The name cookie derives from UNIX objects called magic cookies. These are tokens that are attached to a user or program and change depending on the areas entered by the user or program.... Kevin Dean
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Below is probably more than you want to know, in a form that is complete gibberish, but here it is.
The functional uses of the magic number are numerous, since you can write programs that perform conditional tests on huge numbers of files for exacting info in a data base, preening a disk (via chron), performing sorts based on any of the parameters, etc. In the FreeBSD world, magic numbers are used by the CVS ports system to update and upgrade the listed programs and do maintenance to keep the ports current.
Also I think you can use magic for security by setting up profiles on classes or groups of magic numbers so that files with these parameters are matched to the user's profiles and allow combinations of read, write, execute dependent on the particular user profile. This puts an addition level of security on top of the existing permissions scheme.
I suspect it was this latter use that caught the attention of the M$uck crowd. The encoded number scheme written to your computer from a web server, is later read by `their' server and corresponds to some profile scheme they have created to identify who you are, what you want, what you visited last time, what you should be allowed to visit or do this time, etc, etc.
I turn off cookies and delete any that I have allowed. Some sites, especially those that do credit card sales usually insist you turn cookies `on'. So, if I am buying something, I turn it on. After I finish I turn it off and go delete the cookie.
Chuck
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MAGIC(5) FreeBSD File Formats Manual MAGIC(5)
NAME
magic - file command's magic number file
DESCRIPTION
This manual page documents the format of the magic file as used by the
file(1) command, version 3.22. The file command identifies the type of a
file using, among other tests, a test for whether the file begins with a
certain magic number. The file /usr/share/misc/magic specifies what magic
numbers are to be tested for, what message to print if a particular magic
number is found, and additional information to extract from the file.
Each line of the file specifies a test to be performed. A test compares
the data starting at a particular offset in the file with a 1-byte,
2-byte, or 4-byte numeric value or a string. If the test succeeds, a
message is printed. The line consists of the following fields:
offset A number specifying the offset, in bytes, into the file of the
data which is to be tested.
type The type of the data to be tested. The possible values are:
byte A one-byte value.
short A two-byte value (on most systems) in this machine's na-
tive byte order.
long A four-byte value (on most systems) in this machine's na-
tive byte order.
string A string of bytes.
date A four-byte value interpreted as a unix date.
beshort
A two-byte value (on most systems) in big-endian byte or-
der.
belong A four-byte value (on most systems) in big-endian byte
order.
bedate A four-byte value (on most systems) in big-endian byte
order, interpreted as a unix date.
leshort
A two-byte value (on most systems) in little-endian byte
order.
lelong A four-byte value (on most systems) in little-endian byte
order.
ledate A four-byte value (on most systems) in little-endian byte
order, interpreted as a unix date.
The numeric types may optionally be followed by & and a numeric value, to
specify that the value is to be AND'ed with the numeric value before any
comparisons are done. Prepending a u to the type indicates that ordered
comparisons should be unsigned.
test The value to be compared with the value from the file. If the
type is numeric, this value is specified in C form; if it is a
string, it is specified as a C string with the usual escapes per-
mitted (e.g. \n for new-line).
Numeric values may be preceded by a character indicating the op-
eration to be performed. It may be =, to specify that the value
from the file must equal the specified value, <, to specify that
the value from the file must be less than the specified value, >,
to specify that the value from the file must be greater than the
specified value, &, to specify that the value from the file must
have set all of the bits that are set in the specified value, ^,
to specify that the value from the file must have clear any of
the bits that are set in the specified value, or x, to specify
that any value will match. If the character is omitted, it is
assumed to be =.
Numeric values are specified in C form; e.g. 13 is decimal, 013
is octal, and 0x13 is hexadecimal.
For string values, the byte string from the file must match the
specified byte string. The operators =, < and > (but not &) can
be applied to strings. The length used for matching is that of
the string argument in the magic file. This means that a line
can match any string, and then presumably print that string, by
doing >\0 (because all strings are greater than the null string).
message
The message to be printed if the comparison succeeds. If the
string contains a printf(3) format specification, the value from
the file (with any specified masking performed) is printed using
the message as the format string.
Some file formats contain additional information which is to be printed
along with the file type. A line which begins with the character > indi-
cates additional tests and messages to be printed. The number of > on
the line indicates the level of the test; a line with no > at the begin-
ning is considered to be at level 0. Each line at level n+1 is under the
control of the line at level n most closely preceding it in the magic
file. If the test on a line at level n succeeds, the tests specified in
all the subsequent lines at level n+1 are performed, and the messages
printed if the tests succeed. The next line at level n terminates this.
If the first character following the last > is a ( then the string after
the parenthesis is interpreted as an indirect offset. That means that
the number after the parenthesis is used as an offset in the file. The
value at that offset is read, and is used again as an offset in the file.
Indirect offsets are of the form: (x[.[bsl]][+-][y]). The value of x is
used as an offset in the file. A byte, short or long is read at that
offset depending on the [bsl] type specifier. To that number the value
of y is added and the result is used as an offset in the file. The de-
fault type if one is not specified is long.
Sometimes you do not know the exact offset as this depends on the length
of preceding fields. You can specify an offset relative to the end of
the last uplevel field (of course this may only be done for sublevel
tests, i.e. test beginning with >). Such a relative offset is specified
using & as a prefix to the offset.
BUGS
The formats long, belong, lelong, short, beshort, leshort, date, bedate,
and ledate are system-dependent; perhaps they should be specified as a
number of bytes (2B, 4B, etc), since the files being recognized typically
come from a system on which the lengths are invariant.
There is (currently) no support for specified-endian data to be used in
indirect offsets.
SEE ALSO
file(1)
Public Domain September 3, 1994 2