PKCS#7 Verification program 
===========================

This is a slightly modified version of OpenSSL's verify command. I've added
some options to tell the program where to find the CA's certificate to
verify the signature.

We only need the CA's certificate because the PKCS#7 signature includes the
signing certificate.

The command sintax is: 
verify signature_file.pkcs7 -d data_signed.txt [-cf certsfile | -cd certsdir ] 

- signature_file.pkcs7 is the file containing the PKCS#7 signature. 
  It MUST be delimited by: 
-----BEGIN PKCS7-----
signature.....
-----END PKCS7-----

- The -d option indicates the file that contains the signed data (the data
  we want to verify)

- Options -cf and -cd tells the program where to find the CA's certificate.
  If we use -cf we have to give a SINGLE FILE containing the CA certificate[s]
  This is the common options if we use only one or a small amount of CAs.
  Option -cd gives a dDIRECTORY where we have the CA's certificates each in
  a single file. If we use this options, we have to make the hash links to
  each cert.

  Usually, option -cf is enough and simpler...
  
