WO2000052905A2

WO2000052905A2 - Method and apparatus for enhanced security in a broadband telephony network

Info

Publication number: WO2000052905A2
Application number: PCT/US2000/005520
Authority: WO
Inventors: William A. Aiello; Steven Michael Bellovin; Charles Robert Kalmanek, Jr.; William Todd Marshall; Aviel D. Rubin
Original assignee: At & T Corp.
Priority date: 1999-03-01
Filing date: 2000-03-01
Publication date: 2000-09-08
Also published as: EP1157521A2; BR0008457A; CA2371811A1; WO2000052905A3

Abstract

The broadband telephony interface is provisioned by receiving information authenticating a provisioning server, establishing a communication channel between the user and the provisioning server over which is transmitted authorization information from the user to the provisioning server, and encrypting and transmitting a cryptographic key associated with the user to the provisioning server. The cryptographic key can be a symmetric key or a public key corresponding to a private key stored in the broadband telephony interface. The cryptographic key can be utilized to generate other keys which are utilized to secure communication channels for the telephony service. The broadband telephony interface advantageously can be implemented as untrusted hardware or software that is installed by a customer.

Description

METHOD AND APPARATUS FOR ENHANCED SECURITY IN A BROADBAND TELEPHONY NETWORK

Field of the Invention The present invention relates generally to communication networks, and more particularly to enhanced security in a broadband telephony network.

Background of the Invention Broadband communication networks provide a viable alternative to present local exchange carrier (LEC) loops in providing both voice and data transmission services. A variety of broadband network architectures have emerged as supporting Internet and telephony access: including cable distribution networks, ISDN (Integrated Services Digital Network), broadband ISDN, DSL ("Digital Subscriber Line"), ADSL, etc.

A major concern for such broadband communication networks is the need for adequate security measures. The system architecture must ensure user privacy across the network medium and prevent unauthorized access to services. For example, in the case of cable modems based on the Data Over Cable Service Interface Specification ("DOCSIS", a term referring to the ITU-T J.112 Annex B standard for cable modem systems), security is provided by the DOCSIS Baseline Privacy Interface ("BPI") which addresses some of the vulnerability presented by the shared cable network. BPI provides security mechanisms, including encryption using the Cipher Block Chaining (CBC) mode of the Data Encryption System (DES) and key exchange based on RSA encryption, that defend against an eavesdropping threat in the cable network. The successor to BPI, DOCSIS 1.1 Baseline Privacy Interface Plus ("BPI+") adds authentication based on digital certificates that binds media access control addresses for cable modems to RSA public keys. DOCSIS cable modems must be pre-certified with cryptographic keys and/or certificates installed in the hardware at manufacturing time. DOCSIS cable modems undergo a registration process and a baseline privacy key exchange procedure that is used to establish a secure channel with the cable modem termination system ("CMTS") at the head end. The CMTS verifies a cable modem's public key by verifying the authenticity of the certificate. Use of encryption such as provided by BPI+ is essential for a shared medium access network such as cable.

On the first hop, security measures such as DOCSIS baseline privacy are likely to be adequate. However, the actual path traversed by packets is often complex, and BPI does not provide any data privacy beyond the cable access network. The susceptibility of the public data networks such as the Internet to routing attacks - attacks where the enemy injects false route advertisements possibly to divert traffic to pass an eavesdropping station - is a concern. Quite simply, the science necessary to prevent such attacks does not exist, and it is expected to be a fair number of years before the Internet is adequately protected. In a single, well-managed IP backbone network, it may be possible to take adequate precautions against eavesdropping through good design and rigorous security procedures, though there is still a risk as the equipment and network configuration changes. When traffic traverses more than one backbone (or gets routed over other regional networks of unknown security), however, the potential for attack is greater. In the case of telephony service where ultimate delivery of packets could be via the Internet Protocol to a network not under the control of the service provider, privacy cannot be guaranteed over such paths.

Accordingly, a broadband telephony architecture with enhanced security features is needed, with the overall goals of protecting the privacy of signaling and media traffic and of preventing theft of service.

