Stuck on finding M2 knowing M1 in One Time Pad Lets say I have a plaintext M1 and a plaintext M2, I know both the ciphers C1 and C2 from these M1 and M2. But from M1 I also know a piece of M1 or maybe even the whole text. And I also know C1 XOR C2 = M1 XOR M2. But how can you get M2 if you know M1. So is it possible to do 'something ' to M1 XOR M2 knowing M1 to get M2?
If you know M1 XOR it to both sides of M1 XOR M2 = C1 XOR C2 which gives you M2


  M2=C1 XOR C2 XOR M1


Even if you didn't know M1, if it is long enough it (or M2) might have a common substring, i.e., a crib such as "the", "this". 

You might use the crib as an estimate of part of M1 to obtain a guess for a part of M2 by using the equation above focusing on 3 letter substrings. 

If you obtain as M2 a substring that looks like it could be English, you may have guessed part of the OTP key.