Security analysis of a 2/3-rate double length compression function in the black-box model

In this paper, we propose a 2/3-rate double length compression function and study its security in the black-box model. We prove that to get a collision attack for the compression function requires Ω(2²ⁿ/3) queries, where π is the single length output size. Thus, it has better security than a most secure single length compression function. This construction is more efficient than the construction given in [8]. Also the three computations of underlying compression functions can be done in parallel. The proof idea uses a concept of computable message which can be helpful to study security of other constructions like [8], [14], [16] etc.