-----BEGIN PGP SIGNED MESSAGE----- Jim Gillogly quoted then replied:

...

Brian D Williams <talon57@well.sf.ca.us> writes: I remember awhile back someone posted some clipper documents that were released under FOIA as I recall. The thing that struck me was that the NSA was refering internally to clipper as "The Trapdoor chip." Why refer to it as such if there is no back door?

Those letters made it clear the "trapdoor" was the escrow, and the internal debate was over whether the existence of the escrow would be made public. So far it's been NSA's consistent public position that the escrow is the only way in... and from the FOIA, that's evidently what they're telling the President also.

Most days I'm pretty sure I believe that there aren't any known gotchas in the Skipjack algorithm. If they can really get the escrow, it's ever so much cheaper than doing real cryptanalysis. As Carl Ellison and others point out, that's really one of the big dangers -- if LE doesn't have to break Skipjack to read the traffic, neither do the attackers... and breaking the escrow is probably much cheaper than breaking the algorithm.

My position is that Clipper is iniquitous whether or not there's a[nother] trap door.

Jim Gillogly Mersday, 6 Astron S.R. 1994, 21:58

Hmmmm, lets look at the article I referred to <sounds of digital rummaging> Ahh, here it is: - From toad.com!gnu Mon Jun 21 14:54:03 1993 From: gnu@toad.com (John Gilmore) To: cypherpunks@toad.com Subject: Some FOIA results re Clipper Date: Mon, 21 Jun 1993 14:29:59 -0700 Lee Tien and I have submitted a pile of FOIA requests about Clipper. Here is scanned-in text from some of the more interesting results, courtesy of Lee. Search for "required", for a mention of the proposal to require the use of Clipper. Also note that the role of the "national security community" has been deliberately withheld from the public statements (search for "mentioned"). Most agencies have not yet responded with documents. FBI is claiming it will take them a year, and we are preparing to file suit to force them to do it within 10 days like the law requires. (Our NSA suit over the same thing, is continuing through the gears of the court process.) John Gilmore [This page originally XXXXXXXXXXXXXXX TOP SECRET; now UNCLASSIFIED] OFFICE OF THE ASSISTANT SECRETARY OF DEFENSE WASHINGTON, DC 20301-3040 COMMAND CONTROL COMMUNICATIONS AND INTELLIGENCE MEMORANDUM FOR MS. JOANN H. GRUBE, NSA REPRESENTATIVE/NSC PRD-27 EXPORT CONTROL WORKING GROUP SUBJECT: Comments on PRD-27/NSA Draft (U) (U) Following are comments concerning your proposed memorandum to Jim Lewis, Department of State: XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX blacked out via FOIA (b)(1) exemption. XXXXXXXXXXXXXXXXXXX (U) The assertions in this draft are merely unsupported statements. Recommend that the memorandum provide more empirical evidence to back up its assertions, and that the above comments be reflected in its contents. (signed) Daniel J. Ryan Director, Information Systems Security CLASSIFIED BY: OASD(C3I)/DIR, ISS DECLASSIFY ON: OADR [This page originally XXXXXXXX SECRET; now UNCLASSIFIED] OFFICE OF THE ASSISTANT SECRETARY OF DEFENSE WASHINGTON DC 20301-3040 COMMAND, CONTROL, COMMUNICATIONS AND INTELLIGENCE 30 APR 1993 (stamped) MEMORANDUM FOR THE ACTING ASSISTANT SECRETARY OF DEFENSE (C3I) Subject: PRD/NSC-27 Advanced Telecommunications and Encryption (U) (U) Advances in telecommunications have created the opportunity for public use of encryption to ensure the privacy and integrity of business and personal communications. These same advances threaten the capabilities of law enforcement and national security operations that intercept the communications of narcotraffickers, organized criminals, terrorists, espionage agents of foreign powers and SIGINT targets. Diverse interests are in diametric opposition with regard to industry's right to sell and the public's right to use such capabilities. A highly-emotional, spirited public debate is likely. (U) In its simplest construct, this complex set of issues places the public's right to privacy in opposition to the public's desire for safety. The law enforcement and national security communities argue that if the public's right to privacy prevails and free use of cryptography is allowed, criminals and spies will avoid wiretaps and other intercepts and consequently prosper. They propose that cryptography be made available and required which contains a "trapdoor" that would allow law enforcement and national security officials, under proper supervision, to decrypt enciphered communications. Such cryptography exists, and while there are many practical problems to be solved, this proposal is technically possible to achieve. (U) Opponents of the proposal argue that the public has a right to and an expectation of privacy, that a trapdoor system would be prone to misuse and abuse, and that the proposed solution would not work in any practical sense. They assert that people who are deliberately breaking much more serious laws would not hesitate to use cryptography that does not have a trapdoor, and that secure cryptography will inevitably be supplied by offshore companies. Thus, freedom will be lost and many tax dollars spent to no effect. (U) This situation is complicated by the existence of other interests. For example, there currently exist strict controls on the export of cryptography. The computer industry points out that it has one of the few remaining positive trade balances and that it is vital that the dominance of the American computer industry in