IPSec
A framework of security protocols and algorithms used to secure data at the network layer.
Authentication Header (AH):
Ensures data integrity.
Does not provide confidentiality.
Provides origin authentication.
Uses Keyed-hash mechanisms.
Encapsulating Security Payload (ESP):
Provides data confidentiality (encryption).
Data Integrity.
Does not protect IP header.
AH and ESP uses symmetric secret key algorithms, although public key algorithms are feasible.
Modes
Tunnel.
IPSec gateways provide IPSec services to other hosts in peer-to-peer tunnels.
End-hosts are not aware of IPSec being used to protect their traffic.
IPSec gateways provide transparent protection over untrusted networks.
Transport.
Each end host does IPSec encapsulation of its own data, host-to-host.
IPSec has to be implemented on end-hosts.
The application endpoint must also be the IPSec endpoint.
Header placement
AH
ESP
AH Header
Contains five mandatory fields:
The Next Header field is an 8-bit field that identifies the type of the next payload after the AH.
The Payload Length is an 8-bit field specifying the length of the header (excluding the first 8 bytes) in 4-byte units.
The SPI field contains the negotiated outbound IPsec SPI and is used by the remote peer to identify the SA to which the packet belongs.
The Sequence Number field is a 32-bit field that contains a counter value that increases by one for each sent packet (using the same outbound IPsec SA).
The ICV field has a variable length (multiple of 32 bits) that contains the output of the authentication hash function (or HMAC based on symmetric encryption algorithms) applied to data/headers under protection.
May include padding to ensure that the overall length of the AH header is a multiple of 32 bits in IPv4 or 64 bits in IPv6.
ESP Header and Trailer
Contain five mandatory fields:
The SPI field contains the negotiated outbound IPsec SPI and is used by the remote peer to identify the SA to which the packet belongs.
The Sequence Number field is a 32-bit field that contains a counter value that increases by one for each sent packet (using the same outbound IPsec SA).
The Padding field may contain 0 to 255 zero-bytes to guarantee: (i) a specific payload size imposed by the encryption algorithm (e.g., size multiple of the block cipher size), and (ii) that the Pad Length and Next header fields are right aligned within a 4-byte word.
The Pad Length is an 8-bit field that indicates the number of padding bytes in the Padding field.
The Next Header is an 8-bit field that identifies the type of data contained in the payload data.
May contain two optional fields:
When the encryption algorithm requires an explicit Initialization Vector (IV), this value is sent using the IV field.
Some algorithm modes combine encryption and integrity into a single operation.
The ICV field has a variable length that contains the output of the authentication hash function (or HMAC based on symmetric encryption algorithms) applied to the ESP header, Payload Data, and ESP trailer fields.
The ICV field may include padding.
Security Associations
SAs represent a policy contract between two peers or hosts.
Describe how the peers will use IPSec security services to protect network traffic.
An SA contains the following security parameters:
Authentication/encryption algorithm, key length, and other encryption parameters (e.g. key lifetime, ...).
Session keys for authentication, or HMACs, and encryption, which can be entered manually or negotiated automatically.
A specification of network traffic to which the SA will be applied (e.g. IP traffic or only TELNET sessions).
IPSec AH or ESP encapsulation protocol and tunnel or transport mode.
Last updated