How to create “truly” random and fast files for testing IO performance

#!/bin/sh

for var in {1..1000}
do
    dd if=<(openssl enc -aes-256-ctr \
        -pass pass:"$(dd if=/dev/urandom bs=128 count=1 2>/dev/null | base64)" \
        -nosalt < /dev/zero) \
        bs=1M \
        count=100 \
        iflag=fullblock \
        of=`echo $RANDOM | md5sum | head -c 20; echo;`
done