Криптография 2е издание Протоколы, алгоритмы и исходные тексты на языке С
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CFB-specifically 8-bit CFB-is generally the mode ol choice for encrypting streams of characters when each cha r- acter has to be treated individually, as in a link between a terminal and a host. OFB is most often used in high-speed synchronous systems where error propagation is intolerable. OFB is also the mode of choice if preprocessing is r e- quired. OFB is the mode of choice in a error-prone environment, because it has no error extension. Stay away from the weird modes. One of the four basic modes-ECB, CBC, OFB, and CFB-is suitable for almost any application. These modes are not overly complex and probably do not reduce the security of the system. While it is possible that a complicated mode might increase the security of a system, most likely it just increases the complexity. None of the weird modes has any better error propagation or error recovery characteristics. 9.12 INTERLEAVING With most modes, encryption of a bit (or block) depends on the encryption of the previous bits (or blocks). This can often make it impossible to parallelize encryption. For example, consider a hardware box that does encryption in CBC mode. Even if the box contains four encryption chips, only one can work at any time. The next chip needs the results of the previous chip before it starts working. The solution is to interleave multiple encryption streams. (This is not multiple encryption; that's covered in Se c- tions 15.1 and 15.2). Instead of a single CBC chain, use four. The first, fifth, and every fourth block thereafter are e n- crypted in CBC mode with one IV. The second, sixth, and every fourth block thereafter are encrypted in CBC mode with another IV, and so on. The total IV is much longer than it would have been without interleaving. Think of it as encrypting four different messages with the same key and four different IVs. These messages are all i nterleaved. This trick can also be used to increase the overall speed of hardware encryption. If you have three encryption chips, each c a- pable of encrypting data at 33 megabits/second, you can interleave them to encrypt a single 100 megabit/second data channel. Figure 9.16 shows three parallel streams interleaved in CFB mode. The idea can also work in CBC and OFB modes, and with any number of parallel streams. Just remember that each stream needs its own IV. Don't share. 9.13 BLOCK CIPHERS VERSUS STREAM CIPHERS Although block and stream ciphers are very different, block ciphers can be implemented as stream ciphers and stream ciphers can be implemented as block ciphers. The best definition of the difference I've found is from Ranier Rueppel [1362.]: Block ciphers operate on data with a fixed transformation on large blocks of plaintext data; stream ciphers ope r- ate with a time-varying transformation on individual plaintext digits. Figure 9.16 Interleavingthtee CFB encryptions. In the real world, block ciphers seem to be more general (i.e., they can be used in any of the four modes) and stream ciphers seem to be easier to analyze mathematically. There is a large body of theoretical work on the analysis and design of stream c i- phers-most of it done in Europe, for some reason. They have been used by the world's militaries since the invention of electronics. This seems to be changing; recently a whole slew of theoretical papers have been written on block cipher design. Maybe soon there will be a theory of block cipher design as rich as our current theory of stream cipher d esign. Otherwise, the differences between stream ciphers and block ciphers are in the implementation. Stream ciphers that only e n- crypt and decrypt data one bit at a time are not really suitable for software implementation. Block ciphers can be easier to impl e- ment in software, because they often avoid time-consuming bit manipulations and they operate on data in computer-sized blocks. On the other hand, stream ciphers can be more suitable for hardware implementation because they can be implemented very eff i- ciently in silicon. These are important considerations. It makes sense for a hardware encryption device on a digital communications channel to encrypt the individual bits as they go by. This is what the device sees. On the other hand, it makes no sense for a software encry p- tion device to encrypt each individual bit separately. There are some specific instances where bit- and byte-wise encryption might be necessary in a computer system-encrypting the link between the keyboard and the CPU, for example-but generally the encry p- tion block should be at least the width of the data bus. Глава 10 Using Algorithms Think of security - data security, communications security, information security, whatever - as a chain. The security of the entire system is only as strong as the weakest link. Everything has to be secure: cryptographic algorithms, protocols, key manag e- ment, and more. If your algorithms are great but your random-number generator stinks, any smart cryptanalyst is going to attack your system through the random-number generation. If you patch that hole but forget to securely erase a memory location that contains the key, a cryptanalyst will break your system via that route. If you do everything right and accidentally e-mail a copy of your secure files to The Wall Street Journal, you might as well not have bothered. It's not fair. As the designer of a secure system, you have to think of every possible means of attack and protect against them all, but a cryptanalyst only has to find one hole in your security and exploit it. Cryptography is only a part of security, and often a very small part. It is the mathematics of making a system secure, which is different from actually making a system secure. Cryptography has its "size queens": people who spend so much time arguing about how long a key should be that they forget about everything else. If the secret police want to know what is on your computer, it is far easier for them to