Summary of the Invention

It is an object of the present invention to prevent theft of service. It should not be possible to steal another user's identification information by electronic means or to sell unlimited service by compromising customer premises equipment or injecting messages into the system. Protections should be maintained to limit service to authorized usage subject to proper accounting. It is another object of the present invention to protect the privacy and integrity of signaling and media traffic. It should not be possible to inject signaling traffic into the network that appears to be from another source. It should also not be possible for unauthorized people to eavesdrop on traffic of other users.

This includes traffic analysis by which, for example, an attacker can determine who is talking to whom.

It is another object of the present invention to protect the integrity of the called number. It should not be possible to force a called number to another number. This is necessary to prevent a range of attacks on the service, including one in which an attacker tries to steal business from a competitor by causing calls to be misrouted.

It is another object of the present invention to abide by government wiretap laws, e.g. the Communications Assistance for Law Enforcement Act of

1994 ("CALEA"). This may include supplying signaling information and media streams to the authorities. If encryption keys are mediated by the service provider, they must also be supplied to the authorities.

It is another object of the present invention to discourage denial of service attacks. It should not be easier than it is in the existing PSTN for one user or set of users to prevent other legitimate users from obtaining service.

It is another object of the present invention to provide the correct functionality of conventional telephony features. Subscriber features must function correctly. For example, caller ID information must be included in all calls due to trace requirements. Only users that subscribe to the service should have access to the information. Users should not be able to forge such information in the case of a trace.

It is another object of the present invention to provide an administrative level. There should advantageously be at least two levels of privilege to the system. This is so that decisions such as invoking emergency procedures or downloading new code to customer equipment cannot be performed by all users. For example, in an emergency, administrators must have the ability to preempt a call, while non-administrators should not have this ability. Thus, in accordance with the present invention, an architecture for

95 using a broadband telephony interface ("BTI") is provided with enhanced security features. The BTI registers itself with the network in a secure fashion, so that it can be authenticated and known to the network from then on. A security association is created by having the BTI generate a cryptographic key (symmetric or otherwise) and send it to the network under the public key of the network

100 service provider. The two ends can then use this key to establish a secure connection, and the BTI can send authorization information such as a credit card number over the secure connection. The cryptographic key can then be used to derive subsidiary keys that are used for subsequent communications. By having the BTI generate its own cryptographic key, instead of having a certificate

105 installed at manufacturing time, this allows for the possibility of a BTI implemented as software. The BTI advantageously need not be a trusted or certified box; indeed, a software package executed on a personal computer can fulfill the same functions. This is in contrast to the cable modem, for example, which must be certified to ensure correct behavior and fair access to the medium.

110 These and other advantages of the invention will be apparent to those of ordinary skill in the art by reference to the following detailed description and the accompanying drawings.

Brief Description of the Drawings

115 Fig. 1 is a diagram of a broadband communication network which can be utilized with an embodiment of the present invention.

Fig. 2 is a block diagram of the components of a hardware broadband telephony interface configured for use with a preferred embodiment of the present invention. 120 Fig. 3 is an abstract diagram of a communication provisioning protocol in accordance with a preferred embodiment of the present invention. Detailed Description

With reference to Fig. 1 , a diagram of a broadband communication

125 network is shown which can be utilized with an embodiment of the present invention. A packet-switched IP backbone 100 is shown connected to access networks 150 and 151, here shown as cable distribution networks, and to a more conventional telephony network 135, here shown as the public switched telephone network ("PSTN"). A broadband telephony interface ("BTI") 170 is shown which

130 provide a gateway between one or more telephones 190 and the packet-switched network. The BTI 170 may be physically integrated with a cable modem ("CM") 160, as shown in Fig. 1, to provide the necessary functions to interface between one or more phone lines and the depicted cable access network 150. The cable modem 160 can also be used by other communication devices 180 (in Fig. 1

135 shown for example as a personal computer) to connect to the access networks 150. The access network 150 terminates on a cable modem termination system ("CMTS") 155 at a head end. The CMTS 155 interfaces to an Internet Protocol ("IP") edge router ("ER") 120 that connects to a managed IP backbone 100, which provides the connectivity to other BTIs (e.g. 171 with corresponding edge router

