Cryptography by wuxiangyu



                    SEMINAR    REPORT


                        ABSTRACT                       Page 1

           The requirement of information security within an organization has under gone
   two major changes in the last several decades. Before the widespread use of data
   processing equipment, the security of information felt to be valuable to an organization was
   provided primarily by physical and administrative means. An example of the former is the
   use of rugged filing cabinets with a combination lock for storing sensitive documents. An
   example of the latter is personnel screening procedures used during the hiring process.

   With the introduction of computer, the need for automated tools for protecting files and
   other information stored on the computer became evident. This is especially the case for a
   shared system, such as a time-sharing system, and the need is even more acute for system
   that can be accessed over public telephone network, data network, or the Internet. The
   generic name for the collection of the tools designed to protect data and to thwart hackers is
   computer security.

   The second major change that affected security is the introduction of distributed system and
   the use of network and communication facilities for carrying data between terminal user
   and computer and between computer and computer. Network security measure are needed
   to protect data during their transmission. In fact, the term network security is somewhat
   misleading, because virtually all business, government, and academic organization
   interconnect their data processing equipment with a collection of interconnected networks.
   Such a collection is often referred to as an internet, and the term internet security is used.

   There are no clear boundaries between these two forms of security. For example, one of the
   most publicized types of attack on information system is the computer virus. A virus may
   be introduced into a system physically when it arrives on a diskette and is subsequently
   loaded onto a computer. Viruses may also arrive over an internet. In either case, once the
   virus is resident on a computer security tools are needed to detect and recover from the

           Cryptography is the study of mathematical techniques related to aspects of
   information security, such as confidentially or privacy ,data integrity and entity
   authentication. Cryptography is not only means of providing information security, but
   rather one set of techniques. Confidentially means keeping information secret from all but
   those who authorized to see it. Data integrity means ensuring information has not been
   altered by unauthorized or unknown means. Entity authentication means corroboration of
   the identify of an entity.

          There are some characteristics of cryptographic algorithm. They are level security,
   performance , and ease of implementation. Level security defined by an upper bound on the                                                                          Page 2

   among of work necessary to defeat the objective. Performance refers to the efficiency of an
   algorithm in a particular mode of an operation. Ease of implementation refers to the
   difficulty of realizing the algorithm in practical implementation.

   There are several aspects of security. They are security service, security mechanism, and
   security attack. Security service means a service that enhances the security of the data
   processing system and information transfers of an organization.          Security mechanism
   means that is designed to detect, prevent, or recover from a security attacks. Security attack
   means any action that compromises the security of information owned by an organization.

   Encryption means the process of converting from plaintext to ciphertext. A key is a piece
   of information , usually a number that allows a receiver. Another key also allows a receiver
   to decode messages sent to him or her. There are some types of encryption. They are
   classical techniques, modern techniques, and public-key encryption. In Classical techniques
   there are substitution techniques and transposition techniques. In substitution techniques
   there are Caesar cipher, monoalphabetic cipher and polyalphabetic cipher. In Modern
   techniques there are block cipher , stream cipher and DES algorithm. In Public-key
   encryption the RSA algorithm is there.

          Cryptography has provided us with Digital Signatures that resemble in
   functionality the hand-written signature and Digital Certificates that related to an ID -card
   or some other official documents. There are some application of cryptography. They are
   secure communication, identification, secret sharing, electronic commerce, key recovery
   and remote access.

    Modern cryptography provides essential techniques for securing information and
   protecting data.

                                            INDEX   Subject                                                                                                                                 Page 3

   1           Introduction                                                1
   2           Definition of cryptography                                  1
   3           Categories of cryptographic algorithm                       1
   4           Related Terms of cryptography                               2
   5           Goals of cryptography                                       2
   6           Characteristics of cryptography                             3
   7           Aspects of Security                                         4
   8           The OSI security Architecture
   9           Model For Network Security                                  9
   10          Simplified Model Of Conventional Encryption                11
   11          Classical Encryption Technique                             13
   11.1             Substitution Technique
   11.1             Technique Transposition
   12     Modern Technique                                         15
   12.1             Stream & Block cipher
   12.2             Diffusion & Confusion
   12.3             DES Algorithm
   13     Public-Key Encryption                                    19
   13.1             Principle Of Public-Key Cryptography
   13.2             Public-Key cryptosystem
   13.3             Public-Key cryptosystem : Secrecy
   13.4             Public-Key cryptosystem : Authentication
   13.5             Public-Key cryptosystem : Secrecy & Authentication
   13.6             RSA Algorithm
   14     Advantages & Benefits                                   28
   14.1          ClassicSys as a standard
   14.2           Advantages & Benefits For END-USER…
   14.3          Advantages & Benefits For Authority…
   14.4          Technical Advantages & Benefits…
   15   Comparison between DES, RSA, & SED Algorithm 30
   16     Application Of Cryptography                             31
   17     Conclusion                                              32

                             INTRODUCTION                                                        Page 4

   Due to the rapid growth of digital communication and electronic data exchange information
   security has become a crucial issue in industry, business and administration. Assume a
   sender referred to here and in what follows as Alice (is commonly used) wants to send a
   message m to a receiver referred to as Bob. She uses an insecure communication channel.
   For example, the channel could be a computer network or a telephone line. There is a
   problem if the message contains confidential information. The message could be
   intercepted and read by eavesdropper. Or even worse, some might be able to modify the
   message during transmission, so Bob does not detect the manipulation.

           Cryptography has provided us with digital signature that resemble in functionality
   the hand-written signature and digital certificates that related to an ID CARD or other
   official documents. Modern cryptography provides essential techniques for securing
   information and protecting data.

                         Definition of cryptography
           Cryptography is the study of mathematical techniques related to aspects of
   information security, such as confidentially or privacy, data integrity and entity
   authentication. Cryptography is not the only means of providing information security, but
   rather one set of techniques.

               Categories of cryptographic algorithm
   There are main two types of cryptographic algorithm.
   1: - Symmetric key
   2: - Asymmetric key

                  Symmetric key
                       Sender and Receiver share a key.
                       A secret piece of information used to encrypt or decrypt the message.
                       If a key is secret, than nobody other than sender or receiver can read
                      If Alice and bank each has secret key, than they may send each other
       private message.
                      The task of privately choosing a key before communication, however
       be problematic.                                                                        Page 5

                Asymmetric key

                       Solves the key exchange problem by defining an algorithm which uses
       two keys, each of which can be use to encrypt the message.
                       If one is used to encrypt a message, another key must be used to
       decrypt it.
                       This makes it possible to receive secure message by simply publishing
       key (public key) and keeping another secret (private key).
                       Any one may encrypt a message using public key, but only the owner
       the public key is able to read it.
                       In this way Alice may send private message to owner of a key-pair (the
       bank) by encrypting it using their public-key. Only bank can decrypt it.

                                    Related Terms
   Plaintext: - An original intelligible message or data that is fed into the algorithm as

   Cipher text: - The coded message is known as Cipher text. That is depends on plaintext
   and secret key.

   Encryption: - The process of converting from plaintext to cipher text that is known as

   Decryption: - Restoring the plaintext from cipher text that is known as Decryption.

   Cryptography: - The many schemes used for enciphering constitute the area of study
   known as Cryptography. Such as a scheme is known as Cryptographic system or Cipher.