world markets be preserved. The industry fears that this will be lost if offshore developers incorporate high-quality cryptography into their products while U.S. industry either cannot do so or suffers higher costs or delays due to requirements for export licenses. The industry argues persuasively that overseas markets (much less drug lords or spies) will not look with favor on U.S. products which have known trapdoors when offshore products which do not have them are available. In support of their argument, they note that powerful public-key cryptography developed and patented by RSA using U.S. tax dollars is free to developers in Europe, subject to royalties in the United States, and cannot be exported without expensive and time-late export licenses. These charges are true. (U) The national security community is especially interested in preventing the spread of high-quality encipherment routines overseas, and argues that more extensive use here at home will inevitably result in such a proliferation. Actually, it is too late. The Data Encryption Standard (DES) is already widely available throughout the world in both hardware and software forms, and DES software can be downloaded anywhere in the world from public bulletin boards by anyone with a PC, a MODEM and a telephone. In one recent experiment it took three minutes and fourteen seconds to locate a source-code version of DES on the INTERNET. Widespread availability of DES and RSA will enable offshore developers to provide high-quality encipherment for voice and data communications in competition with U.S. industry's products. XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX blacked out via FOIA exemption (b)(1) XXXXXXXXXXX (U) Despite these concerns, the President has directed that the Attorney General request that manufacturers of communications hardware use the trapdoor chip, and at least AT&T has been reported willing to do so (having been suitably incentivised by promises of Government purchases). The Attorney General has also been directed to create a system for escrow of key material. The Secretary of Commerce has been directed to produce standards based on the use of the trapdoor chip. (U) The President has also directed that the fact that law enforcement officials will have access to the keys will not be concealed from the public. National security officials are not mentioned. (U) The new administration is committed to the development of an information superhighway and a National Information Infrastructure in support of the economy. This worthy goal is independent of arguments as to whether or not law enforcement and national security officials will be able to read at will traffic passing along the information superhighway. A full-scale public debate is needed to ascertain the wishes of U.S. citizens with regard to their privacy, and the impact on public safety of preserving privacy at the expense of wiretapping and communications intercept capabilities of law enforcement and national security personnel. It is not clear what the public will decide. In the meantime, DoD has trapdoor technology and the Government is proceeding with development of the processes needed to apply that technology in order to maintain the capability to perform licit intercept of communications in support of law enforcement and national security. (signed) Ray Pollari Acting DASD (CI & SCM) [This page originally SECRET; now UNCLASSIFIED] ASSISTANT SECRETARY OF DEFENSE WASHINGTON DC 20301-3040 May 3, 1993 COMMAND, CONTROL, COMMUNICATIONS AND INTELLIGENCE EXECUTIVE SUMMARY MEMORANDUM FOR DEPUTY SECRETARY OF DEFENSE FROM: CHARLES A. HAWKINS, JR., ACTING ASD(C3I) (initialed C. Hxxx) SUBJECT: Advanced Telecommunications and Encryption (U) PURPOSE: INFORMATION DISCUSSION:(U) In response to DEPSECDEF's tasking of 21 Apr 93 (TAB A) this information is provided. Advances in telecommunications have created the opportunity for public use of encryption to ensure the privacy and integrity of business and personal communications. These same advances threaten the capabilities of law enforcement and national security operations that intercept the communications of narcotraffickers, organized criminals, terrorists, espionage agents of foreign powers and a broad range of SIGINT targets. Diverse interests are in diametric opposition with regard to industry's right to sell and the public's right to use such capabilities. A highly-emotional, spirited public debate is likely. (U) The law enforcement and national security communities argue that if the public's right to privacy prevails and free use of cryptography is allowed, criminals and spies will avoid wiretaps and other intercepts. They propose that cryptography be made available to the public which contains a "trapdoor" that would allow law enforcement and national security officials, under proper supervision, to decrypt enciphered communications. Such cryptography exists, and while there are many practical problems to be solved, this proposal is technically possible to implement. (U) Opponents of the proposal argue that the public has a right to and expectation of privacy, that such a system would be prone to misuse and abuse, and that the proposed solution would not work in any practical sense. They assert that criminals and spies will not hesitate to use secure cryptography supplied by offshore companies. Thus, the loss of privacy would outweigh any advantages to law enforcement or national security. (U) The computer industry points out that it has one of the few remaining positive trade balances and that it