From: Rick Busdiecker <rfb@lehman.com>
Date: Thu, 4 Jan 1996 17:09:43 +0800
To: "Michael C. Peponis" <mianigand@unique.outlook.net>
Subject: Re: 2047 bit keys in PGP
In-Reply-To: <199601040009.SAA07299@unique.outlook.net>
Message-ID: <9601040052.AA07122@cfdevx1.lehman.com>
MIME-Version: 1.0
Content-Type: text/plain


-----BEGIN PGP SIGNED MESSAGE-----

    From: "Michael C. Peponis" <mianigand@unique.outlook.net>
    Date: Thu, 4 Jan 1996 05:17:35 +0000
    
    > Why is there a limit to the size of the key anyway? It's too bad PGP 
    > doesn't support any size key (within reason).
    
    Within reason is the Key Phrase.  Even with a Pentium 90, I notice a 
    considerable lag in decrypting messages that have been encrypted with 
    a key larger than 2047/8.
    
    Even if you have a fast machine, if the person recieving the message 
    could wait a long time to decrypt you 4096 byte encrypted message.

Another point to realize is that PGP uses a combination of ciphers.
When encrypting, the RSA key is only used to encrypt an IDEA key.
That IDEA key is used to encrypt your message.  Somewhere between 2048
and 4096, you're making the RSA key stronger (harder to brute force)
than the IDEA key.  At that point, the extra time that you're using
for super-big RSA keys is totally wasted.

A similar argument applies to authentication, but then you're
comparing RSA and MD5, although I believe the argument holds for even
smaller RSA keys than in the RSA-IDEA comparison.

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--
Rick Busdiecker                        Please do not send electronic junk mail!
 net: rfb@lehman.com or rfb@cmu.edu    PGP Public Key: 0xDBD9994D
 www: http://www.cs.cmu.edu/afs/cs.cmu.edu/user/rfb/http/home.html
 send mail, subject "send index" for mailbot info, "send pgp key" gets my key
A `hacker' is one who writes code.  Breaking into systems is `cracking'.