Breaking the Counter (CTR) Mode

A block cipher mode is a cryptographic algorithm that employs a symmetric key block cipher algorithm to defend against language and frequency-based attacks. The goal of modes is to randomize the cipher so that for any two identical input blocks the cipher text output is likely to be different. This apparent morphing of the mapping between input and output makes the plain text block appear to be mapped into multiple cipher text blocks, thus confounding cryptologic analysis. Unfortunately, the structure of cryptographic modes introduces opportunities for side-channel attacks that can reveal enough information to uncover the original message without the need to first break the cipher. Of the six generally accepted cryptographic modes (ECB, CBC, PCBC, CTR, OFB, and CFB), the first three modes (ECB, CBC, PCBC) have been shown to be vulnerable to side-channel attacks through the detection of cipher collisions. In this paper, an attack on the CTR mode is presented and demonstrated to work in the example presented.