Unit 10

Coded Language Cipher

About this unit

Decode secret messages, identify substitution rules, and crack symbol-based identification puzzles. This unit tests your ability to detect patterns in encoded data and apply discovered rules to decode new information.

What types of questions will you face?

1Caesar-style letter shift: given example words encoded by shifting letters by N places, decode a ciphertext
2Multi-step transformation: a word is reversed AND each letter shifted — identify the double rule and apply it
3Secret language: four speakers use word-for-word replacements — cross-reference to build the complete vocabulary mapping
4Symbol-attribute identification: items (potions, robots, keycards) each have two-symbol tags — use given examples to decode which symbol means which attribute
5Rule-based letter transformation: each position in the word applies a different rule (first stays, second shifts back 1, etc.)

Skills you will build

Identifying a shift cipher by comparing encoded and decoded letter pairs
Applying multi-step transformations systematically (reverse, then shift)
Building a vocabulary mapping table from limited cross-reference examples
Using process of elimination with symbol-attribute pairs
Checking proposed rules against all given examples before committing

By the end of this unit, you will be able to

Crack any single-rule or double-rule cipher by reverse-engineering from examples
Build a complete word-to-code mapping from a set of example phrases
Decode symbol-based identification puzzles with logical elimination
Apply discovered rules consistently and accurately to new encoded data

Difficulty profile

Medium difficulty (avg 3.00). Shift ciphers are Easy-Medium; the secret language and symbol-attribute puzzles are Medium-Difficult and require careful cross-referencing.

Exam tip: Coded Language Cipher

For shift ciphers: find the shift by comparing the first letter of an example pair (e.g. A\toD means +3). Then verify with a second letter. For secret languages: isolate words that appear in two sentences and match by elimination.

Sample Questions

Lesson 1 of 3Coded Language CipherEasy

Cipher questions reward one habit: find the fixed letter shift from the examples, then apply it letter-by-letter to the target word.

Substitution codes appear regularly on Selective TS — often as a mid-paper puzzle when you compare two known words to lock the rule.

The examiner checks whether you can infer a consistent +N (or −N) shift from paired plaintext/ciphertext and apply it without mixing letters.

Two encoded words are given with their originals. You encode or decode a third word using the same shift, watching wrap-around at A–Z.

Best approach: Align letters of one pair and count the shift (e.g. A→D is +3). Apply that shift to every letter of the new word. Verify with the second example before picking an option.

Question

In a secret code, each letter of the alphabet is replaced by another letter using the same rule throughout. APPLE is written as DSSOH CAKE is written as FDNH Using the same code, what would MANGO be written as?

Using the same code, what would MANGO be written as?

A $PDQJR$
B $NBOHP$
C $OCPIQ$
D $QERKS$

Decided on your answer? Check how you went below.

Correct Answer

PDQJR

Step-by-Step Explanation

Step 1: Find the rule. Compare each letter of APPLE to its coded version DSSOH: Original A P P L E Coded D S S O H Shift +3 +3 +3 +3 +3 Every letter moves 3 places forward in the alphabet. Step 2: Check with CAKE \to FDNH. Original C A K E Coded F D N H Shift +3 +3 +3 +3 Confirmed: the rule is shift every letter forward by 3 . Step 3: Apply the rule to MANGO. Use this section of the shifted alphabet as a guide: Original: A B C D E F G H I J K L M N O P Q R S +3: D E F G H I J K L M N O P Q R S T U V Letter Position +3 Coded letter M 1316 P A 14 D N 1417 Q G 710 J O 1518 R MANGO \to PDQJR ✓ Why the other options are wrong: Option Shift used Result The mistake B — NBOHP +1 M\toN, A\toB, N\toO, G\toH, O\toP Undercounted the shift — used +1 instead of +3. C — OCPIQ +2 M\toO, A\toC, N\toP, G\toI, O\toQ Close but one short — used +2 instead of +3. D — QERKS +4 M\toQ, A\toE, N\toR, G\toK, O\toS Overcounted the shift — used +4 instead of +3. The tell: APPLE has two Ps in a row coded as two Ss in a row — the doubled letter trick is a good way to confirm your shift is consistent before applying it to the new word.

