1 Lecture 15: IPsec AH and ESP IPsec introduction: uses and modes IPsec concepts –security...

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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)

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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)

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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)

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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.)

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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

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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

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AH Format0 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 replayauthentication 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.

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Applying AH toIPv4 Header0 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

• mutable field –ToS, flags, frag.offset, TTL, checksum• mutable, predictable – destination address

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

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Encapsulating Security Payload (ESP)

• provides (optional) encryption and integrity protection

ESP header0 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)

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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

1 Lecture 15: IPsec AH and ESP IPsec introduction: uses and modes IPsec concepts –security...

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