   Cryptanalysis: - Techniques used for deciphering a message without any knowledge of
   enciphering details fall into the area of Cryptanalysis.
   -          Cryptanalysis is what the layperson calls 'Breaking The Code '.

   Cryptology:       - The areas of cryptography and cryptanalysis together are called

                            Goals of cryptography
   The main goals of cryptography are
   1: - Confidentially or privacy                                                                        Page 6

   2: - Data integrity
   3: - Authentication
   4: - Non-repudiation

   1) Confidentially or Privacy: -
           Keeping information secret from all, but those who are authorized to see it.
   Confidentially is the protection of transmitted data from passive attacks. With respect to the
   content of data transmission, several levels of protection can be identified. The broadest
   service protects all user data transmitted between two users over a period of time.

           The aspect of Confidentially is the protection of traffic flow from analysis. This
   requires that an attacker not be able to observe to source and destination, frequency, length
   or any other characteristics of the traffic on a communication facility.

   2) Data Integrity: -

         Ensuring the information has not been altered by unauthorized or unknown means.
   One must have the ability to detect data manipulation by unauthorized parties. Data
   manipulation includes such things as insertion, deletion, and substitution

   3) Authentication: -

           Corroboration of the identify of an entity. Authentication is a service related to
   identification. This function applies to both entities and information.

   4) Non-repudiation: -
          Non-repudiation prevents either sender or receiver from denying a message. Thus,
   when a message is sent, the receiver can prove that the message was in fact send by the
   alleged sender. Similarly, when a message is received, the sender can prove the alleged
   receiver in fact received that message.

         Characteristics of a cryptographic algorithm
   The main characteristics of cryptographic algorithm are
   1: - Level of security
   2: - Performance
   3: - Ease of implementation

   1)             Level Of Security: -
          Typically the level of security is defined by an upper bound on the among of work
   necessary to defeat the objective. This is sometimes called the 'Work Factor'.                                                                          Page 7

           Work Factor could be defined as the minimum amount of work required to compete
   the private key when given the public key, or in the case of the symmetric key scheme to
   determine the secret key.
           A functionality algorithm will need to be combined to meet various information
   security objectives. Which algorithm is most effective for the given objective, will be d
   determined by the basic properties of the algorithm.
   The methods of operations algorithm when applied in various ways and with various inputs
   will typically exhibit different characteristics. Thus, one algorithm could provide very
   different functionality depending on its mode of operation or usage.

        2) Performance :-
   Performance refers to the efficiency of an algorithm in a particular mode of operation . For
   example, the number of bits/sec at which it can encrypt may rate an encryption algorithm.

        3) Ease Of Implementation :-
   This refers to the difficulty of realizing the algorithm in a practical instantiation, and might
   include the complexity of implementing in an either software or a hardware environment.
           The relative importance of various criteria depends to a large extent on the
   application and resources available. For example, in an environment where computing
   power is limited , one may have to trade off very high level of security for better system

   Aspects Of Security
                   To assess the security needs, of an organization effectively and choose
   various security products and policies, the manager responsible for security needs some
   systematic way of defining the requirements for security and characterizing the approaches
   to satisfied those requirements. One approach is to consider three aspects of information
   1)          Security attack
   2)          Security mechanism
   3)          Security service

   1)             Security Attack: -

   Any action that compromises the security of information owned by an organization.

   2)             Security Mechanism: -

   A mechanism that is designed to detect, prevent or recover from a security attack.                                                                            Page 8

   3)             Security Services: -

   A service that enhances the security of the data processing system and the information
   transfers of an organization. The services are intended to counter security attacks, and they
   make use of one or more security mechanism to provide the service.

                       The OSI Security Architecture
           To assess the security needs, of an organization effectively and choose various
   security products and policies, the manager responsible for security needs some systematic
   way of defining the requirements for security and characterizing the approaches to satisfied
   those requirements. This is difficult enough in a centralized data-processing environment;
   with the use of local area and wide area network, the problems are compounded.

   ITU-T (The International Telecommunication Union (ITU) Telecommunication
   Standardization Sector (ITU-T) United Nation (UN) -sponsored agency that develops
   standard, called Recommendations, relating to telecommunication and to Open System
   Interconnection (OSI)) Recommendations X.800, security Architecture for OSI, defines
   such a systematic approach. The OSI security architecture is useful to managers as way of
   organization the task of providing security. Further more, because this architecture was
   developed as international standards, computer and communications vendors have
   developed security feature for their products and services that relate to this structured
   definition of services and mechanisms.

                                   Security Services: -
   X.800 defines a security service as a service provided by a protocol layer of
   communication open system, which ensures adequate security of the system or of data
   X.800 divides these services into five categories and fourteen specific services.

   1)          Authentication
   2)          Access Control
   3)          Data confidentially or Privacy
   4)          Data integrity
   5)          Non- reputation

   1)             Authentication: -

          Corroboration of the identity of an entity. Two specific authentication services are
   defined in the standard.                                                                         Page 9

              Peer Entity Authentication: -

   Used in association with a logical connection to provide confidence in the identity of the
   entities connected.

              Data Origin Authentication: -

          In connection less transfer, provides assurance that the source of received data is as

   2)             Access Control: -
   In the context of network security, access control is the ability to limit and control the
   access to host system and application via communication links. To achieve this, each entity
   trying to gain access must first be identified, or authenticated, so that access rights can be
   tailored to the individual.

   3)             Data Confidentially Or Privacy: -
          The protection of data from unauthorized disclosure. Four specific services of
   confidentially are

              Connection Confidentially: -

   The protection of all user data on a connection.

              Connectionless Confidentially: -

   The protection of all user data in a single data book.

              Selective Field Confidentially: -

   The confidentially of selected fields within the user data on a connection or in a single data

              Traffic - flow confidentiality: -

   The protection of information that might be derived from observation of traffic flow.

   4)             Data Integrity: -
   The assurance that data received is exactly as sent by an authorized entity. That means no
   modification insertion, deletion or replay. There are five types of specific services.

              Connection Integrity With Recovery: -                                                                          Page 10

   Provides for the integrity of all user data on a connection and detects any modification,
   insertion, deletion or reply-of any data within an entries data sequence, with recovery

              Connection Integrity Without Recovery: -

   As above, but provides only detection without recovery.

              Selective-Field Connection Integrity: -

   Provides for the integrity of selected fields within the user data of a data block transferred
   over a connection and takes the form of determination of whether the selected fields have
   been modified, inserted, deleted or replayed.

              Connectionless Integrity: -

   Provides for the integrity of a single connectionless data block and may take the form of
   detection of data modification. Additionally, a limited form of replay detection may be

              Selective-Field Connectionless Integrity: -

   Provides for the integrity of selected fields within a single connectionless data block; takes
   the form of determination of a whether the selected field have been modified.

   5)             Non-repudiation: -

          Provides protection against denial by one of the entities involved in a
   communication of having participated in all or part of the communication. There are two
   types of specific services in Non-repudiation.

              Non-repudiation, origin: -

   Proof that the specific parties sent the massage.

              Non-repudiation, Destination: -

   Proofs that the massage was receive by the specific parties.

