packing into a byte token

dulac has asked for the wisdom of the Perl Monks concerning the following question:

Greetings.

I'm seeking wisdom from the monastery on handling creation of an array of bytes. I've done some research and it appears that I can use the pack command, but I am at a loss on how to obtain the results I seek.

I need to construct a "token" formatted as:

bytes 0 - 3: unsigned 32 bit representation of seconds since the epoch
bytes 4 - 7: unsigned 32 bit number (this represents an employee id)
byte 8: A check byte computed from the exclusive-or ("xor") of all the
+ previous bytes
[download]

I understand I can get the seconds since the epoch using time(), and I have the employee id as a 9 digit number - how do I format these into the above token and how do I create the xor check byte?

Any and all assistance is greatly appreciated.

Comment on packing into a byte token Download Code

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Re: packing into a byte token by BrowserUk (Patriarch) on Sep 21, 2011 at 01:06 UTC
The checksum spec is a little vague. It could be: `my $packed = pack 'NN', time(), 123456789;; my $c = chr(0); $c ^= substr $packed, $_, 1 for 0 .. 7;; $packed .= $c;; print unpack 'C', $packed;; 78 121 55 23 7 91 205 21 147` [download] Or: `my $packed = pack 'NN', time(), 123456789;; my $c = substr $packed, 0, 1; $c ^= substr $packed, $_, 1 for 1 .. 7;; $packed .= $c;; print unpack 'C', $packed;; 78 121 66 151 7 91 205 21 102` [download] Probably the latter. Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice.	[reply] [d/l] [select]
Re: packing into a byte token by jwkrahn (Abbot) on Sep 21, 2011 at 00:41 UTC
`my $seconds_since_the_epoch = pack 'N', time; my $employee_id = pack 'N', 123456789; my $check_byte = $seconds_since_the_epoch ^ $employee_id;` [download]	[reply] [d/l]
Re^2: packing into a byte token by ikegami (Patriarch) on Sep 21, 2011 at 01:45 UTC
Need to xor each byte, so `use List::Util qw( reduce ); my $token = pack 'NN', time, $employee_id; $token .= reduce { $a ^ $b } split //, $token;` [download] Use "V" instead of "N" if you need the opposite byte order.	[reply] [d/l]
Re^3: packing into a byte token by jwkrahn (Abbot) on Sep 21, 2011 at 02:01 UTC
Need to xor each byte Because?	[reply]
Re^4: packing into a byte token by ikegami (Patriarch) on Sep 21, 2011 at 02:09 UTC
Re: packing into a byte token by chrestomanci (Priest) on Sep 21, 2011 at 08:23 UTC
There is an old but a great tutorial in a dusty corner here in the monastery on how to to use pack/unpack that google always finds if you search for perl pack/unpack documentation.	[reply]


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