Chapter 20: IP Security

IPsec - a protocol suite that secures IP communications through authentication, encryption, and key management; supports IPv4 and IPv6.

• Applications:

  • Secure branch office connectivity.

  • Secure remote access for employees/telecommuters.

  • Extranet/intranet partnerships.

  • Enhancing e-commerce security.

• Key Features:

  • Encrypts/authenticates all IP-level traffic.

  • Transparent to end users and applications.

  • Protects routing architecture and communications.

• Benefits:

  • Unified security through firewalls/routers.

  • Resistance to bypass and scalable to organizational needs.

Transport Mode: Secures the payload of IP packets (e.g., between two hosts).

Tunnel Mode: Secures the entire IP packet for scenarios like VPNs.

IP Security Policy Key Concepts:

1. Security Associations (SAs): A one-way logical connection that provides security services (e.g., authentication, confidentiality) to the traffic it carries; Logical separation between different flows of traffic, enabling selective application of security services; SAs support either transport or tunnel mode; 2 SAs are required for bidirectional communication between two entities.

• Uniquely Identified By:

  1. Security Parameters Index (SPI):

     • A 32-bit unique identifier assigned to each SA.

     • Carried in the header of AH or ESP to help the receiver identify the associated SA.

  2. IP Destination Address:

     • Specifies the endpoint of the SA, such as a firewall, router, or host system.

  3. Security Protocol Identifier:

     • Indicates the protocol used for the SA (e.g., AH, ESP).

2. Security Association Database (SAD): A database maintained by the system, containing parameters for active SAs. Each entry in the SAD represents a single SA.

• Key Parameters in Each SAD Entry:

  1. SPI:

     • A unique value assigned by the receiving end of an SA to identify the incoming packet.

  2. Sequence Number Counter:

     • Tracks the sequence number used in the SA, helping detect replayed packets.

  3. Anti-Replay Window:

     • Stores information about received packets to reject duplicates within a predefined window size (default is 64 packets).

  4. Authentication Information:

     • Includes algorithms, keys, and lifetimes used for integrity checks (if AH is active).

  5. Encryption Information:

     • Includes algorithms, keys, and initialization vectors used for confidentiality (if ESP is active).

  6. Lifetime of the SA:

     • Specifies how long the SA remains valid, measured either in time or the number of bytes processed.

  7. Path MTU (Maximum Transmission Unit):

     • Tracks the maximum packet size that can be transmitted without fragmentation.

  8. IPsec Protocol Mode:

     • Specifies whether transport, tunnel, or a wildcard mode is used.

• Purpose:

  • Provides the necessary parameters to process and protect IP traffic for a specific SA.

3. Security Policy Database (SPD): A filtering database that specifies the security policies for outbound and inbound IP traffic.

• Key Elements in Each SPD Entry:

  1. Selectors:

     • Define the subset of traffic that the SPD entry applies to, using fields like:

       • Remote and local IP addresses (ranges or wildcards supported).

       • Next-layer protocol (e.g., TCP, UDP, ICMP).

       • Remote and local ports (single, multiple, or wildcard values).

  2. Policy Action:

     • Specifies how to handle traffic:

       • DISCARD: Drop the packet.

       • BYPASS: Allow the packet without IPsec processing.

       • PROTECT: Apply IPsec processing (authentication/encryption) using the corresponding SA.

• Purpose:

  • Maps traffic to applicable SAs based on selectors.

  • Ensures that only authorized traffic is protected or bypassed, according to security policies.

Processing Flow

1. Outbound Packets

1. Match SPD Entry:

   • Check SPD for a matching entry based on packet attributes (e.g., IP addresses, protocol, port).

2. Apply Policy:

   • If DISCARD, drop the packet.

   • If BYPASS, forward the packet without IPsec processing.

   • If PROTECT, proceed to the SAD to find a matching SA.

3. Process Packet:

   • If no SA exists in the SAD:

     • Invoke Internet Key Exchange (IKE) to establish an SA.

   • Apply security mechanisms (e.g., encryption/authentication) and forward the packet.

2. Inbound Packets

1. IPsec Header Check:

   • Determine whether the packet contains AH or ESP headers or is unprotected.

2. Match SPD Entry:

   • If unprotected, match with the SPD entry to decide whether to BYPASS or DISCARD the packet.

3. Match SAD Entry:

   • If protected, check the SPI in the AH or ESP header to find a matching entry in the SAD.

   • If no match, discard the packet.

4. Process Packet:

   • Apply decryption/authentication based on the SAD parameters.

   • Pass the decrypted/authenticated packet to the next higher layer (TCP, UDP).

Encapsulating Security Payload (ESP)

• ESP Services:

  • Encryption for confidentiality.

  • Data origin authentication.

  • Connectionless integrity.

  • Replay attack protection.

Packet Structure: Fields include Security Parameters Index (SPI), sequence number, payload data, padding, and optional integrity check value (ICV).

2 Modes:

1) Transport Mode: Encrypts payload; IP header is left unprotected.

