AW: [Mailman-Developers] Secure Mailing Lists

[Simone Piunno]

Alle 09:53, venerdì 28 novembre 2003, Dietmar Maurer ha scritto:

> >> 	Actually, you can encrypt the message once, to each of the keys
> >> of each of the people on the list.  You don't have to do multiple
> >> encryptions.  That would save a hell of a lot of processing overhead.

> Sorry, I dont underdstand that suggestion. You want to put several
> encrypted messages into one mail? (I dont know much about public key
> systems). That would increase the size of the message?  Or is it possible 
> to encrypt a message once so that several people can decrypt it?

Basically, public key encryption works like this:

0. you compress the original plaintext file (e.g. gzip), to reduce redundancy
1. you choose a random key, tipically in the range 128->256 bits
2. you encrypt the compressed plaintext file with this key, using a 
traditional simmetric algorithm, (e.g. 3DES)
3. for each receiver you encrypt *that random key* with the receiver's public 
key, using a public key algorithm, (e.g. RSA)
4. you pack everything and send the bundle to all receivers

So the encrypted email size is roughly made up of:

sizeof(compressed_plaintext) + N*[sizeof(random_key) + sizeof(receiver_id)]

Where N is the number of receivers.
So OK, the size grows with the number of receivers, but not that much.  
Moreover, for N < M the encrypted email is smaller that the original 
plaintext (due to compression).  
M depends on achieved compression ratio.

-- 
Simone Piunno, chief architect
Wireless Solutions SPA - DADA group
Europe HQ, via Castiglione 25 Bologna
web:www.wseurope.com tel:+390512966811 fax:+390512966800
God is real, unless declared integer