shell script alpha encryption

letter1=( a b c d e f g h i j k l m n o p q r s t u v w x y z )
for((ie=0;ie<${#letter1[@]};ie++))
do
lett=${letter1[$ie]}
let $lett=$ie
done
letter="$*"
total_letter=`echo "$letter" | wc -c`
for  je in $(seq 0 $total_letter )
do
d=`echo $letter | cut -c $je 2> /dev/null`
if [ -z $d ]
then
echo $d
else
eval echo \${$d} > /tmp/am1
num=`cat /tmp/am1`
if [ $num -gt 22 ]
then
num1=`echo "25 - $num" |bc`
encrypt=$num1
else
encrypt=` echo "$num +3" | bc`
fi
echo ${letter1[$encrypt]}
fi
done