-----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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