UNIT I – Introduction to Security and Classical Encryption

Complete Study Notes with Examples, Explanations, and Best Learning Approach

1. Introduction to Security Attacks, Services, and Mechanisms

Security Attacks

Any action that compromises the security of information.

Type	Description	Example
Passive Attack	Attacker only observes (does not modify) data. Goal: obtain information.	• Eavesdropping on Wi-Fi • Traffic analysis (who talks to whom)
Active Attack	Attacker modifies, deletes, or injects data.	• Masquerade (pretend to be someone else) • Replay attack • Modification of message • Denial of Service (DoS)

Security Services (CIA + AAA)

Service	Meaning	Example
Confidentiality	Data is kept secret from unauthorized parties	Encrypting credit card numbers
Integrity	Data cannot be altered undetectably	Digital signatures, hash functions
Authentication	Verify the identity of sender/receiver	Password, biometric, certificates
Non-repudiation	Sender cannot deny having sent the message	Digital signature with timestamp
Access Control	Only authorized users can access resources	File permissions, firewall rules
Availability	System/data must be available when needed	Protection against DoS attacks

Security Mechanisms

Tools/techniques used to provide the above services:

• Encryption / Decryption
• Hash functions
• Digital signatures
• Authentication protocols
• Access control lists
• Firewalls, IDS/IPS

2. Classical Encryption Techniques

A. Substitution Ciphers

Each letter/plaintext symbol is replaced by another letter/ciphertext symbol.

1. Caesar Cipher (Shift Cipher)
   Key = 3 → A→D, B→E, …, Z→C
   Encryption: C = (P + K) mod 26
   Decryption: P = (C – K) mod 26

   Example
   Plaintext : HELLO
   Key : 3
   Ciphertext: KHOOR

   Very weak – only 25 possible keys.

2. Monoalphabetic Cipher
   Arbitrary fixed substitution (not just shift).
   Example mapping:
   A→X, B→M, C→T, …, Z→Q

   Plaintext : HELLO
   Ciphertext: AXEEH (using some random mapping)

   Still weak – frequency analysis can break it easily (E is most common in English → appears most in ciphertext).

3. Playfair Cipher (Digraph substitution)

   • 5×5 grid with I/J combined
   • Encrypts two letters at a time
     Example Keyword: MONARCHY

   M O N A R
   C H Y B D
   E F G I/J K
   L P Q S T
   U V W X Z
   

   Rules:

   • Same row → right shift
   • Same column → down shift
   • Rectangle → swap columns

   Stronger than monoalphabetic but still breakable.

4. Polyalphabetic Cipher (Best classical substitution)
   Uses multiple substitution alphabets.

   Vigenère Cipher
   Key repeated to match plaintext length.

   Example
   Plaintext : WEAREDISCOVEREDRUNATONCE
   Key : APPLEAPPLEAPPLEAPPLEAPPL
   Ciphertext: CIUIGFKWZVIGBUHXRQEMUWWTP

   Much harder to break with frequency analysis because same letter can encrypt differently.

B. Transposition Ciphers

Letters are rearranged (no letter is replaced, only positions change).

1. Rail Fence Cipher (Depth = 2 or 3)

   Plaintext: MEET ME AFTER THE TOGA PARTY
   Write in zigzag:

   M . E . T . E . F . E . T . T . G . P . R . Y
   . E . M . A . T . R . H . T . O . A . A . T .
   

   Ciphertext: METEFTTGPREY EMA TRHTOAAT

2. Columnar Transposition

   Plaintext : ATTACK AT DAWN
   Key : 3 1 4 2 (means column order 3-1-4-2)

   Write row-wise:

   3 1 4 2
   A T T A
   C K A T
   D A W N
   

   Read by key order: TADA KWTN TACA → TADAKWTNTACA

   Double transposition (repeat twice) makes it much stronger.

C. Cryptanalysis (Code Breaking)

Attack Type	Works on	Method
Brute-force	Any cipher with small key space	Try all keys
Frequency Analysis	Monoalphabetic substitution	Match letter frequencies
Kasiski Examination	Vigenère (polyalphabetic)	Find repeated trigram distance → key length
Known-plaintext attack	Any cipher	You have P and C → deduce key
Chosen-plaintext attack	Block ciphers	Choose P and get C

D. Steganography

Hiding the existence of the message (unlike cryptography which hides the meaning).

