Facilitating legal interception of ip connections

Field of the Invention

The present invention relates to a method and apparatus for facilitating legal interception of IP connections.

Background to the Invention

It is now possible to establish various forms of connection over the internet including data connections as well as voice and video telephony connections. As the speed and extent of the Internet increase, the use of voice and video telephony can be expected to grow. Whilst current technology tends to restrict IP telephony to communications between computer terminals coupled to the Internet, tomorrow's technology will provide for IP telephony between small dedicated telephony terminals, and other mobile devices such as PDAs, palmtop computers etc.

In order to allow such devices to gain widespread acceptance, a key issue which must be addressed is that of security. The two main security concerns are the avoidance of unauthorised eavesdropping, and the need to authenticate terminals involved in a communication (i.e. to ensure that the terminal which a "subscriber" connects to is the terminal which the subscriber intends to connect to and <i>vice versa</i>). However, these concerns are not unique to IP telephony, and are common to many different forms of IP communication.

An Internet Engineering Task Force (IETF) standard known as IPsec (RFC2401) has been defined and provides for the creation of a secure connection between parties using IPv4 and IPv6. In the IPsec model the end points of the secure connection are identified by their IP addresses. In order to allow IPSec packets to be properly encapsulated and decapsulated it is necessary to associate security services and a key between the traffic being transmitted and the remote node which is the intended recipient of the traffic. The construct used for this purpose is a "Security Association" (SA). SAs are negotiated between peer nodes using a mechanism known as "Internet Key Exchange" (IKE), and are allocated an identification known as a "Security Parameter Index" (SPI). The appropriate SA is identified to the receiving node by including the corresponding SPI in the headers of the transmitted data packets. Details of the existing SAs and the respective SPIs are maintained in a Security Association Database (SAD) which is associated with each IPSec node.

As already noted, IPSec SAs are negotiated using the IKE mechanism. More particularly, IPSec SAs make use of IKE phase 2. IKE phase 1 involves the negotiation of an IKE SA. Figure 1 illustrates in general terms the two phases of IKE. When IKE phase 1 is initiated between two nodes, communications are carried out in the open. The mechanisms used must therefore be extremely secure and inevitably computationally intensive. At the end of phase 1 both nodes are authenticated to each other, and a shared secret is established between them. IKE phase 2 makes use of the IKE SA to negotiate one or more IPSec SAs. As the phase 2 negotiations are carried out within a secure "shell", they can be much less computationally intensive than the phase 1 negotiation. Whilst a new IKE SA may be negotiated only infrequently (e.g. one a day or once a week), IPSec SAs may be negotiated every few minutes. IKE phases 1 and 2 are illustrated in more detail in Figures 2 and 3.

IPSec makes use of one or both of the Authentication Header (AH) and Encapsulation Security Payload (ESP) protocols which in turn make use of the corresponding established IPSec SA. Both of these protocols provide for the authentication of sent data packets whilst ESP provides in addition for the encryption of user data. The use of AH and/or ESP is agreed upon by the communicating nodes during the IKE negotiations. It is expected ESP will be preferred for VoIP due to its use of encryption.

The precise way in which IPSec is implemented in a system depends to a large extent upon the security policy of the organisation wishing to employ IPSec. For example, the organisation may specify end-points (e.g. user terminals) to which IP packets may be sent, or from which they may be received, the particular security levels to be used for encrypting packets, etc. Policy is stored in a Security Policy Database (SPD) which is also associated with each IPSec node. Typically, the SPD is distributed amongst a plurality of entities of the IPSec node.

Patent document US-A-5 850 451 (15-Dec-1998) teaches a method and a system for legal interception of data communications, by storing an allocated key-pair both at the terminals and some interception authority.

Summary of the Invention

Traditional circuit switched telephone networks make provision for the legal interception of telephone calls. Such interception must be instigated by the appropriate authorities and is an important weapon against crime. Understandably, it is desirable to make provision for the legal interception of IP connections (whether data, VoIP, video, etc). However, this presents a problem as the IP security protocol which will become the <i>de facto</i> standard is IPSec, and IPSec has been designed to provide terminal-to-terminal security involving strong encryption.

It is an object of the present invention to overcome the problems inherent to IPSec, and in particular to facilitate the legal interception of IP connections.

