Cryptographically secure random numbers are important any time your random numbers are being used in what is meant to be a secure application; for example when generating passwords and cryptographic keys. Tcl's [rand] is not cryptographically secure. However, [Tcllib] does offer [rc4], a stream cipher that is at its core a pseudo-random number generator. There is also the [Random] package which uses the [ISAAC] algorithm; which is considered secure.

These aren't bad solutions, but if you are in a modern Linux based environment your kernel should have support for /dev/random and /dev/urandom. The /dev/random kernel module polls the running system to gather entropy used to generate what are considered true random numbers. The only other true random numbers available to most Tcl applications is random.org, however this service transmits your numbers in the clear which in most instances requiring security is unacceptable. 

/dev/random is powerful and mature, and so it only makes sense to take advantage of when available. The counterpart to /dev/random is /dev/urandom, which stands for "unblocked" random. /dev/random is "blocked" meaning when the pool of available entropy is exhausted you must wait until additional entropy is gathered to get new random data. /dev/urandom does not block, and instead will reuse the entropy pool. While this decreases the quality of the randomness over time, it is still considered a cryptographically secure PRNG. What follows is the example of a function that can generate a 64 bit unsigned integer within a range, using /dev/urandom. Note that /dev/random and /dev/urandom are simply treated as files. 


======
proc randInt { min max } {
    set randDev [open {/dev/urandom} rb]
    set random [read $randDev 8]
    binary scan $random H16 random
    set random [expr (0x$random % (($max-$min) + 1) + $min)]
    close $randDev
    return $random
}
======


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'''[bodangly] - 2011-12-23 19:55:58'''

Good catch RLE, I noticed a slight irregularity in the distribution previously and was trying to figure it out. BINARY told me it required permissions to be set so I changed the above code to use "rb" instead. Edit: Whoops. =\ Wiped out the page and comments.