Overview

Every Cryptographic Message Syntax (CMS) file has two main attributes from the get go: contentType and content.

contentType can be stuff like id-signedData, id-envelopedData, id-encryptedData, etc.

The structure of content depends on the value of contentType.

Within content you'll have a bunch of fields specific to the contentType and either encapContentInfo or encryptedContentInfo. Both of these "content info" fields, in turn, have their own contentType and content fields and within those fields, content is basically akin to a single file.

So like if you wanted to sign multiple files you'd need multiple CMS's, one for each file. Likewise, if you wanted to sign and then encrypt you'd need to manually nest one CMS inside the other.

Reading a CMS

All CMS's are read basically the same way:

use phpseclib4\File\CMS;

$cms = CMS::load(file_get_contents('sample.p7m'));

print_r($cms);

At this point $cms can be an instance of any of the following classes:

• CMS\SignedData
• CMS\EncryptedData
• CMS\DigestedData
• CMS\CompressedData

Each of those classes have their own unique methods (and some common ones, as well).

If you're only dealing with CMS\SignedData then you'll probably want to add an if statement into the code. eg.

if (!$cms instanceof CMS\SignedData) {
    throw new UnexpectedValueException('Expected CMS\\SignedData');
}