Lesson 2 of 3Coded Language CipherEasy

Symbol puzzles look busy, but they almost always use the same two moves: spot the new symbol, then use the shared symbol to lock both attributes.

Two-icon container questions sit in the harder band of Selective ciphers — fast marks if you know newcomer + link, slow traps if you guess.

You must deduce which attribute each symbol represents when two full containers are known and a third shares one symbol and introduces one new symbol.

Six items = 3×3 grid on two attributes (e.g. size × tool). Each crate shows two symbols. Find which robot/card the mystery crate holds.

Best approach: Newcomer → fills the only empty slot on its attribute axis. Shared symbol → must mean the same attribute in both crates; cross-check the known crate to assign Gold/Level etc.

Question

There are six specialised robots in a factory. There are 3 Tall robots and 3 Short robots. For each size, there is one robot equipped with a Laser, one with a Shield, and one with a Claw. Each robot is stored in a crate. Every crate has two symbols stamped on it. One symbol represents the size of the robot and the other represents its equipment, but they can appear in any order. Clues: - The crate for the Tall robot with a Laser has the symbols # and @. - The crate for the Short robot with a Shield has the symbols % and &. - A third crate has the symbols # and %.

Which robot could be stored in the third crate?

A $The Tall robot with a Shield.$
B $The Tall robot with a Laser.$
C $The Short robot with a Shield.$
D $The Short robot with a Claw.$

Decided on your answer? Check how you went below.

Correct Answer

The Tall robot with a Shield.

Step-by-Step Explanation

The Goal We need to figure out what the symbols # and % stand for. The "trap" is that we don't know which symbol is the Size and which is the Equipment. Step 1: The "Split" Method Let's look at what we know from the first two crates and list the possibilities for each symbol. Symbol Must be one of these... Because it came from... # Tall OR Laser The Tall/Laser Crate @ Tall OR Laser The Tall/Laser Crate % Short OR Shield The Short/Shield Crate & Short OR Shield The Short/Shield Crate Step 2: Analysing the Third Crate (# and %) The third crate uses # (from the first group) and % (from the second group). Every crate must have exactly one Size and one Equipment. Let's test the two possible "identities" for #: Possibility 1: If # = Tall, then % must be the Equipment \to % = Shield \to Result: Tall + Shield ✅ (This is Option A!) Possibility 2: If # = Laser, then % must be the Size \to % = Short \to Result: Short + Laser (Not in the options.) Why the other options are wrong: B & C: These are the robots from the first two crates. We already know their symbol pairs (#/@ and %/&). Since the third crate is #/%, it cannot be either of those robots. D: A "Claw" robot would require a symbol we haven't seen yet, because Laser (#/@) and Shield (%/&) already use up those four symbols. Master the Logic: Tips & Shortcuts 1. The "Exclusion" Table Symbol Tall? Short? Laser? Shield? Claw? # YES NO YES NO NO % NO YES NO YES NO 2. The "Shared Link" Shortcut Look for the "Foreign Pair" — a crate that contains one symbol from each known crate: From Crate 1: Tall/Laser From Crate 2: Short/Shield The answer must be a cross-over: Tall/Shield or Short/Laser . 3. Avoid the "Assumption" Pitfall ❌ Wrong: "# is first, so # must be Tall." ✅ Correct: "# could be Tall, or it could be Laser. I need to check both paths."

Lesson 3 of 3Coded Language CipherIntermediate

Jody replies to Todd's question in a unique-symbol code. The reply has six symbols, all different. Two seasons are eliminated because a known letter (T = Λ) doesn't match the code. A third is eliminated because Summer has two M's but the code has no repeated symbols. Only Spring survives.

