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RE: Signing before Encryption and Signing after Encryption
From: "Craig Wright" <cwright () bdosyd com au>
Date: Thu, 23 Mar 2006 07:56:06 +1100


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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Liability limited by a scheme approved under Professional Standards Legislation in respect of matters arising within 
those States and Territories of Australia where such legislation exists.

DISCLAIMER
The information contained in this email and any attachments is confidential. If you are not the intended recipient, you 
must not use or disclose the information. If you have received this email in error, please inform us promptly by reply 
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Any views expressed in this message are those of the individual sender. You may not rely on this message as advice 
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a Partner of BDO.

BDO accepts no liability for any damage caused by this email or its attachments due to viruses, interference, 
interception, corruption or unauthorised access.

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The Norwich University program offers unparalleled Infosec management
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