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RE: Signing before Encryption and Signing after Encryption
From: "John Lightfoot" <jlightfoot () gmail com>
Date: Thu, 23 Mar 2006 10:41:14 -0600

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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---------------------------------------------------------------------------
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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