is vital that the dominance of the American computer industry in world markets be preserved. The industry fears that this will be lost if offshore developers incorporate high-quality cryptography into their products while U.S. industry either cannot do so or suffers higher costs or delays due to requirements for export licenses because of strict controls of export of cryptography. The industry argues persuasively that overseas markets (much less drug lords or spies) will not look with favor on U.S. products which have known trapdoors when offshore products which do not have them are available. CLASSIFIED BY: DASD(CI&SCM) DECLASSIFY ON: OADR [This page originally XXXXXXXX SECRET; now UNCLASSIFIED] (U) The national security community is especially interested in preventing the spread of high-quality encipherment routines overseas, and argues that more extensive use here at home will inevitably result in such a proliferation. This would increase the cost of performing the SIGINT mission or decrease the amount of intelligence, or both. The Data Encryption Standard (DES) is already widely available throughout the world in both hardware and software forms, and DES software can be downloaded anywhere in the world from public bulletin boards by anyone with a PC, a MODEM, and a telephone. Thus far, widespread availability has not led to widespread use. However, widespread availability of DES and RSA will make it possible for offshore developers to provide high- quality encipherment for voice and data communications in competition with U.S. industry's products. XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX blacked out under FOIA exemption (b)(1) XXXXXXXXXXXXXXXXXXXXX (U) The President has directed that the Attorney General request that manufacturers of communications hardware use the trapdoor chip. The Attorney General has also been directed to create a system for escrow of key material. The Secretary of Commerce has been directed to produce standards based on the use of the trapdoor chip. The President has also directed that the fact that law enforcement officials will have access to the keys will not be concealed from the public. National security officials are not mentioned. (U) The new administration is committed to the development of an information superhighway and a National Information Infrastructure in support of the economy. This worthy goal is independent of arguments as to whether or not law enforcement and national security officials will be able to read at will traffic passing along the information superhighway. A full-scale public debate is beginning which will ascertain the wishes of U.S. citizens with regard to their privacy and the impact on public safety of preserving privacy at the expense of wiretapping and communications intercept capabilities of law enforcement and national security personnel. It is not clear what the public will decide. In the meantime, DoD has trapdoor technology and the Government is proceeding with development of the processes needed to apply that technology in order to maintain the capability to perform licit intercept of communications in support of law enforcement and national security. Prepared by: Dan Ryan/ODASD(CI & SCM)/x 41779/28 Apr 93/OSD - ------- End of Forwarded Message Okay, I had to reread it several times, but I see your point! Why do they refer to it as a "trapdoor" when there going in the "frontdoor?" I'm still not convinced that there is no "trapdoor", and will never be as long as the algorithm remains secret. Hey just for snicks, (and new members of the list) lets look at the report on Skipjack again. SKIPJACK Review Interim Report The SKIPJACK Algorithm Ernest F. Brickell, Sandia National Laboratories Dorothy E. Denning, Georgetown University Stephen T. Kent, BBN Communications Corporation David P. Maher, AT&T Walter Tuchman, Amperif Corporation July 28, 1993 (copyright 1993) Executive Summary The objective of the SKIPJACK review was to provide a mechanism whereby persons outside the government could evaluate the strength of the classified encryption algorithm used in the escrowed encryption devices and publicly report their findings. Because SKIPJACK is but one component of a large, complex system, and because the security of communications encrypted with SKIPJACK depends on the security of the system as a whole, the review was extended to encompass other components of the system. The purpose of this Interim Report is to report on our evaluation of the SKIPJACK algorithm. A later Final Report will address the broader system issues. The results of our evaluation of the SKIPJACK algorithm are as follows: 1. Under an assumption that the cost of processing power is halved every eighteen months, it will be 36 years before the cost of breaking SKIPJACK by exhaustive search will be equal to the cost of breaking DES today. Thus, there is no significant risk that SKIPJACK will be broken by exhaustive search in the next 30-40 years. 2. There is no significant risk that SKIPJACK can be broken through a shortcut method of attack. 3. While the internal structure of SKIPJACK must be classified in order to protect law enforcement and national security objectives, the strength of SKIPJACK against a cryptanalytic attack does not depend on the secrecy of the algorithm. 