From: Dave Crocker <dcrocker@brandenburg.com>
Date: Thu, 15 Aug 1996 01:40:24 +0800
To: Mike van der Merwe <mikev@is.co.za>
Subject: Re: PGP...
In-Reply-To: <009A6C9C.E823907E.18@uni-lj.si>
Message-ID: <v03007800ae3784a734eb@[205.214.160.100]>
MIME-Version: 1.0
Content-Type: text/plain


Responses you've gotten have foccussed on defining public versus private
key.  Some have cited how they are used, but I'd like to emphasize this:

Message authentication:

  Make a "hash" of the message, i.e., compute a short, unique tag of the
message.  (A common algorithm for this is MD5.)  Then "encrypt" the hash
using the SENDER's private key.  Anyone wishing to authenticate the message
uses the SENDER's public key to "decrypt" the hash and check it against the
message receive (i.e., recompute a new hash and compare it to the received
one.)


Message privacy:

  Encrypt the message data.  (A common algorithm is DES or, more recently,
triple DES.  PGP uses IDEA.)  To get acceptable performance, encryption is
done using a symmetric key algorithm, rather than a much slower asymmetric
(public) key algorithm.  Then "encrypt" the symmetric key using the
RECEIVER's public key.  The RECEIVER uses their private key to decrypt the
symmetric key and then uses the symmetric key to decrypt the data.


	Note that these two different functions use private keys in an
essential opposite manner, or reverse relationship.

d/


--------------------
Dave Crocker                                            +1 408 246 8253
Brandenburg Consulting                             fax: +1 408 249 6205
675 Spruce Dr.                                 dcrocker@brandenburg.com
Sunnyvale CA 94086 USA                       http://www.brandenburg.com

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