According to a first aspect of the present invention there is provided a method of facilitating the legal interception of IP connections, where two or more terminals can communicate with each other over the Internet using IPSec to provide security, the method comprising: <ul id="ul0001" list-style="none" compact="compact"><li>allocating to each terminal a public/private key pair for use in negotiating IKE and IPSec Security Associations (SAs) with other terminals;</li></ul>    where a terminal is coupled to the Internet via an access network, storing the private key of that terminal within the access network; and    when an IP connection is initiated to or from a terminal on which a legal interception order has been placed, using the private key stored for that terminal within the access network to intercept the connection.

The present invention is applicable in particular to facilitating the interception of IP connections made to or from terminals coupled to telecommunication networks, e.g. wireless telecommunication networks, fixed line telephone networks, and cable networks. Such networks present the authorities with confidential access points to the connections to be intercepted. Where the access network is a wireless telecommunication networks, the private keys for subscribers may be stored at an MSC (GSM network) or at an RNC (UMTS/3GPP network) or their equivalents, or at a server coupled to the MSC/RNC. Where the access network is a fixed line telephone network, the private keys may be stored at the local exchanges of subscribers, or at servers coupled to the local exchanges.

The invention is applicable to IP connections between two or more terminals each of which is coupled to the Internet via an access network holding the necessary private keys. The invention is also applicable to IP connections extending between one of these such terminals and another terminal which handles ISAKMP negotiations itself, without the aid of an access network holding the private key of the terminal.

In certain embodiments of the invention, following the receipt of a request for an IKE phase 1 SA at the access network of a terminal initiating an IP connection, the access network negotiates an ISAKMP SA with a remote node on behalf of the initiating terminal, and passes the SA parameters (including the established shared key) to the initiating terminal over a secure connection. The remote node may be the terminal with which the initiating terminal wishes to establish a connection, or may be a node to which that terminal is coupled.

Once the ISAKMP SA has been established, if the initiating terminal sends a request for the establishment of an IKE phase 2 SA intended for the destination terminal, the access network determines whether or not a legal interception order has been placed on that terminal or on the terminal to which the initiating terminal wishes to connect. If such an order has been placed, the access network interposes itself between the initiating terminal and the remote node during the negotiation of a pair of IKE phase 2 SAs. An important consideration is that neither the initiating terminal, nor the terminal to which the initiating terminal connects, must know that an interception is being made.

In other embodiments of the invention, and in which both of the terminals between which it is desired to establish a connection are coupled to the Internet via the same or different access networks, receipt at an access network (the "first" network) of a request for establishment of an IKE phase 1 from an initiating terminal causes that first network to forward the request to the access network (the "second" network) of the other terminal. The second network then makes a decision on legal interception. If legal interception is required, the second network negotiates an ISAKMP SA directly with the initiating terminal. The first network negotiates a second ISAKMP SA directly with the destination terminal.

When the first access network receives from the initiating terminal a request for the establishment of IPSec SAs, the second access network negotiates a pair of SAs directly with the initiating terminal whilst the first access node negotiates a second pair of SAs directly with the destination terminal. In this way, each of access networks has access to one of the two communication channels. One of the access networks may relay the channel which it is monitoring to the other access network (or to some other node).

In still other embodiments of the present invention, a private key of a terminal to be monitored may be passed from one access network to another so that one access network is in possession of the private keys of both (or all) parties involved in an IP connection. This may be achieved in such a way that the private key is not explicitly disclosed to the receiving access network (e.g. by embedding the key in executable code).

Preferably, IKE Phase 1 (and Phase 2 if appropriate) negotiations handled by an access network are handled by a server. This server may be coupled to or incorporated into a switch of the access network.

Preferably, the public/private key pair allocated to a terminal is stored in a memory of or coupled to the terminal in such a way that the user of the terminal cannot alter the private key without the consent of the operator of the relevant access network.

The invention is applicable in particular to facilitating the interception of data calls although it is also applicable to VoIP calls and other forms of IP telephony including videoconferencing.

According to a second aspect of the present invention there is provided a server for use in intercepting IP connections between two or more terminals, the server comprising a memory for storing the private keys of public/private key pairs of respective terminals, and processing means for identifying when legal interception is to be carried out on a connection to or from a terminal, and for intercepting that connection using the private key of the terminal.

