Classical Encryption Techniques

Symmetric Cipher Model

Symmetric Encryption is one of the oldest and simplest forms of encryption. It operates using a single key for both encrypting and decrypting data, which is why it is also called Single-Key Encryption.

Basic Terminology in Cryptography

• Plain Text: The original message that needs to be exchanged between a sender and a receiver.
  Example: A message like “Meet at 5 PM” before encryption.
• Cipher Text: The encoded version of the plain text, which cannot be easily interpreted by humans.
  Example: The plain text “Meet at 5 PM” might appear as “12A45BXY7” after encryption.
• Encryption (Enciphering): The process of converting plain text into cipher text.
  Output: Converts “Meet at 5 PM” into “12A45BXY7”.
• Decryption (Deciphering): The reverse of encryption, converting cipher text back to plain text.
  Output: Converts “12A45BXY7” back into “Meet at 5 PM”.

The Symmetric Cipher Model

A symmetric cipher model consists of five key components:

1. Plain Text (x): This is the original information or message to be sent to the receiver. It serves as one of the inputs for the encryption algorithm.
   Example: “The account number is 12345.”
2. Secret Key (k): A unique string or value used to encrypt and decrypt the data. This key is independent of the encryption algorithm but determines the transformations and substitutions performed on the plain text.
   Example: A password like “Key2024!”.
3. Encryption Algorithm (E): This algorithm takes the plain text and secret key as inputs, applying substitutions and transformations to create cipher text.
   Formula: E(x,k)=yE(x, k) = yE(x,k)=y
   Example: Using the key “Key2024!” to transform “The account number is 12345” into “ABC987XY65”.
4. Cipher Text (y): This is the unreadable output of the encryption algorithm, ensuring security during transmission. The unique secret key determines the cipher text’s format.
   Example: “The account number is 12345” becomes “ABC987XY65”.
5. Decryption Algorithm (D): This algorithm reverses the encryption process, taking the cipher text and the same secret key to recreate the original plain text. Formula: D(y,k)=xD(y, k) = xD(y,k)=x
   Output: Decoding “ABC987XY65” back into “The account number is 12345”.

Requirements for Encryption

To implement encryption, two primary requirements must be met:

1. Strong Encryption Algorithm: A robust algorithm is essential to produce cipher texts that are resistant to cracking. Even if an attacker gains access to the cipher text, they should not be able to deduce the secret key.
   Example: Using AES (Advanced Encryption Standard) ensures highly secure encryption.
2. Secure Key Sharing: There should be a reliable and secure method for sharing the secret key between the sender and receiver. This prevents attackers from intercepting the key.
   Example: Using a secure channel like an encrypted email or a key management service to share “Key2024!”.

Symmetric Cipher Model

An encryption algorithm, or cipher, transforms plaintext into ciphertext using a secret key. Cryptographic algorithms are divided into Symmetric key cryptography and Asymmetric key cryptography. All encryption methods are based on two key principles: substitution, where each element in the plaintext (such as a bit, letter, or group of bits/letters) is replaced with another element, and transposition, where plaintext elements are rearranged. The primary requirement is that no information is lost during the process.

What is the Substitution Cipher Technique?

In the Substitution Cipher Technique, plaintext characters are substituted with other characters, numbers, or symbols based on a specific key. The identity of the character changes, while its position remains unchanged.

What is the Transposition Cipher Technique?

The Transposition Cipher Technique involves rearranging the positions of letters in the plaintext to produce the ciphertext. Here, the position of the character changes, but its identity stays the same.

Difference Between Substitution Cipher Technique and Transposition Cipher Technique

Substitution Cipher Technique	Transposition Cipher Technique
Plaintext characters are replaced with other characters, numbers, or symbols.	Plaintext characters are rearranged based on their position.
Examples include Monoalphabetic substitution cipher and Polyalphabetic substitution cipher.	Examples include Key-less transposition cipher and Keyed transposition cipher.
The identity of characters changes, but their positions remain the same.	The positions of characters change, but their identities remain the same.
The use of letters with low frequency can help detect the plaintext.	Keys closer to the correct key can reveal the plaintext.
Examples include Caesar Cipher, Monoalphabetic Cipher, and Polyalphabetic Cipher.	Examples include Rail Fence Cipher, Columnar Transposition Cipher, and Route Cipher.
Substitutes plaintext letters or groups of letters with ciphertext based on a specific algorithm or key.	Rearranges the plaintext letters or groups of letters according to a specific algorithm or key.
The frequency distribution of plaintext letters is obscured, but patterns may still be identified through statistical analysis.	The frequency distribution remains unchanged, but the scrambled order makes pattern detection challenging.
Vulnerable to frequency analysis attacks that identify commonly used letters or combinations in the language to deduce the key.	Less prone to frequency analysis but can be attacked through brute force or known plaintext methods.
Easier to understand and implement, making it suitable for simpler applications.	More complex to understand and implement but offers better security for specific use cases.