'Unique-symbol substitution with elimination' is a medium-difficulty cipher variant on NSW Selective TS. Unlike Caesar-shift questions, you don't decode every symbol — you use two short-cut rules (known mappings + uniqueness) to eliminate three options and confirm the fourth.

The examiner tests whether students can (a) extract known letter-symbol pairs from the examples, (b) check each candidate word at the position where a known letter appears, (c) apply the uniqueness rule to eliminate Summer (M repeated but symbols differ), and (d) confirm Spring as the only consistent answer.

Two known coded words give some letter-symbol pairs. A third coded word (often 4–6 symbols long) must be identified from four candidate words. Not all symbols are decodable — the question is solved by ruling out three candidates via contradiction.

Best approach: Step 1: List every known letter-symbol pair from the examples. Step 2: For each candidate word, find positions of known letters and check the code symbol at that position. Mismatch = eliminate. Step 3: Check each candidate for repeated letters. If a letter repeats but the code symbols at those positions differ, eliminate. Step 4: The surviving candidate is the answer.

Question

Jody and Todd frequently message each other in code. They always use the same code, in which each letter of the alphabet is always represented by a unique symbol. For example, Jody is $!@= and Todd is Λ!@@. When Todd asked Jody to name her favourite season of the year, she replied: Δ?#>&\div

What is Jody's favourite season of the year?

A $Spring$
B $Summer$
C $Autumn$
D $Winter$

Decided on your answer? Check how you went below.

Correct Answer

Spring

Step-by-Step Explanation

The correct answer is A — Spring. We solve this by combining two elimination tools: known mappings and the uniqueness rule. Step 1: Extract the known letter-to-symbol mappings Name Letters Code Jody J-O-D-Y $-!-@-= Todd T-O-D-D Λ-!-@-@ Known mappings: J = $, O = !, D = @, Y = =, T = Λ Step 2: Examine the coded reply Reply: Δ ? # > & \div (positions 1-6, all symbols different) Step 3: Eliminate using the known T = Λ rule Autumn = A-U- T -U-M-N \to T is at position 3 \to must be Λ, but code shows #. ✘ Winter = W-I-N- T -E-R \to T is at position 4 \to must be Λ, but code shows >. ✘ Step 4: Eliminate Summer using the uniqueness rule Each letter maps to a UNIQUE symbol, so two identical letters must produce two identical code symbols. Summer = S-U- M-M -E-R \to M at positions 3 and 4 \to those code symbols must match. But code shows # at position 3 and > at position 4. # \neq >. ✘ Step 5: Spring is the only survivor Spring = S-P-R-I-N-G \to all six letters unique, no known letters \to consistent with six different, previously unseen symbols. ✔ Answer: A — Spring. Key lesson: Two elimination tools for unique-symbol ciphers Known symbol rule: If T = Λ, find where T appears in each candidate and check the code at that position. Mismatch \to eliminate. Uniqueness rule: Repeated letters must produce repeated code symbols. If a word has two of the same letter but the code shows different symbols at those spots, that word is eliminated. Together, these two tools usually narrow you down to exactly one answer without needing to decode every symbol.

Up next: Unit 11

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Option	Shift used	Result	The mistake
B — NBOHP	+1	M→N, A→B, N→O, G→H, O→P	Undercounted the shift — used +1 instead of +3.
C — OCPIQ	+2	M→O, A→C, N→P, G→I, O→Q	Close but one short — used +2 instead of +3.
D — QERKS	+4	M→Q, A→E, N→R, G→K, O→S	Overcounted the shift — used +4 instead of +3.

Symbol	Must be one of these...	Because it came from...
#	Tall OR Laser	The Tall/Laser Crate
@	Tall OR Laser	The Tall/Laser Crate
%	Short OR Shield	The Short/Shield Crate
&	Short OR Shield	The Short/Shield Crate

Name	Letters	Code
Jody	J–O–D–Y	$–!–@–=
Todd	T–O–D–D	Λ–!–@–@