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RE: Signing before Encryption and Signing after Encryption
From: "David Gillett" <gillettdavid () fhda edu>
Date: Wed, 22 Mar 2006 13:23:50 -0800

  Does non-repudiation require anything more than assurance that the
private key (a) MUST have been used, and (b) HASN'T been compromised?
Are you just alluding to the measures which support those assertions, 
or to some additional requirement(s) that escapes me?

  [If your private key isn't really private, all bets are off.]

David Gillett


-----Original Message-----
From: Craig Wright [mailto:cwright () bdosyd com au] 
Sent: Wednesday, March 22, 2006 12:56 PM
To: gillettdavid () fhda edu; shyaam () gmail com; 
security-basics () securityfocus com
Subject: RE: Signing before Encryption and Signing after Encryption


True, but the argument was not one as to which is the better method.
There are several secure hashing algorithms.


Further there is more to verification to source than just 
asymmetric keys. Non-repudiation is a complex field in itself 
and requires a entire range of associated infrastructure.

Regards
Craig

-----Original Message-----
From: David Gillett [mailto:gillettdavid () fhda edu]

Sent: 23 March 2006 4:32
To: Craig Wright; shyaam () gmail com; security-basics () securityfocus com
Subject: RE: Signing before Encryption and Signing after Encryption

  The property that a hash match is supposed to verify (is 
this copy the same as the original) is not quite the same as 
the property that a signature verifies (did this document 
come from that source).  There
*are* many applications where one is an acceptable 
alternative to the other.

  However, there have been numerous news items in the last 18 
months about the feasibility of engineering hash collisions 
with several popular algorithms; hashing must be assumed to 
provide weaker verification of its property than might have 
been previously assumed.
(For now, I've recommended that folks using tools that don't yet do
SHA-256 or better should use *both* MD5 and SHA-1 -- I don't 
think anyone has yet described an engineered collision that 
works with both.)


  Engineering hash collisions is apparently easier than 
compromising a properly-secured private key used in a good 
asymmetric algorithm.

David Gillett


-----Original Message-----
From: Craig Wright [mailto:cwright () bdosyd com au]
Sent: Tuesday, March 21, 2006 8:03 PM
To: gillettdavid () fhda edu; shyaam () gmail com;

security-basics () securityfocus com
Subject: RE: Signing before Encryption and Signing after Encryption




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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