1996 Congressional Hearings
Intelligence and Security



JUNE 26, 1996

Thank you for the opportunity to speak with you about the technical impact of encryption policy. It is a privilege to be here, and I hope my perspective will be useful to you.

Let me begin by describing my own background and biases. I am a Principal Research Scientist in the area of computer security and cryptology at AT&T Research in Murray Hill, New Jersey. I also hold a number of ancillary appointments related to computer security; among others, I teach an occasional graduate course in the subject at Columbia University, and I serve as co-chair of the Federal Networking Council Advisory Committee subcommittee on security and privacy (which advises Federal agencies on computer networking issues). However, the views I am presenting here today are my own, and should not be taken to represent those of any organization with which I happen to be affiliated.

I am a computer scientist by training; my Ph.D. is from the Princeton University Computer Science department, and my primary research areas are cryptology, computer security, and large-scale distributed systems. Much of my research focuses on the management of encryption keys in networked computing systems and understanding the risks of using cryptographic techniques to accomplish security objectives. Recent government initiatives in encryption, such as the "Clipper Chip," have naturally been of great interest to me, in no small part because of the policy impact they have on the field in which I work, but also because they present a number of very interesting technical and scientific challenges in their own right.

My testimony today focuses on three areas. First, I will discuss the role and risks of cryptographic techniques for securing the current and future electronic world. Next, I will examine in more detail the security implications of the limitations imposed on US-based cryptographic systems through the government's export policies. Finally, I will discuss the technical aspects of the Administration's current approach to cryptography policy, which promotes "key escrow" systems.


The importance of cryptographic techniques for securing modern computer and communications systems is widely recognized today. Evidence of the scope of this recognition can be found in the increasing number of hardware, software, and system vendors that offer encryption in their products, the increasing demand for high-quality encryption by users in a widening array of applications, and the growing, thriving community of cryptologic researchers of which I am a part. It is vital that those who formulate our nation's policies and official attitude toward encryption understand the nature of the underlying technology and the reasons for its growing importance to our society.

The basic function of cryptography is to separate the security of a message's content from the security of the medium over which it is carried. For example, we might encrypt a cellular telephone conversation to guard against eavesdroppers (allowing the call to be transmitted safely over easily-intercepted radio frequencies), or we might use encryption to verify that documents, such as contracts, have not been tampered with (removing the need to safeguard a copy of the original). The idea that this might be possible is not a new one; history suggests that the desire to protect information is almost as old as the written word itself. Perhaps as a consequence of the invention of the digital computer, our understanding of the theory and practice of cryptography has accelerated in recent years, with a number of new techniques developed and many new applications emerging. Among the most important of the recent techniques is "public key cryptography." It allows secure messages to be exchanged without the need for specific advance arrangements between parties. A related notion is the "digital signature," which allows messages to be "signed" in a way that verifiably associates the signer of a message with its content.

Modern cryptographic techniques are based on the application of simple, if repetitive, mathematical functions, and as such lend themselves nicely to implementation by computer programs. Any information that can be represented digitally can be protected by encryption, including computer files, electronic mail messages, and even audio and video signals such as telephone calls, radio, and television. Encryption can be performed by means of software on general-purpose computers, through special-purpose hardware, or by special programming of microprocessor-based electronic products such as the next generation of cellular telephones. The basic cost of encryption in terms of computational power required is quite low, and the marginal cost of including encryption in a software-based computer program or a programmable electronic product is essentially zero.

Why, then, has encryption recently enjoyed so much attention? The reasons can be found from two perspectives: the technology of modern communication systems, and the new purposes for which we are relying on digital information.

First, the technology and economics of modern communications and computing systems strongly favors media that have little inherent security. For example, wireless telephones have great advantages in convenience and functionality compared with their familiar wired counterparts and are comprising an increasing proportion of the
telephone network. This also makes eavesdropping much easier for curious neighbors, burglars identifying potential targets, and industrial spies seeking to misappropriate trade secrets. Similarly, decentralized computer networks such as the Internet have lower barriers to entry, are much less expensive, are more robust and can be used to accomplish a far greater variety of tasks than the proprietary networks of the past, but, again, at the expense of intrinsic security. The Internet makes it virtually impossible to restrict, or even predict, the path that a particular message will traverse, and there is no way to be certain where a message really originated or whether its content has been altered along the way. It is possible, even common, for electronic mail messages to route through the computers of competitors. This is not a result of sloppy design or poor planning on the part of the Internet's architects; on the contrary, these properties are a direct consequence of the technological advances that make the Internet efficient and useful in the first place.

