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List:       openssl-dev
Subject:    RE: max size of digital signature
From:       Frank Balluffi <frankb () valicert ! com>
Date:       2002-02-26 15:02:49
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Mark Webb asked:

> I have a program that creates a fixed format file.  The file 
> gets created by a 
> client process and verified by the server process.  In order 
> to parse the 
> file, I need to know the size of each field.  The digital 
> signatures that I 
> am creating vary in the 92-96 character range.  Is there a 
> theoretical 
> maximum size of a digital signature?

For RSA, the signature will be the same size as the modulus (commonly
referred to as n), because the signature calculation is mod n. For example,
if you generate an RSA key pair with a 512-bit modulus, the private key will
generate signatures of size 512 bits or 64 octets. If you generate an RSA
key pair with a 1024-bit modulus, the private key will generate signatures
of size 1024 bits or 128 octets.

Because some big number libraries "think" in terms of signed integers, some
big number libraries insert a leading octet with the value 0 if the most
significant bit of a big integer is on. So I would always allocate a few
extra bytes for the buffer to hold the signature.

Frank

> -----Original Message-----
> From: Mark W. Webb [mailto:mark@dolphtech.com]
> Sent: Monday, February 25, 2002 10:27 AM
> To: openssl-dev@openssl.org
> Subject: max size of digital signature
> 
> 
> I have a program that creates a fixed format file.  The file 
> gets created by a 
> client process and verified by the server process.  In order 
> to parse the 
> file, I need to know the size of each field.  The digital 
> signatures that I 
> am creating vary in the 92-96 character range.  Is there a 
> theoretical 
> maximum size of a digital signature?
> 
> thank you.
> -- 
> Mark Webb
> Software Engineer
> Dolphin Technology
> 1300B Floyd Ave
> Rome, NY 13440
> 	
> Phone: (315) 334-4892 x222
> Fax: (315) 339-4846
> Email: mark@dolphtech.com
> ______________________________________________________________________
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> 
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