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

Provided by: tcl-trf-doc_2.1.4-dfsg3-3_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>

Trf transformer commands                              2.1.3                                       crc-zlib(3trf)