Abstract: File comparison employs a single function F to calculate a digital signature from data in a sliding window. The digital signature is both incrementally computable and position sensitive. In particular, F is computable without reprocessing each byte in the array when the window is advanced and facilitates detection of such changes as transposed bytes of data. The function F is defined by two qualities. First, for F(A+B), where A is an array, F(A+B)=F(A)+F(B). Second, given a concatenation operator "!" such that "0!A" indicates an array A with 0 inserted before A, the function F has the property that there is a function G such that F(0!A)=G(F(A'0)). Both polynomials and cyclic redundancy checks ("CRC") may be used as that class of function.