1. Background On April 16, the President announced a new technology initiative aimed at providing a high level of security for sensitive, unclassified communications, while enabling lawfully authorized intercepts of telecommunications by law enforcement officials for criminal investigations. The initiative includes several components: A classified encryption/decryption algorithm called "SKIPJACK." Tamper-resistant cryptographic devices (e.g., electronic chips), each of which contains SKIPJACK, classified control software, a device identification number, a family key used by law enforcement, and a device unique key that unlocks the session key used to encrypt a particular communication. A secure facility for generating device unique keys and programming the devices with the classified algorithms, identifiers, and keys. Two escrow agents that each hold a component of every device unique key. When combined, those two components form the device unique key. A law enforcement access field (LEAF), which enables an authorized law enforcement official to recover the session key. The LEAF is created by a device at the start of an encrypted communication and contains the session key encrypted under the device unique key together with the device identifier, all encrypted under the family key. LEAF decoders that allow an authorized law enforcement official to extract the device identifier and encrypted session key from an intercepted LEAF. The identifier is then sent to the escrow agents, who return the components of the corresponding device unique key. Once obtained, the components are used to reconstruct the device unique key, which is then used to decrypt the session key. This report reviews the security provided by the first component, namely the SKIPJACK algorithm. The review was performed pursuant to the President's direction that "respected experts from outside the government will be offered access to the confidential details of the algorithm to assess its capabilities and publicly report their finding." The Acting Director of the National Institute of Standards and Technology (NIST) sent letters of invitation to potential reviewers. The authors of this report accepted that invitation. We attended an initial meeting at the Institute for Defense Analyses Supercomputing Research Center (SRC) from June 21-23. At that meeting, the designer of SKIPJACK provided a complete, detailed description of the algorithm, the rationale for each feature, and the history of the design. The head of the NSA evaluation team described the evaluation process and its results. Other NSA staff briefed us on the LEAF structure and protocols for use, generation of device keys, protection of the devices against reverse engineering, and NSA's history in the design and evaluation of encryption methods contained in SKIPJACK. Additional NSA and NIST staff were present at the meeting to answer our questions and provide assistance. All staff members were forthcoming in providing us with requested information. At the June meeting, we agreed to integrate our individual evaluations into this joint report. We also agreed to reconvene at SRC from July 19-21 for further discussions and to complete a draft of the report. In the interim, we undertook independent tasks according to our individual interests and availability. Ernest Brickell specified a suite of tests for evaluating SKIPJACK. Dorothy Denning worked at NSA on the refinement and execution of these and other tests that took into account suggestions solicited from Professor Martin Hellman at Stanford University. NSA staff assisted with the programming and execution of these tests. Denning also analyzed the structure of SKIPJACK and its susceptibility to differential cryptanalysis. Stephen Kent visited NSA to explore in more detail how SKIPJACK compared with NSA encryption algorithms that he already knew and that were used to protect classified data. David Maher developed a risk assessment approach while continuing his ongoing work on the use of the encryption chip in the AT&T Telephone Security Device. Walter Tuchman investigated the anti-reverse engineering properties of the chips. We investigated more than just SKIPJACK because the security of communications encrypted with the escrowed encryption technology depends on the security provided by all the components of the initiative, including protection of the keys stored on the devices, protection of the key components stored with the escrow agents, the security provided by the LEAF and LEAF decoder, protection of keys after they have been transmitted to law enforcement under court order, and the resistance of the devices to reverse engineering. In addition, the success of the technology initiative depends on factors besides security, for example, performance of the chips. Because some components of the escrowed encryption system, particularly the key escrow system, are still under design, we decided to issue this Interim Report on the security of the SKIPJACK algorithm and to defer our Final Report until we could complete our evaluation of the system as a whole. 2. Overview of the SKIPJACK Algorithm SKIPJACK is a 64-bit "electronic codebook" algorithm that transforms a 64-bit input block into a 64-bit output block. The transformation is parameterized by an 80-bit key, and involves performing 32 steps or iterations of a complex, nonlinear function. The algorithm can be used in any one of the four operating modes defined in FIPS 81 for use with the Data Encryption Standard (DES). The SKIPJACK algorithm was developed by NSA and is classified SECRET. It is representative of a family of encryption algorithms developed in 1980 as part of the NSA suite of "Type I" algorithms, suitable for protecting all levels of classified data. The specific algorithm, SKIPJACK, is intended to be used with sensitive but unclassified information. The strength of any encryption algorithm depends on its ability to withstand an attack aimed at determining either the key or the unencrypted ("plaintext") communications. There are basically two types of attack, brute-force and shortcut. 