A key cipher confuses letter frequencies, which are essential for solving basic ciphers.
Create an alphanumeric guide, numbering the alphabet sequentially.
A = 1 E = 5 I = 9 M = 13 Q = 17 U = 21 Y = 25
B = 2 F = 6 J = 10 N = 14 R = 18 V = 22 Z = 26
C = 3 G = 7 K = 11 O = 15 S = 19 W = 23
D = 4 H = 8 L = 12 P = 16 T = 20 X = 24
ENCODING THE KEY CIPHER
Using the alphanumeric guide, convert a sentence into numbers.
A FOOL AND HIS MONEY ARE SOON PARTED.
A F O O L A N D H I S M O N E Y A R E S O O N P A R T E D
1 6 15 15 12 1 14 4 8 9 19 13 15 14 5 25 1 18 5 19 15 15 14 16 1 18 20 5 4
Using the alphanumeric guide, convert a key word - FEARLESS - into numbers.
F E A R L E S S
6 5 1 18 12 5 19 19
Repeat the key word for the length of the sentence and then add up the numbers.
A F O O L A N D H I S M O N E Y A R E S O O N P A R T E D
1 6 15 15 12 1 14 4 8 9 19 13 15 14 5 25 1 18 5 19 15 15 14 16 1 18 20 5 4
F E A R L E S S F E A R L E S S F E A R L E S S F E A R L
6 5 1 18 12 5 19 19 6 5 1 18 12 5 19 19 6 5 1 18 12 5 19 19 6 5 1 18 12
7 11 16 33 24 6 33 23 14 14 20 31 27 19 24 44 7 23 6 37 27 20 33 35 7 23 21 23 16
If any total is greater than 26, subtract 26 from that total.
7 11 16 33 24 6 33 23 14 14 20 31 27 19 24 44 7 23 6 37 27 20 33 35 7 23 21 23 16
      -26     -26         -26 -26     -26       -26 -26   -26 -26          
7 11 16 7 24 6 7 23 14 14 20 5 1 19 24 18 7 23 6 11 1 20 7 9 7 23 21 23 16
Using the alphanumeric guide, encode the new letters.
7 11 16 7 24 6 7 23 14 14 20 5 1 19 24 18 7 23 6 11 1 20 7 9 7 23 21 23 16
G K P G X F G W N N T E A S X R G W F K A T G I G W U W P
The sentence reads A FOOL AND HIS MONEY ARE SOON PARTED.
The key cipher reads G KPGX FGW NNT EASXR GWF KATG IGWUWP.
Note the OO in FOOL is now PG. And the OO in SOON is now AT.
DECODING THE KEY CIPHER
The key cipher is deciphered using the key word FEARLESS.
Repeat the key word for the length of the key cipher.
G K P G X F G W N N T E A S X R G W F K A T G I G W U W P
7 11 16 7 24 6 7 23 14 14 20 5 1 19 24 18 7 23 6 11 1 20 7 9 7 23 21 23 16
F E A R L E S S F E A R L E S S F E A R L E S S F E A R L
6 5 1 18 12 5 19 19 6 5 1 18 12 5 19 19 6 5 1 18 12 5 19 19 6 5 1 18 12
Subtract the numbers of the key word from the numbers of the key cipher.
7 11 16 7 24 6 7 23 14 14 20 5 1 19 24 18 7 23 6 11 1 20 7 9 7 23 21 23 16
-6 -5 -1 -18 -12 -5 -19 -19 -6 -5 -1 -18 -12 -5 -19 -19 -6 -5 -1 -18 -12 -5 -19 -19 -6 -5 -1 -18 -12
1 6 15 -11 12 1 -12 4 8 9 19 -13 -11 14 5 -1 1 18 5 -7 -11 15 -12 -10 1 18 20 5 4
If any total is less than 1 (red boxes), add 26 to that total.
1 6 15 -11 12 1 -12 4 8 9 19 -13 -11 14 5 -1 1 18 5 -7 -11 15 -12 -10 1 18 20 5 4
      26     26         26 26     26       26 26   26 26          
1 6 15 15 12 1 14 4 8 9 19 13 15 14 5 25 1 18 5 19 15 15 14 16 1 18 20 5 4
Using the alphanumeric guide, decode the original sentence.
1 6 15 15 12 1 14 4 8 9 19 13 15 14 5 25 1 18 5 19 15 15 14 16 1 18 20 5 4
A F O O L A N D H I S M O N E Y A R E S O O N P A R T E D
A FOOL AND HIS MONEY ARE SOON PARTED.
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