Subject: Re: Encrypt then sign insecure?

From: [email-masked] ("Hal Finney")
Date: Mon, 15 Aug 2005 20:47:24 -0700 (PDT)

Ben Laurie wrote:
> Hal Finney wrote:
> > This paper doesn't apply to systems like OpenPGP which compose public
> > key signatures with public key encryption.  Rather, it investigates the
> > composition of symmetric encryption (e.g. AES) with MAC.
> ...
> This does not seem to me to be true. OpenPGP uses symmetric encryption 
> under the hood, and signs the plaintext rather than the ciphertext. All 
> that is needed is an oracle which will say whether the signature is 
> correct or not.

Krawczyk's paper is about combining MAC and symmetric encryption.
That's not what OpenPGP does.  We don't do MACs.

> Furthermore, OpenPGP does not use CBC, so the security proof from the 
> paper doesn't help.

That's true, but the point is that the paper is not about systems like
OpenPGP at all.

Hal Finney

ietf-openpgp.imc.org>

Ben Laurie wrote:
> Hal Finney wrote:
> > This paper doesn't apply to systems like OpenPGP which compose public
> > key signatures with public key encryption.  Rather, it investigates the
> > composition of symmetric encryption (e.g. AES) with MAC.
> ...
> This does not seem to me to be true. OpenPGP uses symmetric encryption 
> under the hood, and signs the plaintext rather than the ciphertext. All 
> that is needed is an oracle which will say whether the signature is 
> correct or not.

Krawczyk's paper is about combining MAC and symmetric encryption.
That's not what OpenPGP does.  We don't do MACs.

> Furthermore, OpenPGP does not use CBC, so the security proof from the 
> paper doesn't help.

That's true, but the point is that the paper is not about systems like
OpenPGP at all.

Hal Finney