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I decided to post this in spite of an excess on IP of crypto postings over the past few days (additi
From: David Farber <>
Date: Sun, 13 Feb 1994 22:42:43 -0500
Since automatic key management systems permit so many keys, they also reduce the exposure to "known plaintext" attacks. History suggests that codes are most often broken because the user fails to apply them with the necessary rigor and discipline, particularly when choosing, distributing, and installing keys. Automating of these steps provides much of the necessary discipline and rigor. Automatic key distribution and installation increases the effectiveness by protecting the keys from disclosure during distribution, and by making the system resistant to the insertion of keys known to attackers. When keys are installed manually they become known to the human agent who installs them. He is in a position to provide a copy of the key to others. To the extent that this agent is vulnerable to coercion or bribery, the system is weakened by this knowledge. Therefore, the system may be strengthened by automatic mechanisms which provide the agent with beneficial use of the key without granting him knowledge of it. For example, systems available from IBM and Motorola provide for the key to be distributed in smartcards and automatically installed in the target machine. The key can be encrypted in the smartcard or destroyed by the installation process. In either case, the agent can use it, but cannot copy it or give it to another. Just as the use of automata for encoding and decoding reduces the cost and inconvenience of using secret codes, the use of automata for key management reduces the cost and inconvenience of changing the keys frequently. By changing the key frequently, e.g., for each, file, session, message, or transaction, the value to an adversary of obtaining a key is reduced, and the effectiveness of the mechanism is improved. One way of looking at automated key management is that it increases the effective length of the key, or makes it approach the length of the data protected. Asymmetric Key Cryptography However, even though most of the key management can be automated, most such systems require some prearrangement. In any-to-any communications in a large open population, this requirement can quickly become overwhelming. For example, in a population of two hundred people, the number of key pairs and secret exchanges would be in the thousands with many opportunities for keys to be compromised. Moreover, with traditional keys, the initial distribution of keys must be done in such a way as to maintain their secrecy, practically impossible in a large population. These problems are addressed, in part, by public key, or asymmetric key, cryptography. This mechanism was proposed by Whitfield Diffee, Martin Hellman, and Ralph Merkle. It may be the single most innovative idea in modern cryptography. The best known and most widely used implementation is the RSA algorithm invented by Ronald Rivest, Adi Shamir, and Leonard Adelman. [In this mechanism the key has two parts, only one of which must be kept secret. The two parts have the special property that what is encrypted with one can only be decrypted with the other. One half of the key-pair, called the private key, is kept secret and is used only by its owner. The other half, called the public key, is published and is used by all parties that want to communicate with the private key owner. It can be published and does not need to be distributed secretly. Since the public key, by definition, is available to anyone, then anyone can send a message to the owner that only he can read.] With a minimum of pre-arrangement, this function provides the logical analog of an envelope that can only be opened by one person. The larger the communicating population, and the more hostile the environment, the greater is its advantage over symmetric key cryptography. This concealment from all but the intended recipient is the traditional use of cryptography. However, asymmetric key cryptography has another use. A message encrypted using the private key can be read by anyone with access to the public key, but it could only have been encrypted by the owner of the corresponding private key. This use is analogous to a digital signature. It provides confidence that the message originates where it appears to have originated. Since if even a bit of the message is changed it will not decrypt properly, this mechanism also provides confidence that the message has not been either maliciously or accidentally altered. In part, this is also true as between the two parties to a message that is sent using symmetric key cryptography. That is, the recipient of the message knows with a high degree of confidence that it originated with the other holder of the key; he knows it, but he cannot prove it to another. However, with asymmetric key cryptography, he can demonstrate it to a third party. If the owner of the key pair has acknowledged the public part of the key to the third party, then he cannot plausibly deny any message that can be decrypted with it. [The concept of the digital signature is such a novel concept as to easily qualify as an invention on its own. However, it is so closely bound in origin and literature to asymmetric key cryptography that I elect to simply treat them as one.] These two abstractions, the logical envelope and the logical signature, can be composed so as to synthesize any and all of the controls that we have ever been able to achieve by more traditional means. They can be used for payments, contracts, testaments, and high integrity journals and logs. They provide us with a higher degree of security in an electronic environment than we were ever able to achieve in a paper environment. They provide protection in an open environment that is nearly as high as that which we can achieve in an open one. The Impact of the Great Inventions The impact of these inventions is to provide us with secret codes that are cheap enough to be used by default, and arbitrarily strong. Given assumptions about the quantity of data to be protected, the length of time that it must remain secret, its value to an adversary, and the resources available to the adversary, it is possible to apply modern