Second, electronic communication is becoming increasingly critical to the smooth functioning of our society and our economy and even to protect the safety of human life. Communication networks and computer media are rapidly replacing less efficient, traditional modes of interaction whose security properties are far better understood. As
teleconferencing replaces face-to-face meetings, electronic mail replaces letters, electronic payment systems replace cash transactions, and on-line information services replace written reference materials, we gain a great deal in efficiency, but our assumptions about the reliability of very ordinary transactions are often dangerously out-of-date.

Put another way, the trend in communication and computing networks has been away from closed systems in favor of more open ones and the trend in our society is to rely on these new systems for increasingly serious purposes. There is every reason to believe that these trends will continue, and even accelerate, for the foreseeable future. Cryptography plays an important and clear role in helping to provide security assurances that at least mirror what we have come to expect from the older, more familiar communications methods of the not-so-distant past.


The "strength" of an encryption system depends on a number of variables, including the mathematical properties of the underlying encryption function, the quality of the implementation, and the number of different "keys" from which the user is able to choose. It is very important that a cryptosystem and its implementation be of high
quality, since an error or bug in either can expose the data it protects to unexpected vulnerabilities. Although the mathematics of cryptography is not completely understood and cipher design is an exceptionally difficult discipline (there is as yet no general "theory" for designing cipher functions), there are a number of common
cipher systems that have been extensively studied and that are widely trusted as building blocks for secure systems. The implementation of practical systems out of these building blocks, too, is a subtle and difficult art, but commercial experience in this area is beginning to lead to good practices for adding high-quality encryption systems to
software and hardware. Users and developers of secure systems can protect against weaknesses in these areas by choosing only cipher functions that have been carefully studied and by ensuring that their implementation follows good engineering practices.

The most easily quantified variable that contributes to the strength of an encryption system is the size of the pool of potential values from which the cryptographic keys are chosen. Modern ciphers depend on the secrecy of the users' keys, and a system is considered well-designed only if the easiest "attack" involves trying every possible key, one after the other, until the correct one is found. The system is secure only if the number of keys is large enough to make such an attack infeasible. Keys are usually specified as a string of "bits," and adding one bit to the key length doubles the number of possible keys. An important question, then, is the minimum key length sufficient to resist a key search attack in practice.

Last November, I participated in a study, organized by the Business Software Alliance, aimed at examining the computer technology that might be used by an "attacker" in order to determine the minimum length keys that should be used in commercial applications. We followed an unusually conservative methodology in that we assumed that the attacker would have only available standard "off-the-shelf" technology and is constrained to purchase in single-unit quantities with no economies of scale. That is, our methodology would tend to produce a recommendation for shorter keys than would an analysis using the more conventional approach of giving the potential attacker every benefit of the doubt in terms of technological advantages he might enjoy. Nonetheless, we concluded that the key lengths recommended in existing U.S. government standards (e.g., the Data Encryption Standard, with a 56-bit key) for domestic use are far too short and will soon render data protected under them vulnerable to attack with only modest resources. We concluded that keys today should be a bare minimum of 75 bits long, and that systems being fielded today to secure data over the next twenty years must employ keys of at least 90 bits. I have included a copy of our report as an appendix to my testimony.

Attempting to design systems "at the margins" by using the minimum key length needed is a dubious enterprise at best. Because even a slight miscalculation as to the technology and resources available to the potential attacker can make the difference between a secure system and an insecure one, prudent designers specify keys that are longer than the minimum they estimate is needed to resist attack, to provide a margin for error.

