Understanding Cryptographic Ciphers: From Classical Techniques to Modern Encryption

Cryptography is the backbone of modern digital security. From securing messages in ancient times to protecting online banking transactions today, ciphers play a central role in keeping information confidential.

This post walks through the evolution of ciphers, explaining classical methods, modern encryption, stream and block ciphers, and why key management is critical for security.

What Is a Cipher?

A cipher is a technique used to convert plaintext (readable data) into ciphertext (encoded data) so that unauthorized parties cannot understand the message.

Formally:

Ciphertext = Encryption(Plaintext, Key)

Key Components:

• Plaintext: Original, readable message
• Key: Secret value used for encryption/decryption
• Ciphertext: Encrypted, unreadable message
• Encryption Algorithm: Mathematical function that performs the transformation

Ciphers are broadly divided into Classical Ciphers and Modern Ciphers.

Classical Ciphers

Classical ciphers were developed before computers and operate mainly on letters (A–Z). While historically important, they are no longer secure by modern standards.

1. Substitution Ciphers

In substitution ciphers, each letter (or group of letters) in the plaintext is replaced by another letter or symbol using a fixed mapping.

Examples:

• Caesar Cipher
• Vigenère Cipher
• Playfair Cipher
• Hill Cipher

Caesar Cipher Example

How It Works:

• Each letter is shifted by a fixed number of positions in the alphabet
• Shift wraps around (Z → A)

Example:

Plaintext:  HELLO
Key:        shift by 3
Ciphertext: KHOOR

Breaking Down the Shift:

H → K (H + 3)
E → H (E + 3)
L → O (L + 3)
L → O (L + 3)
O → R (O + 3)

2. Transposition Ciphers

Transposition ciphers rearrange the positions of letters rather than changing the letters themselves.

Examples:

• Rail Fence Cipher
• Columnar Transposition Cipher

Example: Rail Fence Cipher

How It Works:

• Write plaintext in a zigzag pattern across multiple "rails"
• Read ciphertext row by row

Example:

Plaintext:  HELLO WORLD

Rail Fence (2 rails):
H . . . O . . . R . .
. E . L . . W . O . L
. . L . . . . . . . D

Ciphertext: HORELWOL LD

Columnar Transposition Example:

Plaintext:  HELLO WORLD

Write in columns (key: 3 columns):
H E L
L O
W O R
L D

Read by columns:
Ciphertext: H L W L E O O D L R

Modern Ciphers

Modern cryptography operates on binary data (0s and 1s) and relies on mathematical algorithms and computational hardness.

Key Differences from Classical Ciphers:

Modern ciphers are classified in two major ways:

Based on the Type of Key

1. Symmetric-Key Ciphers:

Same key for encryption and decryption

Characteristics:

• ✅ Fast and efficient
• ✅ Used for bulk data encryption
• ⚠️ Requires secure key distribution

Examples:

• DES (Data Encryption Standard) — Deprecated
• AES (Advanced Encryption Standard) — Current standard
• 3DES (Triple DES) — Legacy
• ChaCha20 — Modern stream cipher

Use Cases:

• File encryption
• Database encryption
• Disk encryption

2. Asymmetric-Key Ciphers:

Two keys: public key (encryption) and private key (decryption)

Characteristics:

• ⚠️ Slower than symmetric
• ✅ More secure for key exchange
• ✅ No prior key sharing needed

Examples:

• RSA (Rivest-Shamir-Adleman)
• ECC (Elliptic Curve Cryptography)
• Diffie–Hellman (Key exchange)

Use Cases:

• Secure key exchange
• Digital signatures
• SSL/TLS certificates
• Email encryption

Based on the Type of Input Data

1. Block Ciphers:

Encrypt data in fixed-size blocks

Characteristics:

• Fixed block size (typically 64 or 128 bits)
• Process entire blocks at once

Example Block Sizes:

• 64-bit: DES (deprecated)
• 128-bit: AES (standard)

