cloudy understanding of asymmetric cryptography

David SMITH dave.smith at
Thu Mar 26 11:08:22 CET 2009

On Thu, Mar 26, 2009 at 07:51:15PM +1000, Felipe Alvarez wrote:
> _Bob performs symmetric encryption on message with_
> _key "K" (generated randomly). He then encrypts "K" _
> _with Alice's public key, and sends both the symetrically _
> _encrypted message and asymmetrically encrypted key to Alice_
> Is this what happens during most/some/all of public-key
> communications? I had always thought that the message is encrypted
> with public key, and decrypted with secret key. I was not aware that
> key "K" was encrypted with public key, but message encrypted with
> __symmetric_cipher__.

Yes, this is what normally happens with typical usage of gpg.  It's
called a "hybrid" cipher system.

I believe that the reason is that symmetric ciphers are usually more
efficient on computing power than asymmetric ones, so you don't really
want to be encrypting/decrypting lots of data with an asymmetric

> To help my understanding a little futher, if this does not always
> occur, or does not usually occur, when does it occur (not occur)?
> Using what ciphers (algorithms)?

Typical usage for gpg will be ElGamal for the asymmetric public/private
key bit, and AES for the symmetric cipher.

David Smith        | Tel: +44 (0)1454 462380    Home: +44 (0)1454 616963
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