Caesar Cipher

🏛️ Historical Background of the Caesar Cipher

The Caesar Cipher is one of the oldest known encryption techniques, attributed to Julius Caesar. It is a substitution cipher where each letter in the plaintext is shifted by a fixed number of positions down or up the alphabet.

According to Roman historian Suetonius, Caesar would shift each letter in his messages by three positions to keep the content secret. Like, A becomes D, B becomes E, and so on.

This cipher was used by Julius Caesar to protect his private messages from being read by enemies. The simplicity of the cipher made it both fast and easy to use, but also easy to break without modern cryptographic techniques.

“If he had anything confidential to say, he wrote it in cipher...” – Suetonius

This simple technique was effective at the time because few people were literate and cryptographic awareness was low. It represents one of the earliest uses of substitution ciphers in history.

🧠 Fun Fact: Despite its simplicity, the Caesar Cipher is a building block in the evolution of modern cryptography!

🔐 Caesar Cipher Principle

The Caesar Cipher is a type of monoalphabetic substitution cipher where each letter in the plaintext is replaced with a letter a fixed number of positions away in the alphabet. By shifting each letter in a predictable way, the message becomes unreadable to someone who doesn't know the key. Without the shift value, the original meaning is hidden, even though the message still looks like regular letters.

For example, if we use a shift value of 3:

A becomes D (A is moved 3 positions forward)
B becomes E
C becomes F
and so on…

The process is straightforward:

Step 1: Choose a shift key (a number between 1 and 25).
Step 2: Shift each letter in the plaintext by the number of positions indicated by the key.
Step 3: Wrap around if the shift exceeds the letter "Z." For example, with a key of 3, X would become A, Y would become B, and Z would become C.

This method of encryption works by essentially "shifting" the alphabet. If you apply a shift of 3, the new alphabet would look like this:

Original Alphabet: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Shifted Alphabet : D E F G H I J K L M N O P Q R S T U V W X Y Z A B C

Key Characteristics:

Shift Value (Key): A number between 1 and 25, representing the fixed shift in the alphabet.
Key = 3: A → D, B → E, C → F
Key = 5: A → F, B → G, C → H
Alphabet Wrapping: If the shift goes past Z, it wraps back to A.
Key = 3 → X → A, Y → B, Z → C

Mathematical Representation:

Each letter is treated as a number:

A = 0, B = 1, C = 2, ..., Z = 25

🔐 Encryption:

C = (P + K) mod 26

🔓 Decryption:

P = (C - K + 26) mod 26

Where:

C: Ciphertext letter (Result after encryption)
P: Plaintext letter (Original text)
K: Shift key (Number of Positions Shifted)

💡 Did You Know? Julius Caesar used this cipher to protect military messages over 2000 years ago!

✏️ Step-by-Step Guidance to Caesar Cipher

🔐 Caesar Cipher Encryption

Step 1: Choose a Key (Shift Value)

The key determines how many positions each letter in the plaintext will shift in the alphabet. It can be any number between 1 and 25.

Example: Suppose the key is 3.

Step 2: Write Down the Plaintext

The plaintext is the original message you want to encrypt. This can include letters, spaces, and punctuation. Only alphabetic characters are shifted.

Example: Plaintext = HELLO WORLD

Step 3: Substitute Letters Using the Key

                    Key = 3
                    H → K (H = 7 → 7 + 3 = 10 → K)  
                    E → H (E = 4 → 4 + 3 = 7 → H)  
                    L → O (L = 11 → 11 + 3 = 14 → O)  
                    L → O (L = 11 → 11 + 3 = 14 → O)  
                    O → R (O = 14 → 14 + 3 = 17 → R)  

                    W → Z (W = 22 → 22 + 3 = 25 → Z)  
                    O → R (O = 14 → 14 + 3 = 17 → R)  
                    R → U (R = 17 → 17 + 3 = 20 → U)  
                    L → O (L = 11 → 11 + 3 = 14 → O)  
                    D → G (D = 3 → 3 + 3 = 6 → G)

                    🔐 Encrypted Text: KHOOR ZRUOG

🔓 Caesar Cipher Decryption

Step 1: Use the Same Key

The decryption process uses the same key as encryption to reverse the shifts.

Example: Key = 3

Step 2: Write Down the Ciphertext

Example: Ciphertext = KHOOR ZRUOG

Step 3: Reverse the Shift for Each Letter

Find Alphabet Position: A=0, B=1, ..., Z=25
Subtract the Key: Example: K = 10 → 10−3 = 7 → H
Handle Wraparound: If result is negative, add 26
Example: C = 2 → 2−5 = -3 → -3+26 = 23 → X
Find New Letter: Replace the letter with the one at the new position

Note: Non-alphabetic characters (spaces, punctuation) remain unchanged.

                    
                    Key = 3
                    K → H (10 − 3 = 7 → H)  
                    H → E (7 − 3 = 4 → E)  
                    O → L (14 − 3 = 11 → L)  
                    O → L (14 − 3 = 11 → L)  
                    R → O (17 − 3 = 14 → O)  

                    Z → W (25 − 3 = 22 → W)  
                    R → O (17 − 3 = 14 → O)  
                    U → R (20 − 3 = 17 → R)  
                    O → L (14 − 3 = 11 → L)  
                    G → D (6 − 3 = 3 → D)


                    🔓 Decrypted Text: HELLO WORLD

Caesar Cipher Quiz

Hint:

The Caesar Cipher is a substitution cipher where each letter in the plaintext is shifted by a certain number.
Example: A shift of 3 would change A to D, B to E, C to F, and so on.