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Introduction

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- 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

- 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

- 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.

- The cipher text generated by this equation:
- The intended receiver, in possession of the key is able to invert the transformation by this equation:

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)

- 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

- 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.

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)

- 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

- 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