advanced one time pad was re more one time pad stuff o k if you ve read this group for a while you know all about the one time pad but here s a question i haven t seen the one time pad yeilds ideal security but has a well known flaw in authentication suppose you use a random bit stream as the pad and exclusive or as the encryption operation if an adversary knows the plaintext of a message he can change it into any other message here s how it works alice is sending bob a plaintext p under a key stream s alice computes the ciphertext c s xor p and sends it to bob eve knows the plainext p but wants the message to appear as p eve intercepts c and computes c c xor p xor p s xor p eve sends c to bob bob decrypts c by computing c xor s p thus receiving the false message which was substituted by eve now the question is how can this attack be defeated with information theoretic security not just computational security can we define something like ideal authentication which is the analog of ideal security can we obtain ideal authentication if not how much can we limit eve s control over the message if we can achieve ideal authentication does the solution use more key bits or expand the message can we show the solution to be optimal in one or more parameters does anyone know if these questions have been aswered before olson umbc edu
