Showing papers by "Xiaoyang Dong published in 2017"

Improved Conditional Cube Attacks on Keccak Keyed Modes with MILP Method

Abstract: Conditional cube attack is an efficient key-recovery attack on Keccak keyed modes proposed by Huang et al. at EUROCRYPT 2017. By assigning bit conditions, the diffusion of a conditional cube variable is reduced. Then, using a greedy algorithm (Algorithm 4 in Huang et al.’s paper), Huang et al. find some ordinary cube variables, that do not multiply together in the 1st round and do not multiply with the conditional cube variable in the 2nd round. Then the key-recovery attack is launched. The key part of conditional cube attack is to find enough ordinary cube variables. Note that, the greedy algorithm given by Huang et al. adds ordinary cube variable without considering its bad effect, i.e. the new ordinary cube variable may result in that many other variables could not be selected as ordinary cube variable (they multiply with the new ordinary cube variable in the first round).

Cube-like Attack on Round-Reduced Initialization of Ketje Sr

Abstract: This paper studies the Keccak-based authenticated encryption (AE) scheme Ketje Sr against cube-like attacks. Ketje is one of the remaining 16 candidates of third round CAESAR competition, whose primary recommendation is Ketje Sr. Although the cube-like method has been successfully applied to Ketje’s sister ciphers, including Keccak-MAC and Keyak – another Keccak-based AE scheme, similar attacks are missing for Ketje. For Ketje Sr, the state (400-bit) is much smaller than Keccak-MAC and Keyak (1600-bit), thus the 128-bit key and cubes with the same dimension would occupy more lanes in Ketje Sr. Hence, the number of key bits independent of the cube sum is very small, which makes the divide-and-conquer method (it has been applied to 7-round attack on Keccak-MAC by Dinur et al.) can not be translated to Ketje Sr trivially. This property seems to be the barrier for the translation of the previous cube-like attacks to Ketje Sr. In this paper, we evaluate Ketje Sr against the divide-and-conquer method. Firstly, by applying the linear structure technique, we find some 32/64-dimension cubes of Ketje Sr that do not multiply with each other as well as some bits of the key in the first round. In addition, we introduce the new dynamic variable instead of the auxiliary variable (it was used in Dinur et al.’s divide-and-conquer attack to reduce the diffusion of the key) to reduce the diffusion of the key as well as the cube variables. Finally, we successfully launch a 6/7-round 1 key recovery attack on Ketje Sr v1 and v2 (v2 is presented for the 3rd round CAESAR competition.). In 7-round attack, the complexity of online phase for Ketje Sr v1 is 2 113 , while for Ketje Sr v2, it is 2 97 (the preprocessing complexity is the same). We claim 7-round reduced Ketje Sr v2 is weaker than v1 against our attacks. In addition, some results on other Ketje instances and Ketje Sr with smaller nonce are given. Those are the first results on Ketje and bridge the gaps of cryptanalysis between its sister ciphers – Keyak and the Keccak keyed modes.

Conditional Cube Attack on Round-Reduced ASCON

Abstract: This paper evaluates the secure level of authenticated encryption Ascon against cube-like method. Ascon submitted by Dobraunig et al. is one of 16 survivors of the 3rd round CAESAR competition. The cube-like method is first used by Dinur et al. to analyze Keccak keyed modes. At CT-RSA 2015, Dobraunig et al. applied this method to 5/6-round reduced Ascon, whose structure is similar to Keccak keyed modes. However, for Ascon the non-linear layer is more complex and state is much smaller, which make it hard for the attackers to select enough cube variables that do not multiply with each other after the first round. This seems to be the reason why the best previous key-recovery attack is on 6-round Ascon, while for Keccak keyed modes (Keccak-MAC and Keyak) the attacked round is no less than 7-round. In this paper, we generalize the conditional cube attack proposed by Huang et al., and find new cubes depending on some key bit conditions for 5/6-round reduced Ascon, and translate the previous theoretic 6-round attack with 2 66 time complexity to a practical one with 2 40 time complexity. Moreover, we propose the first 7-round key-recovery attack on Ascon. By introducing the cube-like key-subset technique, we divide the full key space into many subsets according to different key conditions. For each key subset, we launch the cube tester to determine if the key falls into it. Finally, we recover the full key space by testing all the key subsets. The total time complexity is about 2 103.9 . In addition, for a weak-key subset, whose size is 2 117 , the attack is more efficient and costs only 2 77 time complexity. Those attacks do not threaten the full round (12 rounds) Ascon.

Quantum Key-recovery Attack on Feistel Structures.

Abstract: Post-quantum cryptography has drawn considerable attention from cryptologists on a global scale. At Asiacrypt 2017, Leander and May combined Grovers and Simons quantum algorithms to break the FX-based block ciphers, which were introduced by Kilian and Rogaway to strengthen DES. In this study, we investigate the Feistel constructions using Grovers and Simons algorithms to generate new quantum key-recovery attacks on different rounds of Feistel constructions. Our attacksrequire $2^{0.25nr-0.75n}$ quantum queries to break an $r$-round Feistel construction.The time complexity of our attacks is less than that observed for quantum brute-force search by a factor of $2^{0.75n}$. When compared with the best classical attacks, i.e., Dinur et al.s attacks at CRYPTO 2015, the time complexity is reduced by a factor of $2^{0.5n}$ without incurring any memory cost.

Conditional Cube Attack on Round-Reduced River Keyak.

Abstract: This paper evaluates the security level of the River Keyak against the cube-like attack. River Keyak is the only lightweight scheme of the Keccak-permutation-based authenticated encryption cipher Keyak, which is one of the 16 survivors of the third round CAESAR competition. Dinur et al. gave the seven-round cube-like attack on Lake Keyak (1600-bit) using the divide-and-conquer method at EUROCRYPT 2015, then Huang et al. improved the result to eight-round using a new conditional cube attack at EUROCRYPT 2017. While for River Keyak, the 800-bit state is so small that the equivalent key (256-bit capacity) occupy double lanes, the attacks can not be applied to the River Keyak trivially. In this paper, we comprehensively explore the conditional cube attack on the small state (800-bit) River Keyak. Firstly, we find a new conditional cube variable which has a much weaker diffusion than Huang et al.’s, this makes the conditional cube attack possible for small state (800-bit) River Keyak. Then we find enough cube variables for six/seven-round River Keyak and successfully launch the key recovery attacks on six/seven-round River Keyak with the time complexity $$2^{33}$$ and $$2^{49},$$ respectively. We also verify the six and seven-round attack on a laptop. Finally, by using linear structure technique with our new conditional cube variable, we greatly increase the freedom degree to find more cube variables for conditional cube attacks as it is complex for 800-bit state to find enough cube variables for eight-round attack. And then we use the new variables by this new method to launch eight-round conditional cube attack with the time complexity $$2^{81}.$$ These are the first cryptanalysis results on round-reduced River Keyak. Our attacks do not threaten the full-round (12) River Keyak.