THE METAPHOR IS THE KEY: CRYPTOGRAPHY,
THE CLIPPER CHIP, AND THE CONSTITUTION

A. Michael Froomkin

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The increases in personal privacy and communications security promised by cryptography come at the expense of those who benefit from insecure communications. If every telephone call is routinely encrypted, domestic law enforcement agencies, such as the FBI and local police forces, will find wiretapping harder or even impossible. If information on computers is routinely encrypted police may find evidence inaccessible or incomprehensible. When sophisticated encryption technologies are used abroad, intelligence agencies such as the NSA, which routinely seek to penetrate the communications of foreign powers, find their missions complicated. To the extent [Page 743]that American citizens are better off because wiretaps help catch and convict criminals, and to the extent that communications intelligence protects the national interest from foreign threats, developments that impede legitimate wiretaps may make us all worse off.

The fear of losing electronic surveillance capabilities because of advances in encryption technology has produced a three-pronged reaction from the law enforcement and intelligence communities. First, their spokespersons have begun a public relations offensive designed to explain why these capabilities matter.{137} Second, they have sought legislation requiring that telephone networks and other similar communications channels be designed in a manner that facilitates wiretapping.{138} Third, they have designed and supported EES, best known in its most famous implementation, the Clipper Chip, which enables the government to keep a copy of the key needed to decrypt all communications using EES. These activities share the premise that it is reasonable for the government to request, and in some cases require, that private persons communicate in a manner that makes interception by the government at least practical and preferably easy.

1. Why the Government Wants EES to Replace DES

The Administration{140} makes two types of arguments in favor of EES. In its hard sell, the Administration, primarily through the [Page 744]FBI, paints a lurid picture of law enforcement stripped of an essential crime-detection and evidentiary tool--wiretapping--while pornographers, drug dealers, terrorists, and child molesters conspire via unbreakable ciphers, storing their records and child pornography in computers that become virtual cryptographic fortresses. Meanwhile, the intelligence agencies, primarily the NSA, quietly murmur that existing policies have proved ineffective in preventing the increasing use of unescrowed encryption, and suggest that their proposals should be adopted to prevent developments that might (or might not, they won't say) undermine the nation's communications intelligence capabilities.

In its soft sell, the government argues that if the NSA has designed a cryptographic system that it is willing to certify as secure and make available to the American public, the government has an obligation to take steps to prevent that cipher from being used against it by criminals and foreign governments. In fact, the current national standard cipher, DES, is strong enough that the U.S. government has sought to prevent its export and may indeed regret having let the algorithm become publicly available.{141} EES, the argument goes, just maintains the status quo. Even if everyone used a Clipper-equipped telephone, telephone conversations would be no less secure against legitimate government wiretapping than they are today, while being more secure against illicit eavesdropping.{142}

a. Domestic Law Enforcement

From the statistics available, it is difficult to determine how [Page 745]much difference wiretaps actually make.{144} The FBI estimates that wiretaps play a role in an average of 2200 convictions per year,{145} but it is unclear how many of these convictions could have been obtained without wiretaps. Despite an almost 50% increase since 1983, court-ordered wiretaps are still relatively rare: only 919 were authorized in 1992 for all federal, state, and local police forces.{146} Of these, only 141 wiretap orders covered electronic devices such as faxes, digital display pagers, voice pagers, cellular phones, or electronic mail. In 1993, the 976 active court-ordered wiretaps allowed police to hear approximately 1.7 million conversations involving nearly 94,000 persons. The listeners described about 20% of the conversations as incriminating.{147} The law enforcement community suggests that wiretaps make the biggest difference in the largest cases because wiretaps have been used to gather evidence in 90% of the terrorism cases brought to trial.{148} The average cost of a wiretap was $57,256 in 1993,{149} so it may be that the biggest cases are the only ones in which the expense of monitoring a telephone line seems justified.{150}