140 121, CMTS 156, access network 151, cable modem 161, communication device 181, and telephone 191) and to gateways 130 to the PSTN 135. A "gate controller" 110 provides authentication, authorization, and call routing functions for calls originated by BTIs. The authentication information used by the gate controller is made available to it by a provisioning process that is described in

145 further detail below. The backbone provides connectivity to a provisioning server 140, which is involved in provisioning the BTI and other network elements. The particular architecture set forth in Fig. 1 is for illustration purposes only and is further described in the following commonly assigned patent applications: Provisional Patent Application entitled "Telephony on a Broadband

150 Network," Serial No. 60/071,346, filed on January 14, 1998; Provisional Patent Application entitled "Telephony Over Broadband Access Networks," Serial No. 60/073,251, filed January 30, 1998; Provisional Patent Application entitled "Distributed Open Signaling Architecture," Serial No. 60/095,288, filed August 4, 1998; and Provisional Patent Application entitled "Distributed Open Signaling

155 Architecture," Serial No. 60/104,878, filed October 20, 1998, the entire contents of which are incorporated herein by reference.

Note that although a limited number of network entities are shown in Fig. 1 for simplicity of presentation, other network entities can obviously be included in the network - such as additional interface units, routers, controllers,

160 and gateways. Although Fig. 1 sets forth a particular broadband telephony architecture, one of ordinary skill in the art would recognize that the security enhancements of the present invention are readily extendible to other architectures. For example, the present invention can be utilized with broadband communication networks that do not use cable access networks but rather use

165 digital subscriber line (DSL), Integrated Services Digital Network (ISDN), or some other access architecture. Moreover, the present invention can be utilized with other packet-switched architectures or with a hybrid network architecture.

Fig. 2 sets forth a simplified block diagram of the components of a BTI, configured for use with the present invention. The BTI performs signaling

170 and call control functions and enables telephony service on the communication network by digitizing, compressing, and packetizing analog signals from a telephone 190 into data packets for transport over the communication network. The functions of a BTI can be implemented in many different ways that would be apparent to one of ordinary skill in the art, including as software executed on a

175 typical computer. Fig. 2 illustrates a hardware embodiment of a BTI 170 that can be a stand-alone device, can be integrated with a telephone 190 to create a standalone telephony device or can be integrated with an access device (e.g. the cable modem 160 in Fig. 1 or a set top box) to form a general network interface unit. The BTI in Fig. 2 comprises a processor 210 and hardware (here shown as a

180 subscriber line interface circuit 250, codec 260, and echo canceler 265) capable of detecting changes in state information (e.g. hook state detection), collecting dialed digits (e.g. dual tone multifrequency (DTMF) signals), and participating in the implementation of telephone features. The processor 210 has access to memory 220 which stores data such as cryptographic keys 222 and the operating system 185 221 and program instructions necessary for the operation of the BTI. For security purposes, it is advantageous for the BTI also to have read only memory 230 which stores code downloading code ("CDC") 232 and the service provider's public key 231, as further discussed below. It is also advantageous for some of the data and code in memory 220 to be stored in some form of non- volatile memory so that

190 such information is not erased if power to the BTI is turned off.

The BTI 170 advantageously should be able to performing probabilistic computation, whether by hardware (e.g. a noisy diode), software (e.g. a pseudo-random generator with a good seed), or some combination. This is necessary for the BTI to be able to generate cryptographic keys and to perform

195 related cryptographic functions such as ElGamal signatures. Without a hardware source of randomness, it will likely be necessary for the BTI 170 to maintain about 200 bytes of state for the lifetime of the device, which will need to change often. One possible embodiment would be to keep the state data in RAM memory which gets copied to nonvolatile flash memory when there is a chance of a loss of

200 power.

As further elaborated on below, the BTI 170 need not necessarily be under the direct control of the service provider, e.g. the entity operating the communication network. The BTI, operated in accordance with the present invention, can be implemented as customer premises equipment that is untrusted

205 and operates based on locally-stored software. The customer, in other words, can purchase the BTI at a local store or can have the device shipped to her home. Where the BTI is implemented as software, it can be simply downloaded and installed on a computer pre-configured for access to the communication network.

