Caesar Cipher (Shift Cipher)

Summary: Each letter is shifted by a fixed number of positions in the alphabet. The classic Caesar cipher uses a shift of 3, but any shift value (1-25) can be used.

Example (Shift 3):

Plain:  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
Cipher: 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

Encryption:

• Plaintext: MEET AT DAWN
• Shift each letter by 3:
  • M -> P, E -> H, E -> H, T -> W
  • A -> D, T -> W
  • D -> G, A -> D, W -> Z, N -> Q
• Ciphertext: PHHW DW GDZQ

Decryption: Shift back by 3 (or shift forward by 23).

Common shifts to try:

• ROT13 (shift 13): Self-reciprocal
• Traditional Caesar (shift 3)
• Brute force: Only 25 possible shifts

Additional Example (Shift 7):

• Plaintext: CODING IS FUN
• Shift alphabet by 7:

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

• Encryption:
  • C->J, O->V, D->K, I->P, N->U, G->N
  • I->P, S->Z
  • F->M, U->B, N->U
• Ciphertext: JVKPUN PZ MBU

Brute Force Decryption Example: Given ciphertext: KHOOR

Try all 26 shifts:

• Shift 0: KHOOR
• Shift 1: JGNNQ
• Shift 2: IFMMP
• Shift 3: HELLO <- Plaintext found!
• Shift 4: GDKKN
• ... (continue if needed)

ROT13 Example:

• Plaintext: HELLO WORLD
• Shift by 13:
  • H->U, E->R, L->Y, L->Y, O->B
  • W->J, O->B, R->E, L->Y, D->Q
• Ciphertext: URYYB JBEYQ
• Decrypt (shift by 13 again): HELLO WORLD
• Note: ROT13 is its own inverse (encoding = decoding)