Impossible Differential Cryptanalysis of reduced-round TEA and XTEA

TEA and XTEA are two lightweight Feistel block ciphers, each of which has a block size of 64 bits and a key size of 128 bits. These two ciphers have ARX structure, i.e. their round functions consist of modular addition, shift and XOR to be exact. Since each operation of TEA and XTEA can be done in a bitwise fashion, we are able to use early abort technique in the impossible differential attack that helps us to remove inappropriate pairs as soon as possible, hence decreasing time complexity. In this paper we present two new 15-round impossible differential characteristics, resulting in the first known impossible differential cryptanalysis mounted on 19 rounds of TEA and 25 rounds of XTEA.