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Download complete ZIPPuzzles · difficulty 1–5
Codes and ciphers
Twenty-five original puzzles in substitution, transposition, and key recovery. Shifts, mirrored alphabets, keyword alphabets, rail fences, repeating keys, and check letters: all workable by hand, each with an explanation that shows the arithmetic rather than simply naming the message.
What this category trains
Substitution and transposition systems
Crib and frequency reasoning
Recovering an unknown key
Systematic checking of every letter
25 puzzles · about 55 minutes end to end, as a guide only · untimed
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The ladder
The puzzles below run in difficulty order, easiest first. Each one checks on its own and shows its explanation straight away, so you get the method for one rung before you attempt the next.
Nothing here is timed and nothing is scored. Read the enciphered text, work the system out on paper, then check yourself against the explanation, which names the technique and shows the arithmetic step by step. Every puzzle is self-contained: the system, and the key or crib you need to recover it, is always given. Alphabet positions run A = 1 to Z = 26, and any shift running past Z wraps back round to A.
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1. One step forward
Every letter of the message was replaced by the letter one place further on in the alphabet. What was the original message?
Each letter moved one place forward.
DPME SBJO
Worked example: A becomes B.
Show a hint
The worked example tells you which direction the encipherer moved. You need to travel the other way.
2. Three places along
Each letter of a short instruction was moved three places forward through the alphabet. What did the instruction say?
Each letter moved three places forward.
DGG VDOW
Worked example: A becomes D.
Show a hint
Work out the size of the step from the worked mapping before you touch the message itself.
3. Numbers for letters
Each letter of a word was replaced by its position in the alphabet, counting A as 1. What was the word?
Alphabet positions, one number per letter.
3 12 5 1 14
Worked example: 1 becomes A.
Show a hint
Write the alphabet out with 1 under A and count along; the gaps between numbers are part of the message.
4. Back to front
Each word of the message was written backwards, but the words were left in their original order. What was the message?
Every word reversed; word order untouched.
NEPO EHT ROOD
Worked example: TAC becomes CAT.
Show a hint
The word boundaries are doing real work here. Ask whether the reversal applies inside each word or across the whole line.
5. Four symbols
A message was written with symbols in place of letters, using this legend: # = A, * = R, + = E, ~ = D. Each group of symbols is one word. What does the message say?
Symbol substitution, legend supplied in the prompt.
# *+~ ~++*
Worked example: * becomes R.
Show a hint
Copy the legend out in front of you and decode one symbol at a time, keeping the groups in the order they are printed.
6. The crib at the front
A shift cipher was used, but the size of the shift was not recorded. You are told that the first word of the message is THE. What does the whole message say?
Shift of unknown size; the first word is known to be THE.
YMJ UFYM NX HQJFW
Show a hint
Line the known word up against the first three ciphertext letters and count the gap between one pair.
7. The alphabet reflected
Each letter was swapped for the letter the same distance from the other end of the alphabet, so the first letter pairs with the last. What was the message?
Mirrored alphabet: first pairs with last, second with second-last, and so on.
DZIN SZMWH
Worked example: A becomes Z.
Show a hint
Write the alphabet forwards on one line and backwards directly beneath it, then read straight down.
8. An alphabet built from a keyword
A substitution alphabet was built by writing the keyword PLANET first, then the remaining letters of the alphabet in their usual order. That row was placed under a normal A to Z row, and each letter of the message was replaced by the letter beneath it. What was the message?
Keyword alphabet: PLANET, then B C D F G H I J K M O Q R S U V W X Y Z.
IEES PS JKKJ
Worked example: P becomes A.
Show a hint
Write both rows out and always start your lookup in the row the ciphertext belongs to.
9. Half the letters gone
The spaces were stripped out of a message and then every second letter was deleted, leaving the first, third, fifth and so on. Which message produces the surviving string?
Spaces removed, then every second letter deleted.
TEODSR
Show a hint
Do not try to fill the gaps in. Encipher each candidate yourself and compare the results letter by letter.
10. A shift for digits
A reference number was disguised by increasing every digit by four, with anything past nine wrapping round to zero, so 8 became 2. What was the original number?
Each digit increased by four, wrapping past nine back to zero.
7150 3826
Show a hint
Handle the digits under four first and decide what wrapping downwards should give before touching the rest.
11. Two rails
The letters of a message were written along two rails, taking the first letter on the top rail, the second on the bottom, the third on the top, and so on. The top rail was then read out, followed by the bottom rail. Which message was enciphered?
Two-rail fence: top rail read out first, then the bottom rail.
SNHLNWEDEPO
Worked example: PLANT becomes PATLN.
Show a hint
Count the letters first. An odd total tells you how many letters ride on each rail before you interleave anything.
12. No crib at all
A shift cipher was used and neither the size of the shift nor any word of the message is known. The word lengths and spacing have been kept. What does the message say?
Shift of unknown size; word lengths preserved.
P ZLL H ZOPW
Show a hint
Start with the words that are only one letter long, then look at the pair of identical letters.
13. The mirror without a map
Every letter was replaced by its partner counted from the opposite end of the alphabet, so the letter at position n becomes the letter at position 27 minus n. What was the message?
Mirrored alphabet, no worked mapping supplied.
