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Lecture 15: IPsec AH and ESP

Lecture 15: IPsec AH and ESP. IPsec introduction: uses and modes IPsec concepts security association security policy database IPsec headers authentication header (AH) encapsulating security payload (ESP). IPsec. IETF standard for real-time communication security

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Lecture 15: IPsec AH and ESP

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  1. Lecture 15: IPsec AH and ESP • IPsec introduction: uses and modes • IPsec concepts • security association • security policy database • IPsec headers • authentication header (AH) • encapsulating security payload (ESP)

  2. IPsec • IETF standard for real-time communication security • designed to work with both IPv4 and IPv6 • Cryptographic protection of the IP traffic (level 3), transparent to the user • main components: • Internet Key Exchange (IKE): IPsec key exchange protocol • Authentication Header (AH): Authentication of the IP packet (optional) • Encapsulating Security Payload (ESP): Encryption/authentication of the IP packet (optional)

  3. Modes and Uses of IPsec • granularity of protection: user, host or network • typical uses: • remote access to network (host-to-gateway) • virtual private networks (gateway-to-gateway) • protocol modes: • transport mode: host applies IPsec to transport layer packet • tunnel mode: gateway applies IPsec to the IP packet of a host from the network (IP in IP tunnel)

  4. Security Association & Policy • Security Association (SA) – IPsec-protected connection (one-way) • specifies the encryption/auth. algorithm, key, etc. • established either “manually” or through IKE • identified by • security parameter index (SPI) • destination IP address • protocol identifier: (AH or ESP – more later) • Security Policy Database – specifies what kind of protection is applied to packets • matches IP packet to SA at source and destination, indexed by SPI • contains • AH information (auth. algorithm, key, key lifetime, etc.) • ESP information (auth./encryption algorithm, key, key lifetime, etc.) • sequence number counter • anti-replay window (at the destination SA) • lifetime of the SA • others (protocol mode, path MTU, etc.)

  5. IPsec Packet Processing Outbound packets: • The proper SA is chosen from the security policy database • From the SA database, the SPI and SA parameters are retrieved • The IPsec protection is performed; packet passed to IP Inbound packets: • By the SPI, the SA is found • IPsec auth./decryption is performed • Packet passed to upper layer protocol application TCP IPsec lower layers

  6. Authentication Header (AH) • provides (optional) integrity protection (authentication) • for packet • some of the fields of IP header • the integrity check is provided by a cryptographic hash • MAC is included in the header • some fields of IP header are modified in transit hence cannot be covered by integrity check; these fields differ in IPv4 and IPv6 • mutable – modifiable in transit, not protected • immutable – constant, protected • mutable but predictable – changes but in a predictable manner – protected with values at receiver

  7. AH Format 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Next Header | Payload Len | RESERVED | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Security Parameters Index (SPI) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sequence Number Field | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + Authentication Data (variable) | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ headers • next header – following header in sequence • payload length – header size (to determine authentication data size) • sequence number – to prevent replay authentication issues • auth. alg.: HMAC (with MD5, SHA1, etc.) CBC-MAC (3DES, RC5, AES, etc.) • typically, IV is included in the payload • authentication covers immutable fields of IP header as well as the payload.

  8. Applying AH toIPv4 Header 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Version| IHL |Type of Service| Total Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identification |Flags| Fragment Offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Time to Live | Protocol | Header Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ BEFORE APPLYING AH ---------------------------- IPv4 |orig IP hdr | | | |(any options)| TCP | Data | ---------------------------- AFTER APPLYING AH --------------------------------- IPv4 |orig IP hdr | | | | |(any options)| AH | TCP | Data | --------------------------------- |<------- authenticated ------->| except for mutable fields • mutable field –ToS, flags, frag.offset, TTL, checksum • mutable, predictable – destination address

  9. Encapsulating Security Payload (ESP) • provides (optional) encryption and integrity protection ESP header 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---- | Security Parameters Index (SPI) | ^Auth. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Cov- | Sequence Number | |erage +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ---- | Payload Data (variable) | | ^ ~ ~ | | | | |Conf. + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Cov- | | Padding (0-255 bytes) | |erage +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | Pad Length | Next Header | v v +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------ | Authentication Data (variable) | ~ ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ • Encryption: usually a block cipher in CBC mode • IV is typically included in the payload (not encrypted)

  10. ESP with IPv4 BEFORE APPLYING ESP ---------------------------- IPv4 |orig IP hdr | | | |(any options)| TCP | Data | ---------------------------- AFTER APPLYING ESP ------------------------------------------------- IPv4 |orig IP hdr | ESP | | | ESP | ESP| |(any options)| Hdr | TCP | Data | Trailer |Auth| ------------------------------------------------- |<----- encrypted ---->| |<------ authenticated ----->| • ESP puts MAC after the payload, why?

  11. AH vs. ESP issues with AH • usefulness is doubtful (authentication is provided by ESP as well) • not efficient to compute (MAC is at the beginning) • wasted (unused) octets in the header • not compatible with network address translation (NAT) which modifies IP address • the need for immutable fields protections is unclear • ESP in tunneling mode can do it

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