cryptography in such a way as to be as strong as required. While it is possible to state a problem in such a way as to defy such a solution, it is difficult to identify such a problem in the real world. That is, It is possible to specify so much data to be encrypted under a single key, of such high value and which must remain safe for such a long time that we cannot say with confidence that the mechanism can stand for that time and cost. For example, we cannot say with confidence how to encrypt several hundred gigabytes worth several trillion dollars and keep it safe for a millennium. On the other hand, we are not aware of any real problems that meet such a specification. Put another way, we can always ensure that the cost of obtaining the information by cryptanalysis is higher than the value of the data or the cost of obtaining it by alternative means. While any code can be broken at some cost, modern codes are economically unbreakable, at least in the sense that the cost of doing so can be made to exceed the value of doing it. A very small increase in the cost to the cryptographer can result in astronomical increases in the cost to a potential adversary. Perhaps just as important, these mechanisms are now sufficiently convenient to use, that, within bounds, they can be widely and easily applied. Given that the more data that is encrypted with a single mechanism, the greater the value in breaking it, the more compromising information is available to an adversary, and that the more a mechanism is used the greater the opportunity for a compromising error in its use, we should continue to apply cryptography only to data that can profit from its use. On the other we need never again be inhibited from using it by issues of cost or convenience. Cryptography and Government Policy It should be obvious to a qualified observer that, announcements here to the contrary not withstanding, we are losing the battle for security and privacy in the computerized and networked world. We could have secret codes imbedded in all software of interest for free. This assertion assumes only that all such software is produced by those represented here, who have already paid for licenses and absorbed much of the necessary development cost, and that the cost of a marginal cycle on the desktop approaches zero. That we do not, is the result of ambivalent government policy. While one agency of government has sponsored the use of standard cryptography, another has tried to undermine confidence in those standards. While one agency has asserted that public standards are essential, another has sponsored secret ones, and a third has used public funds to further such secret standards. While one agency has insisted that trusted codes are essential to world prosperity, another has imposed restrictions on their export and undermined confidence in those that are exported. While one agency recognizes that national security depends upon world prosperity, another believes that signals intelligence is more important. Those of you who have seen my comments in Risks, sci.crypt , and the Communications of the ACM, know my position. It is that the prime mover behind all of these initiatives is NSA, that their motive is the preservation of their jobs and power by protecting the efficiency of signals intelligence, that their strategy is to discourage by every means that they can get away with all private and most commercial use of cryptography. That they have infiltrated the departments of State and Commerce and the White House staff, and that they are using the Department of Justice. While they know that they cannot be fully successful, they also know that they do not have to be. Nor is this simply paranoia on my part. It is the only explanation that accounts for all of the government's actions. It also meets the tests proposed by Machiavelli, Willie Sutton and "Deep Throat." While most of the government confesses that cryptography is essential to personal privacy in the modern era, the administration is not prepared to admit that even the current sparse use is consistent with the government's responsibility to preserve public order. Let me stress that the problem is government policy, not public policy and not administration or congressional policy. This policy has been made in secret and has been resistant to public input. It is the policy of the bureaucracy and not of any individuals. I know most of the players in the development of this policy. I know none that are pursuing a personal agenda, like the results, or are proud of their roles in it. They are simply doing the best that they know how in the face of agency momentum, administration consent, and the absence of congressional guidance. However, the momentum behind these policies is such that the good intentions and professionalism of the individuals is not sufficient to resist it. While the administration has aligned itself with the initiatives, it is not their author. While the initiatives have sponsors within the administration, they were here before the administration and they expect to be here when it is gone. They believe that the policy is important and that the administration is not. While some committees of the congress have held hearings on the issues and even decried the arbitrary actions of the bureaucracy, their hearings always conclude with executive sessions with the NSA and no legislative initiatives to curb the excesses. Forgive me a closing political observation not intended to be partisan. This government is too large, over-zealous and under-effective. It is committed to nothing so much as its own survival. It may be too late to influence it, but if it is not influenced, not only will we not enjoy the fruits of modern cryptography, but we may not enjoy those of telecommunications, trade, our labors, or even those of freedom. Bibliography Ehrsam, W. F., Matyas, S. M., Meyer, C. H., and Tuchman, W. L., "A Cryptographic Key Management System for Implementing the Data Encryption Standard," IBM Systems Journal Vol. 17(2) pp. 106-125 (1978). Kahn, D., The Codebreakers, Macmillan Co., New York (1967). Matyas, S. M., Meyer, C. H., "Generation, Distribution, and Installation of Cryptographic Keys," IBM Systems Journal Vol. 17(2) pp. 126-137 (1978).
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