A substitution cipher replaces letters or symbols. A transposition cipher moves the existing letters into a different order. Knowing which kind you are using helps you follow the steps and spot mistakes in a puzzle or answer sheet.
Compare what happens to the letters
Substitute letters
MEETCaesar; shift 3PHHW| Method | Prepared input | Output | Observation |
|---|---|---|---|
| Caesar, shift 3 | MEET | PHHW | M becomes P; both E letters become H |
| Rail Fence, 2 rails | MEET | MEET | Positions are rearranged, but the repeated E letters hide that change |
| Scytale, 2 columns | ABCD | ACBD | All four letters remain; their order changes |
With two rails, MEET happens to come out as MEET: the repeated E letters hide the change in position. An unchanged result does not always mean that no rearrangement took place. Try ABCD first if you want to see every move clearly.
Check what should stay the same
- A one-to-one substitution keeps the prepared message length and repeated-letter pattern, even though the letters change.
- A pure transposition keeps the same letters with exactly the same counts; only their positions change.
- Check the prepared input first. Some methods remove spaces or merge letters before the cipher itself runs.
Reverse the right operation
To decode Caesar, subtract the shift from each letter’s position. To decode a transposition, rebuild the grid or path and put the letters back in order. The letters have moved; they have not been replaced by partners in an alphabet table.
Check whether the letters still match
A claimed transposition changes BOOK into CQQM. Can it be a rearrangement of those four prepared letters?