PKCS7_sign
p7 = PKCS7_sign(scert, skey, NULL, in, flags);
p7 = PKCS7_sign(NULL, NULL, NULL, in, PKCS7_STREAM | PKCS7_PARTIAL);
PKCS7 *PKCS7_sign(X509 *signcert, EVP_PKEY *pkey, STACK_OF(X509) *certs,
pkcs7 = PKCS7_sign(NULL, NULL, NULL, bio, PKCS7_BINARY | PKCS7_PARTIAL);