12-19-2007
This will not work, a 32 bit CRC will not fit in "4 bytes of hex". While 32 bits is indeed 4 bytes, a byte has 2^8 (256) possible values; base 256 (haha).
You are implying that the spec is transfering a 32 bit CRC value using "hexidecimal". This effectively reduces the resolution for each byte to 16 values. In doing so, to represent the entire range of 32 bit CRC values it will need 8 hexidecimal "bytes".
More simply stated, the biggest value for a 32 bit CRC is 2^32-1 (hex "FFFFFFFF"). This sequence of hexidecimal characters will occupy 8 bytes if translated verbatim.
Given this, I think you are very confused by something.... I am just not sure what. All the "convert the string and CRC to decimal" [sic] talk makes no sense to me. You have an algorithm that generates a CRC from a piece of data. The receiver should use the same alogoirthm, generate a CRC, and compare against the CRC sent with the data.
If the protocol does something more complicated then you need to understand what that is. For example, you mention a "common key" which I presume is the CRC, but I have no idea.
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crc32(n) Cyclic Redundancy Check (crc32) crc32(n)
NAME
crc32 - Perform a 32bit Cyclic Redundancy Check
SYNOPSIS
package require Tcl 8.2
package require crc32 ?1.0?
::crc::crc32 ?-format format? ?-seed value? ?-implementation procname? message
::crc::crc32 ?-format format? ?-seed value? ?-implementation procname? -filename file
DESCRIPTION
This package provides a Tcl-only implementation of the CRC-32 algorithm based upon information provided at http://www.naaccr.org/stan-
dard/crc32/document.html If the Trf package is available then the crc-zlib command is used to perform the calculation.
COMMANDS
::crc::crc32 ?-format format? ?-seed value? ?-implementation procname? message
::crc::crc32 ?-format format? ?-seed value? ?-implementation procname? -filename file
The command takes string data or a file name and returns a checksum value calculated using the CRC-32 algorithm. The result is for-
matted using the format(n) specifier provided or as an unsigned integer (%u) by default.
OPTIONS
-filename name
Return a checksum for the file contents instead of for parameter data.
-format string
Return the checksum using an alternative format template.
-seed value
Select an alternative seed value for the CRC calculation. The default is 0xffffffff. This can be useful for calculating the CRC for
data structures without first converting the whole structure into a string. The CRC of the previous member can be used as the seed
for calculating the CRC of the next member.
Note that as the Trf command crc-zlib cannot accept a seed value, use of this option will force the use of the Tcl only implementa-
tion.
-implementation procname
This hook is provided to allow users to provide their own implementation (perhaps a C compiled extension) or to explicitly request
use of the Tcl only implementation when Trf is installed (by setting -implementation crc::Crc32_tcl. The procedure specfied is
called with two parameters. The first is the data to be checksummed and the second is the seed value. A 32bit integer is expected as
the result.
EXAMPLES
% crc::crc32 "Hello, World!"
3964322768
% crc::crc32 -format 0x%X "Hello, World!"
0xEC4AC3D0
% crc::crc32 -file crc32.tcl
483919716
SEE ALSO
sum(n), cksum(n)
AUTHORS
Pat Thoyts
KEYWORDS
cksum, checksum, crc, crc32, cyclic redundancy check, data integrity, security
crc 1.0 crc32(n)