Features
   * Added partial support for parsing the PKCS7 cryptographic message syntax,
     as defined in RFC 2315. Currently, support is limited to the following:
     - Only the signed data content type, version 1 is supported.
     - Only DER encoding is supported.
     - Only a single digest algorithm per message is supported.
     - Only 0 or 1, certificate is supported per message, which must be in
       X509 format.
     - There is no support for certificate-revocation lists.
     - The authenticated and unauthenticated attribute fields of SignerInfo
       must be empty.
     Many thanks to Daniel Axtens, Nayna Jain, and Nick Child from IBM for
     contributing this feature.