Cipher guides · Substitution

How to Solve the Keyword 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.

Keyword Cipher The alphabet starts with the keyword, then carries on with the letters it hasn't used.

The key, letter by letter

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

A worked example

Message

TREASURE MAP

Key

keyword GOLD

Encoded

TRAGSURA KGP

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 Keyword Cipher. The key: keyword GOLD. The alphabet starts with the keyword, then carries on with the letters it hasn't used. Apply the key to the first three letters and they resolve to 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

There’s a new cipher waiting today

One a day, three guesses, and a streak that breaks the moment you miss one. Today’s is cipher #1.

Today’s cipher All cipher guides