2) Tunnel Mode: Encrypts entire packet; encapsulated with a new IP header.

Encryption: Provides confidentiality by converting plaintext data into unreadable ciphertext.

• Algorithms:

  1. AES (Advanced Encryption Standard):

     • Symmetric encryption.

     • Operates in block modes like CBC (Cipher Block Chaining).

     • Key lengths: 128, 192, or 256 bits.

  2. 3DES (Triple DES):

     • Symmetric encryption.

     • Applies DES encryption three times for higher security.

     • Slower compared to AES.

  3. Authenticated Encryption with GCM (Galois/Counter Mode):

     • Combines encryption and authentication for better performance.

     • Commonly used in modern IPsec implementations.

AuthenticationEnsures data integrity and origin authenticity.

• Algorithms:

  1. HMAC-SHA:

     • Combines a cryptographic hash (SHA-1, SHA-2) with a secret key.

     • Generates a secure message authentication code.

  2. CMAC (Cipher-based MAC):

     • A block cipher-based algorithm providing authentication.

     • Often used with AES.

Padding: Ensures data aligns with encryption block sizes; maintains security by hiding actual data lengths.

• How It Works:

  1. Adds bytes of padding to meet the block size required by the encryption algorithm.

  2. Includes a padding length field to indicate how many bytes were added.

• Example: For AES with a block size of 128 bits (16 bytes), a packet of 14 bytes will need 2 bytes of padding.

Anti-Replay Service: Prevents attackers from capturing and re-sending packets to disrupt or manipulate communications.

• How It Works:

  • Sequence Numbers:

    • Each packet has a unique sequence number within the Security Association (SA).

    • If a sequence number repeats or falls outside the anti-replay window (default size is 64 packets), the packet is discarded.

  • Sliding Window Mechanism:

    • The receiver tracks recently received packets within a moving range.

    • Old or duplicate packets outside the window are rejected.

Transport Mode: Provides end-to-end protection between two hosts.

• How It Works:

  • Encrypts the IP packet payload (e.g., TCP or UDP segments).

  • Leaves the IP header intact for routing.

• Use Case: Secure communication between two endpoints without intermediary network protection.

Tunnel Mode: Provides protection for entire IP packets, often used for Virtual Private Networks (VPNs).

• How It Works:

  • Encapsulates the entire original IP packet (header + payload) inside a new IP packet.

  • A new IP header is added for routing.

• Use Case: Protects communication between gateways or between a host and a gateway (e.g., external users accessing internal networks securely).

Visual Guide Summary

1. Encryption and Authentication Algorithms:

   • Flow of plaintext → encryption → ciphertext → authentication.

   • Algorithms like AES and HMAC-SHA highlighted.

2. Padding:

   • Packet with added padding bytes and padding length field.

   • Alignment to encryption block size.

3. Anti-Replay Service:

   • Timeline showing packet sequence numbers within a sliding window.

   • Marked valid packets, rejected duplicates outside the window.

4. Transport vs. Tunnel Mode:

   • Transport: Original IP header + encrypted payload.

   • Tunnel: New IP header + encrypted original packet (header + payload).

Why Combining Security Associations?

- To provide multiple layers of protection like authentication and confidentiality.

- Combines Authentication Header (AH) and ESP.

• Methods:

  • Transport Adjacency: Uses multiple protocols sequentially without tunneling.

  • Iterated Tunneling: Layers multiple tunnel SAs for added security between endpoints.

• Scenarios:

  • Host-to-host (end-to-end encryption/authentication).

  • Gateway-to-gateway (e.g., VPN).

  • Mixed modes (host to gateway or gateway to gateway).

Internet Key Exchange (IKE) - Automates key management and SA setup for secure communications.

Key Features:

- Diffie–Hellman: Used for secure session key generation.

- Cookies: Thwart denial-of-service attacks by validating participants.

- Nonces: Protect against replay attacks.

IKEv2 Process:

- Initial exchanges:

1. Negotiate cryptographic algorithms and keys.

2. Establish an initial secure channel (IKE SA).

- Follow-up exchanges:

→ Establish additional SAs for traffic.

→ Handle informational updates (e.g., error notifications).

Cryptographic Suites

• Supported Algorithms:

  • Encryption: AES (VPN-B), 3DES (VPN-A).

  • Authentication: HMAC-SHA, CMAC.

  • Key Exchange: Diffie–Hellman groups with modular exponentiation or elliptic curves.

• Suite Examples:

  • VPN-A: Legacy corporate VPNs.

  • VPN-B: Stronger encryption for modern applications.

Key Takeaways and Problem Solving

• IPsec enhances security across various applications by integrating seamlessly with existing networks.

• Efficient policy management through SAD and SPD ensures granular control over traffic processing.

• Modes (transport/tunnel) offer flexibility for end-to-end or gateway-centric implementations.

• Integration of cryptographic protocols like ESP and IKE ensures data integrity and confidentiality.