                                  Security mechanism: -
   As can be seen the mechanism are divided into those that are implemented in a specific
   protocol layer and those that are not specific to any particular protocol layer or security
   service. X.800 distinguishes between reversible encipherment mechanism is simply an                                                                         Page 11

   encryption algorithm that allows the data to be encrypted and subsequently decrypted.
   Irreversible encipherment mechanism includes hash algorithm and used in digital signature
   and message authentication application.

   Security Attacks: -
   A useful means of classifying security attacks, used in x.800, is in term of passive attacks
   and active attacks. A passive attack attempts to learn or make use of information from the
   system but does not affect system resources. An active attack attempts to alter system
   resources or affect their operation.

   Passive Attacks: -

   Passive attacks are in the nature of eavesdropping on, or monitoring of, transmissions. The
   goal of the opponent is to obtain information that is being transmitted. Two types of
   passive attacks are release of message contents and traffic analysis.

           The release of message contents is easily understood. A telephone conversation, an
   electronic mail message, and transferred file may contain sensitive or confidential
   information. We would like to prevent the opponent from learning the contents of these

           A second type of passive attacks, traffic analysis, is subtler. Suppose that we had a
   way of masking the contents of messages or other information traffic so that opponents,
   even if they captured the message, could not extract the information from the message. The
   common technique of masking contents is encryption. If we had encryption protection in
   place, an opponent might still be able to obverse the pattern of these messages. The
   opponent could determine the location and identity of communicating hosts and could
   observe the frequency and length of messages being exchanged. This information might be
   useful in guessing the nature of the communication that was taking place.

           Passive attacks are very difficult to detect because they do not involve any
   alteration of the data. How ever, it is feasible to prevent the success of these attacks,
   usually by means of encryption. Thus, the emphasis in dealing with passive attacks is on
   prevention rather then detection.

   Active Attacks: -

            Active attacks involve some modification of the data stream or the creation of a
   false stream and can be subdivided into four categories: masquerade, replay modification of
   messages, and denial of service.

         A masquerade takes place when one entity pretends to be a different entity. A
   masquerade attack usually includes one of the other forms of active attack.                                                                        Page 12

          Replay involves the passive capture of a data unit and it's subsequent
   retransmission to produce an unauthorized effect.

            Modification of messages simply means that some portion of a legitimate message
   is altered, or that messages are delayed or reordered to produce an unauthorized effect. For
   example, a message meaning "Allow John Smith to read confidential file accounts" is
   modified to mean "Allow Fred Brown to read confidential file accounts".

           The denial of service prevents or inhibits the normal use or management of
   communication facilities. This attack may have a special target; for example an entity may
   suppress all messages directed to particular destination. Another form service denial is the
   disruption of an entire network, either by disabling the network or by overloading it with
   messages so as to degrade performance.

           Active attacks present the opposite characteristics of passive attack where as
   passive attacks are difficult to detect, measures are available to prevent their success. On
   other hand it is quit difficult to prevent active attacks absolutely, because to do so would
   require physical protection of all communications facilities and paths at all times. Instead,
   the goal is to detect than to recover from any disruption or delays caused by them. Because
   the detection as a deterrent effect, it may also contribute to prevention.

                    A Model For Network Security: -
           A model for much of what we will be discussing is captured, in very general terms,
   in figure. A message is to be transferred from one party to another across some sort of
   Internet. The two parties, who are the principals in this transaction, must cooperate for the
   exchange to take place. A logical information channel is established by defining a route
   through the Internet from source to destination and by the cooperative use of
   communication protocol (e.g., TCP/IP) by the two principles.

          Security aspects come in to play when it is necessary or desirable to protect the
   information transmission from an opponent who may present a threat to confidentiality,
   authenticity, and so on. All the techniques for providing security have to components:

               A security-related transformation on the information to be sent. Examples
   include the encryption of the message, which scrambles the message so that it is unreadable
   by the opponent, and the addition of a code based on the contents of the message, which
   can be used to verify the identity of the sender.                                                                        Page 13

                                  Model for Network Security

              Some secret information shared by the two principals and, it is hoped,
   unknown to the opponent. An example is an encryption key used in conjunction with the
   transformation to scramble the message before transmission and unscramble it on

   A trusted third party may be needed to achieve secure transmission. For example, a third
   party may be responsible for distributing the secret information to the two principals while
   keeping it from any opponent. Or a third party may be needed to arbitrate disputes between
   the two principals concerning the authenticity of a message transmission.
           This general model shows that there are four basic tasks in designing a particular
   security service:

               Design an algorithm for performing the security-related transformation. The
   algorithm should be such that an opponent cannot defeat its purpose.
               Generate the secret information to be used with the algorithm
               Develop methods for the distribution and sharing of the secret information.
               Specify of protocol to be used by the two principals that makes use of the
   security algorithm and secret information to achieve a particular security service.
   However, there are other security related situations of interest that do not neatly fit this
   model but that are considered here. A general model of this other situation illustrated by
   figure, which reflects concern for protecting an information system from unwanted access.
   Most readers are familiar with the concerns caused by the existence of hackers, who
   attempt to penetrate systems that can be accessed over a network. The hacker can be some
   one who, with no malign intent, simply get satisfaction from breaking and entering a
   computer system. Or, the intruder can be a disgruntled employee who wishes to do damage,
   or a criminal who seeks to exploit computer assets for financial gain (e.g., obtaining credit
   card numbers or performing illegal money transfers)

   Another type of unwanted access is the placement in a computer system of logic that
   exploits vulnerabilities in the system and that can affect application program as well as
   utility programs such as editor and compilers. Programs can present two kinds of threats:                                                                        Page 14

              Information access threats intercept or modify data on behalf of users who
   should not have access to that data.
              Service threats exploit services flaws in computers to inhibit use by legitimate

                                Network Access Security Model

          Viruses and worms are two examples of software attacks. Such attacks can be
   introduced into a system by means of a disk that contain unwanted logic concealed in
   otherwise useful software.

           The security mechanism needed to coped with unwanted access fall into two broad
   categories. The first categories might be termed a gatekeeper function. It includes
   password-based login procedures that are designed to deny access to all but authorized user
   and screening logic that is designed to detect and reject worms, viruses, and other similar
   attacks. Once is gained, by either an unwanted users or unwanted software, the second line
   of defense consists of a variety of internal controls that monitor activity and analyze stored
   information in an attempt to detect the presence of unwanted intruders.

      Simplified Model of Conventional Encryption: -
   There are two requirements for secure use of conventional encryption:

               We need a strong encryption algorithm. At s minimum, we would like the
   algorithm to be such that an opponent who knows the algorithm and has access to one or
   more cipher text would be unable to decipher the cipher text or figure out the key. This
   requirement is usually stated in a stronger form : The opponent should be unable to decrypt
   cipher text or discover the key even if he or she is in possession of a number of cipher texts
   together with the plaintext that produced each cipher text.
               Sender and receiver must have obtained copies of the secret key in a secure
   fashion and must keep the key secure. If some one can discover the key and knows the
   algorithm, all communication using this key is readable.

   We assume that it is impractical to decrypt a message on the basis of the cipher text
   plus knowledge of the encryption/decryption algorithm. In other words we do not                                                                          Page 15

   need to keep the algorithm secret; we need to keep only the key secret.

