212SEC-4 Chapter 1

Decryption and Cryptography

• Decryption converts ciphertext back to its original plaintext form.
• Key personalities: Dr. Ahmed Ben Hamida, Full Professor and Head of ATMS Lab at King Khalid University; specializes in Signal Processing.

Cryptology Overview

• Cryptology is the science of secret communication.
• It encompasses two main areas:
  • Cryptography: Designs techniques to ensure information secrecy.
  • Cryptanalysis: Studies methods to break cryptographic systems.

Types of Cryptography

• Symmetric Encryption (Secret Key): Uses the same key for both encryption and decryption.
• Asymmetric Encryption (Public Key): Uses a pair of keys; one public for encryption and one private for decryption.

Steganography vs. Cryptography

• Steganography: "Covered writing" that conceals the existence of a message; relies on the secrecy of the method used.
• Cryptography: "Hidden writing" that obscures the meaning of the message; relies on the secrecy of the key rather than the method.

Basic Terminologies in Cryptography

• Plaintext: The original, unencoded message.
• Ciphertext: The encoded or encrypted message.
• Cipher: The algorithm used to transform plaintext into ciphertext.
• Key: Information unique to the sender and receiver, used in the cipher.
• Encipher/Encrypt: The process of converting plaintext to ciphertext.
• Decipher/Decrypt: The process of retrieving plaintext from ciphertext.
• Cryptology: Encompasses the study of both cryptography and cryptanalysis.

Cryptanalysis Techniques

• Linear Cryptanalysis: Analyzes both plaintext and ciphertext to deduce potential keys through simplified cipher methods.

Encryption and Decryption

• Encryption transforms plaintext into ciphertext, making it unreadable.
• The decryption process reverses encryption, converting ciphertext back to plaintext.
• For each key ( k ) in set ( K ), there exists an encryption function ( e_k ) in set ( E ) and a corresponding decryption function ( d_k ) in set ( D ).
• Functions must satisfy the condition ( d_k(e_k(x)) = x ) for every element ( x ) in plaintext ( P ).

Types of Cryptography

• Secret Key Cryptography (SKC) / Symmetric Cryptography:

  • Utilizes a single key for both encryption and decryption.

• Public Key Cryptography (PKC) / Asymmetric Cryptography:

  • Implements paired keys where one key encrypts data while a different key is used for decryption.

• Hash Functions:

  • Employ mathematical transformations for irreversible encryption, creating a fixed-size hash from variable-size input.

Process and Phases

• The sender begins with plaintext, which is then encrypted into ciphertext.
• The ciphertext is transmitted over the Internet to the receiver.
• The receiver decrypts the received ciphertext back into plaintext.

Common Trigrams

• Frequent three-letter combinations in the English language include:
  • THE, ING, AND, HER, ERE, ENT, THA, NTH, WAS, ETH, FOR.
• The trigrams are listed in decreasing order of frequency, highlighting their commonality in text analysis.

Cryptography Overview

• Focus on classic cryptography taught at King Khalid University by Dr. Ahmed AlMokhtar BEN HMIDA.
• Emphasis on confidentiality and integrity of data through shared secret keys.

Course Structure

• Includes topics on cryptographic attacks and implementation failures.
• Total contact hours for the course: 60, with specific topics in chapters 9 and 10 each having 4 hours allocated.

Key Concepts

• Cryptographic attacks are classified into various types, covered in chapter 9 of the course.
• Implementation failures of cryptographic methods, discussed in chapter 10, are critical areas of focus.

Data Protection

• Protected files are analogous to secure communication; confidentiality is maintained as files are stored securely.
• The concept of Alice sending a message to herself highlights the importance of ensuring messages remain untampered over time.

Cryptographic Algorithms

• Modern cryptographic algorithms undergo rigorous cryptanalysis to test their security and effectiveness in finding the key.
• The principle of confusion is fundamental; even minor changes to plaintext should produce unpredictable effects on ciphertext to enhance security.

Cryptography Overview

• Plaintext is unencrypted data, while ciphertext, also known as a cryptogram, is the encrypted version.
• The process of converting plaintext to ciphertext is called encryption.

Key Processes

• Encryption: Transforming plaintext into ciphertext to secure data.
• Decryption: Converting ciphertext back into plaintext.

Key Concepts

• Encryption Key: A critical component used in both encryption and decryption processes to ensure data security.
• Plaintext should be impregnable, preventing easy retrieval from ciphertext.

Types of Attacks in Cryptography

• Listening: Unauthorized interception of communication.
• Modification: Altering data during transmission.
• Production: Creating fake data or messages.
• Replays: Reusing intercepted data to impersonate a legitimate user.

Classic Cryptographic Techniques

• Substitution Cipher: Each letter in the plaintext is replaced with another letter.
• Caesar Cipher: A specific type of substitution cipher that shifts letters by a fixed number, generally without utilizing a key.

Key Space in Substitution Cipher

• The size of the key space for a substitution cipher based on 26 letters is significant as it determines the number of possible unique mappings, impacting security level.

Introduction to Cryptography

• Cryptography involves techniques for secure communication, encompassing both classic and modern methods.
• Focus on the processes of encryption and decryption, vital for maintaining data confidentiality.

Decryption

• Decryption is the process of converting ciphertext back into its original plaintext form.

Block Cipher

• Block ciphers divide text into fixed-size blocks, processing one block at a time.
• Each block produces a corresponding block of encrypted data, using cryptophytic algorithms.

Encryption Schemes

• An encryption scheme consists of both encryption and decryption processes.
• Plaintext is encrypted to produce ciphertext, which then can be decrypted to recover the original message.

Cryptanalysis

• Cryptanalysis refers to the study and practice of breaking encryption techniques to reveal plaintext without accessing the encryption key.

Cryptography Overview

• Plaintext refers to readable data that, through encryption, is transformed into ciphertext (or cryptogram) to ensure confidentiality.
• The process of transforming plaintext into ciphertext is called encryption.
• Decryption is the reverse process, converting ciphertext back to plaintext.
• The confidentiality of plaintext is maintained as it should not be easily derived from ciphertext.

Key Concepts

• Two main types of encryption are asymmetric and symmetric encryption.
• Symmetric encryption utilizes the same key for both encryption (key kA) and decryption (key kB).
• Asymmetric encryption employs different keys for encryption and decryption.

Objectives of Cryptography

• Ensure that plaintext and keys are safeguarded against unauthorized access or easy retrieval.
• Utilize cryptographic techniques to enhance data security and privacy.

Key Definitions

• Cryptology: The study of techniques for secure communication in the presence of third parties.
• Cryptography: The practice of securing information by transforming it into an unreadable format for unauthorized users.
• Cryptanalysis: The study of analyzing information systems while attempting to breach the cryptographic security.

Additional Information

• There are various configurations and contexts under which cryptographic methods are employed.
• This foundational understanding is crucial for advanced studies in cryptography and related fields.