Stealthy Opaque Predicates in Hardware -- Obfuscating Constant Expressions at Negligible Overhead

Opaque predicates are a well-established fundamental building block for software obfuscation. Simplified, an opaque predicate implements an expression that provides constant Boolean output, but appears to have dynamic behavior for static analysis. Even though there has been extensive research regarding opaque predicates in software, techniques for opaque predicates in hardware are barely explored. In this work, we propose a novel technique to instantiate opaque predicates in hardware, such that they (1) are resource-efficient, and (2) are challenging to reverse engineer even with dynamic analysis capabilities. We demonstrate the applicability of opaque predicates in hardware for both, protection of intellectual property and obfuscation of cryptographic hardware Trojans. Our results show that we are able to implement stealthy opaque predicates in hardware with minimal overhead in area and no impact on latency

How to Watermark Cryptographic Functions

We introduce a notion of watermarking for cryptographic functions and propose a concrete scheme for watermarking cryptographic functions. Informally speaking, a digital watermarking scheme for cryptographic functions embeds information, called a \textit{mark}, into functions such as one-way functions and decryption functions of public-key encryption. There are two basic requirements for watermarking schemes. (1) A mark-embedded function must be functionally equivalent to the original function. (2) It must be difficult for adversaries to remove the embedded mark without damaging the original functionality. In spite of its importance and usefulness, there have only been a few theoretical works on watermarking for functions (or programs). Furthermore, we do not have rigorous definitions of watermarking for cryptographic functions and concrete constructions. To solve the above problem, we introduce a notion of watermarking for cryptographic functions and define its security. Furthermore, we present a lossy trapdoor function (LTF) based on the decisional linear (DLIN) problem and a watermarking scheme for the LTF. Our watermarking scheme is secure under the DLIN assumption in the standard model. We use techniques of dual system encryption and dual pairing vector spaces (DPVS) to construct our watermarking scheme. This is a new application of DPVS. Our watermarking for cryptographic functions is a generalized notion of copyrighted functions introduced by Naccache, Shamir, and Stern (PKC 1999) and our scheme is based on an identity-based encryption scheme whose private keys for identities (i.e., decryption functions) are marked, so our technique can be used to construct black-box traitor tracing schemes

Watermarking Public-key Cryptographic Functionalities and Implementations

A watermarking scheme for a public-key cryptographic functionality enables the embedding of a mark in the instance of the secret-key algorithm such that the functionality of the original scheme is maintained, while it is infeasible for an adversary to remove the mark (unremovability) or mark a fresh object without the marking key (unforgeability). Cohen et al. [STOC\u2716] has provided constructions for watermarking arbitrary cryptographic functionalities; the resulting schemes rely on indistinguishability obfuscation (iO) and leave two important open questions: (i) the realization of both unremovability and unforgeability, and (ii) schemes the security of which reduces to simpler hardness assumptions than iO. In this paper we provide a new definitional framework that distinguishes between watermarking cryptographic functionalities and implementations (think of ElGamal encryption being an implementation of the encryption functionality), while at the same time provides a meaningful relaxation of the watermarking model that enables both unremovability and unforgeability under minimal hardness assumptions. In this way we can answer questions regarding the ability to watermark a given implementation of a cryptographic functionality which is more refined compared to the question of whether a watermarked implementation functionality exists. Taking advantage of our new formulation we present the first constructions for watermarking public key encryption that achieve both unremovability and unforgeability under minimal hardness assumptions. Our first construction enables the watermarking of any public-key encryption implementation assuming only the existence of one-way functions for private key detection. Our second construction is at the functionality level and uses a stronger assumption (existence of identity-based encryption (IBE)) but supports public detection of the watermark

