Codes & Ciphers

What are Codes and Ciphers?

A code replaces each letter with another letter, number, or symbol according to a rule. You must find the rule to decode (or encode) a word.

Common Cipher Types

1. Letter Shift (Caesar Cipher)

Each letter shifts forward (or backward) by a fixed number.

Example (+3): A→D, B→E, C→F → "CAT" becomes "FDW"

Example (−1): B→A, C→B, D→C → "DBU" means "CAT"

2. Reverse Alphabet

A↔Z, B↔Y, C↔X, D↔W ...

Example: "CAT" → "XZG"

3. Letters as Numbers

A=1, B=2, C=3 ... Z=26

Example: "3-1-20" = C-A-T = "CAT"

4. Consistent Substitution

A whole word is given with its coded form — find the shift and apply it.

Example: If RAIN is coded as SBJO, how is SNOW coded?
Each letter increases by 1: R→S, A→B, I→J, N→O
Apply to SNOW: S→T, N→O, O→P, W→X → TOPX

How to Decode

1. Find the pattern by comparing the given word to its code.
2. Determine direction: forward shift, backward shift, or reversal.
3. Apply the same rule to encode or decode the target word.
4. Check that your decoded result makes sense as a word.

Key Tips

• Always work out the rule from the sample pair before tackling the question
• For letter-to-number codes, remember A=1 (not A=0)
• When wrapping: Z + 1 = A (use modular arithmetic: position mod 26)
• Reverse alphabet: position of mirror letter = 27 − original position