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TUCoPS :: Crypto
:: cloak.txtThe Cloak Crypto Scheme |

Path: msuinfo!midway!ncar!elroy.jpl.nasa.gov!usc!cs.utexas.edu!peyote!ritter From: ritter@peyote.cactus.org (Terry Ritter) Newsgroups: sci.crypt Subject: Design of Cryptosystem CLOAK Keywords: cipher dynamic substitution additive RNG cloak Message-ID: <3492@peyote.cactus.org> Date: 16 Nov 90 21:32:32 GMT Distribution: usa Organization: Capital Area Central Texas Unix Society, Austin, TX Lines: 294 Apparently something went wrong with the first post -- if this is a duplicate, sorry! If you found the source for the CLOAK cipher a bit much, or even irritating, you are not alone. Gee, folks, I just don't use C much; I prefer Turbo Pascal. A short "theory" document was included in the distribution file CLOAK.ZIP which is on many bulletin boards. But I was worried about message length, and did not include it in the CLOAK posting here. Basically, CLOAK is a stream cipher, with an improved and non- linear combiner design (instead of the usual linear exclusive-OR), and an improved Random Number Generator (with up to 44K of internal state). The sequence of operations is approximately as follows: * A textual User Key is processed into 992 bits * The User Key bits initialize a large RNG * The RNG is extended * A 992-bit "really random" Message Key is created * The RNG is used to XOR-cipher the Message Key * The plaintext Message Key re-inits the RNG * The RNG is extended to operational length * The RNG is used to init the Dynamic Substitution tables * The RNG and Dynamic Substitution encipher plaintext data Since an enciphered Message Key is added to every enciphered file anyway, additional header information is also added. This includes a flag to indicate "enciphered," and values which indicate the cipher technique being used. In this way, the design can be extended in the future, and yet still remain compatible with existing enciphered files. The cryptographic RNG is an Additive design, which is based on some arbitrary primitive mod 2 polynomial; GFSR designs using trinomials of degree 521 have been reported. The CLOAK RNG uses particular primitive 9-nomials found by the author, and includes a selection between degrees 1279, 4253, and 11213 when enciphering. This is a very big RNG. Should you wish to work on breaking CLOAK, please first work on breaking the Dynamic Substitution demo DYNSUB.PAS posted here last year. Using the Turbo internal LCG RNG, without a message key, and just a single level of Dynamic Substitution, an attack should be far easier, and may demonstrate methods which could be used on the larger system. CLOAK DESIGN THEORY Terry Ritter 26 Oct 1990 CLOAK was designed to be a strong cipher. Of course, most cipher programs produce ciphertext which seems to be complex, but very few available ciphers could be expected to win a confrontation with professional cryptanalysts and their fast computers. CLOAK was designed to be as simple as possible, so that the design could be fully revealed. The usual practice is to invent a "proprietary" cipher, and keep the design secret. What this really means is that serious cryptanalysts will know the design, while buyers and users will not; a strange situation indeed. At this point in the public development of cryptography, there is still no practical cipher which is provably secret. Since the design of CLOAK is open to examination, each expert may come to their own conclusion as to its strength. STREAM CIPHERS CLOAK is an example of a stream cipher. The classical Vernam stream cipher simply combines plaintext data with the output from a random number generator in an additive combiner, which is often a single exclusive-OR gate or XOR operation. Unfortunately, additive combiners are inherently susceptible to "known plaintext" attacks. Moreover, most random number generators have only a tiny amount of internal state, and are easy to break. Such a system is really a "toy" cipher. CLOAK improves upon the classical stream cipher design with a completely new class of cryptographic combiner (patent pending), a very large cryptographic random number generator, and "really random" message keys. Alternative design approaches include DES, which is now rather small, and RSA, which is slow and complex. Public-key systems may seem to improve the key-distribution situation, but they also forego the inherent source verification of a secret-key system. This can be overcome only with complex and precise protocols, thus adding even more complexity to the system. CLOAK is a secret-key system, which means that it is necessary to somehow get the secret key to the far end, so that enciphered messages may be deciphered. However, once this is done, only those possessing the secret key may cipher messages in that key. A secret-key system thus mimics the use of a house key, and has similar problems and risks. DYNAMIC SUBSTITUTION COMBINER The new cryptographic combiner can be described as an extension of classical Simple Substitution. In simple substitution, each plaintext data value is translated into a ciphertext value using a substitution table. The innovation of Dynamic Substitution is to re-arrange the content of the substitution table after every substitution operation. In this case, the just-used substitution is exchanged with some substitution selected at random. The result is a non-linear combiner with a substantial amount of internal state. Only non-linear combiners make sense when several are used in sequence; linear additive combiners do not. In CLOAK, I have used a two-level Dynamic Substitution design. The first level randomizes the incoming data, and the second level has 16 separate combiners selected for use at random. Because the second-level combiners are used at random, it should be difficult for a cryptanalyst to work on the content of any particular substitution table. Dynamic Substitution technology is patent-pending. CRYPTOGRAPHIC RANDOM NUMBER GENERATOR The usual computer-language random number generator (RNG) is a 32-bit linear congruential generator (LCG), and can be solved very easily given one, or perhaps a few of the random values. This is a very weak cryptographic design. The generator I selected for use in CLOAK is the Additive RNG, as discussed in Knuth II. The