Dynamic block encryption with self-authenticating key exchange

One of the greatest challenges facing cryptographers is the mechanism used for key exchange. When secret data is transmitted, the chances are that there may be an attacker who will try to intercept and decrypt the message. Having done so, he/she might just gain advantage over the information obtained, or attempt to tamper with the message, and thus, misguiding the recipient. Both cases are equally fatal and may cause great harm as a consequence. In cryptography, there are two commonly used methods of exchanging secret keys between parties. In the first method, symmetric cryptography, the key is sent in advance, over some secure channel, which only the intended recipient can read. The second method of key sharing is by using a public key exchange method, where each party has a private and public key, a public key is shared and a private key is kept locally. In both cases, keys are exchanged between two parties. In this thesis, we propose a method whereby the risk of exchanging keys is minimised. The key is embedded in the encrypted text using a process that we call `chirp coding', and recovered by the recipient using a process that is based on correlation. The `chirp coding parameters' are exchanged between users by employing a USB flash memory retained by each user. If the keys are compromised they are still not usable because an attacker can only have access to part of the key. Alternatively, the software can be configured to operate in a one time parameter mode, in this mode, the parameters are agreed upon in advance. There is no parameter exchange during file transmission, except, of course, the key embedded in ciphertext. The thesis also introduces a method of encryption which utilises dynamic blocks, where the block size is different for each block. Prime numbers are used to drive two random number generators: a Linear Congruential Generator (LCG) which takes in the seed and initialises the system and a Blum-Blum Shum (BBS) generator which is used to generate random streams to encrypt messages, images or video clips for example. In each case, the key created is text dependent and therefore will change as each message is sent. The scheme presented in this research is composed of five basic modules. The first module is the key generation module, where the key to be generated is message dependent. The second module, encryption module, performs data encryption. The third module, key exchange module, embeds the key into the encrypted text. Once this is done, the message is transmitted and the recipient uses the key extraction module to retrieve the key and finally the decryption module is executed to decrypt the message and authenticate it. In addition, the message may be compressed before encryption and decompressed by the recipient after decryption using standard compression tools

Collusion Resistant Watermarking Schemes for Cryptographic Functionalities

A cryptographic watermarking scheme embeds a message into a program while preserving its functionality. Recently, a number of watermarking schemes have been proposed, which are proven secure in the sense that given one marked program, any attempt to remove the embedded message will substantially change its functionality. In this paper, we formally initiate the study of collusion attacks for watermarking schemes, where the attacker’s goal is to remove the embedded messages given multiple copies of the same program, each with a different embedded message. This is motivated by practical scenarios, where a program may be marked multiple times with different messages. The results of this work are twofold. First, we examine existing cryptographic watermarking schemes and observe that all of them are vulnerable to collusion attacks. Second, we construct collusion resistant watermarking schemes for various cryptographic functionalities (e.g., pseudorandom function evaluation, decryption, etc.). To achieve our second result, we present a new primitive called puncturable functional encryption scheme, which may be of independent interest

Digital watermarking in medical images

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University, 05/12/2005.This thesis addresses authenticity and integrity of medical images using watermarking. Hospital Information Systems (HIS), Radiology Information Systems (RIS) and Picture Archiving and Communication Systems (P ACS) now form the information infrastructure for today's healthcare as these provide new ways to store, access and distribute medical data that also involve some security risk. Watermarking can be seen as an additional tool for security measures. As the medical tradition is very strict with the quality of biomedical images, the watermarking method must be reversible or if not, region of Interest (ROI) needs to be defined and left intact. Watermarking should also serve as an integrity control and should be able to authenticate the medical image. Three watermarking techniques were proposed. First, Strict Authentication Watermarking (SAW) embeds the digital signature of the image in the ROI and the image can be reverted back to its original value bit by bit if required. Second, Strict Authentication Watermarking with JPEG Compression (SAW-JPEG) uses the same principal as SAW, but is able to survive some degree of JPEG compression. Third, Authentication Watermarking with Tamper Detection and Recovery (AW-TDR) is able to localise tampering, whilst simultaneously reconstructing the original image