Cipher guides · Substitution

How to Solve the ROT13 Cipher

A substitution cipher swaps every letter for another letter and keeps that swap for the whole message. The text still looks like text — same word lengths, same spacing, same doubled letters — it just isn't in English yet.

How to recognise it

Word shapes survive. If you can see one- and two-letter words, an apostrophe, or the same letter doubled in the middle of a word, you are almost certainly looking at a substitution.

ROT13 A Caesar shift of exactly 13 — which means encoding and decoding are the same move.

The key, letter by letter

A → N
B → O
C → P
D → Q
E → R
F → S
G → T
H → U
I → V
J → W
K → X
L → Y
M → Z
N → A
O → B
P → C
Q → D
R → E
S → F
T → G
U → H
V → I
W → J
X → K
Y → L
Z → M

A worked example

Message

TREASURE MAP

Key

shift 13

Encoded

GERNFHER ZNC

Every letter has been swapped for another one, and the same swap is used all the way through. Letter shapes stay put — word lengths and spacing are still real clues. This one is ROT13. The key: shift 13. A Caesar shift of exactly 13 — which means encoding and decoding are the same move. Take the first letter, G, and walk it back 13 places: it lands on T. Do that to every letter and the message reads T R E… The answer was TREASURE MAP.

Working one out from scratch

Start with the short words and the most common letter. In English that letter is usually E, and a lone letter is usually A or I. Two or three correct guesses tend to collapse the rest of the message on their own.

Now try one

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