A Timing Attack on RC5

This paper describes a known plaintext timing attack on the RC5 encryption algorithm. RC5 is a block cipher designed by Ron Rivest [1] in 1995. It has variable parameters such as the key size, the block length and the number of rounds. Our timing attack works against any version of RC5, but we will focus on the nominal version with 12 rounds, 64 bit blocks and a 128 bit key for simplicity. Timing attacks were invented by Paul Kocher in 1995 [2], and in his paper he mentions "RC5 is at risk on platforms where rotates run in non-constant time". We show how to exploit this property in order to recover the whole secret key in time complexity 2^28 in the best case and with 2^20 known plaintexts. We assume that we can make time measurements which enable us to deduce the total amount of rotations carried out during encryption of a known plaintext under an unknown key