Note on a pair of binary sequences with ideal two-level crosscorrelation

We investigate properties of a pair of binary sequences having ideal two-level crosscorrelation function. The concept of the associated cyclic difference pair is given. Equivalent descriptions via polynomial representation and circulant matrix are given. For a period of every multiple of 4, there exists a binary sequence pair whose in-phase correlation is 4 and all out-of-phase correlations are zero. Based on the result of exhaustive computer search for short lengths we conjecture that the construction given exhausts all possible classes and is unique up to correlation preserving transformations.