How to sum space delimited hex values in a string, and do other stuff?

Question

Hello
I am trying to write a Bourne shell script that takes a non-constant-length character string as input, converts each character to hexidecimal value, sums each value, takes the two's complement of the sum, logically ANDS it with FF, and if any of the resultant "nibbles" are a-f, print/save that nibble as A-F, for later display as a two-character-long character string
For example, assume a variable (as in non-constant) length variable $A
A="w04:0;2"
the road that I started down was to define
B=$( echo $A | od -An -t x1 -N"${#A}" )
# for this example, $B=77 30 34 3a 30 3b 32
It's the middle part where I need some magic. What I want to do is add the whitespace-delimited hex values of this string, for this example
C=77+30+34+3a+30+3b+32
so for this example, $C=1b2
then take two's complement of $C; for this example, $TWOS_COMP=ffff ffff ffff ffe4e
and logically AND it with FF; for this example, almost-final answer is 4e
then save final result as uppercase character string variable (for this example, variable's contents would be 4E)
Is what I'm trying to do intuitively obvious to any of you gurus out there, that could rattle off an answer? I'm okay with trashing my approach thus far and trying something totally different, within the constraint that it run inside a Bourne shell script.
Thanx in advance, if anybody can help!

Answer 1

I've made progress. I've converted each character of a non-constant-length string $A into a space-delimited string of hex values, added them up, and retained the least significant byte (i.e., the 8 least significant bits). Here's how:
B=$( echo $A | od -An -t x1 -N"${#A}" )
D=0
for C in $B; do
  C=0x$C
  D=$(($D + $C))
done
D=$(($D & 16#FF))
D=`echo "ibase=10;obase=16;$D" | bc`
The missing piece now is how to take the two's complement of $D. Anybody out there with any ideas on how to handle the two's complement piece inside a Bourne shell?
Thanx