IPSec using Security Agreement (in 3GPP R5)

In this post I will try to explain how security mechanisms are negotiated between an IMS Client and the Proxy CSCF (only SIP headers) and how to setup SAs using Linux IPSec tools.
The main purpose of Security agreement is to agree on which mechanisms, algorithms or security parameters to use.

There are five main mechanisms used in VoIP networks:

1. digest
2. tls
3. ipsec-ike
4. ipsec-man
5. ipsec-3gpp

We will focus on 3GPP IPSec because the IMS makes it mandatory.

The security mechanism to use is known after the negotiation between the IMS Client and the Proxy CSCF succeeds. This negotiation is performed during the IMS registration and authentication procedures.

Three new SIP header fields have been defined, namely Security-Client, Security-Server and Security-Verify.

Call Flow

IMS Client         P-CSCF              S-CSCF

|                    |                  |

|----(1)REGISTER---->|                  |

|                    |                  |

|                    |---(2)REGISTER--->|

|                    |                  |

|                    |<-----(3) 401 ----|

|                    |                  |

|<----(4) 494/401----|                  |

|                    |                  |

|<==IPSec in place==>|                  |

|                    |                  |

|----(5)REGISTER---->|                  |

|                    |----(6)REGISTER-->|

|                    |                  |

|                    |<---(7) 200 OK----|

|<---(8) 200 OK------|                  |

|                    |                  |

In step (1) the IMS Client sends an unprotected registration request including the security-client header. The Client must indicate that it is able to negotiate security mechanism by adding “Require” and “Proxy-Require” headers. The security-client header includes two ports (client and server ports) that the client wants to negotiate with the proxy CSCF.

In step (2) the Proxy CSCF forwards the request to the Serving CSCF.

In step (3) the Serving CSCF (registrar) challenges the Proxy CSCF. The 401 response is sent to the Proxy CSCF (challenge parameters are under WWW-Authenticated header). The Serving CSCF must include the security-server header.

In step (4) the Proxy CSCF forwards the 401/494 response to the IMS Client. At this stage the Proxy CSCF opens the IPsec security association (SA) for the IMS Client. The IMS Client also setup a SA (this is a temporary SA).

The lifetime of the created SA (between the IMS Client and the Proxy CSCF) is equal to the value of reg-await-auth timer.

In step (5) the IMS Client sends a new registration (to the Proxy CSCF) request including its credentials and copies the content of security-server header to the security-verify header. Before forwarding the request to the Serving CSCF, the Proxy CSCF will check that the previous security-server header and the security-verify headers (added by the IMS Client) are the same. If these values are different, the Proxy CSCF sends an error message to the IMS Client and terminates the created SAs.

In step (6) the Proxy CSCF forwards the request to the Serving CSCF.

In step (7) the Serving CSCF authenticates the IMS Client, and responds with 200 OK.

In step (8) the Proxy CSCF forwards the response to the IMS Client. At this step new SAs will be created. The temporary SAs will be destroyed (or not) by the Proxy CSCF.

Messages

(1)
REGISTER sip:pcscf.open-ims.test SIP/2.0
Security-Client: ipsec-3gpp; alg=hmac-md5-96; spi-c=1111; spi-s=2222; port-c=5062; port-s=5064
Require: sec-agree
Proxy-Require: sec-agree

(4)
SIP/2.0 [494 Security Agreement Required / 401 Unauthorized]
Security-Server: ipsec-3gpp; q=0.1; alg=hmac-md5-96; spi-c=3333; spi-s=4444; port-c=5066; port-s=5068

(5)
REGISTER sip:pcscf.open-ims.test SIP/2.0
Security-Client: ipsec-3gpp; alg=hmac-md5-96; spi-c=1111; spi-s=2222; port-c=5062; port-s=5064
Security-Verify: ipsec-3gpp; q=0.1; alg=hmac-md5-96; spi-c=3333; spi-s=4444; port-c=5066; port-s=5068
Require: sec-agree
Proxy-Require: sec-agree

--> Captures done with Mercuro IMS Client.

Setting up SAs using Linux Tools

Here we suppose that:
- We are using Ubuntu (Linux Kernel 2.6 + KAME-tools)
- the Proxy-CSCF address is '192.168.0.10' and Mercuro IMS Client address is '192.168.0.11'
- for secure ports see above SIP capture
- protocol is esp
- algorithm is 'hmac-md5'
- encrypt-algorithm is 'des-ede3-cbc'
- mode is 'transport'
- confidentiality key is '123456789012123456789012' (see function f2345 in 3GPP milenage algorithms)
- integrity key is '1234567890123456' (see function f2345 in 3GPP milenage algorithms)

1. Install the tools

sudo apt-get install ipsec-tools

2. Edit /etc/ipsec-tools file and add the following script

#Incoming Requests [US <- PC]
spdadd 192.168.0.10/32[5066] 192.168.0.11/32[5064] udp -P in ipsec esp/transport//require;
add 192.168.0.10 192.168.0.11 esp 2222 -m transport -E des-ede3-cbc "123456789012123456789012" -A hmac-md5 "1234567890123456";

#Incoming Replies [UC <- PS]
spdadd 192.168.0.10/32[5068] 192.168.0.11/32[5062] udp -P in ipsec esp/transport//require;
add 192.168.0.10 192.168.0.11 esp 1111 -m transport -E des-ede3-cbc "123456789012123456789012" -A hmac-md5 "1234567890123456";

#Outgoing Requests [UC -> PS]
spdadd 192.168.0.11/32[5062] 192.168.0.10/32[5068] udp -P out ipsec esp/transport//unique:1;
add 192.168.0.11 192.168.0.10 esp 4444 -m transport -u 1 -E des-ede3-cbc "123456789012123456789012" -A hmac-md5 "1234567890123456";

#Outgoing Replies [US -> PC]
spdadd 192.168.0.11/32[5064] 192.168.0.10/32[5066] udp -P out ipsec esp/transport//unique:2;
add 192.168.0.11 192.168.0.10 esp 3333 -m transport -u 2 -E des-ede3-cbc "123456789012123456789012" -A hmac-md5 "1234567890123456";

3. Run the script

sudo /etc/init.d/setkey start

The same can be done under Windows vista using WFP(Windows Filtering Platform) API. For more information on How IPSec (in IMS context) feature could be implemented under Windows you can contact Mercuro Team at [tech dot mercuro -at- inexbee dot com].

For more information visit:
http://www.mercuro.net
http://www.ietf.org/rfc/rfc3329.txt
http://www.arib.or.jp/IMT-2000/V310Sep02/T63/Rel5/33/A33203-520.pdf
http://www.arib.or.jp/IMT-2000/V310Sep02/S3g/Rel5/24/24229-510.pdf
http://www.3gpp.org/FTP/TSG_SA/WG3_Security/TSGS3_24_Helsinki/Docs/PDF/S3-020385.pdf