A timing attack on RC5

Abstract

This paper describes a timing attack on the RC5 block encryption algorithm. The analysis is motivated by the possibility that some implementations of RC5 could result in the data-dependent rotations taking a time that is a function of the data. Assuming that encryption timing measurements can be made which enable the cryptanalyst to deduce the total amount of rotations carried out during an encryption, it is shown that, for the nominal version of RC5, only a few thousand ciphertexts are required to determine 5 bits of the last half-round subkey with high probability. Further, it is shown that it is practical to determine the whole secret key with about 220 encryption timings with a time complexity that can be as low as 228.