Public SourceMage Discussion List

[Ismael Luceno <ismael.luceno AT gmail.com>]

On Tue, 13 Dec 2011 18:11:33 +0900
flux <flux AT sourcemage.org> wrote:
> Ismael Luceno (ismael.luceno AT gmail.com) wrote [11.12.13 12:14]:
<...>
> > So if we just save the hashes separately and sign that file, it
> > would have a similar effect. We should also think about allowing
> > several signatures.
> 
> No, it really wouldn't. You're missing the entire point of signing.
> 
> Signing is never equivalent to hasing. I've already explained this
> point in this thread, and there's plenty of resources on the web
> discussing both.
> 

I'm not saying it's equivalent, I'm just saying S(H(m)) is equivalent
to S(H(H(m1), H(m2), H(m3), ..., H(mN))).

When signing, you must do a digest, otherwise the signature would be
the same size as the file. And it's obvious it's done in this way
because doing it before signing saves a lot of computing power.

But you don't have to believe me... most standards say it explicitly,
like FIPS 186-3 (DSA): 

"A hash function is used in the signature generation process to obtain
a condensed version of the data to be signed; the condensed version of
the data is often called a message digest. The message digest is input
to the digital signature algorithm to generate the digital signature.
The hash functions to be used are specified in the Secure Hash Standard
(SHS), FIPS 180-3.  FIPS approved digital signature algorithms shall be
used with an appropriate hash function that is specified in the SHS.

The digital signature is provided to the intended verifier along with
the signed data. The verifying entity verifies the signature by using
the claimed signatory’s public key and the same hash function that was
used to generate the signature. Similar procedures may be used to
generate and verify signatures for both stored and transmitted data."

So, is there any other excuse to not do it?

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