Access Control with Attribute Certificates

TL;DR

This guide shows you how to use attribute certificates (ACs) to control access based on a subject-object-action model. We’ll cover creating ACs, verifying them, and using them in a simple access control system.

1. Understanding Subject-Object-Action

Before diving into ACs, let’s define the core concept: Subject-Object-Action (SOA). This model describes who (Subject) can do what (Action) to which resource (Object).

• Subject: The user or process requesting access.
• Object: The resource being accessed (e.g., a file, database record, service).
• Action: What the subject wants to do with the object (e.g., read, write, execute).

ACs help us express these permissions in a secure and verifiable way.

2. Setting up Your Environment

You’ll need tools for creating and verifying ACs. OpenSSL is a common choice. Ensure you have it installed on your system.

3. Creating an Attribute Certificate (AC)

1. Create a Configuration File: This file defines the attributes you want to include in your AC. For example, ac_config.txt:

   
   [req]
   prompt = no
   distinguished_name = dn
   
   [dn]
   CN = Attribute Authority
   
   [ca]
   default_days = 3650
   default_md = sha256
   policy = policy_match
   
   [policy_match]
   match.PolicyIdentifier = "1.2.3.4.5"
   

2. Generate a Private Key for the Attribute Authority (AA): This key is used to sign ACs.

   openssl genrsa -out aa.key 2048

3. Create a Certificate Signing Request (CSR) for the AA:

   openssl req -new -key aa.key -config ac_config.txt -out aa.csr

4. Self-Sign the AA Certificate:

   openssl x509 -req -days 3650 -in aa.csr -signkey aa.key -out aa.crt

5. Create an AC for a Subject: This is where you define the permissions.

   openssl ca -config ac_config.txt -extensions att_cert -days 365 -notext -md sha256 -in subject.req -out subject.ac

Important: You’ll need to create a subject.req file for each user/subject you want to issue an AC to. This request will contain information about the subject and their desired attributes (e.g., role, department).

4. Verifying the Attribute Certificate

1. Verify the Signature: Ensure the AC hasn’t been tampered with.

   openssl verify -CAfile aa.crt subject.ac

2. Extract Attributes: Use OpenSSL to read the attributes from the AC.

   openssl x509 -in subject.ac -text -noout | grep "Attribute"

5. Implementing Access Control

1. Access Request: When a user requests access to an object, they present their AC.
2. Verification: Verify the AC’s signature using the AA’s certificate.
3. Attribute Check: Extract the attributes from the verified AC and compare them against the required permissions for the requested action on the specific object.
   • If the user has the necessary attributes, grant access.
   • Otherwise, deny access.

Example (Simplified Python):

import subprocess

def verify_access(ac_file, required_attributes):
    # Verify signature...
    result = subprocess.run(['openssl', 'verify', '-CAfile', 'aa.crt', ac_file], capture_output=True, text=True)
    if result.returncode != 0:
        return False

    # Extract attributes (simplified - use a proper parsing library in production!)
    attributes = subprocess.run(['openssl', 'x509', '-in', ac_file, '-text', '-noout'], capture_output=True, text=True).stdout
    user_attributes = [attr for attr in attributes.splitlines() if "Attribute" in attr]

    # Check if required attributes are present...
    for attr in required_attributes:
        if attr not in user_attributes:
            return False

    return True

6. Considerations

• Attribute Authority Security: The AA’s private key must be protected at all costs.
• Revocation: Implement a mechanism to revoke ACs if they are compromised or no longer valid. Attribute Certificate Revocation Lists (ACRLs) can be used for this purpose.
• Scalability: For large systems, consider using a dedicated attribute authority service and caching mechanisms.