Lesson Plan: Java Caesar Cipher (High School)

Overview

Students will build a Caesar Cipher program in Java that can encrypt and decrypt messages using a shift key. Students will practice string processing, loops, conditionals, methods, and input validation.

Learning Targets

• Explain what a Caesar Cipher is and how shifting works.
• Write Java methods that process strings character-by-character.
• Handle uppercase/lowercase letters while preserving punctuation/spaces.
• Use modular arithmetic to wrap letters (Z→A, z→a).
• Build and test a console program with clear input and output.

Materials

• Computer with Java IDE (Replit, IntelliJ, VS Code, BlueJ, etc.)
• This handout (spec sheet, pseudocode, and rubric)
• Required test cases (included below)

Teacher Hook (5–8 minutes)

• Write on board: HELLO shift 3 → KHOOR
• Ask: “What happens to Z with shift 3?” (wraps to C)
• Explain: Caesar cipher is a substitution cipher; not secure today, but great for learning logic.

Student Tasks (Procedure)

Day 1

1. Read the spec sheet.
2. Write pseudocode (use the structure below).
3. Implement the encrypt() method.
4. Test with required test cases.

Day 2

1. Implement the decrypt() method.
2. Add a menu loop (encrypt / decrypt / quit).
3. Add key normalization (supports negative keys and keys > 26).
4. Final testing + screenshot + submit files.

Spec Sheet (Student-Facing)

Project Name

P_CaesarCipher_Lastname

Goal

Write a Java program that can:

• Encrypt a message using a Caesar shift key
• Decrypt a message using the same shift key
• Preserve spaces and punctuation (do not change them)

Program Requirements

Input

• User chooses:
  • 1) Encrypt
  • 2) Decrypt
  • 3) Quit
• User enters a message (may contain spaces/punctuation).
• User enters an integer key (ex: 3, 13, 25, 29, -2).

Processing Rules

• Only shift letters:
  • A–Z stays uppercase
  • a–z stays lowercase
• Wrap around alphabet:
  • Z shift 1 → A
  • z shift 1 → a
• Non-letters stay the same: spaces, numbers, punctuation unchanged.
• Normalize key so it works for any integer: convert to a value from 0–25.

Output

• Display the final encrypted/decrypted message.

Must Include (Required Programming Concepts)

• At least 2 methods: encrypt() and decrypt()
• At least one loop that processes the string character-by-character
• At least one if/else to detect uppercase/lowercase/non-letter
• Clear console prompts and friendly output
• Good code style (names, indentation, comments)

Required Test Cases (Must show in your output or in a screenshot)

• "HELLO" key 3 → KHOOR
• "abc" key 2 → cde
• "xyz" key 4 → bcd
• "Hello, World!" key 5 → Mjqqt, Btwqi!
• Decrypt "KHOOR" key 3 → HELLO

Pseudocode (Student Version)

START program

LOOP until user chooses Quit
    DISPLAY menu: 1 Encrypt, 2 Decrypt, 3 Quit
    GET userChoice

    IF userChoice is 3
        PRINT goodbye
        EXIT loop
    ENDIF

    PROMPT for message
    READ message (entire line)

    PROMPT for key (integer)
    READ key

    NORMALIZE key:
        key = key % 26
        if key < 0 then key = key + 26

    IF userChoice is 1
        result = encrypt(message, key)
        PRINT result
    ELSE IF userChoice is 2
        result = decrypt(message, key)
        PRINT result
    ELSE
        PRINT invalid choice
    ENDIF
END LOOP

END program


FUNCTION encrypt(text, key)
    CREATE empty result stringbuilder

    FOR each character ch in text
        IF ch is uppercase letter
            shift within 'A'..'Z' using wrap
            append shifted character
        ELSE IF ch is lowercase letter
            shift within 'a'..'z' using wrap
            append shifted character
        ELSE
            append ch unchanged
        ENDIF
    END FOR

    RETURN result string
END FUNCTION


FUNCTION decrypt(text, key)
    // decrypt is encrypt with (26 - key)
    RETURN encrypt(text, 26 - key)
END FUNCTION

Grading Rubric (100 points)

Google Classroom Turn-In (Files Students Submit)

• P_CaesarCipher_Lastname.java — final program
• P_CaesarCipher_Lastname_Pseudocode.mp4 — video of program running
• P_CaesarCipher_Lastname_Screenshot.png — screenshot in Eclipse.

Teacher Notes (Quick Tips)

• If students struggle with wrap-around, remind them: (ch - 'A' + key) % 26
• If they forget to preserve punctuation: only shift letters; everything else stays the same.
• Fast finisher ideas: brute-force all 26 shifts, or add file input/output.