Examples:

• DES (Data Encryption Standard) — 64-bit blocks, deprecated
• AES (Advanced Encryption Standard) — 128-bit blocks, current standard
• 3DES (Triple DES) — 64-bit blocks, legacy

DES encryption:

$C = E_K(P)$

DES decryption:

$P = D_K(C)$

3DES encryption (EDE):

$C = E_{K_3}\big(D_{K_2}(E_{K_1}(P))\big)$

3DES decryption (DED):

$P = D_{K_1}\big(E_{K_2}(D_{K_3}(C))\big)$

➡️ 3DES uses DES internally, but adds:

• More keys
• More computation
• More security

Important backward-compatibility fact:

If:

$K_1 = K_2 = K_3$

Then:

$\text{3DES} = \text{DES}$

For details on AES operational modes, see: Block Cipher Modes of Operation Explained.

How Block Ciphers Work:

Plaintext: "HELLO WORLD" (11 bytes)
Block size: 8 bytes (64 bits)

Block 1: "HELLO WO" → Encrypt → Ciphertext Block 1
Block 2: "RLD" + padding → Encrypt → Ciphertext Block 2

2. Stream Ciphers:

Encrypt data bit-by-bit or byte-by-byte

Characteristics:

• Process data continuously
• Use a keystream combined with plaintext using XOR
• No block size limitation
• Efficient for real-time encryption

Examples:

• RC4 — Deprecated due to vulnerabilities
• ChaCha20 — Modern, secure

How Stream Ciphers Work:

Plaintext:  "HELLO"
Keystream:  "XMCKL"
Ciphertext: XOR(HELLO, XMCKL) = "EQNVZ"

A stream cipher encrypts data by XORing plaintext bits with a keystream generated from a secret key:

C_i = M_i ⊕ K_i

Where:

• C_i = i-th ciphertext bit
• M_i = i-th plaintext bit
• K_i = i-th keystream bit

Unlike OTP, stream ciphers use a pseudo-random number generator (PRNG) to create the keystream from a shorter secret key.

Critical Vulnerability:

If two messages use the same keystream:

C₁ ⊕ C₂ = (M₁ ⊕ K) ⊕ (M₂ ⊕ K)
        = M₁ ⊕ M₂ ⊕ K ⊕ K
        = M₁ ⊕ M₂

The key cancels out, revealing information about both plaintexts. This is a fatal vulnerability.

Attack Scenario:

1. Attacker intercepts two ciphertexts encrypted with same keystream
2. Computes C₁ ⊕ C₂ = M₁ ⊕ M₂
3. Uses language patterns to recover both messages
4. Security completely compromised

Prevention:

• Use unique initialization vectors (IVs) for each message
• Never reuse keys or keystreams
• Use proper key management

Vernam Cipher and One-Time Pad (OTP)

The Vernam Cipher is a symmetric cipher based on the XOR operation:

C = P ⊕ K

Where:

• P = Plaintext
• K = Key
• C = Ciphertext

Conditions for Perfect Security

The Vernam Cipher becomes a One-Time Pad (OTP) if:

1. ✅ The key is as long as the message
2. ✅ The key is truly random
3. ✅ The key is used only once
4. ✅ The key is shared securely

When these conditions are met, OTP is unconditionally secure—it cannot be broken even with infinite computing power.

Mathematical Proof:

• For any ciphertext C and any plaintext P', there exists exactly one key K' = P' ⊕ C
• All possible plaintexts are equally likely
• No statistical test can distinguish the real plaintext

Why Key Management Matters

The strongest algorithm becomes useless if:

• ❌ Keys are reused
• ❌ Keys are predictable
• ❌ Keys are improperly shared
• ❌ Keys are stored insecurely
• ❌ Keys are not rotated regularly

Most real-world cryptographic failures result from poor key management, not weak algorithms.

Cryptography is a constantly evolving field—understanding the fundamentals is the foundation for building and maintaining secure systems.