While solving a (easy...) Project Euler cipher problem this week I repeatedly saw instructions for breaking ciphers after identifying the keylength as:
'shift the ciphertext by that key length and XOR it with itself. This removes the key and leaves you with plaintext XORed with the plaintext shifted the length of the key'
So I can understand how this drops the key out the equation, but I don't see how plaintext XORd with plaintext shifted by keylength is any more tractable. In the many references I saw about this, no one followed up with how to solve the plaintext^shiftedPlaintext piece. It doesn't seem that different than plaintext^keyPhrase really.
What am I missing about why this makes the problem easier to solve? What approaches deal with this part of the problem?