GSV OZHG GIZRM
Show a hint
Subtract each letter's position from 27 and check your rule on the shortest word before doing the rest.
14. Three rails
A message was written in a zig-zag across three rails, running down to the third rail and back up to the first, over and over. The three rails were then read out in order, top to bottom. Which message was enciphered?
Three-rail fence: rails read out top, middle, bottom.
BGKRNTEESIHY
Show a hint
Only every fourth letter lands on the top rail. Extract those three letters for each candidate before doing any more work.
15. Columns out of order
A message was written left to right in rows of four letters, filling three complete rows. The columns were then read downwards and sent in the order 3, 1, 4, 2. What was the message?
Twelve letters in three rows of four; columns sent in the order 3, 1, 4, 2.
OHTCEGSEELTA
Show a hint
Cut the ciphertext into equal blocks first, then label each block with the column number it was sent as.
16. Shifted, then flipped
Two operations were used on this message. Every letter was moved four places forward through the alphabet, and then each word was written backwards. What was the message?
Shift of four forwards, then each word reversed.
HRMJ ILX CIO
Show a hint
Deal with the operation that was applied last before the one applied first, and treat each word separately.
17. Positions plus three
Each letter was turned into its alphabet position, then three was added to every number. The slashes mark the ends of words. What was the message?
Alphabet positions with three added to each; slashes separate words.
21 8 4 7 / 23 11 8 / 16 4 19
Show a hint
Strip the offset off every number first, and keep the letters in a separate column from the arithmetic.
18. Which keyword built the alphabet
A substitution alphabet was built by writing a keyword first, dropping any letter that had already appeared, then adding the unused letters of the alphabet in order; that row sits under a plain A to Z row. The first word of the message is THE. Which keyword was used?
Keyword alphabet of unknown keyword; the message begins with THE.
TEN AIUN OMMR
Show a hint
Write out each candidate alphabet in full, dropping repeats, then check every letter of the known word before deciding.
19. The order the steps were taken
Ignoring spaces, each letter was moved forward by a repeating key of one place then four places, one then four, across the whole message. The resulting ten letters were then written out backwards. What was the message?
Repeating key of one and four places forward, then the whole string reversed.
KBFFLUODEQ
Show a hint
Ask which step was performed last, and remember that a key tied to position stops matching once the letters move.
20. A key of two letters
The key BE was repeated across the message, ignoring spaces, and each key letter's alphabet position was added to the position of the letter beneath it, wrapping past Z back to A. What was the message?
Key BE repeated: add 2 to the first letter, 5 to the second, and so on.
OJGYCYFFYS
Worked example: A under key B becomes C.
Show a hint
Write the repeating key out underneath the ciphertext letter by letter before you subtract anything.
21. The group that fails its check
Each group is four letters followed by a check letter. The check letter's alphabet position equals the total of the four letters' positions, with 26 taken off as often as needed to leave a total between 1 and 26. One group was altered in transmission. Which group is it?
Four data letters plus a check letter in each group.
BEADL CAGEP FLAXM GRIPX
Show a hint
Add the four data letters in each group and reduce the total below 27 before comparing it with the check letter.
22. Five bits per letter
Each letter was written as its alphabet position in binary, using five bits with the largest value on the left, so the bits stand for 16, 8, 4, 2 and 1. What was the word?
Five-bit binary, one group per letter, largest value on the left.
00011 01000 00001 10010 10100
Show a hint
Write 16, 8, 4, 2, 1 above the five columns and add up only the columns holding a one.
23. A key of three letters
The key CAB was repeated across the message, ignoring spaces, and each key letter's alphabet position was added to the letter beneath it, wrapping past Z back to A. What was the message?
Key CAB repeated: add 3, then 1, then 2, and repeat.
KJFHUJHNCS
Show a hint
Number the letters one to ten and write the repeating key underneath before subtracting anything.
24. The message carries its key
One letter of this ciphertext stands on its own and is not part of the message: it is the key. Its alphabet position is the number of places every other letter was moved forward. What was the message?
A shift cipher whose key letter travels inside the ciphertext.
XYNW CQN PJCN I
Show a hint
Decide which letter is doing the work of a key, and remember that whatever it is, it is not part of what you read out.
25. Two systems, one message
This message was enciphered with exactly one of two systems: a mirrored alphabet, where the letter at position n becomes the letter at position 27 minus n, or a shift of five places forward. Decide which was used, then give the message.
One of two named systems was used; the other was not.
HSLIG DZOP
Show a hint
Try both systems on the first three or four letters only, and keep the one that starts producing vowels in sensible places.
Skills these puzzles practise
Each category maps onto the shared aptitude constructs, so a puzzle that interests you has a lesson and a larger practice bank behind it:
Applying symbol, letter, number, and substitution rules.
Inferring rules from shapes, symbols, matrices, and non-verbal patterns.
Comparing strings, records, transactions, forms, and data for mistakes.
Conditional logic, syllogisms, ordering, grouping, argument structure, and constraint problems.
Sustained focus, selective attention, and accuracy under time pressure.
Arithmetic, fractions, percentages, ratios, rates, estimation, word problems, and number relationships.
Identifying general rules from examples, sequences, and observations.
Applying stated rules and constraints to reach valid conclusions.
Data checking and clerical accuracy
Matching, filing, coding, record checking, and identifying discrepancies.