Re: Encrypting outgoing message but not Gcc?

[Nathan Sullivan <alfonso@pants.nu>]

spamtrap@koldfront.dk (Adam Sjøgren) writes:

> On Sat, 06 Dec 2003 13:35:21 -0500, Lloyd wrote:
>
>> Also, I think there's a gap somewhere in my understanding of all
>> this.  How can it be that something encrypted twice, once with the
>> recipient's public key and once with yours, would only need your
>> private key to be decrypted?  Wouldn't there have to be two
>> decryptions, one for each of the previous encryptions?  Or in this
>> case, is the encryption done with some sort of mathematical
>> combination of both public keys, such that either corresponding
>> private key can decrypt it?
>
> I'm not able to adequately and comprehensibly explain the theory, but
> I think it's a proporty of either assymetric key cryptography in
> general or the way PGP/GnuPG is constructed (the latter being less
> likely, probably, perhaps).

It's a property of the way almost all assymetric cryptography is
implemented.  The problem is that public key encryption is grotesquely
slow, so programs like PGP/GPG don't encrypt the entire message with
the public key.  Instead, they encrypt the message with a symmetric
cypher, like IDEA or Blowfish, then encrypt the symmetric key with the
public key for the desired recipient.  If you have multiple
recipients, then multiple copies of the encrypted symmetric key are
included, each encrypted for a specific public key pair.

-- 
	    Nathan Sullivan                     alfonso@pants.nu
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          Conscience is what hurts when everthing else feels so good.