3. Susceptibility to Brute Force Attack by Exhaustive Search In a brute-force attack (also called "exhaustive search"), the adversary essentially tries all possible keys until one is found that decrypts the intercepted communications into a known or meaningful plaintext message. The resources required to perform an exhaustive search depend on the length of the keys, since the number of possible keys is directly related to key length. In particular, a key of length N bits has 2^N possibilities. SKIPJACK uses 80-bit keys, which means there are 2^80 (approximately 10^24) or more than 1 trillion trillion possible keys. An implementation of SKIPJACK optimized for a single processor on the 8-processor Cray YMP performs about 89,000 encryptions per second. At that rate, it would take more than 400 billion years to try all keys. Assuming the use of all 8 processors and aggressive vectorization, the time would be reduced to about a billion years. A more speculative attack using a future, hypothetical, massively parallel machine with 100,000 RISC processors, each of which was capable of 100,000 encryptions per second, would still take about 4 million years. The cost of such a machine might be on the order of $50 million. In an even more speculative attack, a special purpose machine might be built using 1.2 billion $1 chips with a 1 GHz clock. If the algorithm could be pipelined so that one encryption step were performed per clock cycle, then the $1.2 billion machine could exhaust the key space in 1 year. Another way of looking at the problem is by comparing a brute force attack on SKIPJACK with one on DES, which uses 56-bit keys. Given that no one has demonstrated a capability for breaking DES, DES offers a reasonable benchmark. Since SKIPJACK keys are 24 bits longer than DES keys, there are 2^24 times more possibilities. Assuming that the cost of processing power is halved every eighteen months, then it will not be for another 24 * 1.5 = 36 years before the cost of breaking SKIPJACK is equal to the cost of breaking DES today. Given the lack of demonstrated capability for breaking DES, and the expectation that the situation will continue for at least several more years, one can reasonably expect that SKIPJACK will not be broken within the next 30-40 years. Conclusion 1: Under an assumption that the cost of processing power is halved every eighteen months, it will be 36 years before the cost of breaking SKIPJACK by exhaustive search will be equal to the cost of breaking DES today. Thus, there is no significant risk that SKIPJACK will be broken by exhaustive search in the next 30-40 years. 4. Susceptibility to Shortcut Attacks In a shortcut attack, the adversary exploits some property of the encryption algorithm that enables the key or plaintext to be determined in much less time than by exhaustive search. For example, the RSA public-key encryption method is attacked by factoring a public value that is the product of two secret primes into its primes. Most shortcut attacks use probabilistic or statistical methods that exploit a structural weakness, unintentional or intentional (i.e., a "trapdoor"), in the encryption algorithm. In order to determine whether such attacks are possible, it is necessary to thoroughly examine the structure of the algorithm and its statistical properties. In the time available for this review, it was not feasible to conduct an evaluation on the scale that NSA has conducted or that has been conducted on the DES. Such review would require many man-years of effort over a considerable time interval. Instead, we concentrated on reviewing NSA's design and evaluation process. In addition, we conducted several of our own tests. 4.1 NSA's Design and Evaluation Process SKIPJACK was designed using building blocks and techniques that date back more than forty years. Many of the techniques are related to work that was evaluated by some of the world's most accomplished and famous experts in combinatorics and abstract algebra. SKIPJACK's more immediate heritage dates to around 1980, and its initial design to 1987. SKIPJACK was designed to be evaluatable, and the design and evaluation approach was the same used with algorithms that protect the country's most sensitive classified information. The specific structures included in SKIPJACK have a long evaluation history, and the cryptographic properties of those structures had many prior years of intense study before the formal process began in 1987. Thus, an arsenal of tools and data was available. This arsenal was used by dozens of adversarial evaluators whose job was to break SKIPJACK. Many spent at least a full year working on the algorithm. Besides highly experienced evaluators, SKIPJACK was subjected to cryptanalysis by less experienced evaluators who were untainted by past approaches. All known methods of attacks were explored, including differential cryptanalysis. The goal was a design that did not allow a shortcut attack. The design underwent a sequence of iterations based on feedback from the evaluation process. These iterations eliminated properties which, even though they might not allow successful attack, were related to properties that could be indicative of vulnerabilities. The head of the NSA evaluation team confidently concluded "I believe that SKIPJACK can only be broken by brute force there is no better way." In summary, SKIPJACK is based on some of NSA's best technology. Considerable care went into its design and evaluation in accordance with the care given to algorithms that protect classified data. 4.2 Independent Analysis and Testing Our own analysis and testing increased our confidence in the strength of SKIPJACK and its resistance to attack. 