Statistics aside, it seems only logical that the spread of strong, user-friendly cryptography would increase the risk that evil people will be able to frustrate law enforcement attempts to crack their computers or bug their telephones. Whether the risk has yet [Page 746]manifested itself is less clear. For all its predications of disaster in the making, "the FBI has not been able to point to a single instance to date [(September 1994)] where encryption has hampered [its] investigation of a case."{151}

Nevertheless, the fear that rogue cryptography might allow "terrorists, drug dealers, and other criminals"{152} to evade law enforcement seems to supply a large part of the motivation for the Administration's support for EES. One can only sympathize with officials who were, no doubt, asked whether they wished to go down in history as the individuals responsible for letting loose a technology that might someday hamper the investigation of a terrorist threat to a large population center.{153} Faced with the FBI's Manichaean vision of, on the one hand, a world of rampant cryptography in which the bad guys remain impregnable behind cryptological walls and, on the other hand, an ambitious plan to return to the status quo ante in which the police remain able to intercept and understand most if not all electronic communication, it is not surprising that the Clinton Administration opted for what must have appeared to be the safer course.
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b. Intelligence-Gathering

The two functions are related, but different. Acquisition and decryption of foreign communications are the stuff of headlines: listening to the Soviet President's telephone calls made from his limousine or breaking German codes during World War II. Traffic analysis is more subtle, but no less important. It is the study of the sources and recipients of messages, including messages that the eavesdropper cannot understand. In wartime, traffic analysis allows intelligence agencies to deduce lines of command. Changes in the volume and direction of traffic can signal the imminence of operations.{155}

Widespread foreign access to even medium-grade cryptography makes it more difficult for U.S. communications intelligence to select the messages that are worth decrypting, or even worth reading.{156} Worse, it makes traffic analysis much more difficult. So long as most electronic communications are unencrypted, intelligence agencies are able to sort messages in real time, and identify those of interest, or those which warrant further attention.{157} [Page 748]Furthermore, if most traffic is plaintext, then ciphertext cries out for attention--here is someone with something to hide. Even if the message cannot be decrypted quickly, the source can be flagged for traffic analysis, which enables the intelligence agency to build up a picture of the persons with whom the source communicates. If everyone is using strong cryptography, then the most secret messages no longer stand out.

c. Failure of Laws Designed to Prevent the Spread of Strong Cryptography

The ITAR have failed to prevent the spread of strong cryptography. The ITAR prohibit export of cryptographic software,{165} nevertheless software created in the United States routinely and quickly finds its way abroad. For example, when version 2.6 of PGP, a popular military-grade cryptography program, was released in the United States by graduate students at MIT as freeware,{166} a researcher at the Virus Test Center at the University of Hamburg, in Germany, received a copy within days from an anonymous remailer.{167} He then placed it on his internationally-known Internet distribution site.{168} As would-be sellers of cryptographic products have frequently testified to Congress, the major effect of the ITAR is to prevent U.S. companies from competing with those foreign companies that sell sophisticated cryptographic software abroad.{169}

[Page 751]Meanwhile, enforcement of the ITAR has produced absurd results. The State Department has refused to license the export of a floppy disk containing the exact text of several cryptographic routines identical to those previously published in book form.{170} The refusal was all the more bizarre because the book itself was approved for export.{171} The only reasons given by the State Department for its refusal were that "[e]ach source code listing has been partitioned into its own file and has the capability of being compiled into an executable subroutine,"{172} and that the source code is "of such a strategic level as to warrant" continued control.{173} The State Department also concluded that the "public domain" exception to the ITAR{174} did not apply and--most bizarrely of all--that its decision was consistent with the First Amendment.{175}

ii. "Classified at Birth"

The existence of a number of high-level cryptographic algorithms in public circulation, some patented,{181} some not, suggests that the Inventions Secrecy Act has been far from successful at preventing the spread of strong cryptography.{182}

2. How Clipper Works

"Here, here, here be my keys; ascend my chambers;
search, seek, find out."{183}

a. A Tale of Three Keys

From the government's point of view, EES relies on three keys: the session key,{188} the chip key, and the family key. The session key is what SKIPJACK uses to encrypt and decrypt the conversation. Every conversation has a new session key, and any third party seeking to eavesdrop on the conversation would need to have the session key to decrypt the conversation. Oddly, the Clipper Chip does not select the session key; indeed, the Clipper Chips do not care how the telephones do this.

