True, but it's also not GNU anything. Most modern UNIX-like kernels implement some form of /dev/urandom or /dev/random. The actual shell tools used are POSIX.

True, but it's also not GNU anything. Most modern UNIX-like kernels implement some form of /dev/urandom or /dev/random. The actual shell tools used are POSIX. I don't think POSIX even defines any functions/system calls for getting cryptographically secure random numbers, so if you're literally restricting yourself to POSIX constructs, then indeed what OP asks for is impossible :)