Keyword Cipher
The Keyword Cipher is a monoalphabetic substitution cipher where a keyword is used to generate the cipher alphabet. The keyword is written first (with duplicate letters removed), followed by the remaining letters of the alphabet in order. This creates a one-to-one mapping between plaintext and ciphertext letters.
What You'll Learn
Introduction to Keyword Cipher
The Keyword Cipher is a monoalphabetic substitution cipher where a keyword is used to generate the cipher alphabet. The keyword is written first (with duplicate letters removed), followed by the remaining letters of the alphabet in order. This creates a one-to-one mapping between plaintext and ciphertext letters.
Prerequisites
How to Solve Keyword Cipher Problems
Step 1: Write the keyword in uppercase and remove duplicate letters (keep first occurrence)
Step 2: Append the remaining letters of the alphabet in order (excluding those in the keyword)
Step 3: This creates the cipher alphabet (A-Z maps to this string)
Step 4: For encoding, replace each plaintext letter with the cipher letter at the same position
Step 5: For decoding, find the position of the cipher letter in the cipher alphabet and output the plaintext letter at that position
Step 6: Build a mapping table for quick reference
Example Problem
Example: Using keyword 'CIPHER', what is the cipher for 'HELLO'? Solution: Step 1: Keyword: C I P H E R (no duplicates) Step 2: Remaining letters: A B D F G J K L M N O Q S T U V W X Y Z Step 3: Cipher alphabet: C I P H E R A B D F G J K L M N O Q S T U V W X Y Z Step 4: Plaintext A→C, B→I, C→P, D→H, E→E, F→R, G→A, H→B, I→D, J→F, K→G, L→J, M→K, N→L, O→M, P→N, Q→O, R→Q, S→S, T→T, U→U, V→V, W→W, X→X, Y→Y, Z→Z Step 5: HELLO: H→B, E→E, L→J, L→J, O→M → BEJJM Answer: BEJJM
Pro Tips & Tricks
- The keyword determines the entire cipher alphabet
- Duplicate letters in the keyword are ignored (only first occurrence kept)
- The cipher alphabet preserves the order of remaining letters (A-Z)
- Common keywords: 'CIPHER', 'SECRET', 'KEYWORD', 'ALPHABET'
- The letter that appears first in the keyword is often the most frequent
- To decode, create a reverse mapping from cipher letter to plaintext letter
Shortcut Methods to Solve Faster
Common Mistakes to Avoid
Practice Worksheets
Practice makes perfect! Work through these worksheets to master Keyword Cipher. Each worksheet contains 20 questions with detailed explanations. Start from Worksheet 1 and progress through increasing difficulty levels.
Exam Importance
Keyword Cipher is an important topic for various competitive exams. Here's how frequently it appears:
Ready to Master Keyword Cipher?
Start with Worksheet 1 and work your way up to expert level! Each worksheet includes: