crc(n) crc(n) ______________________________________________________________________________
NAME
crc - Message digest "crc"
SYNOPSIS
package require Tcl ?8.2?
package require Trf ?2.1.4?
crc ?options...? ?data?
_________________________________________________________________
DESCRIPTION
The command crc is one of several message digests provided by the pack-
age trf. See trf-intro for an overview of the whole package.
crc ?options...? ?data?
The options listed below are understood by the digest if and
only if the digest is attached to a channel. See section IMME-
DIATE versus ATTACHED for an explanation of the term attached.
-mode absorb|write|transparent
This option has to be present. The specified argument
determines the behaviour of the digest in attached mode.
Beyond the argument values listed above all unique abbre-
viations are recognized too. Their meaning is explained
below:
absorb All data written to the channel is used to calcu-
late the value of the message digest and then
passed unchanged to the next level in the stack of
transformations for the channel the digest is
attached to. When the channel is closed the com-
pleted digest is written out too, essentially
attaching the vlaue of the diggest after the
information actually written to the channel.
When reading from the channel a value for the
digest is computed too, and after closing of the
channel compared to the digest which was attached,
i.e. came behind the actual data. The option
-matchflag has to be specified so that the digest
knows where to store the result of said compari-
son. This result is a string and either "ok", or
"failed".
write All data read from or written to the channel the
digest is attached to is ignored and thrown away.
Only a value for the digest of the data is com-
puted. When the channel is closed the computed
values are stored as ordered through the options
-write-destination, -write-type, -read-destina-
tion, and -read-type.
transparent
This mode is a mixture of both absorb and write
modes. As for absorb all data, read or written,
passes through the digest unchanged. The generated
values for the digest however are handled in the
same way as for write.
-matchflag varname
This option can be used if and only if the option "-mode
absorb" is present. In that situation the argument is the
name of a global or namespaced variable. The digest will
write the result of comparing two digest values into this
variable. The option will be ignored if the channel is
write-only, because in that case there will be no compar-
ison of digest values.
-write-type variable|channel
This option can be used for digests in mode write or
transparent. Beyond the values listed above all their
unique abbreviations are also allowed as argument values.
The option determines the type of the argument to option
-write-destination. It defaults to variable.
-read-type variable|channel
Like option -write-type, but for option -read-destina-
tion.
-write-destination data
This option can be used for digests in mode write or
transparent. The value data is either the name of a
global (or namespaced) variable or the handle of a
writable channel, dependent on the value of option
-write-type. The message digest computed for data written
to the attached channel is written into it after the
attached channel was closed. The option is ignored if
the channel is read-only.
Note that using a variable may yield incorrect results
under tcl 7.6, due to embedded \0's.
-read-destination data
This option can be used for digests in mode write or
transparent. The value data is either the name of a
global (or namespaced) variable or the handle of a
writable channel, dependent on the value of option -read-
type. The message digest computed for data read from the
attached channel is written into it after the attached
channel was closed. The option is ignored if the channel
is write-only.
Note that using a variable may yield incorrect results
under tcl 7.6, due to embedded \0's.
The options listed below are always understood by the digest, attached
versus immediate does not matter. See section IMMEDIATE versus ATTACHED
for explanations of these two terms.
-attach channel
The presence/absence of this option determines the main
operation mode of the transformation.
If present the transformation will be stacked onto the
channel whose handle was given to the option and run in
attached mode. More about this in section IMMEDIATE ver-
sus ATTACHED.
If the option is absent the transformation is used in
immediate mode and the options listed below are recog-
nized. More about this in section IMMEDIATE versus
ATTACHED.
-in channel
This options is legal if and only if the transformation
is used in immediate mode. It provides the handle of the
channel the data to transform has to be read from.
If the transformation is in immediate mode and this
option is absent the data to transform is expected as the
last argument to the transformation.
-out channel
This options is legal if and only if the transformation
is used in immediate mode. It provides the handle of the
channel the generated transformation result is written
to.
If the transformation is in immediate mode and this
option is absent the generated data is returned as the
result of the command itself.
NOTES
This command uses the same CRC polynomial as the CRC algorithm used by
PGP (http://www.pgp.com).
IMMEDIATE VERSUS ATTACHED
The transformation distinguishes between two main ways of using it.
These are the immediate and attached operation modes.
For the attached mode the option -attach is used to associate the
transformation with an existing channel. During the execution of the
command no transformation is performed, instead the channel is changed
in such a way, that from then on all data written to or read from it
passes through the transformation and is modified by it according to
the definition above. This attachment can be revoked by executing the
command unstack for the chosen channel. This is the only way to do this
at the Tcl level.
In the second mode, which can be detected by the absence of option
-attach, the transformation immediately takes data from either its com-
mandline or a channel, transforms it, and returns the result either as
result of the command, or writes it into a channel. The mode is named
after the immediate nature of its execution.
Where the data is taken from, and delivered to, is governed by the
presence and absence of the options -in and -out. It should be noted
that this ability to immediately read from and/or write to a channel is
an historic artifact which was introduced at the beginning of Trf's
life when Tcl version 7.6 was current as this and earlier versions have
trouble to deal with \0 characters embedded into either input or out-
put.
SEE ALSO
adler(n), crc(n), crc-zlib(n), haval(n), md2(n), md5(n), md5_otp(n),
ripemd-128(n), ripemd-160(n), sha(n), sha1(n), sha1_otp(n), trf-intro(n)
KEYWORDS
authentication, crc, hash, hashing, mac, message digest, pgp
COPYRIGHT
Copyright (c) 1996-2003, Andreas Kupries <andreas_kupries@users.sourceforge.net>
Trf transformer commands 2.1.4 crc(n)
Mac OS X 10.8 - Generated Wed Sep 5 19:26:51 CDT 2012