strength of an RNG is at least partially related to the amount of its internal state, and the ones designed for statistical service are rather small. But the Additive generator is easily customized, and can be made one or more bytes wide. For CLOAK, the width has been set at 32 bits. The Additive generator can also be made as long as desired, provided that one can find primitive mod 2 polynomials of large degree. For CLOAK primitives have been found at degrees 1279, 4253 and 11213. Thus, the amount internal state can be 11213 times as large as the usual computer language LCG RNG. There are billions of acceptable polynomials; the particular ones selected for use in CLOAK are protected by copyright. JITTERIZER Despite its size, the Additive generator is nonetheless a linear system, and thus "easily" solved (assuming a cyptanalyst gets past the combiners). Naturally, the manipulation of 11,000 equations in 11,000 unknowns of 32-bits each might be expected to slow cryptanalysis somewhat. Nevertheless, it is still necessary to add some sort of isolation mechanism to hide the true sequence from analysis. For CLOAK, I have devised a mechanism which periodically deletes some of the Additive RNG result values from the output stream. A random number of values are deleted after a random number of steps. In addition, each group of values is given a separate value offset. This should make cryptanalysis a real exercise; certainly the jitterizer cannot be expected to make cryptanalysis any easier. THE MESSAGE KEY An RNG will produce a particular sequence of random numbers after being initialized from a particular key. Thus, when the usual stream cipher is used twice with the same key, the messages will be combined with the same sequence. But one of the properties of additive combiners is that repeated sequences can "canceled out," thus leaving a combination of two plaintext messages. Such a combination is likely to be broken easily. Consequently, it is important that the same sequence not be used for each message. This could be done, for example, by having a different User Key for each message. But this is surely unacceptable, for who could remember that many keys? Who could transfer that many keys to the far end with any degree of secrecy? An alternative is to use a different key for each message, but do so only internally. Each Message Key is enciphered and included in the message header; the User Key is used to encipher and thus protect the message key. Each message key is a "really random" value which is created dynamically for each message. Because the message key is a truly arbitrary value, and since most cryptanalysis methods require that a message "make sense," an enciphered message key will be very difficult to crack. Another advantage of the message key is that it can be made quite large, as well as random. CLOAK uses a message key consisting of 31 values of 32 bits each; a 124-byte message key (992 bits). This is enough to directly initialize a degree-31 Additive RNG, which is then stepped until enough data are produced for a degree-127 RNG, etc. Thus, the "really random" message key automatically satisfies many of the questions about a statistically correct initialization of the RNG. It also reduces the need for extreme length in the user key. THE USER KEY Although the User Key is used only to protect the message key, it must initialize an RNG in order to do so. This initialization should be as arbitrary as possible. I decided to use LFSR or CRC technology to perform a polynomial division of the textual key, and use remainders as the initialization value. The user key is processed by 32 degree-31 polynomials, thus producing 32 different results of 31 bits each. By eliminating the unused bits, 31 values of 32 bits each are produced; this is exactly the right amount of data to initialize a degree-31 Additive RNG. By stepping the Additive RNG, it will eventually fill enough to initialize a degree-127 RNG, and so on. CLOAK collects the user key from the keyboard into a string with a maximum length of 255 bytes; thus, up to 2040 bits of randomness are collected. This is then reduced (or expanded) to the 992 bits needed for degree-31 Additive RNG initialization. CLOAK does not insist on a long user key, but the serious user will create very unique keys of 30 characters or longer. There are millions of acceptable degree-31 primitive mod 2 polynomials; the particular ones selected for CLOAK are protected by copyright. SELECTED BIBLIOGRAPHY . Beker, H. and F. Piper. 1982. Cipher Systems. John Wiley: New York. . Blum, L., M. Blum and M. Shub. 1986. A Simple Unpredictable Pseudo-Random Number Generator. SIAM Journal of Computing. 15: 364-383. . Chaitin, G. 1975. Randomness and mathematical proof. Scientific American. 232(5): 47-52. . Ciarcia, S. 1986. Build a Hardware Data Encryptor. Byte. Sept: 97-111. . Devours, C., D. Kahn, L. Kruh, G. Mellen, B. Winkel. 1987. Cryptology Yesterday, Today, and Tomorrow. Artech House: Norwood, MA. . Geffe, P. 1973. How to protect data with ciphers that are really hard to break. Electronics. 46(1): 99-101. . Golomb, S. 1982 (original 1967). Shift Register Sequences. Aegean Park Press: POB 2837, Laguna Hills, CA 92653. . Kahn, D. 1967. The Codebreakers. Macmillan: New York. . Knuth, D. The Art of Computer Programming, Vol. 2, Seminumerical Algorithms. 2nd ed. Addison-Wesley: Reading, Massachusetts. . Kochanski, M. 1987. A Survey of Data Insecurity Packages. Cryptologia. XI(1): 1-15. . Kochanski, M. 1988. Another Data Insecurity Package. Cryptologia. XII(3): 165-173. . L'Ecuyer, P. and R. Proulx. 1989. About Polynomial- Time "Unpredictable" Generators. Proceedings of the 1989 Winter Simulation Conference. 467-476. . Lu, S. 1979. The Existence of Good Cryptosystems for Key Rates Greater than Message Redundancy. IEEE Transactions on Information Theory. IT-25(4): 475-477. . MacWilliams, F. and N. Sloane. 1977. The Theory of Error-Correcting Codes. North Holland: Amsterdam / New York. . Marsaglia, G. 1984. A current view of random number generators. Proceedings of the Sixteenth Symposium on the Interface Between Computer Science and Statistics. 3-10. . Marsaglia, G. and L. Tsay. 1985. Matrices and the Structure of Random Number Sequences. 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