Cisco Cisco IOS Software Release 12.4(23)
Appendix A—IPSec Operation
IPSec Security Associations
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Security Target For Cisco IOS IPSec
Figure 9
IPSec Encapsulating Security Payload
IPSec Security Associations
IPSec provides many options for performing network encryption and authentication. The TOE requires
encryption, integrity and authentication. When the security service is determined, the two
communicating nodes must determine exactly which algorithms to use (the TOE uses 3DES or AES for
encryption; and SHA-1 for integrity). After deciding on the algorithms, the two devices must share
session keys. The security association is the method that IPSec uses to track all the particulars
concerning a given IPSec communication session. A Security Association (SA) is a relationship between
two or more IPSec devices that describes how the entities will use security services to communicate
securely.
encryption, integrity and authentication. When the security service is determined, the two
communicating nodes must determine exactly which algorithms to use (the TOE uses 3DES or AES for
encryption; and SHA-1 for integrity). After deciding on the algorithms, the two devices must share
session keys. The security association is the method that IPSec uses to track all the particulars
concerning a given IPSec communication session. A Security Association (SA) is a relationship between
two or more IPSec devices that describes how the entities will use security services to communicate
securely.
An IPSec security association is unidirectional, meaning that for each pair of communicating IPSec
devices there are at least two security connections - one from A to B and one from B to A. The security
association is uniquely identified by a randomly chosen unique number called the security parameter
index (SPI) and the destination IP address of the destination. When a system sends a packet that requires
IPSec protection, it looks up the security association in its database, applies the specified processing,
and then inserts the SPI from the security association into the IPSec header. When the IPSec peer
receives the packet, it looks up the security association in its database by destination address and SPI
and then processes the packet as required.
devices there are at least two security connections - one from A to B and one from B to A. The security
association is uniquely identified by a randomly chosen unique number called the security parameter
index (SPI) and the destination IP address of the destination. When a system sends a packet that requires
IPSec protection, it looks up the security association in its database, applies the specified processing,
and then inserts the SPI from the security association into the IPSec header. When the IPSec peer
receives the packet, it looks up the security association in its database by destination address and SPI
and then processes the packet as required.
A special bi-directional SA, known as the IKE SA is used to establish and manage all IPSec SA’s.
IPSec Operation
Authentication
IKE creates an authenticated, secure tunnel between two IPSec entities (such as the TOE) called the IKE
SA, which is then used to negotiate the security associations for IPSec used to protect the packet flow.
This process requires that the two entities authenticate themselves to each other and establish shared
keys. IKE supports multiple authentication methods. The two entities must agree on a common
authentication protocol through a negotiation process. The following mechanisms are supported in the
TOE:
SA, which is then used to negotiate the security associations for IPSec used to protect the packet flow.
This process requires that the two entities authenticate themselves to each other and establish shared
keys. IKE supports multiple authentication methods. The two entities must agree on a common
authentication protocol through a negotiation process. The following mechanisms are supported in the
TOE:
Pre-shared key
The same key is pre-installed on each device. IKE peers authenticate each other by computing and
sending a keyed hash of data that includes the preshared key. If the receiving peer is able to
independently create the same hash using its preshared key, it knows that both parties must share the
same secret, thus authenticating the other party
sending a keyed hash of data that includes the preshared key. If the receiving peer is able to
independently create the same hash using its preshared key, it knows that both parties must share the
same secret, thus authenticating the other party
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IP
header
Data
IP
header
Data
Encrypted
New IP
header
ESP
header