Sharif Digital Repository

Collision attack is one of the usual attacks in cryptanalysis of hash functions. One method for applying this attack is linear approximation of equivalent compress function. Finding collisions for linearized compress function, having low Hamming weight, with a large probability could provide collisions for the main compress function. For this purpose, the problem of finding collision for used compress function is modeled by that of the linearized one, having low Hamming weight differences with a large probability. The problem of finding collisions with low Hamming weight differences is equivalent to the problem of finding codewords with low Hamming weights in linear binary code. Canteaut and... 

In order to provide both security and availability for a given secret, one way is to distribute it among a number of parties called participants. The distribution should be accomplished in such a way that any subset of participants, the size of which is at least equal to a given number, be able to reconstruct the secret, using their shares. More specifically, a (t, n)-threshold secret sharing scheme refers to the procedure of assigning each of the n participants a private share, such that every subset of at least t participants could recover the secret. Due to the possibility of quantum attacks in future, we need to construct secure channels for transmitting secret shares. Such channels... 

Code-based cryptosystems could be a suitable alternative to the cryptosystems based on number theory. It is shown that cryptosystems based on descrete logarithm and factoring is vulnerable to the Shor’s algorithm running on quantum computers, while code-based cryptosystemsare thought to be secure against this cryptanalysis. Despite its security, large key size and low transmission rate keep thesecryptosystems impractical. Reliability is one of our inevitable desires in communication systems along with security.In order to fulfill these desires, joint encryption-encoding schemes has been released.Using LDPC codes in joint encryption-encoding schemes, as an alternative to classical linear... 

Honey Encryption (HE) has been introduced to provide resilience for cryptographic algorithms with low min-entropy keys, against brute-force attacks. Using spurious keys, we have designed an encryption framework consisting of a Distribution Transforming Encoder (DTE), which uses a Password Based Encryption (PBE) algorithm in CTR mode of operation. The proposed DTE block benefits from an efficient error-correcting code, acting as a many-to-one mapping scheme. The proposed framework gets some spurious key-plausible looking plaintext tuples as input and outputs a ciphertext. The structure of the proposed scheme is designed to increase the uncertainty of attacker about the correct plaintext, even... 

The Internet of Things is one of the most common technologies in today's world where its security is an important issue. Lightweight authentication protocols are crucial for privacy preserving in Internet of Things (IoT). IoT authentication protocols must meet security requirements and resist against cryptographic threats. On the other hand, these protocols should be implementable for devices with constrained memory and computational power in this area and should not impose a heavy computational load on such devices. In some IoT devices, such as RFIDs, resource constraints are critical, such that using standard hash functions is not practical. In this applications, security requirements is... 

Secure communication among members of a group requires a shared cryptographic key. To address this issue, group key agreement and exchange schemes are introduced. In group key exchange schemes, a trusted center generates a shared key for the group and sends it securely to the group members. But in group key agreement schemes, all members of the group are involved in generating group keys. One of the useful ways in these schemes is to use secret sharing to share keys. A public key infrastructure is used to secure the distribution of the shares in secret sharing schemes. As quantum computers threat the classic cryptographic algorithms that are based on the difficulty of factoring large numbers... 

The need to protect the key in cryptosystems has been a motivation of studying secret sharing schemes. A secret sharing scheme is a method for sharing a secret data (key) by distributing some values, called shares, to a number of participants in such a way that only some authorized subset of them can recover the secret. In a threshold secret sharing scheme, authorized subsets are those whose size are at least a given value called threshold of the scheme. Increasing the attacker capabilities in achieving the participants’ shares, requires an increase in the threshold parameter. In a changeable threshold secret sharing scheme, participants ate able to compute new shares from their old shares... 

DeepFake is a type of forgery that uses deep learning algorithms to make changes to audio and video content that the audience is unable to detect. Nowadays, due to the threats posed by the use of DeepFake to move people's faces in video, researchers' attention has been drawn to designing methods to detect this type of forgery. Detection methods are usually classified into two types. The first case is the extraction of features to detect forgery distortions, for example, the extraction of facial orientations to detect inconsistencies. The second case is the use of deep learning networks for feature extraction and classification, of which the EfficientNet network is an example. Despite the... 

large-scale data collection from multiple sources to a single entity, such as a cloud provider, poses a challenging problem for implementing centralized machine learning algorithms. Constraints such as privacy protection and restrictive access policies that prevent accessing personally identifiable information hinder the development of centralized machine learning algorithms in important and sensitive domains like healthcare. However, from early disease detection to discovering new drugs, leveraging artificial intelligence in this domain is a fun-damental necessity. As a potential solution, federated learning has been proposed, allowing data owners (users) to jointly train a shared machine... 

Steganography is the art and technique of concealing secret message in ordinary data cover, transmitted over a public channel, in a way that eavesdroppers, as well as the channel users, cannot detect the presence of the secret message. However, steganalysis tries to detect this type of covert communication using some effective analysis techniques. Steganalysis is often based on statistical properties of the suspicious signal that are expected to change due to the message embedding process. Secret message (which mostly is a set of pseudo-random bits because of cryptography) affects the statistical features of the cover signal. So far, many steganalysis techniques have been reported that are... 

Identity verification is a key part of identity reidentification process. Nowadays, Identity reidentification using face-based algorithms are popular in learning and vision area due to their generality and accessibility of this body organ. Using a fake image, occlusions on face and appearance changes like makeup can cause distortion in face verification systems which can be a drop in function of such systems. Most of these face verification models like DeepFace, FaceNet, ArcFace and SphereFace use convolution networks as their major architecture. These models, in addition to their large storage consuming and high computational complexity, due to using face as their major feature, are not... 

Managing health data and providing remote treatment services to patients has always faced security and privacy challenges. Because of the importance and sensitivity of this data, the use of centralized networks such as cloud space and trustin a cloud server can be dangerous, due to the lack of complete trust in the cloud server. To prevent the threats of centralized systems, we provide a decentralized blockchain-based system in which miners, in addition to authenticating physicians and patients, are also responsible for searching encrypted data and all user activities can be tracked in the distributed ledger. The proposed scheme preserves data privacy, and there is no need to trust a server.... 

Internet is the largest multi-purpose, self-reliant, complex and distributed computer network across the globe. The nodes of this network are placed in every place like homes, offices, military camps, schools and all other locations by utilizing many different communication protocols, media and capacities. Furthermore, by rapid technology development, there are many newly born applications utilizing internet in new ways such as Internet of Things systems. The explosive growth of technology causing accelerated addition of new nodes to the internet, and any new single node may have several new unpatched vulnerabilities. This is a serious issue for managing this vast chaotic configuration and... 

A threshold secret sharing scheme is a method for sharing a secret among a group of players, called participants. Each participant is given a private value, as his/her share. The secret can only be recovered when a certain number of participants pool their shares together. The correct recovery of the secret in these schemes depends on the honest behavior of the participants. However, in the real world, this assumption may not hold well. Dishonest participants could submit fraudulent shares in the process of recovering the secret so that honest participants cannot obtain the proper secret. A robust secret sharing scheme still makes it possible to recover the secret correctly, despite the...