   This feature of symmetric encryption is what makes it feasible for widespread use. The fact
   that the algorithm need not be kept secret means that manufacturers can end has developed
   low-cost chip implementations of data encryption algorithms. These chips are widely
   available and incorporated into a number of products. With the use of symmetric
   encryption, the principal security problem is maintaining the secrecy of the key.

                             Secret key shared by                         Secret key shared by
                             sender & receiver.                           sender & receiver.


       Plaintext                                                                                       Plaintext
       Input                                                                                           Output

                                Encryption                                  Decryption
                                Algorithm                                   Algorithm

                         Simplified Model of Conventional Encryption

   Cryptography: -
   Cryptographic systems are characterized along three independent dimensions.

               The type of operations used for transforming plain text to cipher text. All
   encryption algorithms are based on two general principles: substitution, in which each
   element in the plaintext (bit, letter, group of bits or letters) is mapped in to another element,
   and transposition, in which elements in the plaintext are rearranged. The fundamental
   requirement is that no information be lost. Most systems, referred to as product systems,
   involve multiple stages of substitutions and transpositions.

              The number of keys used. If both sender and receiver use the same key, the
   system is referred to as symmetric, single-key, secret-key, or conventional encryption. If
   the sender and receiver each use a different key, the system is referred to as asymmetric,
   two-key, or public-key encryption.                                                                            Page 16

                The way in which the plaintext is processed. A block cipher processes the
   input one block of elements at a time, producing an output block for each input block. A
   stream cipher processed the input elements continuously, producing output one element at a
   time, as it goes along.
   Cryptanalysis: -
   There are two general approaches to attacking a conventional encryption scheme:

                 Cryptanalysis: -

   Cryptanalytic attacks rely on the nature of the algorithm plus perhaps some knowledge of
   the general characteristics of the plaintext or even some sample plaintext-cipher text pairs.
   This type of attack exploits the characteristics of the algorithm to attempt to deduce a
   specific plaintext or to deduce the key being used. If the attack succeeds in deducing the
   key, the effect is catastrophic: All future and past messages encrypted with that key are

                 Brute-force attack: -

   The attacker tries every possible key on a piece of cipher text until an intelligible
   translation into plaintext is obtained. On average, half of all possible keys must be tried to
   achieve success.

                  Classical Encryption Techniques: -
   A study of these techniques unable us to illustrate the basic approaches to symmetric
   encryption used today and the types of cryptanalytic that must be anticipated.
          The two basic building blocks of all encryption techniques are substitution and
   transposition. We examine these in the next two sections. Finally, we discuss a system that
   combines both substitution and transposition.

   Substitution Techniques: -
           A substitution technique is one in which the letters of plaintext are replaced by
   other letters or by numbers or symbols. If the plaintext is viewed as a sequence of bits, then
   substitution involves replacing plaintext bit patterns with cipher text bit patterns.

   Caesar Cipher: -

          The earliest known use of a substitution cipher, and the simplest, was by Julius
   Caesar. The Caesar cipher involves replacing each letter of the alphabet with the letter
   standing three places further down the alphabet. For example                                                                         Page 17

          Plain: meet me after the toga party

   Note that the alphabet is wrapped around, so that the latter following Z is
   A. We can define the transformation by listing all possibilities, as follow:

           Plain: a b c d e f g h I j k l m n o p q r s t u v w x y z
           Cipher: D E F G H I J K L M N O P Q R S T U V W X Y Z A B C
    Let us assign a numeric equivalent to each letter:
   Then the algorithm can be expressed as follows. For each plaintext letter p, substitute the
   cipher letter C:
                                        C = E (p) = (P+3) mod (26)
           A shift may be of any amount, so that the general Caesar algorithm is
                                           C = E (p) = (p+k) mod (26)
      Where k takes on a value in the range 1 to 25. The decryption algorithm is simply
                                           P = D(C) = (C-k) mod (26)

   Transposition Techniques: -

                   All the techniques examined so far involve the substitution of a cipher text
   symbol for a plaintext symbol. A very different kind of mapping is achieved by performing
   some sort of permutation on the plaintext letters. This technique is referred to as a
   transposition cipher.

                   The simplest such cipher is the rail fence technique, in which the plaintext is
   written down as a sequence of diagonals and then read off as a sequence of rows. For,
   example, to encipher the message " meet me after the toga party " with a rail fence of depth
   2, we write the following.
   The encrypted message is
   This sort of thing would be trivial to crypt analyze. A more complex scheme is to write the
   messages in a rectangle, row by row, and read the message off, column by column, but
   permute the order of the columns. The order of the columns then becomes, the key to the
   algorithm. For example,
                                  Key:          4312567
                                  Plaintext:     at ta ckp
                                               os t p on e
                                               du n t I l t
                                               woa m x y z

                       Cipher                                                                text:
          TTNAAPTMTSUOAODWCOIXKNLYPETZ                                                                          Page 18

   A pure transposition cipher is easily recognized because it has the same letter frequencies
   as the original plaintext. For the type of columnar transposition just shown, cryptanalysis is
   fairly straightforward and involves laying out the cipher text in a matrix and playing around
   with column positions. Digram and trigram frequency tables can be useful.
            The transposition cipher can be made significantly more secure by performing more
   than one stage of transposition. The result is a more complex permutation that is not easily
   reconstructed. Thus, if the foregoing message is re-encrypted using the algorithm.
                                   Key:           4 3 1 2 5 6 7
                                   Input:          t t n a a p t
                                                  m t s u o a o
                                                  d w c o I x k
                                                  n l y p e t z

                                 Output:       NSCYAUOPTTWLTMDNAOIEPAXTTOKZ

   Modern Techniques: -
                  Virtually all-symmetric block encryption algorithm in current use is based
   on a structure referred to as a Feistel block cipher. We begin with a comparison of stream
   ciphers and block ciphers.

   Stream ciphers: -

   A stream cipher is one that encrypts a digital data stream one bit or one byte at a time.
   Example of classical stream ciphers is auto keyed Vigenere cipher and the Vernam cipher.

   Block ciphers: -

          A block cipher is one in which a block of plaintext is treated as a whole and used to
   produced a cipher text block of equal length. Typically, a block size of 64 or 128 bits is
   used. Using some of the modes of operation explained later in this chapter, a block cipher
   can be used to achieve the same effect as a stream cipher. Far more effort has gone into
   analyzing block ciphers. In general, they seem applicable to a broader range of applications
   than stream ciphers. The vast majority of network-based symmetric cryptographic
   applications make use of block ciphers.

   Diffusion and Confusion: -

          The terms diffusion and confusion were introduced by Claude Shannon to capture
   the two basic building blocks for any cryptographic system. Shannon's concern was to
   thwart cryptanalysis based on statistical analysis. The reasoning is as follows. Assume the                                                                         Page 19

   attacker has some knowledge of the statistical characteristics of the plaintext. For example,
   in a human -readable message in some language, the frequency distribution of the various
   letters may be known. Or there may be words or phrases likely to appear in the message. If
   these statistics are in any way reflected in the cipher text, the cryptanalyst may be able to
   deduce the encryption key, or part of the key, or at least a set of keys likely to contain the
   exact key.

