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RE: Signing before Encryption and Signing after Encryption
From: "Craig Wright" <cwright () bdosyd com au>
Date: Wed, 22 Mar 2006 15:02:38 +1100


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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