Re: Signing before Encryption and Signing after Encryption

["Gregory Rubin" <grrubin () gmail com>]

True.  Signatures don't really require asymmetric keys.  An example of
this is an HMAC and variants thereof.  Right now, I'm commonly signing
URLs using the following system (so I have no excuse for forgetting
it):

BaseUrl = http://www.foo.com/one/two?three=four
Secret = SharedSecret
Signature = md5(Secret + BaseURL)

New URL = BaseURL + "&hash=" + Signature

Greg Rubin

On 3/21/06, Craig Wright <cwright () bdosyd com au> wrote:
> 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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