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applied cryptography - protocols, algorithms, and source code in c

If all the networks users trust one another and all

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Unformatted text preview: Which is better, public-key cryptography or symmetric cryptography? This question doesn’t make any sense, but has been debated 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 algorithms 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 attacks. 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 Communications Channels This is the classic Alice and Bob problem: Alice wants to send Bob a secure message. What does she do? She encrypts the message. In theory, this encryption can take place at any layer in the OSI (Open Systems Int...
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