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 ?