Codes & Ciphers
Decoding and encoding messages using letter and number rules
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
- Find the pattern by comparing the given word to its code.
- Determine direction: forward shift, backward shift, or reversal.
- Apply the same rule to encode or decode the target word.
- 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
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In the reverse alphabet code (A↔Z, B↔Y ...), what letter codes for M?
Show answer
Answer: N
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Each letter shifts back 3 (D→A, E→B ...). 'FDW' decodes to ___.
Show answer
Answer: CAT
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If A = 1, B = 2, then C = ?
Show answer
Answer: 3
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Each letter shifts forward 2 (A→C, B→D ...). Decode 'EQI'.
Show answer
Answer: COG
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