diceware

#!/bin/sh
#
# Allows a user to generate diceware passwords.
#
# USAGE: ./diceware <wordlist> <amount words> [<separator>]
#
# This script uses hexdump, /dev/urandom, and grep.
#
# Licensed under WTFPL: http://sam.zoy.org/wtfpl/
#

echoWord() {
	echo $2
}

generateWord() {
	local randomInt
	while :
	do
		# A word list consists of exactly 7776 entries (6^5), since there are 5 dices that need to be rolled.
		# Since the script deals with bytes, the minimum amount of bytes to retrieve is 2 ((2^8)^2 being 65536).
		#
		# That number then has to be converted to 8192, which 1. is the nearest number that's a power of 2
		# and 2. will not introduce any bias when mapping 0-65535 -> 0-8191.
		#
		# This means that in some cases when the number is larger than 7776, it needs to be retried. Simply mapping
		# 0-65535 to 0-7775 introduces a bias in word selection, albeit not a fairly large one.
		#
		randomInt=$((`hexdump -n2 -e '/2 "%u"' /dev/urandom` % 8192))
		if [ "$randomInt" -le 7775 ]; then
			break
		fi
	done

	# The word list contains dice outcomes. Simply convert the number back to base 6 (starting with 1 instead of 0)
	local generated
	local x=0
	while :
	do
		local number=$(($randomInt % 6 + 1))
		randomInt=$(($randomInt / 6))
		generated="$number$generated"
		x=$(($x + 1))
		if [ "$x" = "5" ]; then
			break
		fi
	done

	echoWord `grep "$generated" "$1"`
}

if [ "$2" = "" ]; then
	echo "Usage $0 <wordlist> <amount words> [<separator>]" 1>&2
	exit 1
fi

separator=$3
if [ "$separator" = "" ]; then
	separator=" "
fi

x=0
while :
do
	if [ "$x" -ge "$2" ]; then
		break
	fi

	word=`generateWord "$1"`
	if [ "$result" = "" ]; then
		result=$word
	else
		result="$result$separator$word"
	fi

	x=$(($x+1))
done

echo "$result"