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:
- you compress the original plaintext file (e.g. gzip), to reduce redundancy
- you choose a random key, tipically in the range 128->256 bits
- you encrypt the compressed plaintext file with this key, using a traditional simmetric algorithm, (e.g. 3DES)
- for each receiver you encrypt *that random key* with the receiver's public key, using a public key algorithm, (e.g. RSA)
- 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