MISTY1 is a block cipher designed by Matsui in 1997. It was well evaluated and standardized by projects, such as CRYPTREC, ISO/IEC, and NESSIE. In this paper, we propose a key recovery attack on the ...full MISTY1, i.e., we show that 8-round MISTY1 with 5 FL layers does not have 128-bit security. Many attacks against MISTY1 have been proposed, but there is no attack against the full MISTY1. Therefore, our attack is the first cryptanalysis against the full MISTY1. We construct a new integral characteristic by using the propagation characteristic of the division property, which was proposed in EUROCRYPT 2015. We first improve the division property by optimizing the division property for a public S-box and then construct a 6-round integral characteristic on MISTY1. Finally, we recover the secret key of the full MISTY1 with
2
63.58
chosen plaintexts and
2
121
time complexity. Moreover, if we use
2
63.994
chosen plaintexts, the time complexity for our attack is reduced to
2
108.3
. Note that our cryptanalysis is a theoretical attack. Therefore, the practical use of MISTY1 will not be affected by our attack.
In this paper, we introduce a new type of attack, called
nonlinear invariant attack
. As application examples, we present new attacks that are able to distinguish the full versions of the (tweakable) ...block ciphers Scream, iScream and Midori64 in a weak-key setting. Those attacks require only a handful of plaintext–ciphertext pairs and have minimal computational costs. Moreover, the nonlinear invariant attack on the underlying (tweakable) block cipher can be extended to a ciphertext-only attack in well-known modes of operation such as CBC or CTR. The plaintext of the authenticated encryption schemes SCREAM and iSCREAM can be practically recovered only from the ciphertexts in the nonce-respecting setting. This is the first result breaking a security claim of SCREAM. Moreover, the plaintext in Midori64 with well-known modes of operation can practically be recovered. All of our attacks are experimentally verified.
Piccolo is a lightweight block cipher proposed by Sony Corporation in 2011. The designers showed two key modes, Piccolo-80 and Piccolo-128, which use an 80-bit secret key and a 128-bit one, ...respectively. Isobe and Shibutani estimated the security of Piccolo-80, and they showed that 14-round (reduced) Piccolo-80 w/o whitening keys is vulnerable against the Meet-in-the-Middle attack. The time complexity of their attack is about 273, but unfortunately it requires 264 texts, namely, the full code book. In this paper, we propose a new impossible differential attack against 14-round Piccolo-80 w/o whitening keys, and it can recover the secret key without relying on the full code book. The time complexity is 268 and it uses 262.2 distinct know plaintexts.
The cube attack is a powerful cryptanalytic technique and is especially powerful against stream ciphers. Since we need to analyze the complicated structure of a stream cipher in the cube attack, the ...cube attack basically analyzes it by regarding it as a blackbox. Therefore, the cube attack is an experimental attack, and we cannot evaluate the security when the size of cube exceeds an experimental range, e.g., 40. In this paper, we propose cube attacks on non-blackbox polynomials. Our attacks are developed by using the division property, which is recently applied to various block ciphers. The clear advantage is that we can exploit large cube sizes because it never regards the cipher as a blackbox. We apply the new cube attack to Trivium , Grain128a, ACORN and Kreyvium. As a result, the secret keys of 832-round Trivium , 183-round Grain128a, 704-round ACORN and 872-round Kreyvium are recovered. These attacks are the current best key-recovery attack against these ciphers.
In this paper, we deal with upper bounds for the security of some Feistel networks. Such a topic has been discussed since the introduction of Luby-Rackoff construction. The Luby-Rackoff construction ...is unrealistic because its round functions must be chosen at random from the set of all functions. Knudsen dealt with a more practical construction whose round functions are chosen at random from a family of 2k randomly chosen functions, and showed an upper bound for the security by demonstrating generic key recovery attacks. However it is still difficult for designers to choose functions randomly. Then, this paper considers the security of some Feistel networks which have more efficient and practical round functions, and such Feistel networks are indeed used by some Feistel ciphers in practice. We show new properties using the relationship between plaintexts and ciphertexts. We propose new generic key recovery attacks by using our properties, and confirm the feasibility by implementing the attack on Feistel ciphers with small block sizes. As a result, we conclude that efficient and practical 6-round Feistel networks are not secure.
A division property is a generic tool to search for integral distinguishers, and automatic tools such as MILP or SAT/SMT allow us to evaluate the propagation efficiently. In the application to stream ...ciphers, it enables us to estimate the security of cube attacks theoretically, and it leads to the best key-recovery attacks against well-known stream ciphers. However, it was reported that some of the key-recovery attacks based on the division property degenerate to distinguishing attacks due to the inaccuracy of the division property. Three-subset division property (without unknown subset) is a promising method to solve this inaccuracy problem, and a new algorithm using automatic tools for the three-subset division property was recently proposed at Asiacrypt2019. In this paper, we first show that this state-of-the-art algorithm is not always efficient and we cannot improve the existing key-recovery attacks. Then, we focus on the three-subset division property without unknown subset and propose another new efficient algorithm using automatic tools. Our algorithm is more efficient than existing algorithms, and it can improve existing key-recovery attacks. In the application to
Trivium
, we show a 842-round key-recovery attack. We also show that a 855-round key-recovery attack, which was proposed at CRYPTO2018, has a critical flaw and does not work. As a result, our 842-round attack becomes the best key-recovery attack. In the application to Grain-128AEAD, we show that the known 184-round key-recovery attack degenerates to a distinguishing attack. Then, the distinguishing attacks are improved up to 189 rounds, and we also show the best key-recovery attack against 190 rounds. In the application to
ACORN
, we prove that the 772-round key-recovery attack at ISC2019 is in fact a constant-sum distinguisher. We then give new key-recovery attacks mounting to 773-, 774- and 775-round
ACORN
. We verify the current best key-recovery attack on 892-round Kreyvium and recover the exact superpoly. We further propose a new attack mounting to 893 rounds.
We present several improvements to the framework of differential-linear attacks with a special focus on ARX ciphers. As a demonstration of their impact, we apply them to Chaskey and ChaCha and we are ...able to significantly improve upon the best attacks published so far.
ASCON is one of the elegant designs of authenticated encryption with associated data (AEAD) that was selected as the first choice for lightweight applications in the CAESAR competition, which also ...has been submitted to NIST lightweight cryptography standardization. ASCON has been in the literature for a while; however, there has been no successful AEAD which is secure and at the same time lighter than ASCON. In this article, we have overcome the challenge of constructing a permutation that is lighter than the ASCON permutation while ensuring a similar performance, and based on which we achieve a more lightweight AEAD which we call Sycon. Extensive security analysis of Sycon confirms that it provides the same level of security as that of ASCON. Our hardware implementation result shows that the Sycon permutation has 5.35% reduced area, compared to the ASCON permutation. This leads to a remarkable area reduction for Sycon AEAD which is about 14.95% as compared to ASCON AEAD. We regard Sycon as a new milestone as it is the lightest among all the AEADs belonging to the ASCON family.
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