luo, Lai, and You proposed a new WBAC-oriented AES implementation, and claimed that their implementation is secure against both Billet et al. 's attack and De Mulder et al. 's attack. In this study, based on the existing table-composition-targeting cryptanalysis techniques, the authors show that the secret key of the Luo-Lai-You (LLY) implementation can be recovered with a time complexity of about 2 44 . Furthermore, the authors propose a new white-box AES implementation based on table lookups, which is shown to be resistant against the existing table-composition-targeting white-box attacks. The authors, key-embedding tables are obfuscated with large affine mappings, which cannot be cancelled out by table compositions of the existing cryptanalysis techniques. Although their implementation requires twice as much memory as the LLY WBAES to store the tables, its speed is about 63 times of the latter.

Towards practical white-box lightweight block cipher implementations for IoTs

According to the Kerckhoffs’s principle, the security of a system should be only depended on the security of its secret key. To build the trusted computing base, Secure Element (SE) and Trusted Execution Environment (TEE) have been proposed for secure computing and authentication. But users still need to believe that SE and TEE-supported hardware will not be evil or intruded. In order to totally remove the dependence of extra hardware, white-box cryptography was introduced by Chow et al. (2002) which gives a software solution for AES implementations in an extremely hostile environment. After Chow et al.’s seminal paper, many white-box implementations were proposed on different block ciphers. In IoTs applications, SE and TEE might have the practical issues if the implementation costs are constrained. In this paper, we first discuss the practical issues that relate to white-box block cipher implementations from lightweight block ciphers. Furthermore, we give the white-box implementations of KLEIN, Present and LBlock as the typical candidates that represent the Substitution-Permutation Network (SPN) and Feistel structures. Finally the performance and the costs are compared with the white-box AES implementation. The comparison shows that white-box implementations are not only related to block and key lengths, but also the structure of the cipher and its white-box implementation methodology strongly affect the implementation costs.

A white-box AES-like implementation based on key-dependent substitution-linear transformations

It is becoming increasingly common to deploy cryptographic algorithms within software applications which are executed in untrusted environments owned and controlled by a possibly malicious party. White-box cryptography aims to protect the secret key in such an environment. Chow et al. developed a white-box AES implementation in 2002 by hiding secret keys into lookup tables. Afterwards, some improvements were proposed. However, all the published schemes have been shown to be insecure. AES was originally designed without consideration of execution in a white-box attack context. Because of the fixed confusion and diffusion operations, it is easy to break AES’s white-box version. In this paper, we propose an AES-like cipher by replacing AES’s S-boxes and MixColumn matrices with key-dependent components while keeping their good cryptographic properties. We show that the white-box implementation of our AES-like cipher can resist current known attacks.

Conditional Re-encoding Method for Cryptanalysis-Resistant White-Box AES

Conventional cryptographic algorithms are not sufficient to protect secret keys and data in white-box environments, where an attacker has full visibility and control over an executing software code. For this reason, cryptographic algorithms have been redesigned to be resistant to white-box attacks. The first white-box AES (WB-AES) implementation was thought to provide reliable security in that all brute force attacks are infeasible even in white-box environments; however, this proved not to be the case. In particular, Billet and others presented a cryptanalysis of WB-AES with 230 time complexity, and Michiels and others generalized it for all substitution-linear transformation ciphers. Recently, a collision-based cryptanalysis was also reported. In this paper, we revisit Chow and others’ first WB-AES implementation and present a conditional re-encoding method for cryptanalysis protection. The experimental results show that there is approximately a 57% increase in the memory requirement and a 20% increase in execution speed.