CMS/PKCS7 message functions.

CMS is defined in RFC 3369 and is an continuation of the RSA Labs standard PKCS7. The basic messages in CMS is

- SignedData Data signed with private key (RSA, DSA, ECDSA) or secret (symmetric) key
- EnvelopedData Data encrypted with private key (RSA)
- EncryptedData Data encrypted with secret (symmetric) key.
- ContentInfo Wrapper structure including type and data.

See the library functions here: hx509 CMS/pkcs7 functions