   Other than recourse to ideal systems, Shannon suggests two methods for frustrating
   statistical cryptanalysis: diffusion and confusion. In diffusion, the statistical structure of the
   plaintext is dissipated into long-range statistics of the cipher text. This is achieved by
   having each plaintext digit affect the value of many cipher text digits, which is equivalent
   to saying that ciphertext digit is affected by many plaintext digits. An example of diffusion
   is to encrypt a message M = m1, m2, m3,… of characters with an averaging operation :
                        Yn =  mn + i (mod 26)

   Adding k successive letters to get a ciphertext letter Yn. One can show that the statistical
   structure of the plaintext has been dissipated. Thus the letter frequencies in the ciphertext
   will be more nearly equal than in the plaintext; the Digram frequencies will also be more
   nearly equal, and so on. In a binary block cipher, diffusion can be achieved by repeatedly
   performing some permutation of the sata followed by applying a function to that
   permutation; the effect is that bits from different positions in the original plaintext
   contribute to a single bit of ciphertext.

            Every block cipher involves a transformation of a block of plaintext into a block of
   ciphertext, where the transformation depends on the key. The mechanism of diffusion seeks
   to make the statistical relationship between the plaintext and ciphertext as complex as
   possible in order to thwart attempts to deduce that key. On the other hand, confusion seeks
   to make the relationship between the statistics of the ciphertext and the value of the
   encryption key as complex as possible, again to thwart attempts to discover the key. Thus,
   even if the attacker can get some handle on the statistics of the ciphertext, where the
   transformation depends on the key. The mechanism of diffusion seeks to make the
   statistical relationship between the plaintext and ciphertext as complex as possible in order
   to thwart attempts to deduce that key. On the other hand, confusion seeks to make the
   relationship between the statistics of the ciphertext and the value of the encryption key as
   complex as possible, again to thwart attempts to discover the key. Thus, even if the attacker
   can get some handle on the statistics of this, as Federal Information Processing Standards
   46 (FIPS pub 46). The algorithm itself is referred to as the Data Encryption Algorithm
   (DEA). For EDS, data are encrypted in 640bit blocks using a 56-bit key. The algorithm
   transforms 64-bit input in a series of steps into a 64-bit output. The same steps, with the
   same key, are used to reverse the encryption.

          The DES enjoys widespread use. It has also been the subject of much controversy
   concerning how secure the DES is,. To appreciate the nature of the controversy, let us
   quickly review the history of the DES.                                                                             Page 20

           In the late 1960s, IBM set up a research project in computer cryptography led by
   Horst Feistel. The project concluded in 1971 with the development of an algorithm with the
   designation LUCIFER (FEIS73), which was sold to Lloyd's of London for use in a cash-
   dispensing system, also developed by IBM LUCIFER is a Feistel block cipher that operates
   on blocks of 64 bits, using a key also of 128 bits. Because of the promising results
   produced by the LUCIFER project, IBM embarked on an effort to develop a marketable
   commercial encryption product that ideally could be implemented on a single chip. The
   effort was headed by Walter Tuchman and Cart Meyer, and if involved not only IBM
   researchers but also out-side consultants and technical advice from NSA. The outcome of
   this effort was a refined version of LUCIFER that was more resistant to cryptanalysis but
   that had a reduced key size of 56 bits, to fit on a single chip.

           In 1973, the National Bureau of Standards (NBS) issued a request for proposals for
   a national cipher standard. IBM submitted the results of its Tuchman-Meyer project. This
   was by far the best algorithm proposed and was adopted in 1977 as the Data Encryption

           Before its adoption as a standard, the proposed DES was subjected to intense
   criticism, which has not subsided to this day. Two areas drew the critics’ fire. First, the key
   length in IBM's original LUCIFER algorithm was 128 bits, but that of the proposed system
   was only 56 bits, an enormous reduction in key size of 72 bits. Critics feared that this
   key length was too short to withstand brute-force attacks. The second area of concern was
   that the design criteria for the internal structure of DES, the S-boxes, were classified. Thus,
   users could not be sure that the internal structure of DES was free of any hidden weak
   points that would enable NSA to decipher messages without benefit of the key. Subsequent
   events, particularly the recent work on differential cryptanalysis, seem to indicate that DES
   has a very strong internal structure. Furthermore, according to IBM participants, the only
   changes that were made to the proposal were changed to the S-boxes, suggested by NSA,
   that removed vulnerabilities identified in the course of the evaluation process.

           Whatever the merits of the case, DES has flourished and is widely used, especially
   in financial applications. In 1994, NIST reaffirmed DES for federal use for another five
   years; NIST recommended the use of DES for applications other than the protection of
   classified information. In 1999, NIST issued a new version of its standard that indicated
   that DES should only be used for legacy systems and that triple DES (which in essence
   involves repeating the DES algorithm three times on the on plaintext using two or three
   different keys to produce the ciphertext) be used.

   DES Encryption: -                                                                          Page 21

   The overall scheme for DES encryption is illustrated in figure. As with any encryption
   scheme, there are two inputs to the encryption function: the plaintext to be encrypted and
   the key. In this case, the plaintext must be 64 bits in length and the key is 56 bits in length.

   General Depiction of DES Encryption Algorithm

   Looking at the left-hand side of the figure, we can see that the processing of the plaintext
   proceeds in three phases. First, the 64-bit plaintext passes through an initial permutation
   (IP) that rearranges the bits to produce the permuted input. This is followed by a phase
   consisting of 16 rounds of the same function, which involves both permutation and
   substitution functions. The output of the last (16) round consists of 64 bits that are a
   function of the input plaintext and the key. The left and right halves of the output are
   swapped to produce the preoutput. Finally, the preoutput is passed through a permutation                                                                           Page 22

   that is the inverse of the initial permutation function, to produce the 64-bit ciphertext. With
   the exception of the initial and final permutation, DES has the exact structure of a Feistel

            The right-hand portion of figure shown the way in which the 56-bit key is used.
   Initially, the key is passed through a permutation function. Then, for each of the 16 rounds,
   a subkey (Ki) is produced by the combination of a left circular shift and a permutation. The
   permutation function is the same for each round, but a different subkey is produced because
   of the repeated iteration of the key bits.

                          Public-key cryptography: -
           The development of public-key cryptography is the greatest and perhaps the only
   true revolution in the entire history of cryptography. From its earliest beginning to modern
   times, virtually all cryptographic system have been based on the elementary tools of
   substitution and permutation.

   Principle of Public-key cryptosystem: -
          The concept of public-key cryptography evolved from an attempt to attack two of
   the most difficult problems associated with symmetric encryption. The first problem is that
   of key distribution.

          As we have seen, key distribution under symmetric encryption requires either

                  That to communicants already share a key, which some how has been
   distributed to them; or
                  The use of a key distribution center Whitfield Diffie. One of the discoverers
   of public-key encryption (along with Martin Hellman, both at Stanford University at the
   time), reasoned that this second requirement negated the very essence of cryptography, the
   ability to maintain total secrecy over your own communication. As Diffie put to (DIFF88),
   " what good would it do after all to develop impenetrable cryptosystems, if their users were
   forced to share their keys with a KDC that could be compromised by either burglary or
   subpoena? "

   The second problem that Diffie pondered, and one that was apparently unrelated to the first
   was that of " digital signatures ". If the use of cryptography was to become widespread, not
   just in military situations but for commercial and private purposes, then electronic message
   and documents would need the equivalent of signatures used in paper documents. That is,
   could a method be devised that would stipulate, to the satisfaction of all parties that a
   digital message had been sent by a particular person? This is a somewhat broader
   requirement than that of authentication, and its characteristics and ramifications are
   In the next subsection, we look at the overall framework for public-key cryptography. Then
   we examine the requirements for the encryption/decryption algorithm that is at the heart of
   the scheme.                                                                          Page 23

   Public-key cryptosystems: -
           The public-key algorithms rely on one key for encryption and a different but related
   key for decryption. These algorithms have the following important characteristics:
                 It is computationally infeasible to determine the decryption key given only
   knowledge of the cryptographic algorithm and the encryption key.

