Algorithm Identifier ASN.1 Structure

The ALGORITHM_IDENTIFIER in PKCS#7 (and CMS—Cryptographic Message Syntax) is an ASN.1 (Abstract Syntax Notation One) structure that describes the algorithm used for cryptographic operations, such as signing, encryption, or hashing.

Format of ALGORITHM_IDENTIFIER

ALGORITHM_IDENTIFIER ::= SEQUENCE {
algorithm        OBJECT IDENTIFIER,
parameters       ANY DEFINED BY algorithm OPTIONAL
}

Explanation of Fields

  1. algorithm:
    • Type: OBJECT IDENTIFIER (OID)
    • Description: Identifies the algorithm used (e.g., RSA, DSA, SHA256). The OID points to a standard or custom-defined algorithm.
    • Example: 1.2.840.113549.1.1.11 (OID for SHA256withRSA).
  2. parameters:
    • Type: ANY
    • Description: Contains any optional parameters required by the algorithm. This field is optional and depends on the algorithm used. For example, in case of RSA encryption, this might be empty, but for EC (Elliptic Curve) algorithms, it could contain curve parameters.
    • Example: For RSA, it might be empty, but for EC algorithms, it could contain the curve parameters in DER-encoded format.

Example (for RSA Signature with SHA256)

ALGORITHM_IDENTIFIER ::= SEQUENCE {
algorithm        OBJECT IDENTIFIER {1 2 840 113549 1 1 11},   -- SHA256withRSA
parameters       NULL
}

Key Points

  • algorithm specifies which cryptographic algorithm is used.
  • parameters holds additional algorithm-specific information, if needed.

Summary:

The ALGORITHM_IDENTIFIER ASN.1 structure in PKCS#7/CMS defines the cryptographic algorithm (via an OID) and optionally its parameters, used for signing, encryption, or hashing in the PKCS7 message format.