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RE: Signing before Encryption and Signing after Encryption
From: "David Gillett" <gillettdavid () fhda edu>
Date: Wed, 22 Mar 2006 09:32:11 -0800

  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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