History of the Internet

The Internet’s inception dates back to the 1960s with the creation of the first functional model, ARPANET (Advanced Research Projects Agency Network). It enabled multiple computers to operate within a single network, which was a groundbreaking achievement. ARPANET employed packet switching to facilitate communication among computers on the same network. In October 1969, the first message was successfully transmitted between two computers using ARPANET. This laid the foundation for continuous technological advancements.

How is the Internet Set Up?

The Internet relies on physical infrastructure, such as optical fiber cables and copper wires, alongside networking mediums like LAN (Local Area Network), WAN (Wide Area Network), and MAN (Metropolitan Area Network). Even wireless technologies like 2G, 3G, 4G, and WiFi depend on these physical setups. A governing authority called ICANN (Internet Corporation for Assigned Names and Numbers), based in the USA, oversees Internet protocols and IP address management.

How Does the Internet Work?

The Internet operates using a system of clients and servers. A client could be a laptop directly connected to the Internet, while servers are powerful computers indirectly connected to it, storing vast amounts of website data. Servers use ISPs (Internet Service Providers) to connect to the Internet and are identified by their unique IP addresses.

Since remembering numeric IP addresses can be challenging, domain names are assigned to websites. When a user searches for a domain name in their browser, the request is sent to a server. The server translates the domain name into an IP address, as it cannot process the domain name directly. This translation occurs via the DNS server (Domain Name System), akin to looking up someone’s details in a directory.

For example, searching for a URL like www.example.com prompts the browser to retrieve its IP address and forward the request to the relevant server. The server then processes this request and delivers the required website content to the user.

For wireless Internet like 4G, data travels through optical cables, reaching cellular towers and subsequently reaching devices via electromagnetic signals. Similarly, routers convert light signals into electrical signals, enabling Internet connectivity through ethernet cables.

What is an IP Address?

An IP Address (Internet Protocol Address) is a unique identifier assigned to each device connected to the Internet, enabling data flow management. Just as Aadhaar or passport numbers identify individuals, IP addresses distinguish computers, routers, and websites.

An IP address consists of four numerical blocks, such as 192.168.0.1, with each block ranging from 0 to 255. The total range spans from 0.0.0.0 to 255.255.255.255. IP addresses are classified into four main types:

1. Static IP Address
2. Dynamic IP Address
3. Private IP Address
4. Public IP Address

World Wide Web (WWW)

The World Wide Web is a vast collection of interconnected webpages and documents that can be accessed using URLs. For instance, the URL www.wikipedia.org leads to the Wikipedia website, housing numerous web pages and related documents.

Hypertext links connect documents, enabling users to navigate between related content. Timothy Berners-Lee initiated the WWW project in 1989 to facilitate collaboration among researchers at CERN. The World Wide Web Consortium (W3C) oversees its further development.

Difference Between World Wide Web and the Internet

World Wide Web	Internet
Repository of webpages/documents	Network connecting global computers
Software-oriented	Hardware-oriented
Subset of the Internet	Superset encompassing the WWW
Uses HTTP	Uses IP protocols

Uses of the Internet

1. E-commerce: Platforms like eBay and Etsy simplify shopping and transactions.
2. Digital Payments: Apps like PhonePe and UPI gateways promote cashless transactions.
3. Education: Online learning platforms enable access to quality courses.
4. Social Networking: Sites like Facebook and WhatsApp foster global communication.
5. Entertainment: Streaming movies, music, and gaming are popular online activities.

Advantages of the Internet

• Facilitates online banking, freelancing, and remote jobs.
• Offers countless entertainment options like web series and gaming.
• Provides effective tools for communication via emails and video calls.

Disadvantages of the Internet

• Excessive use leads to time wastage and health issues.
• Cybercrimes like phishing and data breaches pose significant risks.
• Overdependence can negatively impact children’s growth and development.

Difference between Block Cipher and Transposition Cipher

1. Block Cipher:

Block Cipher is a symmetric key cipher used to convert plaintext into ciphertext. It applies either a substitution process or sometimes a permutation process, where a block of plaintext is replaced with a corresponding arbitrary block of ciphertext.

2. Transposition Cipher:

Transposition Cipher involves rearranging the positions of characters in the plaintext. It changes the arrangement of characters while preserving their identities.

Differences Between Block Cipher and Transposition Cipher

Block Cipher	Transposition Cipher
A block of plaintext is treated as a single unit.	Plaintext is written as a sequence of characters.
Produces a ciphertext block of the same length as the plaintext block.	Reads sequences in the form of rows.
An error in transmitting one block does not affect other blocks.	An error in one letter impacts the entire ciphertext.
The encryption process is relatively slow.	The encryption process is relatively fast.
Security depends on the design of the encryption function.	Security can be enhanced by performing multiple transpositions.
Plaintext is divided into blocks, and the algorithm operates on each block independently.	Plaintext is divided into letters, and the algorithm operates on each letter individually.
The complexity of the encryption is straightforward.	The transposition process involves greater complexity.
Characters lose their identity during encryption.	Characters retain their identity throughout the process.