Under Seal

Switch to End Device with Dynamic Key Distribution

The supplicant needs a MACsec profile. You can create this in the associated profile editor for the Cisco Secure Client by selecting MKA as the key management approach in the Networks/Security area and the encryption algorithm for MACsec. This setting must be compatible with the switch and be configured symmetrically. In this example, I choose AES128-GCM. Next, import the profile on the client. The authenticator (switch) requires a little more manual work: First, you need to define the RADIUS servers with IP addresses, ports, and a shared secret. Second, reference them in the settings for authentication, authorization, and accounting (AAA) and enable IEEE 802.1X globally if you have not already done so.

Additionally, you need a MACsec-specific MKA policy. The key server priority is set in the policy. Also, you need to specify the encryption algorithm to match that of the supplicant (i.e., AES128-GCM for the example in Listing 1) and set the Confidentiality Offset to 0 bytes in the MKA policy. This setting tells MACsec to encrypt the whole frame. Where switches are coupled, a certain portion of the frame could remain unencrypted (e.g., to allow virtual local area network (VLAN) tags to be evaluated for transit components) (Figure 3). Replay protection defines the extent to which the frame order can deviate from the regular order.

radius server macsec1
  address ipv4 192.0.2.1 auth-port 1812 acct-port 1813
key t0ps3cr3t
!
radius server macsec2
  address ipv4 192.0.2.2 auth-port 1812 acct-port 1813
key t0ps3cr3t
!
aaa group server radius macsec
  server name macsec1
  server name macsec2
!
aaa new-model
aaa authentication dot1x default group macsec
aaa authorization network default group macsec
aaa accounting dot1x default start-stop group macsec
!
dot1x system-auth-control
!
mka policy ITA_MKA
  key-server priority 100
  macsec-cipher-suite gcm -aes-128
  confidentiality-offset 0
  replay-protection window-size 10
!
interface GigabitEthernet2/0/1
  description ITA_MACsec_Client
  switchport mode access
  switchport access vlan 10
  macsec
  authentication host-mode multi-auth
  authentication order dot1x
  authentication port-control auto
  dot1x pae authenticator
  authentication linksec policy must-secure
  mka policy ITA_MKA
  spanning-tree portfast

Figure 3: MACsec frame setup with encrypted VLAN tag and payload. In addition to classic Ethernet headers, the 802.1AE header (security tag) and an ICV are inserted.

Finally, set up the physical port, which is used to define both the legacy 802.1X settings and a reference to the MKA policy; MACsec is enabled, and a linksec policy with must-secure enforces encryption. On the authentication server, you first need to define an authorization profile with the must-secure policy, which you then reference in an authorization policy for the desired supplicants on the basis of their properties from the authentication.

Switch to Switch in Static CAK Mode

The setup for MACsec between two switches is somewhat simpler than dynamic key distribution in the case of a switch to end device. The configuration is the same on both switches except for the key server priority. First, define a keychain for MACsec with a key number, which acts as the CKN. Inside the key number, the CKN creates the algorithm for the key and the key string, which acts as the CAK. Second, define the MKA policy (which I will not explain again here). Finally, bind the MKA policy and the keychain as a pre-shared key and enable MACsec on the physical interface (Listing 2).

key chain ITA macsec
  key 1000
  cryptographic-algorithm aes-256-cmac
  key-string 12345678911234567890123456789012
!
mka policy ITA_MKA_Switch
  key-server priority 100
  macsec-cipher-suite gcm-aes-256
  confidentiality-offset 30
!
interface TenGigabitEthernet1/0/1
  description ITA_MACsec_Client
  switchport mode trunk
  macsec network-link
  mka policy ITA_MKA_Switch
  mka pre-shared-key key-chain ITA

Conclusions

MACsec cannot replace end-to-end encryption; however, when it comes to encrypting a defined link with high performance, MACsec is a very good choice. The same applies to securing Layer 2 protocols between client and switch or between two switches. Despite the latest revision in 2018, supplicant availability is still fairly low.