Suppose Alice wants to have a secure conversation with Bob. Alice calls Bob, then pushes the red button. At this point, the two Clipperphones have to agree to a session key according to a method selected by the manufacturer. The maker of the Clipperphone is free to use as secure a method as she likes. The two Clipperphones might, for example, use a supersecure method of agreeing on the session key which is so safe that two strangers who have never met before can agree on a session key in public while being overheard, and yet anyone who overhears what they say will still be unable to work out what the key is.{189} Assume that Alice and Bob use telephones that have this supersecure selection method built in. Once the two telephones agree on the session key, each phone feeds the key to its Clipper Chip.{190} As soon as the Clipper Chips are [Page 755]told the session key, they begin the Clipper telephone session. The first step in a Clipper telephone session is to undermine the eavesdropper-proof creation of the session key by transmitting the session key in encrypted form for the benefit of any public servants who may be listening.

At the start of every Clipper session, a Clipper Chip sends a stream of data called a Law Enforcement Access Field (LEAF).{191} Unless Bob's Clipper Chip receives a valid LEAF from Alice's chip, Bob's chip will not talk with it.{192} As can be seen from the Figure on page 756, the LEAF is built in layers. At the center lies the session key. The chip encrypts the session key with the unique chip key. It then appends the sending chip's serial number and a checksum, then reencrypts the data with the family key, which is a master key held by the government.{193} [Page 756]

In short, eavesdroppers seeking access to the session key must use two keys to decrypt the LEAF: the family key (which is common to all chips) and the chip key (which is different for every chip). Assuming that the family key will be in fairly wide circulation,{194} the security of the Clipper Chip stands or falls on the security of the master list of chip keys. This list, or the two lists of key segments, would be of enormous value to any attacker, such as a foreign government bent on industrial espionage. The way in which the keys are created, and the method by which they are held and released, are critical elements of the user's security.

When a public servant engaged in a lawful wiretap first comes across a Clipper session, she records it, including the LEAF. The public servant must now acquire the family key if she does not already possess it. According to NIST, the family keys will not be transmitted to law enforcement personnel, but will instead be stored [Page 757]in special circuit boards capable of being installed in ordinary PCs.{195} Once decrypted with the family key, the LEAF reveals the serial number of the Clipper Chip and also reveals the encrypted session key. The public servant must then contact the two escrow agencies, giving them the chip's serial number and a legally valid reason for the wiretap, usually in the form of a warrant from a state court, a federal court, or the special Foreign Intelligence Surveillance Act (FISA) court.{196} The requestor must "certify that [the] necessary legal authorization for interception has been obtained to conduct electronic surveillance regarding these communications."{197} How this certification operates when the legal basis [Page 758]is "exigent circumstances" (which is determined by the same officer who would be requesting the key segment), is not explained,{198} perhaps because warrantless wiretaps based on exigent circumstances are relatively rare.{199} There remains some doubt as to how the NSA and other agencies in the national security community will obtain keys. It is notable that in a recent meeting involving the FBI, the NSA, and AT&T's Bell Labs, "the NSA did not answer a question as to whether the national security community would obtain keys from the same escrow mechanism for their (legally authorized) intelligence gathering or whether some other mechanism would exist for them to get the keys."{200}

