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Authentication and encryption
Authentication algorithms
IPsec uses hash algorithms to perform authentication. A hash algorithm produces a fixed-length digest for
an arbitrary-length message. IPsec peers respectively calculate message digests for each packet. The
receiver compares the local digest with that received from the sender. If the digests are identical, the
receiver considers the packet intact and the sender's identity valid. IPsec uses the Hash-based Message
Authentication Code (HMAC) based authentication algorithms, including HMAC-MD5 and
HMAC-SHA1. Compared with HMAC-SHA1, HMAC-MD5 is faster but less secure.
Encryption algorithms
IPsec uses symmetric encryption algorithms, which encrypt and decrypt data by using the same keys. The
following encryption algorithms are available for IPsec on the device:
•
DES
—Encrypts a 64-bit plaintext block with a 56-bit key. DES is the least secure but the fastest
algorithm.
•
3DES
—Encrypts plaintext data with three 56-bit DES keys. The key length totals up to 168 bits. It
provides moderate security strength and is slower than DES.
•
AES
—Encrypts plaintext data with a 128-bit, 192-bit, or 256-bit key. AES provides the highest
security strength and is slower than 3DES.
IPsec implementation
To implement IPsec protection for packets between two peers, complete the following tasks on each peer:
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Configure an IPsec policy, which defines the range of packets to be protected by IPsec and the
security parameters used for the protection.
•
Apply the IPsec policy to an interface or an application.
When you apply an IPsec policy to an interface, you implement IPsec based on the interface. Packets
received and sent by the interface are protected according to the IPsec policy. When you apply an IPsec
policy to an application, you implement IPsec based on the application. Packets of the application are
protected according to the IPsec policy, regardless of the receiving and sending interface of the packets.
IPsec protects packets as follows:
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When an IPsec peer identifies the packets to be protected according to the IPsec policy, it sets up
an IPsec tunnel and sends the packet to the remote peer through the tunnel. The IPsec tunnel can be
manually configured beforehand, or it can be set up through IKE negotiation triggered by the
packet. The IPsec tunnels are actually the IPsec SAs. The inbound packets are protected by the
inbound SA, and the outbound packets are protected by the outbound SA.
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When the remote IPsec peer receives the packet, it drops, de-encapsulates, or directly forwards the
packet according to the configured IPsec policy.
Interface-based IPsec supports setting up IPsec tunnels based on ACLs.
ACL-based IPsec
To implement ACL-based IPsec, configure an ACL to define the data flows to be protected, reference the
ACL in an IPsec policy, and then apply the IPsec policy to an interface. When packets sent by the
interface match the permit rule of the ACL, the packets are protected by the outbound IPsec SA and
encapsulated with IPsec. When the interface receives an IPsec packet whose destination address is the
IP address of the local device, it searches for the inbound IPsec SA according to the SPI carried in the
