PKCS 7—An Overview

The low-level message functions adhere to RSA PKCS #7 Cryptographic Message Syntax Standard, published by RSA Laboratories, a division of RSA Data Security, Inc. The specification is available on their web site, http://www.rsa.com. To most effectively use the low-level message functions, you should become familiar with this specification. A few of its definitions are highlighted here.

The PKCS #7 standard describes a general syntax for data that may have cryptography applied to it, such as digital signatures and digital envelopes. The syntax admits recursion, so that, for example, one envelope can be nested inside another, or one party can sign digital data that has already been put into an envelope. It also allows arbitrary attributes, such as signing time, to be authenticated along with the content of a message. Further, it provides for other attributes, such as countersignatures, to be associated with a signature.

The type of data contained in a PKCS #7 message is called its content type. There are two classes of content types—Base and Enhanced.

• Base. Content types in this class contain only data with no cryptographic enhancements. Presently there is only one content type in this class, the Data content type.
• Enhanced. Content types in this class contain data of some type (possibly encrypted), and other cryptographic enhancements (such as hashes or signatures).

The content in the Enhanced class employs encapsulation, giving rise to the terms, outer content (the one containing the enhancements) and inner content (the one being enhanced). For example, an Enhanced class might contain the Data content type (Base class) that has a signature included with it. In this case, the Data content type is the inner content and the combination of the Data content type and the signature forms the outer content.

The content types defined in PKCS #7 standard are:

• Data. An octet (BYTE) string.
• Signed Data. Consists of content of any type and encrypted message digests of the content for zero or more signers.
• Enveloped Data. Consists of encrypted content of any type and encrypted content-encryption keys for one or more recipients. The combination of encrypted content and encrypted content-encryption key for a recipient is a digital envelope for that recipient.
• Signed-and-Enveloped Data. Consists of encrypted content of any type, encrypted content-encryption keys for one or more recipients, and doubly encrypted message digests for one or more signers. The double encryption consists of an encryption with a signer's private key followed by an encryption with the content-encryption key.
• Digested Data. Consists of content of any type and a message digest of the content.

  Note  Throughout the PKCS #7 specification, the terms digest and digested are used to refer to the value produced from applying a hashing algorithm to data. Throughout this manual we have used the terms "hash" and "hashed" for the same purpose. For example, the data type used with the low-level message functions is called hashed rather than digested. For the purposes of this document the two sets of terms may be used interchangeably.

• Encrypted Data. Consists of encrypted content of any type. Unlike the enveloped-data content type, the encrypted-data content type has neither recipients nor encrypted content-encryption keys. Keys are assumed to be managed by other means.