   In addition, some algorithms, such as RSA, also exhibit the following characteristics:

                  Either of the two related keys can be used for encryption , with other used
   for decryption.

   A public-key encryption scheme has six ingredients.

                  Plaintext: - This is the readable message or data that is fed into the
   algorithm as input.
                  Encryption algorithm: - The encryption algorithm performs various
   transformations on the plaintext.
                  Public and private key: - This is a pair of keys that have been selected so
   that if one is used for encryption, the other is used for decryption. The exact
   transformations performed by the encryption algorithm depend on the public or private key
   that is provided as input.
                  Ciphertext: - This is the scrambled message produced as input. It depends
   on the plaintext and the key. For a given message, two different keys will produce two
   different ciphertexts.
                  Decryption algorithm: - This algorithm accepts the ciphertext and the
   matching key and produces the original plaintext.

   The essential steps are the following:

                  Each user generates a pair of keys to be used for the encryption and
   decryption of messages.
                  Each user places one of the two keys in a public register or other accessible
   file. This is the public key. The companion key is kept private. As figure suggests, each
   user maintains a collection of public keys obtained from others.
                  If Bob wishes to send a confidential message to Alice, Bob encrypts the
   message using Alice's public key.
                  When Alice receives the message, she decrypts it using her private key. No
   other recipient can decrypt the message because only Alice knows Alice's private key.

   With this approach, all participants have access to public keys, and private keys, are
   generated locally by each participant and therefore need never be distributed. As long as a
   system controls its private key, its incoming communication is secure. At any time, a
   system can change its private key and publish the companion public key to replace its old
   public key.                                                                           Page 24

   Table shows some of the important aspects of symmetric and public-key encryption. To
   discriminate between the two, we will generally refer to the key used in symmetric
   encryption as a secret key. The two keys used for public-key encryption are referred to the                                                                      Page 25

   public key and private key. Invariably, the private key is kept secret, but it is referred to as
   a private key than a secret key to avoid confusion with symmetric encryption.

                  Conventional Encryption                         Public-key Encryption

           Needed to work :-                               Needed to Work :-

           1) The same algorithm with the same key          1) One algorithm is used for encryption
              is used for encryption and decryption.           and decryption with a pair of keys,
                                                               one for encryption and one for

           2) The sender and receiver must share            2) The sender and receiver must each
              the algorithm and the key.                       Have one of the matched pair of
                                                               keys(not the same one ).

           Needed for Security :-                          Needed for Security :-

           1) The key must be kept secret.                  1) One of the two keys must be kept

           2) It may be impossible or at least              2) It may be impossible or at least
               impractical to decipher a message if            impractical to decipher a message
              no other information is available.              If no other information is available.

           3) Knowledge of the algorithm plus              3) Knowledge of the algorithm plus of
              samples of ciphertext must be                   the keys plus samples of ciphertext
              insufficient to determine the key.              must be insufficient to determine the
                                                             other key.

   Let us take a closer look at the essential elements of a public-key encryption scheme, using
   figure. There is some source A that produces a message in plaintext, X =
   X[X1,X2,…..Xm]. The M elements of X are letters in some finite alphabet.
           The message is intended for destination B. B generates a related pair of keys: a
   public key, Kub, and a private key, KRb. KRb is known only to B, whereas Kub is publicly
   available and therefore accessible by A.
   With the message X and the encryption key KUb as input, A forms the ciphertext Y = Y
   [Y1, Y2…YN]:

                    Y = EKUb (X)                                                                           Page 26

   The intended receiver, in possessing of the matching private key, is able to invert the

                          X = DKRb(Y)

                           Source A                           Cryptanalyst         Destination B

                          Source A                                                Destination
                       Source                                                                           Destination
                                                 Encryption                  Decryption

                                B’s public key   Algori                      Algori       B’s private
                                                 thm                         thm          key

                                                                             Key pair

                                Public - key cryptosystem: secrecy

   An opponent, observing Y and having access to KUb, but not having access to KRb or X,
   must attempt to recover X and/or KRb. It is assumed that the opponent does have
   knowledge of the encryption (E) and decryption (D) algorithms. If the opponent is
   interested only in this particular message, then the focus of effort is to recover X, by
   generating a plaintext estimate X^. Often, however, the opponent is interested in being able
   to read future messages as well, in which case an attempt is made to recover KRb by
   generating an estimate K^Rb.

          We mentioned earlier that either of the two related keys can be used for encryption,
   with the other being used for decryption. This enables a rather different cryptographic
   scheme to be implemented. Whereas the scheme illustrated in Figure provides
   confidentiality, Figure shows the use of public-key encryption to provide authentication:
                                 Y = EKRa (X)
                                 X = DKUa (Y)

           In this case, A prepares a message to B and encrypts it using A's private key before
   transmitting it. B can decrypt the message using A's public key. Because the message was
   encrypted using A's private key, only A could have prepared the message. Therefore, the
   entire encrypted message serves as a digital signature. In addition, it is impossible to alter                                                                                         Page 27

   the message without access to A's private key, so the message is authenticated both in
   terms of source and in terms of data integrity.

                    Source A                          Cryptanalyst             Destination B

                   Source A                                                   Destination B

               Source                   Encryption                     Decryption
                                         Algorithm                      Algorithm                     Destination

                         A’s private                                                   A’s public key

                                         Key pair

                           Public -key Cryptosystem: Authentication

   In the preceding scheme, the entire message is encrypted, which, although validating both
   author and contents, requires a great deal of storage. Each document must be kept in
   plaintext to be used for practical purposes. A copy also must be stored in ciphertext so that
   the origin and contents can be verified in case of a dispute. A more efficient way of
   achieving the same results is to encrypt a small block of bits that is function of the
   document. Such a block, called an authenticator, must have the property that it is infeasible
   to change the document without changing           the authenticator. If the authenticator is
   encrypted with the sender's private key, it serves as a signature that verifies origin, content,
   and sequencing.

   It is important to emphasize that the encryption process just described does not provide
   confidentiality. That is, the message being sent is safe from alteration but not from
   eavesdropping. This is obvious in the case of a signature based on a portion of the message,
   because the rest of the message is transmitted in the clear. Even in the case of complete
   encryption, as shown in figure, there is no protection of confidentiality because any
   observer can decrypt the message by using the sender's public key.
           It is, however, possible to provide both the authentication function and
   confidentiality by a double use of the public-key scheme.
                          Z = EKUb [ EKRa(X) ]
                          X = DKUa [ DKRb(z) ]                                                                           Page 28

                                      Source A                                              Destination B

                            Encry.               Encry.                    Decry.               Decry.
            Source          Algori-              Algori-                                                    Dest.
                                                                           Algori-              Algori-
                            them                 them                      them                 them

                                                                                      B’s private key

                                                    B’s public key         Key Pair
        A’s private
                            Key Pair        A’s public key

                      Public _ key cryptosystem: Secrecy and Authentication

           In this case, we being as before by encrypting a message, using the sender's private
   key. This provides the digital signature. Next, we encrypt again, using the receiver's public
   key. Only the intended receiver, who alone has the matching private key, can decrypt the
   final ciphertext. Thus, confidentiality is provided. The disadvantage of this approach is that
   the public-key algorithm, which is complex, must be exercised four times rather than two
   in each communication.

