Re: Signing before Encryption and Signing after Encryption

["Gregory Rubin" <grrubin () gmail com>]

The problem with signing after encryption is that it is possible to
strip the signature off and claim a message as your own.  Consider the
following situation (encrypted text is signed, not the plain text).

Alice wants to authenticate herself to Bob, so she composes a message
with secret information, encrypts it to Bob's public key, and then
signs it with her private key and sends it off.

Mallory intercepts the message. She can't read the message, but she
can strip off the signature so she now just has the message encrypted
to Bob's key.  She now signs the message with her key and sends it on.

Bob now receives the secret information in an encrypted message with a
valid signature by Mallory.  Ergo, he believes that Mallory knows this
secret information.  This is a problem.

If the signature is on the plain text as opposed to the private text,
this attack fails.

Greg Rubin


On 3/21/06, David Gillett <gillettdavid () fhda edu> wrote:
>   Signing requires a private key -- therefore, it *must* be
> Asymmetric.  Asymmetric is typically much slower than Symmetric,
> so you get things like SSL that use Asymmetric to protect the
> exchange of the Symmetric key used for actual payload encryption.
>
>   Signing after encryption allows the signature to be verified
> before/without decrypting the payload.  There are a variety of
> circumstances in which that could be useful, which are blocked
> if the signing is done first.  I can't think of any where the
> opposite is true.
>
> David Gillett, CISSP
>
>
> > -----Original Message-----
> > From: shyaam () gmail com [mailto:shyaam () gmail com]
> > Sent: Tuesday, March 21, 2006 9:28 AM
> > To: security-basics () securityfocus com
> > Subject: Signing before Encryption and Signing after Encryption
> >
> > Hello All,
> > I was asked a question in an interview. I would like to know
> > more about this. I am sorry if it is really basic question.
> >
> > What are the tradeoffs between Signing before Encryption and
> > Signing after Encryption? Please do let me know on either
> > case when you use a Symmetric Key and an Asymmetric key.
> >
> > I am sure that this is a very basic question. I appologize again.
> >
> > Kind Regards,
> > Shyaam
> >
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EARN A MASTER OF SCIENCE IN INFORMATION ASSURANCE - ONLINE
The Norwich University program offers unparalleled Infosec management
education and the case study affords you unmatched consulting experience.
Tailor your education to your own professional goals with degree
customizations including Emergency Management, Business Continuity Planning,
Computer Emergency Response Teams, and Digital Investigations.

http://www.msia.norwich.edu/secfocus
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