From: Eric Blossom <eb@sr.hp.com>
Date: Fri, 6 May 94 14:38:49 PDT
To: hfinney@shell.portal.com
Subject: Lady Ada's Cryptophone
In-Reply-To: <199405060512.WAA15752@jobe.shell.portal.com>
Message-ID: <9405062138.AA03452@srlr14.sr.hp.com>
MIME-Version: 1.0
Content-Type: text/plain


>
>>    Each phone shall have a button (hard or soft)
>> which can be pressed by the caller at any time.  Pressing
>> it will cause a new TDES key to be generated and exchanged.
>> [Should it generate a new n and g for D-H, or just create
>> a new x and demand a new Y?]  Paranoid users can press
>> this button every few seconds if they wish.  (In my
>> humble opinion, even a single-DES phone is quite secure
>> if it has this feature.)
>
>It might be possible to compute the DH in the background while the
>conversation is going on, but if the computer is also compressing,
>uncompressing, encrypting and decrypting at the same time, that's
>not going to be easy.
>

Another thing to remember is that out of the DH you're going to get
somewhere in the neighborhood of 1000 - 2000 bits of secret.  Assuming
triple DES you only eat up 3*56 = 168 (you may actually use 3*64 = 192 just
to keep life simple) of these bits.  You can rekey 5-10 times without
having to re-exponentiate.

Eric Blossom