International Association for Cryptologic Research

International Association
for Cryptologic Research


Shibam Ghosh


Partial Sums Meet FFT: Improved Attack on 6-Round AES
The partial sums cryptanalytic technique was introduced in 2000 by Ferguson et al., who used it to break 6-round AES with time complexity of $2^{52}$ S-box computations -- a record that has not been beaten ever since. In 2014, Todo and Aoki showed that for 6-round AES, partial sums can be replaced by a technique based on the Fast Fourier Transform (FFT), leading to an attack with a comparable complexity. In this paper we show that the partial sums technique can be combined with an FFT-based technique, to get the best of the two worlds. Using our combined technique, we obtain an attack on 6-round AES with complexity of about $2^{46.4}$ additions. We fully implemented the attack experimentally, along with the partial sums attack and the Todo-Aoki attack, and confirmed that our attack improves the best known attack on 6-round AES by a factor of more than 32. We expect that our technique can be used to significantly enhance numerous attacks that exploit the partial sums technique. To demonstrate this, we use our technique to improve the best known attack on 7-round Kuznyechik by a factor of more than 80, and to reduce the complexity of the best known attack on the full MISTY1 from $2^{69.5}$ to $2^{67}$.
Attacking the IETF/ISO Standard for Internal Re-keying CTR-ACPKM
Encrypting too much data using the same key is a bad practice from a security perspective. Hence, it is customary to perform re-keying after a given amount of data is transmitted. While in many cases, the re-keying is done using a fresh execution of some key exchange protocol (e.g., in IKE or TLS), there are scenarios where internal re-keying, i.e., without exchange of information, is performed, mostly due to performance reasons.Originally suggested by Abdalla and Bellare, there are several proposals on how to perform this internal re-keying mechanism. For example, Liliya et al. offered the CryptoPro Key Meshing (CPKM) to be used together with GOST 28147-89 (known as the GOST block cipher). Later, ISO and the IETF adopted the Advanced CryptoPro Key Meshing (ACKPM) in ISO 10116 and RFC 8645, respectively.In this paper, we study the security of ACPKM and CPKM. We show that the internal re-keying suffers from an entropy loss in successive repetitions of the rekeying mechanism. We show some attacks based on this issue. The most prominent one has time and data complexities of O(2κ/2) and success rate of O(2−κ/4) for a κ-bit key.Furthermore, we show that a malicious block cipher designer or a faulty implementation can exploit the ACPKM (or the original CPKM) mechanism to significantly hinder the security of a protocol employing ACPKM (or CPKM). Namely, we show that in such cases, the entropy of the re-keyed key can be greatly reduced.
Practical Related-Key Forgery Attacks on Full-Round TinyJAMBU-192/256
TinyJAMBU is one of the finalists in the NIST lightweight cryptography competition. It is considered to be one of the more efficient ciphers in the competition and has undergone extensive analysis in recent years as both the keyed permutation as well as the mode are new designs. In this paper we present a related-key forgery attack on the updated TinyJAMBU-v2 scheme with 256- and 192-bit keys. We introduce a high probability related-key differential attack where the differences are only introduced into the key state. Therefore, the characteristic is applicable to the TinyJAMBU mode and can be used to mount a forgery attack. The time and data complexity of the forgery are 233 using 214 related-keys for the 256-bit key version, and 243 using 216 related-keys for the 192-bit key version.For the 128-bit key we construct a related-key differential characteristic on the full keyed permutation of TinyJAMBU with a probability of 2−16. We extend the relatedkey differential characteristics on TinyJAMBU to practical-time key-recovery attacks that extract the full key from the keyed permutation with a time and data complexity of 224, 221, and 219 for respectively the 128-, 192-, and 256-bit key variants.All characteristics are experimentally verified and we provide key nonce pairs that produce the same tag to show the feasibility of the forgery attack. We note that the designers do not claim related-key security, however, the attacks proposed in this paper suggest that the scheme is not key-commiting, which has been recently identified as a favorable property for AEAD schemes.
The QARMAv2 Family of Tweakable Block Ciphers
We introduce the QARMAv2 family of tweakable block ciphers. It is a redesign of QARMA (from FSE 2017) to improve its security bounds and allow for longer tweaks, while keeping similar latency and area. The wider tweak input caters to both specific use cases and the design of modes of operation with higher security bounds. This is achieved through new key and tweak schedules, revised S-Box and linear layer choices, and a more comprehensive security analysis. QARMAv2 offers competitive latency and area in fully unrolled hardware implementations.Some of our results may be of independent interest. These include: new MILP models of certain classes of diffusion matrices; the comparative analysis of a full reflection cipher against an iterative half-cipher; our boomerang attack framework; and an improved approach to doubling the width of a block cipher.