Cryptographically secure random numbers are difficult to achieve, but important for things such as password and session key generation. You probably want a strong random number generator for something such as casino games involving real money. There are some solutions available in Tcl already, particularly in the Random package which provides a C implementation of the ISAAC algorithm. ISAAC is considered a cryptographically secure algorithm. Tcllib provides the rc4 package, a stream cipher that is essentially a random number generator.
However, if you are in a recent Linux environment chances are your kernel has support for /dev/random and its counterpart /dev/urandom. These kernel modules poll drivers and the running system to collect entropy used in generating true random numbers. The only difference between /dev/random and /dev/urandom is that /dev/urandom will start generating numbers with repeating entropy once the buffer has been overrun, as random is blocked and urandom is not. So for something such as a long-term cryptographic key it is usually recommended to generate using /dev/random, however, urandom is still considered a cryptographically secure pseudo-random number generator. For times when you need to generate random numbers often, /dev/urandom is more convenient than /dev/random. If you want a "truly" random number though, use /dev/random. The only other "true" random numbers available to most Tcl applications would be random.org, and relying on a third-party that transmits your numbers in the clear kind of defeats the purpose of a secure RNG. So, lets take advantage of these powerful, mature tools that Linux offers us. Below is an example of a function to generate a random unsigned integer up to 64 bits using /dev/urandom.
======
proc randInt { min max } {
set rand [open {/dev/urandom}]
set random [read $rand 8 ]
binary scan $random H16 random
set random [expr (0x$random % $max) + $min]
set random [expr $random > $max ? ($random % $max) + $min : $random]
close $rand
return $random
}
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<> Mathematics