The escrow agents have no duty to make any independent inquiries as to the adequacy of the certification before releasing the key segments.{201} Once satisfied that the wiretap request appears legitimate (in that it comes from someone authorized to make a request and contains her certification that adequate legal authority exists), the escrow agents are required to disclose the key segments for the key for which the serial number was submitted. The public servant requesting the key fragments puts them together and uses [Page 759]the reconstituted chip key to decrypt the session key. Armed with the decrypted session key, the public servant can at last decrypt the conversation. Because the presence of the Clipper Chip has no effect on the applicable constitutional and statutory rules, the public servant remains obligated to minimize the intrusion.{202}

In summary, a public servant might decrypt an EES message as follows:

Public servant
(1) intercepts the message, including the LEAF (128-bit LEAF encrypted with the family key);
(2) decrypts the LEAF with the family key (32-bit chip ID, 80-bit session key encrypted with chip key, 16-bit checksum);
(3) contacts her escrow agents, reports the chip ID, and avers existence of the legal authority for the wiretap;
(4) receives two 80-bit key segments;
(5) XORs{203} the key segments to produce an 80-bit chip key;
(6) decrypts the encrypted session key with the chip key;
(7) decrypts the entire message with her decrypted session key.

b. The Escrow Agents' Critical Role

After the keys are generated, the escrow agents will be given a disk containing lists of chip serial numbers and an associated 80-bit number which represents half the information needed to recreate a chip's key. Both key segments must be combined to retrieve the chip key, and neither segment alone provides the holder with any information as to the chip key's contents.{207}

Although the escrow agents do not check the bona fides of any requests for key fragments, they do require a substantial amount of paperwork before releasing a key. The escrow agents are also required to keep detailed records of key segment requests and releases. The existence of this paper trail should provide a significant disincentive to rogue wiretapping requests by agents in the field. Similarly, NIST has announced an elaborate system of safeguards to protect each Clipper Chip's unique key. The scheme [Page 761]involves complex rationing of information and mutual monitoring by the escrow agents from the moment the Clipper Chip is created. Further security attends the inscription of the key upon a Clipper Chip, its subsequent division into two key segments, and ultimate safeguarding by the two escrow agents.{208}

The security precautions introduced by NIST in late 1994 are complex. To the nonspecialist they appear sufficient to prevent security breaches at the time the keys are "burned in" and to prevent surreptitious copying or theft of the key list from the escrow agents. But no amount of technical ingenuity will suffice to protect the key fragments from a change in the legal rules governing the escrow agents. Thus, even if the technical procedures are sound, the President could direct the Attorney General to change her rules regarding the escrow procedures. Because these rules were issued without notice or comment, affect no private rights, and (like all procedural rules) can therefore be amended or rescinded at any time without public notice, there is no legal obstacle to a secret amendment or supplement to the existing rules permitting or requiring that the keys be released to whomever, or according to whatever, the President directs. Because the President's order would be lawful, none of the security precautions outlined by NIST would protect the users of the EES system from disclosure of the key segments by the escrow agents. Nothing in the EES proposal explicitly states that the NSA will not keep a set of keys; indeed, the only way to acquire a set of EES-compliant chips is to have the device that incorporates them tested and approved by the NSA. Similarly, although the specifications for the decrypt processor call for it to delete keys when a warrant expires and to automatically send a confirmation message to the key escrow agents, the interim model (there is only one) in use by law enforcement organizations relies on manual deletion.{209} [Page 762]

c. Limited Recourse for Improper Key Disclosure

If the intercepted communication were an e-mail or a file transfer, rather than a telephone call, the chip owner subject to an illegal or inadvertent disclosure by the escrow agents may be in a particularly weak position if the information ever makes its way to court: many Title III protections granted to voice communications do not apply to transfers of digitized data.{217}

Shortly before the 103d Congress adjourned, Congressman George Brown introduced the Encryption Standards and Procedures Act of 1994,{218} which would have waived the sovereign immunity of the United States for "willful" but unauthorized disclosures of key fragments by its officials--and excluded liability in all other circumstances.{219} In the absence of similar legislation, however, there [Page 764]may currently be no monetary remedy even for a "willful" disclosure.