Current U.S. policy encourages the designers of encryption systems to take exactly the opposite approach. Encryption systems designed for export from the United States at present generally must use keys no more than 40 bits long. Such systems provide essentially no cryptographic security, except against the most casual "hacker." Examples of 40 bit systems being "broken" through the use of spare computer time on university computer networks are commonplace. Unfortunately, it is not only users outside the U.S. who must make do with the inferior security provided by such short keys. Because of the difficulty of maintaining multiple versions of software, one for domestic sale and one for export, and the need for common interoperability standards, many US-based products are available only with export-length keys.

There is no technical, performance, or economic benefit to employing keys shorter than needed. Unlike, for example, the locks used to protect our homes, very secure cryptographic systems with long keys are no more expensive to produce or any harder to design or use than weaker systems with shorter keys. The only reason vendors design systems with short keys is to comply with export requirements.

The key length figures and analysis in this section are based on so-called "secret key" cryptosystems. For technical reasons, current public key cryptosystems employ much longer keys than secret key systems to achieve equivalent security (public keys are measured in hundreds or thousands of bits). However, virtually all systems that use public key cryptography also rely on secret key cryptography, and so the overall strength of any system is limited by the weakest encryption function and key length in it.


A number of recent Administration initiatives have proposed that future cryptosystems include special "key escrow" provisions to facilitate access to encrypted data by law enforcement and intelligence agencies. In a such systems, copies of keys are automatically deposited, in advance, with third parties who can use them to arrange for law enforcement access if required in the future. Several key escrow systems have been proposed by the Administration, differing in the details of how keys are escrowed, and who the third party key holders are. In the first proposal, called the "Clipper chip," the system is embedded in a special tamper-resistant hardware-based cryptosystem and copies of keys are held by federal agencies. In the more recent "public key infrastructure" proposal, keys are escrowed at the time a new public key is generated and are held by the organization (public or private) responsible for certification of the public key.

Although the various key escrow proposals differ in the details of how they accomplish their objective, there are a number of very serious fundamental problems and risks associated with all of them.

There are some appropriate commercial applications of key escrow techniques. A properly designed cryptosystem makes it essentially impossible to recover encrypted data without the correct key. This can be a double-edge sword; the cost of keeping unauthorized parties out is that if keys are lost or unavailable at the time they are needed, the owner of encrypted data will be unable to make use of his own information. This problem, of balancing secrecy with assurances of continued availability, remains an area of active research, and commercial solutions are starting to emerge. The Administration's initiatives do not address this problem especially well, however.

The first problem with key escrow is the great increase in engineering complexity that such systems entail. The design and implementation of even the simplest encryption systems is an extraordinarily difficult and delicate process. Very small changes can introduce fatal security flaws that often can be exploited by an attacker. Ordinary (non-escrowed) encryption systems have conceptually rather simple requirements (for example, the secure transmission of data between two parties) and yet, because there is no general theory for designing them, we still often discover exploitable flaws in fielded systems. Key escrow renders even the specification of the problem itself far more complex, making it virtually impossible to assure that such systems work as they are intended to. It is possible, even likely, that lurking in any key escrow system are one or more design weaknesses that allow recovery of data by unauthorized parties. The commercial and academic world simply does not have the tools to analyze or design the complex systems that arise from escrow.

Key escrow is so difficult that even systems designed by the classified world can have subtle problems that are only discovered later. In 1994 I discovered a new type of "protocol failure" in the Escrowed Encryption Standard, the system on which the Clipper chip is based. The failure allows, contrary to the design objectives of the system, a rogue user to circumvent the escrow system in a way that makes the data unrecoverable by the government. Other weaknesses have been discovered since then that make it possible, for example, to create incriminating messages that appear to have originated from a particular user.

It should be noted that these weaknesses have been discovered in spite of the fact that most of the details of the standard are classified and were not included in the analysis that led to the discovery of the flaws. But these problems did not come about because of incompetence on the part of the system's designers. Indeed, the U.S. National Security Agency is likely the most advanced cryptographic enterprise in the world, and is justifiably entrusted with developing the cryptographic systems that safeguard the government's most important military and state secrets. The reason the Escrowed Encryption Standard has flaws that are still being discovered is that key escrow is an extremely difficult technical problem, with requirements unlike anything previously encountered.

