ForgedAttributes: An Existential Forgery Vulnerability of CMS and PKCS#7 Signatures

This work describes an existential signature forgery vulnerability of the current CMS and PKCS#7 signature standards. The vulnerability results from an ambiguity of how to process the signed message in the signature verification process. Specifically, the absence or presence of the so called SignedAttributes field determines whether the signature message digest receives as input the message directly or the SignedAttributes, a DER-encoded structure which contains a digest of the message. If an attacker takes a CMS or PKCS#7 signed message M which was originally signed with SignedAttributes present, then he can craft a new message M 0 that was never signed by the signer and has the DER-encoded SignedAttributes of the original message as its content and verifies correctly against the original signature of M . Due to the limited flexibility of the forged message and the limited control the attacker has over it, the fraction of vulnerable systems must be assumed to be small but due to the wide deployment of the affected protocols, such instances cannot be excluded. We propose a countermeasure based on attack-detection that prevents the attack reliably

Fast and Secure Root Finding for Code-based Cryptosystems

In this work we analyze five previously published respectively trivial approaches and two new hybrid variants for the task of finding the roots of the error locator polynomial during the decryption operation of code-based encryption schemes. We compare the performance of these algorithms and show that optimizations concerning finite field element representations play a key role for the speed of software implementations. Furthermore, we point out a number of timing attack vulnerabilities that can arise in root-finding algorithms, some aimed at recovering the message, others at the secret support. We give experimental results of software implementations showing that manifestations of these vulnerabilities are present in straightforward implementations of most of the root-finding variants presented in this work. As a result, we find that one of the variants provides security with respect to all vulnerabilities as well as competitive computation time for code parameters that minimize the public key size

Efficiency and Implementation Security of Code-based Cryptosystems

This thesis studies efficiency and security problems of implementations of code-based cryptosystems. These cryptosystems, though not currently used in the field, are of great scientific interest, since no quantum algorithm is known that breaks them essentially faster than any known classical algorithm. This qualifies them as cryptographic schemes for the quantum-computer era, where the currently used cryptographic schemes are rendered insecure. Concerning the efficiency of these schemes, we propose a solution for the handling of the public keys, which are, compared to the currently used schemes, of an enormous size. Here, the focus lies on resource-constrained devices, which are not capable of storing a code-based public key of communication partner in their volatile memory. Furthermore, we show a solution for the decryption without the parity check matrix with a passable speed penalty. This is also of great importance, since this matrix is of a size that is comparable to that of the public key. Thus, the employment of this matrix on memory-constrained devices is not possible or incurs a large cost. Subsequently, we present an analysis of improvements to the generally most time-consuming part of the decryption operation, which is the determination of the roots of the error locator polynomial. We compare a number of known algorithmic variants and new combinations thereof in terms of running time and memory demands. Though the speed of pure software implementations must be seen as one of the strong sides of code-based schemes, the optimisation of their running time on resource-constrained devices and servers is of great relevance. The second essential part of the thesis studies the side channel security of these schemes. A side channel vulnerability is given when an attacker is able to retrieve information about the secrets involved in a cryptographic operation by measuring physical quantities such as the running time or the power consumption during that operation. Specifically, we consider attacks on the decryption operation, which either target the message or the secret key. In most cases, concrete countermeasures are proposed and evaluated. In this context, we show a number of timing vulnerabilities that are linked to the algorithmic variants for the root-finding of the error locator polynomial mentioned above. Furthermore, we show a timing attack against a vulnerability in the Extended Euclidean Algorithm that is used to solve the so-called key equation during the decryption operation, which aims at the recovery of the message. We also present a related practical power analysis attack. Concluding, we present a practical timing attack that targets the secret key, which is based on the combination of three vulnerabilities, located within the syndrome inversion, a further suboperation of the decryption, and the already mentioned solving of the key equation. We compare the attacks that aim at the recovery of the message with the analogous attacks against the RSA cryptosystem and derive a general methodology for the discovery of the underlying vulnerabilities in cryptosystems with specific properties. Furthermore, we present two implementations of the code-based McEliece cryptosystem: a smart card implementation and flexible implementation, which is based on a previous open-source implementation. The previously existing open-source implementation was extended to be platform independent and optimised for resource-constrained devices. In addition, we added all algorithmic variants presented in this thesis, and we present all relevant performance data such as running time, code size and memory consumption for these variants on an embedded platform. Moreover, we implemented all side channel countermeasures developed in this work. Concluding, we present open research questions, which will become relevant once efficient and secure implementations of code-based cryptosystems are evaluated by the industry for an actual application

How to implement the public Key Operations in Code-based Cryptography on Memory-constrained Devices

While it is generally believed that due to their large public key sizes code based public key schemes cannot be conveniently used when memory-constrained devices are involved, we propose an approach for Public Key Infrastructure (PKI) scenarios which totally eliminates the need to store public keys of communication partners. Instead, all the necessary computation steps are performed during the transmission of the key. We show the feasibility of the approach through an example implementation and give arguments that it will be possible for a smart card controller to carry out the associated computations to sustain the transmission rates of possible future high speed contactless interfaces