210 Provisioning

In accordance with a preferred embodiment of the present invention, Fig. 3 illustrates security protocols to be utilized in the provisioning of a user who wishes to utilize the network. The following notation and abbreviations are used in the discussion:

215

In the case of a cable access network 150, it is assumed that the cable modem 160 has undergone DOCSIS registration and the baseline privacy key exchange prior to the provisioning process described below. The cable modem 160 thus has a secure channel with the CMTS 155 at the head end. It is assumed that the network infrastructure beyond the head end is a managed backbone for which reasonable security precautions have been taken, e.g. to secure servers. The provisioning server 140 itself, since it manages keys, must be very well secured. Nevertheless, it is assumed that the BTI 170 cannot trust

225 signals on the cable. That is, the threat model includes the possibility that a hostile intruder can masquerade as the cable head-end and fool the BTI into believing it is communicating with a legitimate service provider.

In accordance with an embodiment of the present invention, cryptographic means are advantageously utilized to authenticate the service

230 provider. The objective of the provisioning process is for the service provider to securely establish an association between a customer account and a cryptographic key, where the key is available only to the BTI (and the provisioning server). The key can be used to authenticate key exchanges later. In practice, where the key is a symmetric key, this means that the two sides share a string of random bits that

235 can be used as encryption and keys for message authentication codes (MAC). It is common to use different keys to encrypt and MAC in each direction , so if a 128- bit cipher is used, the provisioning scenario will result in at least 512 bits of shared random bits. Note that the cryptographic key can be a public key rather than a symmetric key, where the corresponding private key is stored in the BTI.

240 The service provider, SR, is assumed to have a public/private key pair. The private key is stored in a safe place and there are strict procedures for accessing this key. The public key, Ksp₊, is stored in the memory of the BTI or built into the BTI, for example by burning the key into read only memory. If this public key turns out to be source of attack (e.g. attackers successfully substitute a

245 rogue key into BTIs in a particular area and manage to spook the provisioning server), then the key can be further protected by storing it in tamper-resistant storage. It is advantageous that there be a public key infrastructure whereby the service provider issues public key certificates for the provisioning servers, e.g. RS. There can be several layers of hierarchy in practice. It is assumed that the private

250 key for the provisioning server is stored somewhere inside the network, and that when the BTI sends a message to the provisioning server, it is communicating with a secure location inside the network. The user obtains and installs the BTI, whether by merely plugging the device in or by installing software on a computer. The user picks up the phone

255 and dials a provisioning number (e.g. 611) to enable registration. In accordance with a preferred embodiment of the present invention, the following messages, as illustrated in Fig. 3, then take place. It should be noted that, in addition to or in lieu of the signatures indicated in Fig. 3, the values of the messages can be digitally signed or hashed, using a message authentication code (MAC) with each

260 message. Different keys can be utilized in each direction with the protocol. Such details are not included for simplicity of exposition and would be known to one of ordinary skill in the art.

At step 301, the BTI 170 receives the provisioning number:

265 U ^ BTI : 611

The BTI issues a SETUP message to the gate controller 110, which routes the call to a provisioning server 140 and returns a SETUP_ACK message containing the IP address of the provisioning server. The authentication information in the 270 SETUP message from the BTI can be null.

At step 302, the BTI 170 announces its existence to the provisioning server 140

BTI -» PS : yo!

275 and requests the certificate and public key from the provisioning server 140. At step 303, the provisioning server 140 provides its public key and certificate:

PS - BTI : K_PS+,CERT(PS,K_PS+). *iy-

280

Certificates are convenient here because they allow the BTI to store a public key, here the service provider's public key, and have confidence in another public key (here, the provisioning server's public key) if it carries a certificate signed by the private key corresponding to the service provider's public key stored in the BTI. 285 At step 304, the BTI 170 generates random symmetric keys, SK,

AK, and Rand transmits the following message to the provisioning server 170:

BTI -» PS : {SK, AK,N₀}κ, {info}_SK,{K}_K^

290 K is used to encrypt the message that is sent with a symmetric cipher. K itself is encrypted with the public key of PS to make sure nobody else can read it. SK is a session key that will be used for future communication with the provisioning server 170 for the remainder of the provisioning. AK is a symmetric key that is used to secure the audio channel. In practice, AK may actually be a master key

295 used to generate the actual keys used for encryption and to generate message authentication codes (MAC). No is a random nonce (a one-time identifier) used to prevent replay attacks (it is possible to avoid using nonces by including a hash of the received (challenged) message in every response). The reply from PS will contain No as well to link the two messages together, info contains the

300 information in the message, such as the network address (Media Access Control address, IP address, etc.) of the broadband telephony interface 170.

