1. Cryptography:
1.1. Plaintext An original message
1.2. Ciphertext The coded message
1.3. Enciphering/encryption The process of converting from plaintext to ciphertext
1.4. Deciphering/decryption Restoring the plaintext from the ciphertext
1.5. Cryptography The area of study of the many schemes used for encryption
1.6. Cryptographic system/cipher/Algorithm A scheme
1.7. Cryptanalysis Techniques used for deciphering a message without any knowledge of the enciphering details
1.8. Cryptology The areas of cryptography and cryptanalysis
2. Cryptographic Systems :
2.1. The type of operations used for transforming plaintext to ciphertext :
2.1.1. Substitution
2.1.2. Transposition
2.2. The number of keys used :
2.2.1. Symmetric, single-key, secret-key, conventional encryption /Classical Encryption
2.2.2. Asymmetric, two-key, or public-key encryption
2.3. The way in which the plaintext is processed:
2.3.1. Block cipher
2.3.2. Stream cipher
3. Requirements for secure use of Symmetric Encryption :
3.1. 1- A strong encryption algorithm
3.2. 2- Sender and receiver must have obtained copies of the secret key in a secure fashion and must keep the key secure
4. Cryptanalysis and Brute-Force Attack :
4.1. Cryptanalysis
4.1.1. - Attack exploits the characteristics of the algorithm to attempt to deduce a specific plaintext or to deduce the key being used.
4.1.2. - The traces of structure or pattern in the plaintext may survive encryption and be discernible in the ciphertext
4.1.3. - The mathematical properties of the pair of keys may make it possible for one of the two keys to be deduced from the other.
4.2. Brute-force attack
4.2.1. Involves trying every possible key on a piece of ciphertext until an intelligible translation into plaintext is obtained.
4.2.2. some degree of knowledge about the expected plaintext is needed (compression, language, numbers..), and some means of automatically distinguishing plaintext from garble is also needed
5. encryption techniques:
5.1. substitution.
5.1.1. - Is one in which the letters of plaintext are replaced by other letters or by numbers or symbols
5.1.2. - If the plaintext is viewed as a sequence of bits, then substitution involves replacing plaintext bit patterns with ciphertext bit patterns
5.2. transposition.
5.3. Rotor machines that combines both substitution and transposition.
6. transposition cipher :
6.1. Rail Fence Cipher
6.1.1. - Plaintext is written down as a sequence of diagonals and then read off as a sequence of rows
6.2. Row Transposition Cipher
6.2.1. - Write the message in a rectangle, row by row, and read the message off, column by column, but permute the order of the columns
6.2.1.1. The order of the columns then becomes the key to the algorithm
7. strategies to keep information secret :
7.1. 1- hide the existence of the information
7.2. 2- make the information unintelligible
8. Cryptography is everywhere (applications) :
8.1. Secure communication:
8.1.1. – web traffic: HTTPS
8.1.2. – wireless traffic: 802.11i WPA2
8.2. Encrypting files on disk: TrueCrypt
8.3. User authentication
8.4. Digital signatures
8.5. Anonymous communication
8.6. Anonymous digital cash
9. Encryption Scheme Security :
9.1. 1- Unconditionally secure
9.1.1. - if the ciphertext generated by the scheme does not contain enough information to determine uniquely the corresponding plaintext. No matter how much time an opponent has, it is impossible for him or her to decrypt the ciphertext simply because the required information is not there.
9.1.2. - One time pad.
9.2. 2- Computationally secure
9.2.1. if either of the following criteria are met:
9.2.1.1. - The cost of breaking the cipher exceeds the value of the encrypted information
9.2.1.2. - The time required to break the cipher exceeds the useful lifetime of the information
9.3. 3- Semantic Security
10. Types on Encrypted Messages of Attacks :
10.1. ciphertext only
10.1.1. known to cryptanalyst :
10.1.1.1. - ancryption algorithm
10.1.1.2. - ciphertext
10.2. known plaintext
10.2.1. known to cryptanalyst :
10.2.1.1. - ancryption algorithm
10.2.1.2. - ciphertext
10.2.1.3. - one or more plaintext - ciphertext pairs formed with the secret key
10.3. chosen plaintext
10.3.1. known to cryptanalyst :
10.3.1.1. - ancryption algorithm
10.3.1.2. - ciphertext
10.3.1.3. - plaintext massage chosen by cryptanalyst together with its corresponding ciphertext generated with the secret key
10.4. chosen ciphertext
10.4.1. known to cryptanalyst :
10.4.1.1. - ancryption algorithm
10.4.1.2. - ciphertext
10.4.1.3. - ciphertext massage chosen by cryptanalyst together with its corresponding decrypted plaintext generated with the secret key
10.5. chosen text
10.5.1. known to cryptanalyst :
10.5.1.1. - ancryption algorithm
10.5.1.2. - ciphertext
10.5.1.3. - plaintext massage chosen by cryptanalyst together with its corresponding ciphertext generated with the secret key
10.5.1.4. - ciphertext massage chosen by cryptanalyst together with its corresponding decrypted plaintext generated with the secret key
11. substitution cipher :
11.1. Caesar Cipher
11.1.1. - Involves replacing each letter of the alphabet with the letter standing three places further down the alphabet
11.1.2. - Alphabet is wrapped around so that the letter following Z is A
11.2. Monoalphabetic Cipher
11.2.1. - Permutation Of a finite set of elements S is an ordered sequence of all the elements of S , with each element appearing exactly once
11.2.2. - If the “cipher” line can be any permutation of the 26 alphabetic characters, then there are 26!
11.2.3. - Easy to break because they reflect the frequency data of the original alphabet
11.2.4. - Countermeasure is to provide multiple substitutes (homophones) for a single letter
11.2.5. - Digram Two-letter combination Most common is th
11.2.6. - Trigram Three-letter combination Most frequent is the
11.3. Playfair Cipher
11.3.1. - Treats digrams in the plaintext as single units and translates these units into ciphertext digrams
11.3.2. - Based on the use of a 5 x 5 matrix of letters constructed using a keyword
11.3.3. - Playfair Key Matrix
11.3.3.1. Fill in letters of keyword (minus duplicates) from left to right and from top to bottom, then fill in the remainder of the matrix with the remaining letters in alphabetic order
11.4. Polyalphabetic Ciphers
11.4.1. - Improves on the simple monoalphabetic technique by using different monoalphabetic substitutions as one proceeds through the plaintext message
11.4.2. - All these techniques have the following features in common:
11.4.2.1. 1- A set of related monoalphabetic substitution rules is used
11.4.2.2. 2- A key determines which particular rule is chosen for a given transformation
11.5. Vigenère Cipher
11.5.1. - Best known and one of the simplest polyalphabetic substitution ciphers
11.5.2. - In this scheme the set of related monoalphabetic substitution rules consists of the 26 Caesar ciphers with shifts of 0 through 25
11.5.3. - Each cipher is denoted by a key letter which is the ciphertext letter that substitutes for the plaintext letter a
11.5.4. - To encrypt a message, a key is needed that is as long as the message. Usually, the key is a repeating keyword