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Introduction. Conventional encryption, also referred to symmetric encryption, secret key, or single key encryption. An encryption scheme has five ingredients: Plaintext: this is the original message Encryption Algorithm: performs various transformation and substitution on the plaintext.

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Presentation Transcript
Introduction
Introduction

  • Conventional encryption, also referred to symmetric encryption, secret key, or single key encryption.

  • An encryption scheme has five ingredients:

    • Plaintext: this is the original message

    • Encryption Algorithm: performs various transformation and substitution on the plaintext


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  • Secret Key: The exact substitution and transformations performs by the algorithm depends on the Key

  • Cipher Text: this is the scrambled message produced as output.

  • Decryption Algorithm: this is the encryption algorithm run in reverse



Essential elements of a conventional encryption
Essential Elements of a Conventional Encryption

  • Source for a message which produces a message in plaintext.

    M elements could be a 26 capital letters or nowadays the binary alphabet {0,1}

  • A key of the form:

    is generated at the source and is delivered to the

    destination by the mean of secure channel.


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  • The cipher text generated by this equation:

  • The intended receiver, in possession of the key is able to invert the transformation by this equation:


Cryptography
Cryptography

Generally classified along three independent Dimensions:

  • The type of operations used for transforming plain text to cipher text (substitution, and transposition {permutation, and matrix})

  • The number of keys used

    • Symmetric (single key)

    • Public key or asymmetric (two keys)

  • The way in which the plaintext is processed

    • Block (cipher one block at a time)

    • Stream (cipher the input continuously)


Cryptanalysis
Cryptanalysis

  • The process of attempting to discover the plain text or key is known as cryptanalysis.

  • Brute–Force approach: knowing the algorithm and cipher text and trying all possible keys.

  • Chosen Plaintext: if the opponent knows the placement of certain key words in the header of file


Average time required for key exchange
Average Time Required for Key Exchange

  • An encryption scheme is computationally secure if :

    • The cost of breaking the cipher text exceeds the value of the encrypted information.

    • The time required to break the cipher exceeds the life time of the information.


Classical encryption techniques
Classical Encryption Techniques

One useful classification is:

  • Substitution Technique: the letters of plaintext are replaced by other letters or by numbers, examples are (Caesar cipher and Mono-alphabetic ciphers, Hill cipher and Polyalphabetic cipher).

  • Transposition techniques: performing some permutation on the plaintext letters such as Rail Fence algorithm and rotor machines.

    (go to the net and find information about them)


Caesar cipher technique
Caesar Cipher Technique

  • Each letter of the alphabet is replaced by the letter stands three places further down the alphabet:

  • Note that the alphabet is wrapped around, so that the letter following Z is A


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  • Then have Caesar cipher as:

    • C = E(p) = (p + k) mod (26)

    • p = D(C) = (C – k) mod (26)

  • The important characteristics of this technique:

    • The encryption and decryption algorithms are known.

    • There are only 25 key to try which is far from security.

    • The language of the plaintext is known



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