AW: [Mailman-Developers] Secure Mailing Lists
On Thu, 2003-11-27 at 17:07, Brad Knowles wrote:
At 1:12 PM -0500 2003/11/27, Barry Warsaw wrote:
- Obviously you're going to do personalized deliveries, so for any such list you'll probably want to disable digests.
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.
You could easily handle a digest the same way.
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?
- Dietmar
At 9:53 AM +0100 2003/11/28, Dietmar Maurer wrote:
Sorry, I dont underdstand that suggestion. You want to put several encrypted messages into one mail?
No.(I dont know much about public keysystems). That would increase the size of the message? Or is it possible to encrypt a message once so that several people can decrypt it?
Your latter question gets to the point. You encrypt the message once with the session symmetric key, and then you encrypt the session symmetric key once for each recipient public key. Each recipient uses their private key to decrypt the session symmetric key, which is then used to decrypt the message.
Fortunately, PGP makes all this transparent to the recipients. The resulting message is somewhat larger, because you've encrypted the session symmetric key for each recipient public key, but this is usually a relatively small expansion and since PGP has built-in compression, this is not usually too much of a loss.
If you had a large number of recipients, this might become more of an issue. In that case, you might want to do this function in smaller batches.
The only disadvantage with this approach is that you can see what keyids that a message is encrypted for, and this allows someone to do traffic analysis (see who is talking to whom). If this was an issue of concern, then this is something that should be configurable on a per-list basis.
-- Brad Knowles, <brad.knowles@skynet.be>
"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety." -Benjamin Franklin, Historical Review of Pennsylvania.
GCS/IT d+(-) s:+(++)>: a C++(+++)$ UMBSHI++++$ P+>++ L+ !E-(---) W+++(--) N+ !w--- O- M++ V PS++(+++) PE- Y+(++) PGP>+++ t+(+++) 5++(+++) X++(+++) R+(+++) tv+(+++) b+(++++) DI+(++++) D+(++) G+(++++) e++>++++ h--- r---(+++)* z(+++)
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
participants (3)
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Brad Knowles -
Dietmar Maurer -
Simone Piunno