You shouldn't use urandom for crypto purposes. /dev/random is generated (on most platforms) as cryptographic strength numbers (usually from hardware), but can block if it runs out of data. /dev/urandom was created with the guarantee to never block and will use /dev/random's pool of numbers initially but can start outputting lower entropy numbers if /dev/random blocks.

/dev/random is better than urandom for this sort of thing. On linux:

dd if=/dev/random bs=64 count=1 2>/dev/null | hexdump