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Contributed by SafeNet Corporation

This paper explores the mechanics and policies that are necessary to protect information transmitted over an untrusted high speed wide area ATM network. The network model assumes a set of local area networks that are physically secure, interconnected by an untrusted wide area network. The threat model assumes an outsider threat such that security demands strong encryption of the data as well as access control between the untrusted wide area network and trusted local area networks. This paper details the security assumptions and requirements for this type of network. The paper then presents a set of reference networks and discusses the cryptographic requirements necessary to achieve a high level of information privacy, provide access control, scale efficiently as the network grows in size and speed, and operate transparently to the end user.

The commercialization of the Internet and the advent of broadband networking technologies have ushered in the �Information Revolution.� As with all advances in information exchange, the benefits of rapid communication also carry the liability associated with security for both the information and the information infrastructure.

With the emergence of broadband services such as Asynchronous Transfer Mode (ATM), the same network can be used for voice, video, data, transaction processing, and a host of other integrated services. These network services provide the means for information exchange. For businesses, this information often represents intellectual property, the �family jewels,� which must be protected.

Scope

The concept of network security means a lot of different things to different people; thus it is important to define the terms associated with security. For our purposes, network security specifically refers to the means to assure privacy of data and to control access from untrusted networks. The term privacy means that data transmitted on an untrusted network provides as little information as possible to unauthorized third parties. Privacy is achieved by encryption of data.

Access control refers to the policy of allowing communication between certain entities and disallowing communication between other entities. This means that access control can be used to prohibit network access between untrusted networks and trusted networks. One of the issues associated with access control is identification of the entities that are providing privacy and access control services. These entities are referred to as cryptographic units.

Before one designs, builds, and deploys a network security system, it is essential to answer the following questions:

� What information are we trying to protect?

The basic assumption of this paper is that the threat to networking security is an outsider threat. In other words, any security breach will occur outside the security perimeter. This outside threat is technically sophisticated with access to the best commercially available technology, and is tenacious enough to break weak security procedures. A corporate espionage operation is an example of such a threat.

For network communication there are two major types of security breaches that are applicable for this paper, wire tapping and information mis-routing. Both of these concerns involve a third party gaining access, either accidentally or maliciously, to sensitive information.

In terms of networking security, it is important to decide whom can be trusted with what type of information and what should be the access control strategy. The security perimeter is the boundary between those people and equipment that can be trusted and the rest of the world. An encrypting firewall between the trusted network and the untrusted network is one approach to providing this security perimeter. In general, the smaller the security perimeter, the more expensive it is to achieve security per user since it requires an encrypting firewall for each security perimeter. However, the information security for the overall network is improved. The larger the security perimeter, the lower the cost per user, but at the liability of needing to trust a larger number of people and equipment.

This paper presents a set of examples to illustrate methods to define the security perimeter for various network configurations and security requirements using the encrypting firewall security model. The basic model assumes that groups sharing common infrastructure such as a LAN or a building constitute a trusted set of users. This basic model attempts to balance risk against cost and complexity.

Before discussing how to deploy security within a network, it makes sense to define the requirements for an Encrypting ATM Firewall (referred to as a cryptographic unit). The following items compose the functions necessary for a cryptographic unit to operate as an encrypting firewall:

� Ensure privacy of the data.

� Provide access control to the trusted network.

� Be transparent to the network and users.

� Communicate with a certificate authority for credential and policy information.

� Provide Operations, Administrations, Maintenance, and Provisioning (OAMP) functions.

The most important function for an encrypting security device is to provide privacy for the data in transit over the untrusted network. This means that the data must be scrambled with a sufficiently robust cryptographic algorithm, such that no information is available to a potential adversary other than the existence of a message and possibly its length.

This means that the payload of every cell transiting the untrusted network must be encrypted with a strong cryptographic algorithm. Furthermore, the algorithm should ensure that patterns in the plain text do not show up as patterns in the encrypted text, also referred to as cipher text.

The second major function of an encrypting firewall is to provide access control between the trusted network and the untrusted network. The amount of access control is a policy issue dictated by the security administrator. The most restrictive (and secure) access control policy is to allow connections between trusted networks via the untrusted network only if that connection is part of an authenticated security context.

One method to administer access control is to require the establishment of a security context on a per-Virtual Circuit (VC) basis is during call setup. When a host-to-host connection request is first made across the untrusted network, the cryptographic units must first set up a security context before the connection is allowed to be completed between the hosts. The security context establishes the identity of the cryptographic units and sets up the necessary session keys for encrypting the particular VC. This approach also permits security for Point-to-Multipoint (PMP) connection.

