symmetric encryption broken

[Michael H. Warfield]

On Fri, Mar 09, 2001 at 11:00:57PM +0100, Florian Weimer wrote:
> "Matthias Urlichs" <smurf at noris.de> writes:

> > Some ciphers are symmetric. If you DES-encrypt something, and then "encrypt"
> > the ciphertext again, you magically get the plaintext back.

> The OpenPGP message format removes such properties from the underlying
> ciphers if they are present, so as a GnuPG user, you won't notice
> them.

	I don't think any of the supported algorithms have such a
properties anyways.  DES/3DES, Cast5, Blowfish, and Idea all certainly
don't, I just double checked.  I'm not as sure about the Rijndahl varients
or TwoFish, I don't have the specs at my fingertips right now, but I would
be very much surprised if they did.  In almost every case, any system
based on a Fiestel network (DES, and Blowfish for sure) would require
reversing the subkeys for decryption and that's NOT symetrical in this
sense.  Idea is not based on Fiestel networks but shares that same
property.

	Mike
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