International Association for Cryptologic Research

International Association
for Cryptologic Research


Poorvi L. Vora


Pseudo-Linear Approximations for ARX Ciphers: With Application to Threefish
Kerry A. McKay Poorvi L. Vora
The operations addition modulo 2^n and exclusive-or have recently been combined to obtain an efficient mechanism for nonlinearity in block cipher design. In this paper, we show that ciphers using this approach may be approximated by pseudo-linear expressions relating groups of contiguous bits of the round key, round input, and round output. The bias of an approximation can be large enough for known plaintext attacks. We demonstrate an application of this concept to a reduced-round version of the Threefish block cipher, a component of the Skein entry in the secure hash function competition.
Related-Key Statistical Cryptanalysis
Darakhshan J. Mir Poorvi L. Vora
This paper presents the Cryptanalytic Channel Model (CCM). The model treats statistical key recovery as communication over a low capacity channel, where the channel and the encoding are determined by the cipher and the specific attack. A new attack, related-key recovery -- the use of $n$ related keys generated from $k$ independent ones -- is defined for all ciphers vulnerable to single-key recovery. It is shown to correspond to the use of a concatenated code over the channel, where the relationship among the keys determines the outer code, and the cipher and the attack the inner code. It is shown that there exists a relationship among keys for which the communication complexity per bit of independent key is finite, for any probability of key recovery error. This may be compared to the unbounded communication complexity per bit of the single-key-recovery attack. The practical implications of this result are demonstrated through experiments on reduced-round DES.
High Integrity Elections
This paper describes in detail a voting scheme which allows voters to be sure that whatever they see in the booth will be included correctly in the outcome. It presents a rigorous and understandable model of requirements for election systems, states formally the properties of the system, and proves them. As a step towards understanding the full 2D voting system, it also presents a simpler 1D system.
David Chaum's Voter Verification using Encrypted Paper Receipts
Poorvi L. Vora
In this document, we provide an exposition of David Chaum's voter verification method that uses encrypted paper receipts. This document provides simply an exposition of the protocol, and does not address any of the proofs covered in Chaum's papers.