Examples:

• Invisible ink
• LSB (Least Significant Bit) in image/audio pixels
• Hiding text in whitespace of documents

Cryptography + Steganography = very powerful.

E. Stream vs Block Ciphers

Feature	Stream Cipher	Block Cipher
Unit of encryption	1 bit/byte at a time	Fixed block (64/128/256 bits)
Example
Example	RC4, Salsa20, A5/1 (GSM)	DES, AES, Blowfish
Speed	Very fast	Slower (but secure)
Error propagation	One bit error affects one bit	One bit error corrupts whole block
Usage	Real-time (voice, video)	File/database encryption

3. Modern Block Ciphers

A. Principles of Modern Block Ciphers

Designed according to two principles by Claude Shannon (1949):

1. Confusion – Make relationship between plaintext, key, and ciphertext as complex as possible (achieved by substitution/S-boxes).
2. Diffusion – Each plaintext bit should affect many ciphertext bits; each key bit should affect many ciphertext bits (achieved by permutation/P-boxes).

Good cipher alternates confusion and diffusion layers many times.

B. Feistel Structure (Basis of DES, Lucifer, Blowfish, etc.)

• Block split into Left (L) and Right (R) halves
• Each round:

  Li   = Ri-1
  Ri   = Li-1 ⊕ f(Ri-1, Ki)
  

• f = round function (confusion + diffusion)

Advantages:

• Encryption and decryption almost same (just reverse key order)
• Proven design for many ciphers

C. Data Encryption Standard (DES)

• Block size: 64 bits
• Key size: 56 bits (64 bits with 8 parity bits parity)
• 16 Feistel rounds
• Adopted as US federal standard in 1977

DES Round Structure

IP → 16 rounds → FP (Final Permutation)
Each round:
  - Expansion (32→48 bits)
  - XOR with 48-bit round key
  - 8 S-boxes (6→4 bits each) → 32 bits
  - P-box permutation

Strength of DES

• 56-bit key → 2⁵⁶ ≈ 7.2 × 10¹⁶ keys
• 1998: EFF built “Deep Crack” machine – broke DES in <3 days
• Today: DES broken in hours on normal PCs or seconds on cloud GPUs → DES is completely insecure today

Differential Cryptanalysis (Biham & Shamir, 1990)

• Chosen-plaintext attack
• Studies how differences in plaintext pairs propagate through rounds
• DES can be broken with 2⁴⁷ chosen plaintexts (theoretical)
• But DES was actually designed to resist it (NSA influence)

Block Cipher Modes of Operation

How to encrypt data longer than one block.

Mode	Full Name	Features	Use Case
ECB	Electronic Code Book	Each block encrypted independently → identical blocks → identical ciphertext	Not recommended (leaky)
CBC	Cipher Block Chaining	XOR with previous ciphertext block + IV	Most common, secure
CFB	Cipher Feedback	Turns block cipher into stream cipher	Stream data
OFB	Output Feedback	Also stream mode, but no error propagation	Noisy channels
CTR	Counter	Parallelizable, turns block cipher into stream, no padding needed	Modern favorite (AES-GCM)

Triple DES (3DES or TDEA)

Because single DES is weak → run DES three times.

Most secure variant: E-D-E with three different keys (168-bit key)

Ciphertext = EK3(DK2(EK1(Plaintext)))

Effective key length ≈ 112 bits (due to meet-in-the-middle attack).
Still used in banking (EMV, older ATMs), but being replaced by AES.

Summary Table

Topic	Key Point
Classical Substitution	Caesar → Mono → Playfair → Vigenère (best)
Classical Transposition	Rail fence, Columnar (rearrange letters)
Cryptanalysis	Frequency analysis breaks monoalphabetic easily
Stream vs Block	Stream = bit-by-bit, Block = fixed chunks
Shannon’s Principles	Confusion + Diffusion
Feistel Structure	Basis of DES, reversible encryption/decryption
DES	64-bit block, 56-bit key, 16 rounds – now broken
Triple DES	3×DES with 112–168 bit security – slow but safe
Best Mode Today	CBC or CTR (with authentication → GCM)

These notes + solving 20–30 numerical/handwritten encryption examples of Caesar, Vigenère, Playfair, Rail Fence, and DES round calculations will give you complete mastery of Unit I.

Happy learning!