Related-key rectangle attack on 36 rounds of the XTEA block cipher

XTEA is a 64-round block cipher with a 64-bit block size and a 128-bit user key, which was designed as a short C program that would run safely on most computers. In this paper, we present a related-key rectangle attack on a series of inner 36 rounds of XTEA without making a weak key assumption, and a related-key rectangle attack on the first 36 rounds of XTEA under certain weak key assumptions. These are better than any previously published cryptanalytic results on XTEA in terms of the numbers of attacked rounds.