break into your house and install a camera that can record what is on your computer screen than it is for them to cryptanalyze your hard drive. Additionally, the traditional view of computer cryptography as "spy versus spy" technology is becoming increasingly ina p- propriate. Over 99 percent of the cryptography used in the world is not protecting military secrets; it's in applications such as bank cards, pay-TV, road tolls, office building and computer access tokens, lottery terminals, and prepayment electricity meters [43,44]. In these applications, the role of cryptography is to make petty crime slightly more difficult; the paradigm of the well-funded a d- versary with a rabbit warren of cryptanalysts and roomsful of computers just doesn't apply. Most of those applications have used lousy cryptography, but successful attacks against them had nothing to do with cry p- tanalysts. They involved crooked employees, clever sting operations, stupid implementations, integration blunders, and random idiocies. (I strongly recommend Ross Anderson's paper, "Why Cryptosytems Fail" [44]; it should be required reading for anyone involved in this field.) Even the NSA has admitted that most security failures in its area of interest are due to failures in impl e- mentation, and not failures in algorithms or protocols [1119]. In these instances it didn't matter how good the cryptography was; the successful attacks bypassed it completely. 10.1 CHOOSING AN ALGORITHM When it comes to evaluating and choosing algorithms, people have several alternatives: - They can choose a published algorithm, based on the belief that a published algorithm has been scrutinized by many cry p- tographers; if no one has broken the algorithm yet, then it must be pretty good. - They can trust a manufacturer, based on the belief that a well-known manufacturer has a reputation to uphold and is u n- likely to risk that reputation by selling equipment or programs with inferior algorithms. - They can trust a private consultant, based on the belief that an impartial consultant is best equipped to make a reliable evaluation of different algorithms. - They can trust the government, based on the belief that the government is trustworthy and wouldn't steer its citizens wrong. - They can write their own algorithms, based on the belief that their cryptographic ability is second-to-none and that they should trust nobody but themselves. Any of these alternatives is problematic, but the first seems to be the most sensible. Putting your trust in a single manufa c- turer, consultant, or government is asking for trouble. Most people who call themselves security consultants (even those from big- name firms usually don't know anything about encryption. Most security product manufacturers are no better. The NSA has some of the world's best cryptographers working for it, but they're not telling all they know. They have their own interests to further which are not congruent with those of their citizens. And even if you're a genius, writing your own algorithm and then using it without any peer review is just plain foolish. The algorithms in this book are public. Most have appeared in the open literature and many have been cryptanalyzed by e x- perts in the field. I list all published results, both positive and negative. I don't have access to the cryptanalysts done by any of the myriad military security organizations in the world Which are probably better than the academic institutionsДthey've been doing it longer and are better funded), so it is possible that these algorithms are easier to break than it appears. Even so, it is far more likely that they are more secure than an algorithm designed and implemented in secret in some corporate basement. The hole in all this reasoning is that we don't know the abilities of the various military cryptanalysts organizations. What algorithms can the NSA break? For the majority of us, there's really no way of knowing. If you are arrested with a DES-encrypted computer hard drive, the FBI is unlikely to introduce the decrypted plaintext at your trial; the fact that they can break an algorithm is often a bigger secret than any information that is recovered. During WWII, the Allies were forbidden from using decrypted German Ultra traffic unless they could have plausibly gotten the information elsewhere. The only way to get the NSA to admit to the ability to break a given algorithm is to encrypt something so valuable that its public dissemination is worth the admission. Or, better yet, create a really funny joke and send it via encrypted e-mail to shady characters in shadowy countries. NSA employees are people, too; I doubt even they can keep a good joke secret. A good working assumption is that the NSA can read any message that it chooses, but that it cannot read all messages that it chooses. The NSA is limited by resources, and has to pick and choose among its various targets. Another good assumption is that they prefer breaking knuckles to breaking codes; this preference is so strong that they will only resort to breaking codes when they wish to preserve the secret that they have read the message. In any case, the best most of us can do is to choose among public a l- gorithms that have withstood a reasonable amount of public scrutiny and cryptanalysts. Algorithms for Export Algorithms for export out of the United States must be approved by the U.S. government (actually, by the NSA (see Section 25.1). It is widely believed that these export-approved algorithms can be broken by the NSA. Although no one has admitted this on the record, these are some of the things the NSA is rumored to privately suggest to companies wishing to export their crypt o- graphic products: - Leak a key bit once in a while, embedded in the ciphertext. - "Dumb down" the effective key to something in the 30-bit range. For example, while the algorithm might accept a 100-bit key, most of those keys might be equivalent. - Use a fixed IV, or encrypt a fixed header at the beginning of each encrypted message. This facilitates a known-plaintext attack. - Generate a few random bytes, encrypt them with the key, and then put both the plaintext and the ciphertext of those ra n- dom bytes at the beginning of the encrypted message. This also facilitates a known- plaintext attack. NSA gets a copy of the source code, but the algorithm's details remain secret from everyone else. Certainly no one adve r- tises any of these deliberate weaknesses, but beware if you buy a U.S. encryption product that has been approved for export. 