Consider the following enhancement of the Vigenre cipher. We assume that the plain- text is a case-insensitive English text using only the 26 letters (without space or any other symbol). To encrypt a plaintext of length n, one first uniformly randomly generates a string over the alpha- bet (A..Z] of length 17, and then inserts this string into the beginning of the plaintext. That is, we first construct a string x = X 1X2 ... Xn+17, such that X1 X17 is the string we have generated, and In+17 is the original plaintext string. We then construct a string y as follows: Yi = Xi for 1