Just how does naive XOR encryption show its weaknesses? Basically, it does very little to thwart frequency analysis. Suppose we use 8-byte blocks of plain text and a corresponding 8-byte long encryption key. (It doesn't make much difference if blocks are longer; the same argument applies, although this requires more known cipher text.) Find some cipher text and simply temporarily ignore everything except bytes 1, 9, 17, etc., of the cipher text.

This plain text, corresponding to this first-of-each-block cipher text, will still have the same frequency regularities as the whole plain text. And each identical plain text byte will be transformed into the same cipher text byte. So by knowing that the letter "E" makes up about 13% of plain text (assuming it is English prose), all we need do is look for a cipher text byte value occurring at this same frequency (we simplify here by ignoring case and punctuation, but this is not important for the concept). Once we find these corresponding plain text and cipher text bytes, the key byte is given instantly by KB = PB XOR CB, or in the example: KB = 'E' XOR 'q'. Once we know the key bytes, we can decipher all the cipher text values whose plain text is not an "E" without further work. Repeat the procedure for cipher text bytes [2,10,18,...] and [3,11,19,...] and so on.