   Application for Public-Key Cryptosystems: -
   Before proceeding, we need to clarify one aspect of public-key cryptosystems that is
   otherwise likely to lead to confusion, Public-key systems are characterized by the use of a
   cryptographic type of algorithm with two keys, one held private and one available publicly.
   Depending on the application, the sender uses either the sender's private key or the
   receiver's public key, or both, to perform some type of cryptosystems into three categories.                                                                         Page 29

                 Encryption/decryption:

   The sender encrypts a message with the recipient's public key.

                    Digital signature:

   The sender " signs " a message with its private key. Signature is achieved by a
   cryptographic algorithm applied to the message of to a small block of data that is a function
   of the message.

                 Key exchange:

   Two sides cooperate to exchange a session key. Several different approaches are possible,
   involving the private key(s)of one both parties.

    Some algorithms are suitable for all three applications, whereas others can be used only for
   one or two of these applications.

   The RSA Algorithm: -

         The pioneering paper by Diffie and Hellman [DIFF 76 b] introduce a new
   Approach to cryptography and, in effect challenged cryptologists to come up with a
   cryptographic algorithm that met the requirements for public - key systems. One of the first
   of the responses to the challenge was developed in 1977 by Ron Rivest, Adi Shamir, and
   Len Adleman at MIT and first published in 1978 [RIVE 78] the Rivest - Shamir- Adleman
   (RSA) scheme has since that time reigned supreme as the most widely accepted and
   implemented general - purpose approach to public - key encryption.

                  The RSA scheme is a block cipher in which the plaintext and ciphertext are
   integers between 0 and n -1 for some n. A typical size for n is 1024 bits, or 309 decimal
   digits. We examine RSA in this section in some detail, beginning with an explanation of
   the algorithm. Then we examine some of the computational and cryptanalytical
   implications of RSA.

   Description of the Algorithm: -

           The scheme developed by Rivest, Shamir, and Adleman makes use of an expression
   with exponential. Plaintext is encrypted in blocks, with each block having a binary value
   less than some number n. That is the block size must be less than or equal to log2(n); in
   practice, the block size is k bits, where 2k < n < 2k+1. Encryption and decryption are of
   the following forms, for some plaintext block M and ciphertext block C.
                          C = Me mod n
                          M = Cd mod n = (Me) d mod n = Med mod n                                                                        Page 30

   Both sender and receiver must know the value of n. The sender knows the value of e, and
   only the receiver knows the value of d. Thus, this is a public-key encryption algorithm with
   a public key of KU = {e,n} and a private key of KR ={d,n}. For the algorithm to be
   satisfactory for public-key encryption, the following requirements must be meet:
   1 -> it is possible to find value of e, d, n such that Med = M mod n for all M < n.
   2 -> it relatively easy to calculate Me and Cd for all values of M < n.
   3 -> it is infeasible to determine d given e and n.

    For now, we focus on the first requirement and consider the other questions later. We need
   to find a relationship of the form
                                   Med = M mod n
   A corollary to Euler's theorem, fits the bill: Given two prime numbers, p and q and two
   integers n and m, such that n = pq and 0 < m< n, and arbitrary integer k, the following
   relationship holds:
                           Mk(n) + 1 = mk (p-1)(q-1)+1 = m mod n
   Where (n) is the Euler totient function which is the number of positive integers less then n
   and relatively prime to n. for p, q prime, (pq) = (p-1)(q-1). Thus we can achieve the
   desired relationship if
                           Ed = k(n) + 1
   This is equivalent to saying:
                           Ed = 1 mod (n)
                            D = e-1 mod (n)
   That is e and d are multiplicative inverses mod (n). Note that according to the rules of
   modular arithmetic, this is true only if d (and therefore e) is relatively prime to (n),
   Equivalently, gcd ((n), d) = 1
   We are now ready to state the RSA scheme. The ingredients are the following:
   P, q, two prime numbers                              (private, chosen)
        n = pq                                                  (public, calculated)
   e, with gcd((n),e) = 1; 1<e<(n)                    (public, chosen)
   d = e-1 mod (n)                                     (private, calculated)

   The private key consists of {d, n} and the public key consists of {e, n}. Suppose that user
   A has published its public key and that user B wishes to send the message M to A. then B
   calculates C = Me (mod m) and transmits C. on receipt of this ciphertext, user A decrypts
   by calculating M = Cd (mod m).

   It is worthwhile to summarize the justification for this algorithm. We have chosen e and d
   such that
                         d = e-1 mod (n)
                  ed = 1 mod (n)
   Therefore, ed is of the form k(n)+1. But by the corollary to Euler’s theorem, provided
   here, given two prime numbers p and q, and integers n = pq and M with
   0 < M < m:
                         Mk(n) + 1 = Mk (p-1)(q-1)+1 = M mod n                                                                        Page 31

   So, Med = M mod n.
   Now                C = Me mod n
                      M = Cd mod n = (Me) d mod n = Med mod n = m mod n

                          Advantages & Benefits: -
   ClassicSys as a standard...
   Besides ClassicSys ciphering at high speed, two more advantages make
   Classic prime candidate for THE standard application in cryptography :

   1.         ClassicSys uses only 1 secret key to meet ALL the cryptographic needs of an
   user such as :

                  To authenticate himself
                  To authenticate messages with a time reference
                  To generate all the Session Keys he needs for Email (as one possible
                  To generate several keys for other applications: banking, electronic
   commerce, electronic voting, casino games at home, ...
   2. ClassicSys is designed in such a way that there is no valid reason to forbid it's
   use in any country in the world. ClassicSys gives all the required guarantees to its
   users and their government : secret keys must not be divulged and Security Services
   can always decipher suspect messages.

   Advantages & benefits for the End-User ...
   ClassicSys offers more than the known advantages of encryption solutions:

                 Very high speed of encryption (see below).
                 The chip contains the SED algorithm and all the other features of
   ClassicSys. One system covers all cryptographic needs, for all applications.
                 New applications can be added without updating the chip.
                 ClassicSys works is fully automated, requests to the TA are returned
   directly, without human intervention.
                 Private Keys are completely unknown to everybody, even the Trust
   Authority's manager! All keys are written into chips and are not accessible to humans or
   other machines. This guarantees the privacy of all the end-users.
                 Once an end-user has received the information to generate his Application
   Keys, he does not need the intervention of the TA anymore. Email for example, users do
   not need the TA to exchange messages between themselves.
                 ClassicSys acts like a public key cryptosystem : every end-user has one
   public ID number, which is used in a similar way to public keys. Email for example, when                                                                   Page 32

   somebody wants to communicate with another end-user, he sends to the TA his ID number
   and the one from his correspondent. In return he receives information from the TA to
   generate their Session Key.

