Construction of the skipping steps for preimage attack of MD5

The construction of the skipping steps for preimage attack of MD5 is proposed and analyzed in this paper. With the help of branch-and-selection program, the possibility of the local collision in the skipping steps merely is 2^-5. We analyze the key points of construction the skipping steps and the relationship between the initial structure and the skipping steps. According to Yu Sasaki and Kazumaro Aoki´s thought, the skipping steps and the initial structure are reciprocal: the initial structure separates the one-block MD5 algorithm into two parts and generates two neutral message words which are independent of each part respectively; the skipping steps skip from the one part to the other in order to keep the dependent property of the two neutral message words because of the permutation of the message words. The skipping steps and the initial structure work together to connect the two parts in one-block MD5 algorithm and reduce the complexity of preimage attack. Therefore, the complexity of preimage attack of MD5 can be reduced to 2^123.4 in theory.