Consider a variant of the one-time pad with message space {0,1}n , where the key space is restricted to all n-bit strings with an even number of 1’s. Prove that this cipher is not secure by showing an adversary whose semantic security advantage is 1.

Community Answer

   Our One Time Pad(OTP)is secure and that the chance of AA to succeedis 2−n2−n (for INFO(1n,m)=mINFO(1n,m)=m),because SS also has a chance of 2−n2−n to pickthe correct binary string. But as we are tryingto proove the security, we can't argue that way.And as soon as we can no longer use that knowledge, the proof nolonger works.semantic security of the one ... See the full answer