   Advantages & benefits for the Authority...
                  ClassicSys enables the TA and National Security Service (NSS) to act
   completely separately, under different authorities, as required by our Democracies.
   Requests from the NSS to the TA are recorded encrypted by the TA (TA doesn't know the
   ID of Alice or Bob in a suspect message). This guarantees the confidentiality of the NSS's
   investigation, however, the recorded provides an audit trail for any Competent
   Investigating Authority. Optimum ClassicSys operation should have the TA and NSS
   under different authorities, but every country can implement it as seen fit.
                  ClassicSys enables the NSS to decrypt the content of suspect incoming and
   outgoing international messages, without the necessity for users to deposit their private
   secret keys in the corresponding countries (as with the RSA).
                  Only the NSS is able to request necessary information to the TA to
   investigate suspect messages.
                  Each country remains independent regarding the deciphering of the
   incoming and outgoing messages: each message contains the necessary information to be
   deciphered by the 2 National Security Services.
                  Each Trust Authority has its own Private Key. Consequently they can only
   compute Private Keys for domestic users.

   Technical advantages & benefits
   ClassicSys is easy to implement in integrated circuits because:
                 It uses only XOR and branching functions
                 No reporting arithmetic bits are needed
                 Programming can be done with a polynomial structure.
                 The length of the blocks of key and data are identical and equal to 128 bits
   (16 bytes).

   Security of ClassicSys is enhanced compared to other systems because:
                   Deciphering is not the reverse of ciphering
                   The ciphering and deciphering keys are different
                   All the PrivateKeys (end-users, TAs, NSS’s) are included in an IC and
   therefore not accessible.
   There is no known way to reconstruct, by cryptanalysis, the secret key, knowing a
   clear and it's corresponding encrypted message.

   Differential cryptanalysis is not suitable to the SED algorithm. On average, there is
   only one key corresponding to a clear and its associated encrypted text and therefore, each
   bit of the key has equal weight in the algorithm.                                                                      Page 33

   Only 1 secret key of 128 bits is enough to meet all the cryptographic needs of an
   end-user such as :
                 To generate all the Session Keys he needs
                 To authenticate himself
                 To authenticate messages with a time reference
                 To generate several keys for other applications (banking, electronic
          commerce, electronic voting, casino games at home,...)

   Unlike the RSA algorithm, where every key requires a determined space, the SED
   algorithm can use every block contained in the space 2128.

   The SED algorithm is very fast for the following reasons:
                  The length of the blocks (key and data) is small (128 bits against more than
   512 bits) but long enough to disable every exhaustive cryptanalysis.
                  On average. It is possible to compute at 1/3 of the clock frequency (8 to 10

   The SED algorithm is completely transparent. Due to the theory of Multiplicative
   Groups we can confirm that there is no Trojan Horse in the SED algorithm.

   The SED algorithm permits chained mode ciphering, allowing reduction of the
   authentication information to one block of 128 bits, whatever the length of the data
   to authenticate.

   Comparison between the DES, the RSA and the SED

   The table below compares the important features of the DES, the RSA and the SED
   algorithms, used within global cryptographic systems.

   Feature              DES                    RSA                      SED
   Speed                high                  low                       high
   Deposit of keys      needed                needed                    not needed
   Country independence no                    no                        yes
   Trojan Horse         not proved            no                        no
   Data block length    64 bits minimum       512 bits                  128 bits
   Key length           56 bits minimum        512 bits                 128 bits
   Use of data space    full, 64 bits (2^64), variable, limited,         full 128 bits
                        8 bytes                not defined               (2^128), 16
   Ciphering & deciphering
   key                     same                    different           different
   Ciphering & deciphering
   algorithm                different                same               different
   Algorithm contains only
   XOR and branching         no                       no                yes                                                                         Page 34

   Average number of key
   For one pair E&C=1    probably not                 probably yes        yes
   cryptanalysis method  differential method           product             no known
                                                     factorization        method

   Global system including
   algorithm               not suitable                not suitable        ClassicSys

   Application: -
   Cryptography is extremely useful; there is a multitude of applications, many of which are
   currently in use. A typical application of cryptography is a system built out of the basic
   techniques. Such systems can be of various levels of complexity. Some of the more simple
   applications are secure communication, identification, authentication, and secret sharing.
   More complicated applications include systems for electronic commerce, certification,
   secure electronic mail, key recovery, and secure computer access.

   In general, the less complex the application, the more quickly it becomes a reality.
   Identification and authentication schemes exist widely, while electronic commerce systems
   are just beginning to be established. However, there are exceptions to this rule; namely, the
   adoption rate may depend on the level of demand. For example, SSL-encapsulated HTTP
   (see Question 5.1.2) gained a lot more usage much more quickly than simpler link-layer
   encryption has ever achieved. The adoption rate may depend on the level of demand.

   Secure Communication
   Secure communication is the most straightforward use of cryptography. Two people may
   communicate securely by encrypting the messages sent between them. This can be done in
   such a way that a third party eavesdropping may never be able to decipher the messages.
   While secure communication has existed for centuries, the key management problem has
   prevented it from becoming commonplace. Thanks to the development of public-key
   cryptography, the tools exist to create a large-scale network of people who can
   communicate securely with one another even if they had never communicated before.

   Identification and Authentication
   Identification and authentication are two widely used applications of cryptography.
   Identification is the process of verifying someone's or something's identity. For example,
   when withdrawing money from a bank, a teller asks to see identification (for example, a
   driver's license) to verify the identity of the owner of the account. This same process can be
   done electronically using cryptography. Every automatic teller machine (ATM) card is
   associated with a ``secret'' personal identification number (PIN), which binds the owner to
   the card and thus to the account. When the card is inserted into the ATM, the machine
   prompts the cardholder for the PIN. If the correct PIN is entered, the machine identifies that
   person as the rightful owner and grants access. Another important application of                                                                         Page 35

   cryptography is authentication. Authentication is similar to identification, in that both allow
   an entity access to resources (such as an Internet account), but authentication is broader
   because it does not necessarily involve identifying a person or entity. Authentication
   merely determines whether that person or entity is authorized for whatever is in question.
   For more information on authentication and identification, see Question 2.2.5.

   Secret Sharing
   Another application of cryptography, called secret sharing, allows the trust of a secret to be
   distributed among a group of people. For example, in a (k, n)-threshold scheme,
   information about a secret is distributed in such a way that any k out of the n people (k £ n)
   have enough information to determine the secret, but any set of k-1 people do not. In any
   secret sharing scheme, there are designated sets of people whose cumulative information
   suffices to determine the secret. In some implementations of secret sharing schemes, each
   participant receives the secret after it has been generate.

         This document's some topics are just picked up by some of reference book and
   some excellent web sight which give me good explore such references are following.
            Cryptography And Network Security (William Stallings).
            Computer Network ( Andrew S. Tanenbaum).

   Conclusion :-
           By analysis of this report and their subtopics which are mentioned above, which are
   inherently guides us about various cryptographic techniques used in data security. By using
   of encryption techniques a fair unit of confidentiality, authentication, integrity, access
   control and availability of data is maintained. Using cryptography Electronic Mail Security,
   Mail Security, IP Security, Web security can be achieved.                                                                          Page 36
Cryptography   Page 37

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