Ubuntu Manpage: crc-zlib - Message digest "crc-zlib"

Provided by: tcl-trf-doc_2.1.4-dfsg2-1_all

NAME

       crc-zlib - Message digest "crc-zlib"

SYNOPSIS

       package require Tcl  ?8.2?

       package require Trf  ?2.1.3?

       crc-zlib ?options...? ?data?

DESCRIPTION

The command crc-zlib is one of several message digests provided by the package trf. See
trf-intro for an overview of the whole package.

crc-zlib ?options...? ?data?
The options listed below are understood by the digest if and only if the digest is
attached to a channel. See section IMMEDIATE 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 abbreviations are
recognized too. Their meaning is explained below:

absorb All data written to the channel is used to calculate 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 completed 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 comparison. 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 computed. When the channel is closed the computed values are
stored as ordered through the options -write-destination, -write-
type, -read-destination, 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 comparison 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-destination.

-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 versus ATTACHED.

If the option is absent the transformation is used in immediate mode and the
options listed below are recognized. 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 the zlib
       compression library (http://www.gzip.org/zlib/).

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 commandline 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 output.

KEYWORDS

       authentication, crc, crc-zlib, hash, hashing, mac, message digest, zip, zlib

COPYRIGHT

       Copyright (c) 1996-2003, Andreas Kupries <andreas_kupries@users.sourceforge.net>