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List: ms-cryptoapi
Subject: Re: Signing a message
From: Greg Haskins <greg () PHONEMISER ! COM>
Date: 1998-07-29 14:56:21
[Download RAW message or body]
Pretty much everything everyone has said is correct. The output length
of a typical signature algorithm is dependant on a few variables,
including (but not limited to) 1) Algorithm, 2) Key length, 3) Length of
message. The output size is not necessarily a direct function of the
key size because you sometimes have to consider the length of the input
as well. For instance, the RSA signature scheme produces a signature
equal to the length of the signed message(1). The ElGamal scheme
produces a signature equal to twice the length of the signed message.
The DSA standard incorporates SHA (160 bit output hash) with ElGamal
which ends up yielding two SHA outputs for a total of 320 bits.
Hash functions are introduced so that the output is not as much of a
burden on both transmission bandwidth and CPU usage. If you use RSA,
for instance, without a hash, your message length effectively doubles
because you must transmit a message of length M and a signature of
length M(1). This is bad news for all of you people still using 28.8K
modems ;). In addition, modular exponentiation in the field Zp where p
is in the order of 2^512 - 2^2048 is pretty nasty on a message of length
M (where M is sufficiently large) as opposed to a nice little 160 bit
hash output.
(Note 1: The signature of M may actually be a little larger due to the
need to divide the message into blocks that fit within the group Zn
where n = p*q during the segmentation phase)
-greg
-------------------------------------------------------------
Gregory Haskins
Server Development
MediaCom Corporation
Bedford MA, USA
------------------------------------------------------------
Phonebills getting high?
http://www.phonemiser.com
> -----Original Message-----
> From: DolanP@DELUXE-DATA.CO.UK [SMTP:DolanP@DELUXE-DATA.CO.UK]
> Sent: Wednesday, July 29, 1998 7:38 AM
> To: CryptoAPI@DISCUSS.MICROSOFT.COM
> Subject: Re: Signing a message
>
> My undertsanding is that the size of a signature is a function of the
> private key size and not the hash size (ie. the signature will be the
> same
> size regardless of the hashing algorithm).
>
> Regards,
> Patrick Dolan.
>
>
>
>
>
> Rob Mig <rjmr@MAILCITY.COM> on 29/07/98 01:36:20
>
> Please respond to Microsoft Cryptographic API
> <CryptoAPI@DISCUSS.MICROSOFT.COM>
>
> To: CryptoAPI@DISCUSS.MICROSOFT.COM
> cc: (bcc: Patrick Dolan/UK/Deluxe Data/Deluxe Corporation)
> Subject: Re: Signing a message
>
>
>
>
> That's right, the product of the hashing is a digest calculated from
> the
> original message that has exactly the same lenght for any message if
> you
> use the same hashing algorithm.
> --
>
> On Tue, 28 Jul 1998 10:33:53 Bill Brice wrote:
> >Larry,
> >
> >It is my understanding that a signature (a hash of a data object)
> >is the same size regardless of the input data object size. This
> >is true per hash algorithm. i.e. all SHA-1 hashes are the same
> >size, all MD5 hashes are the same size, but different than SHA-1.
> >Someone correct me if I am wrong. Hope this helps.
> >
> >Bill Brice, Chief PKI Architect
> >International DataTrust
> >
> >
> >> -----Original Message-----
> >> From: Morroni, Lawrence A [mailto:Lawrence.Morroni@UNISYS.COM]
> >> Sent: Tuesday, July 28, 1998 10:16 AM
> >> To: CryptoAPI@DISCUSS.MICROSOFT.COM
> >> Subject: Signing a message
> >>
> >>
> >> Is it normal for two messages of varying length to be the same
> exact
> >> size after being signed?
> >>
> >> ========================
> >> ------------------------------------------
> >> Larry A Morroni Jr.
> >> lawrence.morroni@unisys.com
> >> UNISYS
> >> ------------------------------------------
> >> ========================
> >>
> >> ----------------------------------------------------------------
> >> Users Guide http://www.microsoft.com/workshop/essentials/mail.asp
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> >
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> >> lawrence.morroni@unisys.com
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> >Morroni, Lawrence A [mailto:Lawrence.Morroni@UNISYS.COM]
> >> Sent: Tuesday, July 28, 1998 10:16 AM
> >> To: CryptoAPI@DISCUSS.MICROSOFT.COM
> >> Subject: Signing a message
> >>
> >>
> >> Is it normal for two messages of varying length to be the same
> exact
> >> size after being signed?
> >>
> >> ========================
> >> ----------------------0D
> > Keiko
> >
>
>
> Now MailCity offers forwarding so you can check your MailCity messages
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