Brief Description of the Drawings

<ul id="ul0002" list-style="none" compact="compact"><li>Figure 1 illustrates at a general level the signalling between two nodes during a secure data connection establishment process;</li><li>Figure 2 illustrates at a more detailed level the signalling involved in an IKE phase 1 of the process of Figure 1;</li><li>Figure 3 illustrates a Quick Mode message exchange of an IKE phase 2 of the process of Figure 1;</li><li>Figure 4 illustrates schematically a system for providing IP telephony between user terminals;</li><li>Figure 5 is a signalling diagram illustrating a first method of facilitating legal interception of an IP connection between two of the terminals in Figure 4; and</li><li>Figure 6 is a signalling diagram illustrating a second method of facilitating legal interception of an IP connection between two of the terminals in Figure 4.</li></ul>

Detailed Description of a Preferred Embodiment

The method which will now be described makes use of features described in the following documents: [IPsec] RFC 2401, Security Architecture for the Internet Protocol, November 1998; [IKE] RFC 2409, The Internet Key Exchange (IKE), November 1998; [ISAKMP] RFC 2408, Internet Security Association and Key Management Protocol, November 1998; IETF Draft ''Running IKE Phase 2 over Artificial Kerberos IKE SA", draft-ietf-kink-ike-over-kkmp-00.txt; RFC 2459, Internet X.509 Public Key Infrastructure Certificate and CRL Profile. Reference should be made to these documents for a fuller understanding of the method.

Figure 4 illustrates a scenario in which a pair of mobile wireless terminals T1,T2 are coupled to the Internet via respective access networks 1,2. For the purpose of this example, the access networks 1,2 are considered to be mobile telecommunication networks, with the wireless terminals T1,T2 being mobile telephones (or other mobile wireless terminals having mobile telephone functionality). It is noted that whilst current digital networks (such as GSM, PDS, and digital AMP networks) allow user terminals to access the Internet, Internet access will be greatly enhanced by the introduction of data services (such as GPRS) and as third future generation mobile networks (such a UMTS) are introduced. The terminals T1,T2 are assumed to be specifically designed to facilitate IP data calls (and possibly IP telephony applications including VoIP and video telephony).

Illustrated in Figure 4 are a pair of interception servers S1,S2 associated with respective access networks 1,2. Each server S1,S2 is coupled to that "switch" (e.g. MSC or RNC - not shown in Figure 4) of the associated access network 1,2 which handles IP calls made to or from terminals connected to the network. As explained above, in order to make use of IPSec to secure IP connections, a terminal should be in possession of a public/private key pair (P_priv/P_publ). This key pair may be stored in a fixed memory of the terminal or may be stored in the terminal's SIM card. It is assumed to have been stored there by the operator of the access network to which the terminal's owner subscribes, prior to the terminal being presented to the owner. Once stored in the terminal (or SIM card) it is not possible for the owner to change the key pair without the consent of the network operator. The operator retains a copy of the subscriber's public/private key pair and stores this in a look-up table of the interception server S1,S2. The server also has a knowledge of the terminal's capabilities including the available cryptographic algorithms and the default security policy. The server maintains a record of those subscribers on whom legal interception orders have been placed by the authorities.

When a terminal (say T1 in Figure 4) wishes to establish an IP connection to another terminal it must first obtain the IP address of the other terminal T2. If the initiating terminal T1 does not know this IP address from the outset, it must obtain it, e.g. by sending a user entered URL to a Domain Name Server (DNS) which can resolve the address and return it to the initiating terminal. As already outlined above, in order to secure the connection, the initiating terminal T1 must then negotiate a pair of IPSec SAs with the other (or "destination") terminal T2. Figure 5 illustrates the exchange of signalling messages required to establish a pair of SAs.

Firstly, the initiating terminal T1 sends a Request ISAKMP SA message to its access network 1. This message contains the identity of the initiating and destination terminals (which may be the terminals' IMSI codes or telephone numbers). The request is handled by the access network's interception server S1. The server checks whether or not it holds the private key for initiating terminal T1. If not, the call connection attempt is rejected. If the server does hold the appropriate private key, the server S1 handles the negotiation of the ISAKMP SA (IKE phase 1) on behalf of the terminal T1 (the terminal T1 does not require IKE Phase 1 capabilities). In the same way, the interception server S2 in the access network 2 of the "destination" terminal T2 handles the ISAKMP SA negotiation on behalf of that terminal T2. This is possible because the interception servers S1,S2 know the private keys of the respective terminals T1,T2. Upon completion of the negotiation, the relevant ISAKMP SA parameters are returned by the interception servers to their respective terminals. As the access networks are private networks, typically using security protocols to secure transmission over the radio interface, there is no danger of the returned parameters falling into the wrong hands. (As an alternative to IKE Phase 1, Kerberos authentication may be used.)

