From: Mike N. 

On Wed, 29 Mar 2006 07:31:21 +0300, "Antti Kurenniemi"
 wrote:

>But doesn't the first part, the identifying, sort of fail if you always sign
>your messages? I mean, what's to stop me from copying your signature and
>sending a message as Chris, which someone would the look up and confirm that
>yes, this is indeed from Chris?

   Because the signature also includes a hash of the message content.  The
worst you could do is replay the message exactly, but it would still be the
original message with the same signature.   Any change to the message
content will cause the signature to fail.

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