gcc modexp.cpp -o modexp.exe
gcc sha256.c -o sha256.exe

#  generate a rsa key  ( needs to be more than 256 bits )
openssl genrsa -out tst.pvk 260

# extract the pubkey, exponent, and priv key.
pq=`openssl asn1parse -in tst.pvk | awk "NR==3"|cut -d: -f4`
e=`openssl asn1parse -in tst.pvk | awk "NR==4"|cut -d: -f4`
d=`openssl asn1parse -in tst.pvk | awk "NR==5"|cut -d: -f4`

# generate a message:
msg=`./sha256.exe modexp.cpp`
echo ..... hash
echo $msg

# calculate  rsa_crypt(sha256(), privkey)
encrypted=`echo "16 o 16 i $msg $d $pq |p"|dc|tr -d "\134\n"`
echo .... privkey encrypted hash
echo $encrypted

# verify that we can decrypt this:
echo .... pubkey decrypted hash
./modexp.exe $encrypted $e $pq | sed s/^0*//