 Symmetric encryption is a form of cryptosystem in which encryption and decryption are performed using the same key. It is also known as conventional encryption.

The two types of attack on an encryption algorithm are:

Cryptanalysis:
 Based on properties of the encryption algorithm.
 Some knowledge of the general characteristics of the plaintext or even some sample plaintextciphertext pairs.
 Bruteforce: involves trying all possible keys.


Traditional symmetric ciphers use:
 Substitution techniques: map plaintext elements (characters, bits) into ciphertext elements.
 Transposition techniques: systematically transpose the positions of plaintext elements.
 Rotor machines are sophisticated precomputer hardware devices that use substitution techniques.
 Steganography is a technique for hiding a secret message within a larger one in such a way that others cannot discern the presence or contents of the hidden message.
 Cryptanalysis are
techniques used for deciphering a message without any knowledge of the enciphering details.

Requirements for secure use of conventional encryption:

We need a strong encryption algorithm.
 The opponent should be unable to decrypt ciphertext or discover the key even if he or she is in possession of a number of ciphertexts together with the plaintext that produced each ciphertext.
 Sender and receiver must have obtained copies of the secret key in a secure fashion and must keep the key secure.


Cryptographic systems are characterized along three independent dimensions:

The type of operations used for transforming plaintext to ciphertext (Substitution, Transposition).
 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, singlekey, secretkey, or conventional encryption.
 If the sender and receiver use different keys, the system is referred to as asymmetric, twokey, or publickey encryption.

The way in which the plaintext is processed.
 Stream Cipher: symmetric encryption algorithm in which ciphertext output is produced bitbybit or bytebybyte from a stream of plaintext input.
 Block Cipher symmetric encryption algorithm in which a block of plaintext bits (typically 64 or 128) is transformed as a whole into a ciphertext block of the same length


Types of attacks on encrypted messages:

Ciphertext only:
 Encryption algorithm.
 Ciphertext.

Known plaintext:
 Encryption algorithm.
 Ciphertext.
 One or more plaintextciphertext pairs formed with the secret key.

Chosen plaintext:
 Encryption algorithm.
 Ciphertext.
 Plaintext message chosen by cryptanalyst, together with its corresponding ciphertext generated with the secret key.

Chosen ciphertext.
 Encryption algorithm.
 Ciphertext.
 Purported ciphertext chosen by cryptanalyst, together with its corresponding decrypted plaintext generated with the secret key.

Chosen text:
 Encryption algorithm.
 Ciphertext.
 Plaintext message chosen by cryptanalyst, together with its corresponding ciphertext generated with the secret key.
 Purported ciphertext chosen by cryptanalyst, together with its corresponding decrypted plaintext generated with the secret key.

 Generally, an encryption algorithm is designed to withstand a knownplaintext attack.

Encryption scheme types:

Unconditionally secure:
 If the ciphertext generated by the scheme does not contain enough information to determine uniquely the corresponding plaintext, no matter how much ciphertext is available.
 Conditionally secure.

 All forms of cryptanalysis for symmetric encryption schemes are designed to exploit the fact that traces of structure or pattern in the plaintext may survive encryption and be discernible in the ciphertext.
 Cryptanalysis for publickey schemes proceeds from a fundamentally different premise, namely, that the mathematical properties of the pair of keys may make it possible for one of the two keys to be deduced from the other.

Well encryption algorithm characteristics:
 The cost of breaking the cipher exceeds the value of the encrypted information.
 The time required to break the cipher exceeds the useful lifetime of the information.
 Unfortunately, there is no encryption algorithm that is unconditionally secure.

Substitution Techniques:
 Caesar Cipher.
 Monoalphabetic Ciphers.

Playfair Ciphers.
 Hill Cipher.

Polyalphabetic Ciphers.

Features of polyalphabetic ciphers:
 A set of related monoalphabetic substitution rules is used.
 A key determines which particular rule is chosen for a given transformation.

 47How to know that the plaintext was encrypted using monoalphabetic or Vigenère cipher.

Types of
Polyalphabetic Ciphers: AutoKey: completing key length with first characters from plaintext.
 Repeating Key: repeat key to the length of plaintext.


OneTime Pad.

Why OneTime Pad is completely secure?
 Randomness in generating key.
 Production of random output that bears no statistical relationship to the plaintext.

Why OneTime Pad is not a practical solution?
 There is the practical problem of making large quantities of random keys.
 The problem of key distribution and protection.
 Because of these difficulties, the onetime pad is of limited utility, and is useful primarily for lowbandwidth channels requiring very high security.


Transposition Techniques:
 Rail Fence Cipher.
 Rectangular Cipher.

Multiple Stages of Encryption:
 Rotor Machines.
 The significance of the rotor machine today is that it points the way to the most widely used cipher ever: the Data Encryption Standard (DES).

Steganography Techniques:

Character marking:
 Selected letters of printed or typewritten text are overwritten in pencil. The marks are ordinarily not visible unless the paper is held at an angle to bright light.

Invisible ink:
 A number of substances can be used for writing but leave no visible trace until heat or some chemical is applied to the paper.

 Small pin punctures on selected letters are ordinarily not visible unless the paper is held up in front of a light.

Typewriter correction ribbon:
 Used between lines typed with a black ribbon, the results of typing with the correction tape are visible only under a strong light.


Questions:

What is meant by “precomputer”?
 ما قبل عصر الحاسوب
 What’s meant by “digrams”?
 Reasons for preferring Playfair Cipher.
 Encryption/Decryption of algorithms should be summarized in points.

 53First paragraph.
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