At step 305, the provisioning server 140 acknowledges the registration request and proves knowledge of the session key:

305 PS ^ BTI : {ACK,N₀,N,} SK

At this point, the session key is "good" and the network associates it with the particular IP endpoint.

At step 306, the BTI 170 sets up a voice connection with the 310 provisioning server 140 and uses the audio channel key, AK, to secure the voice path. In practice, the audio stream should be encrypted and protected using message authentication codes. For simplicity, the secure messages, M, on the audio channel are represented as {M}_AK- A this point, the BTI 170 completes the setup of the voice connection to the provisioning server 140. 315 At step 307, the provisioning server 140 prompts the user for her authentication information:

PS -> U : {"enter auth info"}^

320 The authentication information can be implemented in many different ways. For example, the authentication information can be a work order number that has been given to a customer (or to an installer) after the customer has subscribed for the service. The work order must be supplied when the BTI is provisioned to identify the customer account. As another example, the authentication information can be

325 a credit card number, address, etc. that is provided by the user who subscribes for the service during the provisioning call itself. The audio stream is secured using AK from the PS 140 to the BTI 170, which converts it to an analog voice for the user.

At step 308, the user speaks or dials her authentication information

330 in response to the prompt:

U -Ξ> BTI : auth info

At step 309, the BTI 140 sends the authentication information over the secure 335 audio channel to the provisioning server 140:

BTI - PS : {auth info}^

At step 310, the BTI 140 generates a public/private key pair for the 340 user and sends the public key, Ky₊, to the provisioning server 140.

BTI -^ PS .- N,,^,^} SK The provisioning server 140 associates the authentication information sent over 345 the secure audio channel with the public key, Ku₊, sent over the secure control channel. The PS 140 can do this because (a) it is aware that both came from the same network address and (b) it successfully authenticates and decrypts both the audio and control channel information using the keys, AK and SK, which the PS knows are associated with the same broadband telephony interface 170. The PS 350 140 stores the BTI's public key for later usage and acknowledges receipt:

At this point, the user is provisioned. The BTI 170 and the provisioning server 355 140 share a long-term symmetric key that the provisioning server can associate with the subscriber account. In practice, the BTI and PS may share up to 512 random bits to comprise four 128-bit encryption and MACing keys, as described above. In future sessions, the BTI 170 can generate a session key, sign it, and send it under the public key of the provisioning server 140 or the long-term key it 360 shares with the server in a similar manner. No interaction from the user is necessary to establish these future session keys.

The foregoing Detailed Description is to be understood as being in every respect illustrative and exemplary, but not restrictive, and the scope of the

365 invention disclosed herein is not to be determined from the Detailed Description, but rather from the claims as interpreted according to the full breadth permitted by the patent laws. It is to be understood that the embodiments shown and described herein are only illustrative of the principles of the present invention and that various modifications may be implemented by those skilled in the art without

370 departing from the scope and spirit of the invention.

Claims

What is claimed is:

1. A method of provisioning a user's broadband telephony interface comprising the steps of: receiving information authenticating a provisioning server; establishing a communication channel between the user and the provisioning server over which is transmitted authorization information from the user to the provisioning server; and encrypting and transmitting a cryptographic key associated with the user to the provisioning server.

2. The method of claim 1 wherein the communication channel is a voice channel connection.

3. The method of claim 2 wherein the communication channel is encrypted using an audio channel key which is encrypted and transmitted to the provisioning server prior to establishing the communication channel.

