lightweight rsa signature verification using openssl for keygeneration, 
and 'dc' for the rsa calculations.

AUTHOR: itsme@xs4all.nl

FILES:

create_new_key.sh 
sign_file.sh
verify_file.sh
sha256.c

EXAMPLE:

** first compile the file hashing tool
bash$ gcc sha256.c -o sha256

** then generate a new key
bash$ ./create_new_key.sh  512
Generating RSA private key, 512 bit long modulus
...............++++++++++++
..............++++++++++++
e is 65537 (0x10001)

** this creates 2 files:
bash$ ls -l key.*
-rw-rw-rw-  1 itsme None 129 May 31 00:41 key.pub
-rw-rw-rw-  1 itsme None 497 May 31 00:41 key.pvk

** how to sign a file:
bash$ ./sign_file.sh README

** this creates a .sig file:
bash$ cat README.sig
1B343DB8A0B60625BDD863EA31559951464D22F3......

** to verify a file, file.sig, key.pub combination:
bash$ ./verify_file.sh  README

** create an invalid signature:
bash$ echo 12345 > README.sig

** see how it fails:
bash$ ./verify_file.sh  README
signature mismatch
bash$