very short plaintexts symmetrically encrypted

Werner Koch wk at
Tue Jan 12 09:35:17 CET 2010

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



Die Gedanken sind frei.  Ausnahmen regelt ein Bundesgesetz.

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