4.2.1 Randomness and Correlation Tests A strong encryption algorithm will behave like a random function of the key and plaintext so that it is impossible to determine any of the key bits or plaintext bits from the ciphertext bits (except by exhaustive search). We ran two sets of tests aimed at determining whether SKIPJACK is a good pseudo random number generator. These tests were run on a Cray YMP at NSA. The results showed that SKIPJACK behaves like a random function and that ciphertext bits are not correlated with either key bits or plaintext bits. Appendix A gives more details. 4.2.2 Differential Cryptanalysis Differential cryptanalysis is a powerful method of attack that exploits structural properties in an encryption algorithm. The method involves analyzing the structure of the algorithm in order to determine the effect of particular differences in plaintext pairs on the differences of their corresponding ciphertext pairs, where the differences are represented by the exclusive-or of the pair. If it is possible to exploit these differential effects in order to determine a key in less time than with exhaustive search, an encryption algorithm is said to be susceptible to differential cryptanalysis. However, an actual attack using differential cryptanalysis may require substantially more chosen plaintext than can be practically acquired. We examined the internal structure of SKIPJACK to determine its susceptibility to differential cryptanalysis. We concluded it was not possible to perform an attack based on differential cryptanalysis in less time than with exhaustive search. 4.2.3 Weak Key Test Some algorithms have "weak keys" that might permit a shortcut solution. DES has a few weak keys, which follow from a pattern of symmetry in the algorithm. We saw no pattern of symmetry in the SKIPJACK algorithm which could lead to weak keys. We also experimentally tested the all "0" key (all 80 bits are "0") and the all "1" key to see if they were weak and found they were not. 4.2.4 Symmetry Under Complementation Test The DES satisfies the property that for a given plaintext-ciphertext pair and associated key, encryption of the one's complement of the plaintext with the one's complement of the key yields the one's complement of the ciphertext. This "complementation property" shortens an attack by exhaustive search by a factor of two since half the keys can be tested by computing complements in lieu of performing a more costly encryption. We tested SKIPJACK for this property and found that it did not hold. 4.2.5 Comparison with Classified Algorithms We compared the structure of SKIPJACK to that of NSA Type I algorithms used in current and near-future devices designed to protect classified data. This analysis was conducted with the close assistance of the cryptographer who developed SKIPJACK and included an in-depth discussion of design rationale for all of the algorithms involved. Based on this comparative, structural analysis of SKIPJACK against these other algorithms, and a detailed discussion of the similarities and differences between these algorithms, our confidence in the basic soundness of SKIPJACK was further increased. Conclusion 2: There is no significant risk that SKIPJACK can be broken through a shortcut method of attack. 5. Secrecy of the Algorithm The SKIPJACK algorithm is sensitive for several reasons. Disclosure of the algorithm would permit the construction of devices that fail to properly implement the LEAF, while still interoperating with legitimate SKIPJACK devices. Such devices would provide high quality cryptographic security without preserving the law enforcement access capability that distinguishes this cryptographic initiative. Additionally, the SKIPJACK algorithm is classified SECRET NOT RELEASABLE TO FOREIGN NATIONALS. This classification reflects the high quality of the algorithm, i.e., it incorporates design techniques that are representative of algorithms used to protect classified information. Disclosure of the algorithm would permit analysis that could result in discovery of these classified design techniques, and this would be detrimental to national security. However, while full exposure of the internal details of SKIPJACK would jeopardize law enforcement and national security objectives, it would not jeopardize the security of encrypted communications. This is because a shortcut attack is not feasible even with full knowledge of the algorithm. Indeed, our analysis of the susceptibility of SKIPJACK to a brute force or shortcut attack was based on the assumption that the algorithm was known. Conclusion 3: While the internal structure of SKIPJACK must be classified in order to protect law enforcement and national security objectives, the strength of SKIPJACK against a cryptanalytic attack does not depend on the secrecy of the algorithm. - ------- End of forwarded message ------- Brian Williams Extropian Cypherpatriot "Cryptocosmology: Sufficently advanced comunication is indistinguishable from noise." --Steve Witham "Have you ever had your phones tapped by the government? YOU WILL and the company that'll bring it to you.... 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