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Code versus Ciphers

Updated Jan 30, 2024 ·

Code​

A system that substitutes one word or phrase for another. Codes are intended to provide secrecy and/or efficiency. An example of a code is the "Ten" code which is used by the police and other organizations who communicate by radio

The Ten-Code, also known as ten signals, is a set of code words used to represent common phrases in voice communication, particularly in law enforcement and emergency services.

Here are some examples:

  • 10-1: Signal Weak
  • 10-2: Signal Good
  • 10-3: Stop Transmitting
  • 10-4: Acknowledgment (OK)
  • 10-5: Relay
  • 10-6: Busy
  • 10-7: Out of Service
  • 10-8: In Service
  • 10-9: Repeat
  • 10-10: Fight in Progress

Ciphers​

A cipher is an algorithm used to encrypt and decrypt information, transforming plain text into coded (cipher) text and vice versa.

  • Ciphers ensure data confidentiality
  • Central to encryption protocols.

Types of Ciphers:

  • Stream Cipher
  • Block Cipher

Stream Ciphers​

Stream ciphers encrypt data one bit or byte at a time, making them fast and suitable for continuous data flow.

  • Encrypts data in a *continuous stream- instead of fixed blocks
  • Uses a key stream generator to create random-looking bits
  • The key stream is XORed with the plaintext to get the ciphertext

Common features:

  • Same key is used for both encryption and decryption
  • Often implemented in hardware for faster performance
  • Works well for real-time data like voice or video streams
  • Becomes weak if the same key stream is reused or not random enough

Examples:

  • RC4
  • Salsa20
  • ChaCha20

Stream ciphers are useful for real-time communication, where speed and efficiency are key, but they must use unique and random key streams to stay secure.

Block Ciphers​

Block ciphers divide data into fixed-size blocks and encrypt each block separately using the same key.

  • Encrypts data in blocks, such as 64 or 128 bits
  • Each block of plaintext becomes one block of ciphertext
  • Can perform multiple rounds of transformations like substitution and permutation

Common features:

  • Easier to implement and generally more secure than stream ciphers
  • Often used in software-based encryption systems
  • Common in protocols and file encryption where data comes in chunks
  • Requires padding if data doesn't perfectly fit into blocks
  • Can run in different modes to provide flexibility and added protection

Examples:

  • AES
  • DES
  • 3DES

Cryptography Techniques​

Cryptography protects information by transforming readable data into a secret form and back again. These transformations keep data private and secure during transmission and storage.

  • Encryption turns readable data into unreadable form
  • Decryption converts it back into its original form
  • Two main techniques are substitution and transposition ciphers

Both methods aim to hide the original message while keeping it recoverable only with the right key.

Substitution Cipher​

A substitution cipher replaces each part of the message with another symbol or letter based on a fixed rule.

  • Each letter or group in plaintext is replaced with another
  • A simple example is shifting letters along the alphabet
  • Can use a single alphabet or multiple shifting patterns

Example: In a basic shift of 3, A → D, B → E, C → F.

This idea shows how substitution hides meaning by changing characters, not order.

Transposition Cipher​

A transposition cipher hides the message by rearranging characters instead of changing them.

  • The order of letters changes while letters stay the same
  • One example is the rail fence cipher, written in a zigzag pattern
  • Another is the columnar transposition, which uses grids and keys to reorder text

This technique scrambles the structure of data, showing that security can come from arrangement, not replacement.

Common Ciphers​

Caesar Cipher​

The Caesar Cipher shifts letters of the alphabet by a fixed number of positions. It’s one of the oldest and simplest ciphers.

  • Each letter shifts a set number of places
  • Shifts can be from 1 to 25 positions
  • Example: With a shift of 3, A → D, B → E, C → F
  • ROT2 means a shift of two positions
  • ROT3 means a shift of three positions

Although it's historically significant, the Caesar Cipher is not used for serious encryption today due to its simplicity and vulnerability to straightforward cryptanalysis

Vigenere Cipher​

The Vigenère Cipher improves on the Caesar Cipher by using a repeating keyword to determine the shifts for each letter.

  • A keyword defines how much to shift each letter
  • The key repeats to match the message length
  • Decryption uses the same key to reverse the shifts
  • Strengths: Adds complexity and resists frequency analysis
  • Weaknesses: Patterns in long texts can still reveal the key

Example: Using the key LEMON, the word HELLO becomes OIWWC. The Vigenère Cipher shows how layering keys adds strength to encryption.

Book Cipher (Running Key Cipher)​

The Book Cipher, also called the Running Key Cipher, is a type of polyalphabetic substitution cipher that uses text from a book or document as the key. In this cipher, each letter of the plaintext is shifted based on the corresponding letter in the key text

  • The book or text is known to both sender and receiver
  • The longer the key text, the harder it is to break
  • The same book and edition must be used for encryption and decryption

Example:

If the message is "HELLO" and the book text starts with "WORL...", each plaintext letter is shifted by the alphabet position of the corresponding book letter.

Strengths

  • Offers stronger security than short-key ciphers like Vigenère
  • Key is hard to guess if the book is unknown

Weaknesses

  • Vulnerable if the book is identified or reused
  • Requires both parties to have identical copies of the text

This cipher shows how using long, natural-language keys can strengthen substitution methods while keeping encryption manual and simple.

Exlusive OR (XOR)​

XOR is a basic operation that compares bits and returns 1 if they’re different, or 0 if they’re the same.

  • If bits are different, the result is 1
  • If bits are the same, the result is 0
  • Reversing is easy — XOR with the same key again restores the original

Example:

1 XOR 0 = 1  
0 XOR 0 = 0  
1 XOR 1 = 0

Applications

  • Encryption: Used in stream and block ciphers
  • One-Time Pad: A perfect encryption if the key is random and used once
  • Error Detection: Helps identify bit errors in data

Security Notes

  • Key secrecy is critical for XOR security
  • Reusing keys makes it vulnerable to attacks