very short plaintexts symmetrically encrypted

[Werner Koch]

On Mon, 11 Jan 2010 13:06:03 -0500, lists.gnupg-users at mephisto.fastmail.net wrote:

> Forgive me, but how is a MitM attack possible against a symmetric cypher
> using a shared, secret key?

For example by swapping messages.  Two messages are sent on two
out-of-band events one which says Yes and the other says No.  If you
can mount an active MitM attack you can revert the meaning.  A MitM
may also inject faults to make the received message look like a
transmission error and thereby triggering another message.

Right, you can counter such attacks by adding more information to the
message.  However, the original post was about two short messages.


Salam-Shalom,

   Werner

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