4. The method of claim 3 wherein the cryptographic key associated with the user is encrypted using a session key which is encrypted and transmitted to the provisioning server prior to establishing the communication channel.

5. The method of claim 4 wherein the session key and the audio channel key are encrypted using a cryptographic key that is encrypted using a cryptographic key associated with the provisioning server and transmitted to the provisioning server with the encrypted session and audio channel key.

6. The method of claim 5 wherein the cryptographic key associated with the provisioning server is received with the information authenticating the provisioning server.

7. The method of claim 6 wherein a random nonce is included with the encrypted session key.

8. The method of claim 1 wherein the information authenticating the provisioning server is a digital certificate.

9. The method of claim 1 wherein the cryptographic key associated with the user is a symmetric key.

10. The method of claim 1 wherein the cryptographic key associated with the user is a public key corresponding to a private key stored in the broadband telephony interface.

1 1. The method of claim 1 wherein a hash is included with each transmission.

12. A broadband telephony interface comprising: a first interface to a user telephone; a second interface to a communication network with access to a provisioning server; memory for storing cryptographic keys; a processor connected to the memory and the first and second interfaces for executing program instructions, the program instructions causing the processor to perform the steps of: receiving information authenticating the provisioning server; establishing a communication channel between the user telephone and the provisioning server over which is transmitted authorization information from the user to the provisioning server; and encrypting and transmitting a cryptographic key associated with the user to the provisioning server.

13. The broadband telephony interface of claim 12 wherein the communication channel is a voice channel connection.

14. The broadband telephony interface of claim 13 wherein the communication channel is encrypted using an audio channel key which is encrypted and transmitted to the provisioning server prior to establishing the communication channel.

15. The broadband telephony interface of claim 14 wherein the cryptographic key associated with the user is encrypted using a session key which is encrypted and transmitted to the provisioning server prior to establishing the communication channel.

16. The broadband telephony interface of claim 15 wherein the session key and the audio channel key are encrypted using a cryptographic key that is encrypted using a cryptographic key associated with the provisioning server and transmitted to the provisioning server with the encrypted session and audio channel key.

17. The broadband telephony interface of claim 16 wherein the cryptographic key associated with the provisioning server is received with the information authenticating the provisioning server.

18. The broadband telephony interface of claim 17 wherein a random nonce is included with the encrypted session key.

19. The broadband telephony interface of claim 12 wherein the information authenticating the provisioning server is a digital certificate.

20. The broadband telephony interface of claim 12 wherein the cryptographic key associated with the user is a symmetric key.

21. The broadband telephony interface of claim 12 wherein the cryptographic key associated with the user is a public key corresponding to a private key stored in the broadband telephony interface.

22. The broadband telephony interface of claim 12 wherein a hash is included with each transmission.

23. A method of operating a provisioning server comprising the steps of: receiving a request to be provisioned from a broadband telephony interface; transmitting authentication information to the broadband telephony interface; receiving authorization information over a communication channel established between a user of the broadband telephony interface and the provisioning server; and receiving an encrypted cryptographic key associated with the user from the broadband telephony interface.

24. The method of claim 23 wherein the communication channel is a voice channel connection.

25. The method of claim 24 wherein the communication channel is encrypted using an audio channel key which is received from the broadband telephony interface prior to establishing the communication channel.

26. The method of claim 25 wherein the cryptographic key associated with the user is encrypted using a session key which is received from the broadband telephony interface prior to establishing the communication channel.

27. The method of claim 26 wherein a cryptographic key associated with the provisioning server is transmitted to the broadband telephony interface and the session key and the audio channel key are received encrypted using the cryptographic key associated with the provisioning server.

28. The method of claim 27 wherein the cryptographic key associated with the provisioning server is transmitted with the authentication information to the broadband telephony interface.

29. The method of claim 28 wherein a random nonce is included with encrypted session key and audio channel key.

30. The method of claim 23 wherein the authentication information is a digital certificate.

31. The method of claim 23 wherein the cryptographic key associated with the user is a symmetric key.

32. The method of claim 23 wherein the cryptographic key associated with the user is a public key corresponding to a private key stored in the broadband telephony interface.

33. The method of claim 23 wherein a hash is included with each transmission.