This paper proposes a new block cipher called HARPOCRATES, which is different from traditional SPN, Feistel, or ARX designs. The new design structure that we use is called the substitution convolution network. The novelty of the approach lies in that the substitution function does not use fixed S-boxes. Instead, it uses a key-driven lookup table storing a permutation of all 8-bit values. If the lookup table is sufficiently randomly shuffled, the round sub-operations achieve good confusion and diffusion to the cipher. While designing the cipher, the security, cost, and performances are balanced, keeping the requirements of encryption of data-at-rest in mind. The round sub-operations are massively parallelizable and designed such that a single active bit may make the entire state (an $8 \times 16$8x16 binary matrix) active in one round. We analyze the security of the cipher against linear, differential, and impossible differential cryptanalysis. The cipher's resistance against many other attacks like algebraic attacks, structural attacks, and weak keys are also shown. We implemented the cipher in software and hardware; found that the software implementation of the cipher results in better throughput than many well-known ciphers. Although HARPOCRATES is appropriate for the encryption of data-at-rest, it is also well-suited in data-in-transit environments.

Advanced Encryption Standard (AES) is a well-known, extensively used, broadly studied, and industry-standard symmetric key-based block cipher. Hence cryptanalysis of AES is a felicitous job for the cryptography researchers. Impossible differential cryptanalysis (IDC) is a powerful attack technique on symmetric-key ciphers. In this paper, we report a cluster-based parallel attack to retrieve the full key for all versions of five-round AES (128, 192, 256) and partial key recovery of six-round AES (192, 256) using IDC. The full key of AES-128 can be determined in only 6.5 min. To recover the full key of five-round AES-192/256, a total of 10 minutes is required. Next we find the second round key of six-round AES-(192, 256). It takes a total of 12.5 min. To the best of our knowledge, our results outperformed the existing best results with respect to timing and as well as the success probability for the full/partial key recovery of five/six rounds of AES.

This work investigates whether a modified substitution cipher can have any cryptographic importance in today’s scenario. A classical substitution cipher is a confusion-only cipher. Due to the absence of a proper diffusion mechanism, this cipher is vulnerable to frequency attacks. We propose a block cipher model based on the substitution cipher in which we introduce a diffusion mechanism and multiple encryption rounds. The key indirectly used in the encipherment process is supplied to a deterministic random bit generator to shuffle a look-up table (LUT) using a uniform shuffling algorithm randomly. After studying the cryptographic properties of randomly generated LUTs, we perform a case study of an imaginary block cipher assumed to have a diffusion function equipped with a perfect  diffusion  property. Though, a randomly generated permutation may possess weaker cryptographic properties with a high probability. From the investigation, we find that with a robust diffusion mechanism, the proposed modified substitution cipher can have sufficient potentiality against several popularly known attacks.