internet security

 THE DATA ENCRYPTION STANDARD The Data Encryption Standard (DES), a system developed for the U.S. government, was intended for use by the general public. It has been officially accepted as a cryptographic standard both in the United States and abroad. The DES algorithm is a careful and complex combination of two fundamental building blocks of encryption: substitution and transposition. The algorithm derives its strength from repeated application of these two techniques, one on top of the other, for a total of 16 cycles. The sheer complexity of tracing a single bit through 16 iterations of substitutions and  transpositions has so far stopped researchers in the public from identifying more than a handful of general properties of the algorithm. The algorithm begins by encrypting the plaintext as blocks of 64 bits. The key is 64 bits long, but in fact it can be any 56-bit number. (The extra 8 bits are often used as check digits and do not affect encryption in normal i...

PRIVATE KEY CRYPTO SYSTEM Symmetric encryption (also called   private-key encryption   or   secret-key encryption ) involves   using the same key for encryption and decryption. Encryption involves applying an operation (an algorithm) to the data to be encrypted using the private key to make them unintelligible. The slightest algorithm (such as an exclusive OR) can make the system nearly tamper proof (there being so such thing as absolute security). However, in the 1940s, Claude Shannon  proved that to be completely secure, private-key systems need to use keys that are at least as long as the message to be encrypted. Moreover, symmetric encryption requires that a secure channel be used to exchange the key, which seriously diminishes the usefulness of this kind of encryption system. The main disadvantage of a secret-key cryptosystem is related to the exchange of keys. Symmetric encryption is based on the exchange of a secret (keys). The pr...

MAKING GOOD ENCRYPTION ALGORITHM So far, the encryption algorithms we have seen are trivial, intended primarily to demonstrate the concepts of substitution and permutation. At the same time, we have examined several approaches cryptanalysts use to attack encryption algorithms. Now we examine algorithms that are widely used in the commercial world. For each type of encryption we considered, has the advantages and disadvantages. But there is a broader question: What does it mean for a cipher to be "good"? The meaning of good depends on the intended use of the cipher. A cipher to be used by military personnel in the field has different requirements from one to be used in a secure installation with substantial computer support. In this section, we look more closely at the different characteristics of ciphers. Shannon's Characteristics of "Good" Ciphers In 1949, Claude Shannon [SHA49] proposed several characteristics that identify a good cipher. ...

ELEMENTARY CRYPTOGRAPHY: SUBSTITUTION CIPHER Encryption is the process of encoding a message so that its meaning is not obvious; decryption is the reverse process, transforming an encrypted message back into its normal, original form. Alternatively, the terms encode and decode or encipher and decipher are used instead of encrypt and decrypt. That is, we say that we encode, encrypt, or encipher the original message to hide its meaning. Then, we decode, decrypt, or decipher it to reveal the original message. A system for encryption and decryption is called a crypto system. The original form of a message is known as plaintext, and the encrypted form is called cipher text. For convenience, we denote a plaintext message P  as a sequence of individual characters P  = < p 1, p 2, …, pn >. Similarly, cipher text is written as C  = < c 1, c 2, …, cm >. Plain text Encryption Cipher text Cipher text Decryption plain text ...