Stochastic Timing Analysis of the AES Cipher Algorithm over a Correlated Fading Channel

Broadcast and multicast services (BCMCS) in CDMA2000 wireless telecommunications networks restrict high- quality multimedia services to their intended recipients by encrypting content. The air specification of BCMCS includes the Advance Encryption Standard (AES) block cipher in the security layer of the broadcast protocol suite. We surveyed the AES block cipher algorithm and characterized the time it requires. We also profiled the execution time of each transformation that comprises the AES cipher. Drawing on these results, we proposed a analytic model that can predict the time required to decrypt encrypted contents at a mobile by considering cross-layer information, such as the characteristics of error control using Reed-Solomon coding in the MAC layer and varying conditions of the fading channel in the physical layer. This model will enable us to estimate the buffer size required to ensure seamless multimedia services. Index Terms—AES cipher, fading channel, timing analysis. I. INTRODUCTION The aim of BCMCS (1), (2) is to provide broadcast and multicast services to subscribers, who receive content from content sources (CSs), via a cellular serving system. The content may be IP multimedia messages, such as broadcast multimedia messages. A CS may be part of the serving net- work, or an independent entity. If access to BCMCS requires a subscription, then the content can be encrypted so that it can only be viewed or otherwise processed by authorized users (3). In order to prevent illegal access, the BCMCS access network encrypts the BCMCS content with the Advanced Encryption Standard (AES (4)-(6)), also known as Rijndael, which uses defined encryption procedures (7). The AES is an iterated block cipher with a fixed block size and a variable key length. It has been adopted as an encryption standard by the U.S. government, extensively analyzed and is now used worldwide. The AES was announced by the National Institute of Standards and Technology (NIST) as U.S. FIPS PUB 197 (FIPS 197) in 2001, after its predecessor, Data Encryption Standard (DES (8)), was found to be too weak, because of its small key size, in the context of advances in processor power. In this paper, we analyze the AES block cipher in detail, and characterize the execution time required by an AES cipher in the context of the flow diagram of the algorithm. Based on this fundamental analysis, we propose a method of analyzing the delay incurred by the operation of the security protocol in the BCMCS environment, and show how it depends on the configuration of the error control performed in the MAC layer. This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2007-357-D00174). The BCMCS MAC protocol uses an error control based on forward error-correction (FEC) to serve all subscribers reliably (9), (10), even in the presence of poor channel con- ditions. This FEC mechanism uses Reed-Solomon (RS) (11), (12) codes on top of the turbo codes to reduce the packet error-rate at mobile receivers, which experience fading in the physical channel. A fading channel has a memory property, which allows state transitions to be represented by a Markov process (13)-(15), and this in turn permits a function to be constructed which can predict the number of errors in a particular RS codeword of a receiver, and hence the expected number of residual packet errors after RS decoding at the mobile. This allows us to formulate and evaluate a metric for the delay incurred by AES deciphering at the security layer of a receiving mobile, for different settings of the RS error control at the MAC layer, and for different conditions of fading on the broadcast physical channel.