Cryptography Module 2

Cryptography

• The art or science of transforming an intelligible message into an unintelligible one, and then retransforming it back to its original form.

Terminology

• Plaintext: The original intelligible message.
• Cipher text: The transformed message.
• Cipher: An algorithm for transforming an intelligible message into an unintelligible one.
• Key: Critical information used by the cipher, known only to the sender and receiver.
• Encipher (encode): The process of converting plaintext to cipher text using a cipher and a key.
• Decipher (decode): The process of converting cipher text back into plaintext using a cipher and a key.
• Cryptanalysis: The study of principles and methods of transforming an unintelligible message back into an intelligible one without knowledge of the key (also called code breaking).
• Cryptology: Both cryptography and cryptanalysis.
• Code: An algorithm for transforming an intelligible message into an unintelligible one using a code-book.

Classification of Cryptographic Systems

• Classified along 3 independent dimensions: • Type of operations used for transforming plaintext to cipher text. • Type of operations used for transforming plaintext to cipher text. • The way in which the plaintext is processed.

Types of Operations

• Substitution: Each element in the plaintext is mapped into another element.
• Transposition: Elements in the plaintext are rearranged.

Type of Keys

• Symmetric key (or single key or conventional encryption): The sender and receiver use the same key.
• Public key encryption: The sender and receiver use different keys.

Way of Processing

• Block cipher: Processes the input and block of elements at a time, producing output block for each input block.
• Stream cipher: Processes the input elements continuously, producing output element one at a time, as it goes along.

History of Cryptography

• Began primarily as a way for two parties to communicate securely, even if their messages might be read by an eavesdropper.

Applications of Cryptography

• Attesting the identity of the organization operating a web server.
• Digitally signing electronic documents.
• Protecting the confidentiality of online banking and shopping transactions.
• Protecting the confidentiality of files stored on a hard drive.
• Protecting the confidentiality of packets sent over a wireless network.

Categories of Cryptography

• Symmetric key cryptography.
• Asymmetric key cryptography.

Symmetric Key Cryptography

• An encryption system in which the sender and receiver of a message share a single, common key.
• The most popular symmetric–key system is the Data Encryption Standard (DES).

Types of Symmetric Key Cryptography

• Classical cryptography: • Transposition ciphers: A method of encryption by which the positions held by units of plaintext are shifted according to a regular system. • Substitution ciphers: A method of encryption by which units of plaintext are replaced with ciphertext, according to a fixed system.
• Modern cryptography: • Stream ciphers: The same plaintext bit or byte will encrypt to a different bit or byte every time it is encrypted. • Block ciphers: An encryption method that applies a deterministic algorithm along with a symmetric key to encrypt a block of text.

Asymmetric Key Cryptography

• The encryption process where different keys are used for encrypting and decrypting the information.
• Keys are different but are mathematically related, such that retrieving the plain text by decrypting ciphertext is feasible.