IPsec Overview
Configuring IPsec
page 18-6
OmniSwitch AOS Release 7 Network Configuration Guide
June 2013
Unlike AH which only authenticates the data, ESP encrypts data and also optionally authenticates it. It
provides these services by encrypting the original payload and encapsulating the packet between a header
and a trailer, as shown in the figure below.
IP Packet protected by ESP
ESP is identified by a value of 50 in the IPv6 header. The ESP header is inserted after the IPv6 header and
before the upper layer protocol header. The Security Parameter Index (SPI) in the ESP header is a 32-bit
value that, combined with the destination address and protocol in the preceding IPv6 header, identifies the
security association (SA) to be used to process the packet. SPI helps distinguish multiple SA’s configured
for the same source and destination combination. The payload data field carries the data that is being
encrypted by ESP. The Authentication digest in the ESP header is used to verify data integrity. Authenti-
cation is always applied after encryption, so a check for validity of the data is done upon receipt of the
packet and before decryption.
Encryption Algorithms
There are several different encryption algorithms that can be used in IPsec. However, the most commonly
used algorithms are “AES” and “3DES”. These algorithms are used for encrypting IPv6 packets.
•
Advanced Encryption Standard - Cipher Block Chaining - (AES-CBC)
The AES-CBC mode comprises three different key lengths; AES-128, AES-192 and AES-256. Each block
of plaintext is XOR'd with the previous encrypted block before being encrypted again.
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Triple DES (3DES)
A mode of the DES encryption algorithm that encrypts data three times. Three 64-bit keys are used,
instead of one, for an overall key length of 192 bits (the first encryption is encrypted with second key, and
the resulting cipher text is again encrypted with a third key). 3DES is a more powerful version of DES.
Authentication Header (AH)
An Authentication Header (AH) provides connectionless integrity and data origin authentication. This
protocol permits communicating parties to verify that data was not modified in transit and that it was genu-
inely transmitted from the apparent source. AH helps verify the authenticity/integrity of the content and
origin of a packet. It can optionally protect against replay attacks by using the sliding window technique
and discarding old packets. It authenticates the packet by calculating the checksum via hash-based
message authentication code (HMAC) using a secret key and either HMAC-MD-5 or HMAC-SHA1 hash
functions.
16
24
32-bit
Security association identifier (SPI)
Sequence Number
Payload data (variable length)
Padding (0-255 bytes)
Pad Length
Next Header
Authentication Data (variable)