10.2 PUBLIC-KEY CRYPTOGRAPHY VERSUS SYMMETRIC CRYPTOGRAPHY Which is better, public-key cryptography or symmetric cryptography? This question doesn't make any sense, but has been d e- bated since public-key cryptography was invented. The debate assumes that the two types of cryptography can be compared on an equal footing. They can't. Needham and Schroeder [1159] pointed out that the number and length of messages are far greater with public-key alg o- rithms than with symmetric algorithms. Their conclusion was that the symmetric algorithm was more efficient than the public-key algorithm. While true, this analysis overlooks the significant security benefits of public-key cryptography. Whitfield Diffie writes 492,494]: In viewing public-key cryptography as a new form of cryptosystem rather than a new form of key management, I set the stage for criticism on grounds of both security and performance. Opponents were quick to point out that the RSA system ran about one- thousandth as fast as DES and required keys about ten times as large. Although it had been obvious from the beginning that the use of public key systems could be limited to exchanging keys for conventional [symmetric] cryptography, it was not immediately clear that this was necessary. In this context, the proposal to build hybrid systems [879] was hailed as a discovery in its own right. Public-key cryptography and symmetric cryptography are different sorts of animals; they solve different sorts of problems. Symmetric cryptography is best for encrypting data. It is orders of magnitude faster and is not susceptible to chosen-ciphertext a t- tacks. Public-key cryptography can do things that symmetric cryptography can't; it is best for key management and a myriad of protocols discussed in Part I. Other primitives were discussed in Part I: one-way hash functions, message authentication codes, and so on. Table 10.1 lists different types of algorithms and their properties [804]. 10.3 ENCRYPTING COMMUN1CAT10NS CHANNELS This is the classic Alice and Bob problem: Alice wants to send Bob a secure message. What does she do? She encrypts the me s- sage. In theory, this encryption can take place at any layer in the OSI (Open Systems Interconnect) communications model. (See the OSI security architecture standard for more information [305].) In practice, it takes place either at the lowest layers (one and two) or at higher layers. If it takes place at the lowest layers, it is called link-by-link encryption; everything going through a pa r- ticular data link is encrypted. If it takes place at higher layers, it is called end-to-end encryption; the data are encrypted selectively and stay encrypted until they are decrypted by the intended final recipient. Each approach has its own benefits and drawbacks. Link-by Link Encryption The easiest place to add encryption is at the physical layer (see Figure 10. 1). This is called link-by-link encryption. The i n- terfaces to the physical layer are generally standardized and it is easy to connect hardware encryption devices at this point. These devices encrypt all data passing through them, including data, routing information, and protocol information. They can be used on any type of digital communication link. On the other hand, any intelligent switching or storing nodes between the sender and the receiver need to decrypt the data stream before processing it. This type of encryption is very effective. Because everything is encrypted, a crypt- analyst can get no information about the structure of the information. He has no idea who is talking to whom, how long the messages they are sending are, what times of day they communicate, and so on. This is called traffic-flow security: the enemy is not only denied access to the information, but also access to the knowledge of where and how much information is flowing. Security does not depend on any traffic management techniques. Key management is also simple; only the two endpoints of the line need a common key, and they can change their key independently from the rest of the network. Imagine a synchronous communications line, encrypted using 1-bit CFB. After initialization, the line can run indefinitely, r e- covering automatically from bit or synchronization errors. The line encrypts whenever messages are sent from one end to the other; otherwise it just encrypts and decrypts random data. Eve has no idea when messages are being sent and when they are not; she has no idea when messages begin and end. All she sees is an endless stream of random-looking bits. If the communications line is asynchronous, the same 1-bit CFB mode can be used. The difference is that the adversary can get information about the rate of transmission. If this information must be concealed, make some provision for passing dummy messages during idle times. The biggest problem with encryption at the physical layer is that each physical link in the network needs to be encrypted: Leaving any link unencrypted jeopardizes the security of the entire network. If the network is large, the cost may quickly become prohibitive for this kind of encryption. Additionally, every node in the network must be protected, since it processes unencrypted data. If all the network's users trust one another, and all nodes are in secure locations, this may be tolerable. But this is unlikely. Even in a single corporation, information might have to be kept secret within a department. If the network accidentally misroutes information, anyone can read it. Table 10.2 summarizes the pros and cons of link-by-link encryption. |