Re: Using a Magic Value in Place of Authentication

"Ilmari Karonen" <usenet2@xxxxxxxxxxxxxx> wrote in message
That sounds as if you're interpreting "E(c || m)" as "m encrypted with
scheme E using key k". I was assuming the OP meant it in the
conventional sense, where "||" means concatenation, such that "E(c ||
m)" would mean "the concatenation of c and m, encrypted with the
scheme E", where the key is considered part of the definition of E.
(One would usually write E_K(m) to specify encryption with the key K,
but omitting the subscript is reasonable if the specific key used is
not relevant.)

Yes you are correct || does mean concatenation and there must be some sort
of key somewhere. But it doesn't negate the fact that given appropriate
values to k c and m it is possible to produce the same cyphertext on
multiple occasions assuming a 1 to 1 trasformation, if the cyphertext is for
example "ABCD" with one k, c and m it is possible, although somewhat
unlikely depending on the algorithm being used that an entirely different k
c and m can also produce a cyphertext of "ABCD" otherwise it would give
information to an attacker such that "ABCD" is the cyphertext on this
message, and it will never occur again for any other message. If the
attacker had some chosen plaintext, then that would lead to a partial if not
a total mapping of the key.