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Communications of the ACM

Tapping on My Network Door


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Readers of this column are familiar with the risks of illegal monitoring of Internet traffic. Less familiar, but perhaps just as serious, are the risks introduced when law enforcement taps that same traffic legally.

Ironically, as insecure as the Internet may be in general, monitoring a particular user's traffic as part of a legal wiretap isn't so simple, with failure modes that can be surprisingly serious. Packets from one user are quickly mixed in with those of others; even the closest thing the Internet has to a telephone numberthe IP addressoften changes from one session to the next and is generally not authenticated. An Internet wiretap by its nature involves complex software that must reliably capture and reassemble the suspect's packets from a stream shared with many other users. Sometimes an Internet service provider (ISP) is able to provide a properly filtered traffic stream; more often, there is no mechanism available to separate out the targeted packets.

Enter Carnivore. If an ISP can't provide exactly the traffic covered by a court order, the FBI offers its own packet sniffer, a PC running special software designed especially for wiretap interception. The Carnivore computer (so named, according to press reports, for its ability to "get to the meat" of the traffic) is connected to the ISP's network segment expected to carry the target's traffic. A dial-up link allows FBI agents to control and configure the system remotely.

Needless to say, any wiretapping system (whether supplied by an ISP or the FBI) relied upon to extract legal evidence from a shared, public network link must be audited for correctness and must employ strong safeguards against failure and abuse. The stringent requirements for accuracy and operational robustness provide especially fertile ground for many familiar risks.

First, there is the problem of extracting exactly (no more and no less) the intended traffic. Standard network monitoring techniques provide only an approximation of what was actually sent or received by any particular computer. For wiretaps, the results could be quite misleading. If a single packet is dropped, repeated, or miscategorized (common occurrences in practice), an intercepted message could be dramatically misinterpreted. Nor is it always clear "who said what." Dynamic IP addresses make it necessary to capture and interpret accurately not only user traffic, but also the messages that identify the address currently in use by the target. Furthermore, it is frequently possible for a third party to alter, forge, or misroute packets before they reach the monitoring point. This usually cannot be detected by the monitor. Correctly reconstructing higher-level transactions, such as email, adds still more problems.

The general-purpose nature of Carnivore entails its own risks. ISPs vary greatly in their architecture and configuration; a new component that works correctly in one might fail badlysilently or destructivelyin another. Carnivore's remote control features are of special concern, given the potential for damage should a criminal gain control of an installed system. ISPs are understandably reluctant to allow such devices to be installed deep within their infrastructures.

Complicating matters further are the various kinds of authorized wiretaps, with different legal standards for each. Because Carnivore is a general-purpose black box, an ISP (or a court) cannot independently verify that any particular installation has been configured to collect only the traffic for which it is legally authorized.

Internet wiretaps raise many difficult questions, both legal and technical. The legal issues are being debated in Congress, in the courts, and in the press. The technical issues include the familiar (and difficult) problems of software correctness, complex system robustness, user interfaces, audit, accountability, and security.

Unfortunately, there's no systematic way to be sure that any system as complex and sensitive as Carnivore works as intended. A first step, the best our community has yet found for this situation, is to subject the source code and system details to wide scrutiny. Focused reviews by outside experts should be part of this process, as should opening the code to the public. While the details of particular wiretaps may properly be kept secret, there's no reason for the wiretapping mechanism itself to be concealed. The observation that sunshine is the best disinfectant applies at least as well to software as it does to government.

Even if we could guarantee the correctness of software, difficult systems issues still remain. Software alone cannot ensure that the reviewed code is what is actually used, that filters and configuration files match court orders, that evidence is not tampered with, and so on.

Ultimately, it comes down to trustof those who operate and control the system and of the software itself. Trusting a law enforcement agent to be honest and faithful to duty in a free society is one thing. Trusting complex, black-box software to be correct and operationally faithful to specifications, however, is quite another.

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Authors

Matt Blaze (mab@research.att.com) and Steven M. Bellovin (smb@research.att.com) are researchers at AT&T Labs in Florham Park, NJ.


©2000 ACM  0002-0782/00/1000  $5.00

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