Re: Signing before Encryption and Signing after Encryption

[Greg Rubin <grrubin () gmail com>]

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It verifies a different thing.  It doesn't verify that a specific
person signed it, just that a trusted person signed it.  If there are
only two people in the circuit, then I know that the other person must
have signed it if I didn't.  You also have to remember that you can't
verify this signature either if you don't have access to the secret key.

Greg Rubin

John Lightfoot wrote:
> I don't understand how a signature can work with a shared key.  If
> two people share a key, how can you tell which one of them signed
> it?
>
> -----Original Message----- From: Gregory Rubin
> [mailto:grrubin () gmail com] Sent: Wednesday, March 22, 2006 12:55 PM
>  To: Craig Wright Cc: gillettdavid () fhda edu; shyaam () gmail com;
> security-basics () securityfocus com Subject: Re: Signing before
> Encryption and Signing after Encryption
>
> True.  Signatures don't really require asymmetric keys.  An example
> of this is an HMAC and variants thereof.  Right now, I'm commonly
> signing URLs using the following system (so I have no excuse for
> forgetting it):
>
> BaseUrl = http://www.foo.com/one/two?three=four Secret =
> SharedSecret Signature = md5(Secret + BaseURL)
>
> New URL = BaseURL + "&hash=" + Signature
>
> Greg Rubin
>
> On 3/21/06, Craig Wright <cwright () bdosyd com au> wrote:
> > Hello, Just to be difficult....
> >
> > David stated "Signing requires a private key". This is correct
> > through feasibility, but it is not technically correct as there
> > are signature schemes that only require symmetric keys. Signing
> > with symmetric keys is a lot more complex and thus more prone to
> > error and has a range of key management issues. This does not
> > mean that it is not possible.
> >
> > In fact there are scheme to sign a message using only Hashing
> > algorithms. The simplest of these is to hash the document and
> > keep a list of document hashes (similar to software). A user
> > could check the list to see if the message was valid or if
> > tampering had occurred. A third party could keep the hash tables
> > to ensure that the lists where accurate.
> >
> > So signing does not require a private key - it just makes it
> > easier. Next it also depends on non-repudiation/repudiation
> > issues. It is easy to sign a document and have a verification
> > that it is unaltered but with no proof that the original signer
> > could not come back and accuse the receiver of forging the
> > document.
> >
> > An example symmetric scheme could be:
> >
> > Alice encrypts a message using a symmetric key known to Bob (and
> > Alice only) Alice hashes the encrypted message Alice encrypts the
> > (encrypted) message and hash using a symmetric key known to Jim
> > but unknown to Bob Bob receives the hashed and encrypted message.
> >
> >
> > If Bob alters the message - the hash will not work. Alice can not
> > lie as Jim has a copy. Key management is a bugger, but still
> > possible (though unlikely)
> >
> > ANSI X9.17 Notarised Symmetric Keys may be used to sign.
> >
> > Regards Craig S Wright
> >
> > PS There are also hybrid ciphers for signing which are based on a
> >  combination of all the above - but this for another post
> >
> >
> > -----Original Message----- From: David Gillett
> > [mailto:gillettdavid () fhda edu] Sent: 22 March 2006 6:21 To:
> > shyaam () gmail com; security-basics () securityfocus com Subject: RE:
> > Signing before Encryption and Signing after Encryption
> >
> > 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
> >
> >
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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. 

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