The third major requirement for a cryptographic firewall is transparency, with respect to the end-user and the network. In short, for users within trusted networks the existence of the cryptographic firewall should not be visible. The cryptographic firewall must be compatible with the ATM standards with respect to user cells and signaling channels. For user cells, only the payload of the cell is encrypted. This means the ATM header information remains in the clear and thus the encryption does not affect transit of the cell through the untrusted network. For signaling channels, the cryptographic firewall is responsible for intercepting call setup messages from the trusted network to the untrusted network. The cryptographic firewall must then comply with the signaling standards in order to interoperate seamlessly between the trusted network and the untrusted network.

A Certificate Authority is generally used to vouch for the authenticity of a public key and its association with other identifying information. In the context of a network security system, it can also play a critical role in vouching for the identity of cryptographic units and other entities. As part of establishing a security context, cryptographic units need to exchange certificates signed by a trusted certificate authority. A security context between cryptographic units is only valid if both cryptographic units can validate the credentials of the other cryptographic unit.

It is anticipated that security policies will be administered through existing network management protocols. The certificate authority may also play a role in authenticating management agents, policy changes, and so forth. Some of policy issues that are relevant to a network security system include:

� time limits on session keys,

� revocation of certificates,

� certificate lifetimes,

� distribution of public keys associated with a cryptographic unit,

� access between hosts on trusted networks and hosts on untrusted networks,

� audits and response to unusual events, and

� policies concerning establishment of Permanent Virtual Circuits (PVCs) and Permanent Virtual Paths (PVPs).

This section illustrates the mechanism of defining a security perimeter for networks. These networks are referred to as reference networks and represent typical configurations for deploying security. As part of defining these reference networks it is necessary to discuss the particular components that make up the security system and how each of these components fits into a network based security system.

The basic components of a network based security system are the cryptographic unit and the certificate authority. The cryptographic unit performs the following functions � Encryption and decryption of ATM cell payload on a per-cell basis; 

� Access control based on call setup within a switched virtual circuit network;

� Enforcement of policies and confirmation of credentials for each secure session.

According to the criteria for trusted networks, the security overlay must be transparent. In order to talk to Host B across the untrusted network, Host A simply attempts to establish a call using standard ATM signaling. The cryptographic units transparently establish a security context through the untrusted network according to the following steps.

The basic deployment of a security firewall as a network overlay calls for a trusted Local Area Network connected to an untrusted Wide Area Network by means of a high speed ATM link. The security perimeter is then at the Point-of-Presence (POP) between the untrusted wide area network and the trusted local area network. This section presents two basic configurations for a trusted local area network interconnection to the wide area network.

The first configuration, assumes a local ATM environment where all endusers are interconnected locally via ATM switches. The security perimeter is between the local root ATM switch and the untrusted wide area network. This configuration assumes that the local network is trusted and the wide area network is untrusted. All local traffic in this configuration would remain in the clear. Traffic entering or leaving the security perimeter would be subject to encryption and access control policies. This configuration represents both a cost effective and security effective solution that scales well with the requirements of the local area network.

The second configuration is very similar to the first configuration but allows for backward compatibility with existing legacy networks. In the case, the trusted local network is IP-based, supporting many different network types. (Note that this is strictly a private IP network which is not connected to the Internet.) The IP router with ATM capability provides the interface between IP traffic on the legacy networks and the ATM network. The ATM link between the IP router and the wide area network contains the cryptographic unit that provides encryption of the ATM cell payload (and therefore IP datagrams). This configuration is cost effective, security effective, and scalable in the same manner as the first configuration.

The two configurations illustrated in the previous section represent building blocks for developing security within a Wide Area Network. The basic idea for security over a wide area network is that security perimeters are established at the interface between local area networks and the wide area network according to Figures 2 and 3. These local area networks then become �trusted islands.� The assumption is that there is no requirement for trust within the untrusted network. Thus, even if the untrusted network is compromised, there is no compromise to the privacy of the data nor can there be unauthorized access to the trusted ATM networks. This is because all data between the trusted islands is encrypted and access control is accomplished at the point-of-presence to the wide area network.

The point-of-presence of the untrusted wide area network is typically an Add/Drop multiplexor that connects the local area network to a high bandwidth local loop that goes to the central office of the local carrier. Within the untrusted wide area network, traffic will be switched through a hierarchy of switches starting with the local central office and then (possibly) through the Inter-exchange Carriers (IEC). A customer has no control over the switching or security of switched traffic within the untrusted network. Therefore, it is on the trusted network side of the point-of-presence that a security firewall must be installed.

This paper explored the mechanisms and policies that are necessary to protect information transmitted over an untrusted high speed wide area ATM network. One basic assumption for establishing a trusted network is the development of a threat model. The model chosen is an outsider threat, technically capable and properly motivated such that strong encryption and access control are necessary to protect the security of the data. The second basic assumption is that the local area network is physically secure and trusted. Thus, it makes sense from a security as well as economic standpoint to provide encryption and access control at the point-of-presence to the untrusted wide area network.

[1] Scott Lane and Data Deluca. "Proposed Encryption Algorithm and Mode of Operation," ATM Forum/96-0104, February 1996.

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