A second problem with key escrow arises from the difficulty of operating a key escrow center in a secure manner. According to the Administration (for example, see the May 20, 1996 White House draft report "Enabling Privacy, Commerce, Security and Public Safety in the Global Information Infrastructure"), key escrow centers must be prepared to respond to law enforcement requests for escrowed data 24 hours a day, completing transactions within two hours of receiving each request. There are thousands of law enforcement agencies in the United States authorized to perform electronic surveillance, and the escrow center must be prepared to identify and respond to any of them within this time frame. If the escrow center is also a commercial operation providing data recovery services, it may also have tens of thousands of additional private sector customers that it must be prepared to serve and respond to. There are few, if any, secure systems that operate effectively on such a scale and under such tightly-constrained response time. The argument, advanced by the Administration, that escrow centers can use the same procedures that protect classified data is a curious one, since classified information is by its nature available to a far smaller and more carefully-controlled potential audience than are escrowed keys. It is simply inevitable that escrow centers that meet the government's requirements will make mistakes in giving out the wrong keys from time to time or will be vulnerable to fraudulent key requests. Key escrow, by its nature, makes encrypted data less secure because the escrow center introduces a new target for attack.

A third problem with the Administration's key escrow proposals is that they fail to distinguish between cryptographic keys for which recovery might be required and those for which recoverability is never needed. There are many different kinds of encryption keys, but for the purposes of discussing key escrow it is sufficient to divide keys into three categories. The first includes keys used to encrypt stored information, which must be available throughout the lifetime of the data. The owner of the data has an obvious interest in ensuring the continued availability of such keys, and might choose to rely on a commercial service to store "backup" copies of them. A second category of key includes those used to encrypt real-time communications such as telephone calls. Here, the key has no value to its owner once the transaction for which it was used has completed. If a key is lost or destroyed in the middle of a conversation, a new one can be established in its place without permanent loss of information. For these keys, the owner has no use for recoverability; it is of value only to law enforcement and others who wish to obtain access to a conversation without the knowledge or cooperation of the parties. Finally, there are the keys used not for secrecy but for signature and authentication, to insure that messages indeed originated from a particular party. There is never a need for anyone, law enforcement or the key owner, to recover such keys, since their purpose is not to obscure content but rather to establish authorship. If the owner looses a signature key, a new one can be generated easily at any time.

Unfortunately, however, the current Administration proposal exposes all three types of keys equally to the risks introduced by the escrow system, even though recoverability is not required for all of them. Partly this is because there is no intrinsic difference in the structure of the different types of keys; they are usually indistinguishable from one another outside of the application in which they are used.

Finally, there is the problem that criminals can circumvent almost any escrow system to avoid exposure to law enforcement monitoring. All key escrow systems are vulnerable to so-called "superencryption," in which a user first encrypts data with an unescrowed key prior to processing it with the escrowed system. Most escrow systems are also vulnerable to still other techniques that make it especially easy to render escrowed keys useless to law enforcement. The ease of avoiding law enforcement when convenient raises an obvious question as to whether the reduced security and high cost of setting up an escrow system will yield any appreciable public safety benefit in practice.


The wide availability of encryption is vitally important to the future growth of our global information infrastructure. In many cases, encryption offers the only viable option for securing the rapidly increasing range of human, economic and social activities taking place over emerging communication networks. It is no exaggeration to say that the availability of encryption in the commercial marketplace is and will continue to be necessary to protect national security. Unfortunately, current policy, through export controls and ambiguous standards, discourages, rather than promotes, the use of encryption.

Current encryption policy is enormously frustrating to almost everyone working in the field. Export controls make it difficult to deploy effective cryptography even domestically, and we can do little more than watch as our foreign colleagues and competitors, not constrained by these rules, are matching our expertise and obtaining an ever-increasing share of the market. A large part of the problem is that the current regulations were written as if to cover hardware but are applied to software, including software in the public domain or aimed at the mass market. The PRO-CODE bill goes a long way toward moving the regulations in line with the realities of the technology.

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