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Cryptographic Key Management

  • Definition: The secure use of cryptographic key algorithms depends on the protection of cryptographic keys.

  • Key Protection: All keys must be protected against modification, and secret/private keys must be safeguarded against disclosure.

  • Process: Cryptographic key management involves the generation, protection, storage, exchange, replacement, and usage of keys.

  • Monitoring: Includes monitoring and recording of each key's access, usage, and context.

  • Key Management System: Comprises key servers, user procedures, and protocols.

Symmetric Key Distribution Technique

  • Key Distribution: The means of delivering a key to two parties securely without allowing others to see the key.

  • Frequent Key Changes: Desirable to limit potential data compromise if an attacker gains access to the key.

Symmetric Key Distribution Options

  • Methods for A and B:

    • A selects a key and delivers it to B.

    • A third party selects the key and delivers it to both A and B.

    • A party transmits a new key to the other, encrypted with the old key.

    • A third-party Key Distribution Center (KDC) can deliver a key over encrypted links.

Scale of Symmetric Key Distribution

  • Endpoints: If n endpoints (e.g., users, processes) are needed, each requires n – 1 secret keys.

  • Key Delivery: Requires O(n²) courier deliveries to distribute keys for n endpoints.

  • Lifecycle: Keys have an expiry date and need periodic key rollover; managing large networks (e.g., 1000 nodes) can require up to half a million keys.

Key Distribution Methodologies

Key Translation Center (KTC)

  • A KTC transfers symmetric keys between two entities, managing encryption for communication:

    • A generates a session key.

    • A encrypts this key with a master key for the KTC.

    • The KTC manages the decryption and re-encryption of session keys for secure delivery.

Key Distribution Center (KDC)

  • A KDC generates and distributes session keys:

    • A requests a key from the KDC.

    • The KDC securely encrypts and distributes the session key to both parties.

  • This centralizes key management and makes distribution more manageable.

Hierarchical Key Distribution

  • Key Hierarchy: Higher-level keys are used infrequently to prevent cryptanalysis, encrypting more frequently used lower-level keys.

  • Ephemeral Keys: Short-lived keys used for single transactions to enhance security.

Asymmetric Encryption for Secret Key Distribution

  1. Initial Setup: A communicates with B using a public/private key pair.

  2. Session Key Generation: B generates a temporary session key and sends it encrypted to A.

  3. Post-Session: Both A and B discard keys after the session.

Public Key Distribution Techniques

  • Techniques for distributing public keys include:

    1. Public Announcement: Participants share keys widely but are prone to forgery.

    2. Publicly Available Directory: Managed by a trusted authority; risks if authority is compromised.

    3. Public-Key Authority:

      • A requests B's public key from the authority.

      • Authority exchanges keys, which can become a bottleneck and vulnerable.

    4. Public-Key Certificates: Issued by a Certification Authority (CA), enhancing both security and verifiability.

X.509 Certificates

  • Overview: Defines authentication services and is widely adopted for formatting public-key certificates.

    • Originally issued in 1988, with significant revisions, emphasizing RSA.

X.509 Certificate Structure

  • Fields include version, serial number, signature algorithm, issuer name, period of validity, and subject name.

  • Extensions in Version 3: Custom additional fields for enhanced functionality.

Revocation of Certificates

  • Certificates typically have a set validity period; a new certificate is issued before the old one expires.

  • Reasons for revocation include private key compromise or if the user is no longer certified.

Example of Certificate Compromise

  • Microsoft Security Bulletin MS01-027: Incident where counterfeit certificates were issued, posing security risks to Microsoft users, enabling attackers to sign harmful code.