At this stage, the interception servers S1,S2 know the parameters which will be used to secure the subsequent negotiation of IPSec SAs between the terminals (i.e. IKE Phase 2). When terminal T1 wishes to begin a secure communication with terminal T2, it sends a Begin IPSec SA negotiation request to terminal T2. This message is intercepted by the access network 1 and the interception server S1 is notified. The request includes in its header portion the relevant ISAKMP SA data. The server S1 then checks its records to see if either the initiating or destination terminals have legal interception orders placed on them. If they do not, the IPSec SA negotiation continues without the involvement of interception server S1. It is noted that once the IPSec SAs have been negotiated in this way, it is not possible for the access network to intercept (and decrypt) traffic carried using the IPSec SAs. If on the other hand the interception server S1 determines that a legal interception order has been placed on one of the terminals T1 or T2, the server S1 interposes itself between terminals T1 and T2 during the negotiation of the pair of IPSec SAs (similarly server S2 may interpose itself in the IPSec SA negotiation if that server decides that it too must intercept the connection). As far as terminals T1 and T2 are concerned, they are negotiating the IPSec SAs with each other and are not aware of the role of the server(s) in the process. Subsequently, encrypted traffic may be intercepted at the server S1, decrypted using the negotiated IPSec SA parameters known to server S1, and routed to an appropriate monitoring point. An important feature is that neither of the terminals is allowed to run the full ISAKMP negotiation.

There is illustrated in Figure 6 signalling associated with an alternative interception scheme. In this scheme, successful legal interception requires the active participation of both interception servers S1,S2 which again hold the private keys of their respective subscribers, and also assumes that the terminals involved in the connection are capable of handling the IKE phase 1 negotiations (nb. this latter requirement does not exist for the embodiment of Figure 5). When an interception server (S1 in Figure 6) receives a request for an ISAKMP SA negotiation from an originating terminal T1, that interception server S1 forwards the request to the destination side interception server S2. If the initiating side interception server S1 determines that legal interception is required, a flag is set in the sent request.

When the destination side interception server S2 receives the request it checks the status of the interception flag. If that flag is set, or if the receiving interception server S2 determines from its own records that legal interception is required, the server S2 performs the first ISAKMP SA negotiation (including the Diffie-Hellman exchange) directly with the initiating terminal T1 on behalf of the destination terminal T2. The destination side interception server S2 also notifies the originating side server S1 that it should negotiate a second ISAKMP SA directly with the destination terminal T2.

When the initiating side access network 1 subsequently intercepts an IPSec SA negotiation request sent from the initiating terminal T1 (and intended for the destination terminal T2), the request is forwarded to the destination side interception server S2. The destination side interception server S2 represents itself to terminal T1 as if it were the destination terminal T2 for the purpose of the IPSec phase 2 negotiation. A pair of IPSec SAs are established between terminal T1 and the interception server S2. At the same time, the destination side interception server S2 instructs the initiating side interception server S1 to negotiate a second pair of IPSec SA with the destination terminal T2. Server S1 therefore represents itself to the destination terminal T2 as if it were terminal T1 during the IPSec phase 2 negotiation. A second pair of IPSec SAs are established between server S1 and terminal T2.

It is necessary to echo data between the servers S1 and S2 in order to give the servers access to both channels. Thus, data originating from terminal T1 is reflected by server S2 to server S1. Server S1 then encrypts the data using the appropriate IPSec SA and sends it to terminal T2. Similarly, data originating from terminal T2 is reflected by server S1 to server S2, with server S2 encrypting the data using the appropriate IPSec SA and sending it to terminal T1.

It is noted that with the embodiment of Figure 6 there is no need for "artificial" ISAKMP SA transfers (between servers and terminals). Rather, the initiating terminals IKE signalling will be processed by the destination side interception server and <i>vice versa.</i>

It will be appreciated by the person of skill in the art that various modifications may be made to the above described embodiments without departing from the scope of the present invention. For example, in the embodiment described with reference to Figure 6, the additional delay which results from echoing one monitored channel back from one interception server to the other may be avoided by executing a secure IKE proxy server at the interception server which is monitoring the connection. This proxy server is sent from the other interception server and contains the private key of the terminal for which it is responsible. The private key is incorporated into the proxy server in such a way that it is not disclosed to the receiving interception server. However, when the proxy server is executed, it allows the interception server to conduct the IKE phase 1 and 2 negotiations directly with the initiating and destination terminals. Thus, the interception server will have access to both traffic channels.