[Day 3] 2. Encryption Methods

Here’s a corrected and clarified version of the information:

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Types of Encryption

There are two primary types of encryption techniques:

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1. Substitution Cipher

In a substitution cipher, each character in the plaintext is replaced by another character based on a specific rule.

Example:

Copy
Plaintext:  ABCDEFGHIJKLMNOPQRSTUVWXYZ  
Ciphertext: ZYXWVUTSRQPONMLKJIHGFEDCBA

There are two main types of substitution ciphers:

1. Mono-alphabetic Substitution Cipher:
   Each character is replaced by a single, unique character using a fixed substitution rule.
   Examples:

   • Atbash Cipher: Reverses the alphabet (as in the example above).
   • Caesar Cipher: Shifts each character by a fixed number of places in the alphabet (e.g., shifting "A" by 3 becomes "D").

2. Poly-alphabetic Substitution Cipher:
   Uses multiple substitution alphabets, meaning the same plaintext letter may be encoded differently depending on its position or a key.
   Examples:

   • Vigenère Cipher: A key is repeated to determine which alphabet is used to encrypt each letter. For example, with the plaintext "HELLO" and the key "KEY", the key repeats as "KEYKE" to encode the message.
   • Homophonic Substitution Cipher: Each plaintext character can be substituted by multiple possible symbols or numbers, reducing predictability. For example, "A" could map to "1," "17," or "99," but no symbol or number should always map to the same plaintext character.

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2. Transposition Cipher

In a transposition cipher, the characters of the plaintext are rearranged in a specific pattern or order without altering the characters themselves.

Example: For the plaintext "HELLO", using a simple columnar transposition with 3 columns:

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Plaintext:  H E L L O
Arranged:   H L E L O (rearrange by reading vertically)
Ciphertext: HLELO

No new characters are introduced; the original characters are merely shuffled to form the ciphertext.

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Key Differences Between Substitution and Transposition Ciphers

|Feature|Substitution Cipher|Transposition Cipher| |---|---|---| |Operation|Replaces characters with other characters|Rearranges the order of characters| |Character Integrity|Characters may be altered (e.g., A → D)|Original characters remain unchanged| |Examples|Atbash, Caesar, Vigenère|Columnar Transposition, Rail Fence|

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