On the Subject of the Autokey CM

A machine that knows no bounds when it comes to ciphers.

The keyword on screen 1 will be referred to as the Autokey Word.

If the Submit Button is white, follow the instructions under Encrypt Instructions. Otherwise follow the instructions under Decrypt Instructions.

Encrypt Instructions

  • Take the sum of the alphabetic positions (A1Z26) of the Nth letter of the encrypted word and the Nth letter of the Autokey Word.
  • If the resulting number is above 26, subtract 26 until the number is between 1 - 26.
  • Turn the number back into a letter (A1Z26) to get your Nth decrypted letter.
  • Write the Nth letter of the encrypted word at the end of the Autokey Word.
  • Do this for each letter of the encrypted word to get a decrypted word.

Decrypt Instructions

  • Take the alphabetic position (A1Z26) of the Nth letter of the encrypted word and subtract it by the alphabetic position (A1Z26) of the Nth letter of the Autokey Word.
  • If the resulting number is below 1, add 26 until the number is between 1 - 26.
  • Turn the number back into a letter (A1Z26) to get your Nth decrypted letter.
  • Write the Nth letter of the decrypted word at the end of the Autokey Word.
  • Do this for each letter of the encrypted word to get a decrypted word.

Example 1

Encrypted Word: JDAHYY
Keyword: CURL
Using Encrypt Rules

J, C → 10 + 3 → 13 → M
D, U → 4 + 21 → 25 → Y
A, R → 1 + 18 → 19 → S
H, L → 8 + 12 → 20 → T
Y, J → 25 + 10 → 35 - 26 → 9 → I
Y, D → 25 + 4 → 29 - 26 → 3 → C

Decrypted Word: MYSTIC

Example 2

Encrypted Word: EZGABWG
Keyword: HUNG
Using Decrypt Rules

E, H → 5 - 8 → -3 + 26 → 23 → W
Z, U → 26 - 21 → 5 → E
G, N → 7 - 14 → -7 + 26 → 19 → S
A, G → 1 - 7 → -6 + 26 → 20 → T
B, W → 2 - 23 → -21 + 26 → 5 → E
W, E → 23 - 5 → 18 → R
G, S → 7 - 19 → -12 + 26 → 14 → N

Decrypted Word: WESTERN

You can use this table as a way to get your decrypted letters. To do so, follow the instructions below:

  • Use the keyword's letter as the row.
  • If you are adding, use the encrypted word's letter as the column. The intersection of the 2 will be your decrypted letter.
  • If you are subtracting, find the encrypted word's letter within the row. The column that the letter is in becomes your decrypted letter.
ABCDEFGHIJKLMNOPQRSTUVWXYZ
ABCDEFGHIJKLMNOPQRSTUVWXYZA
BCDEFGHIJKLMNOPQRSTUVWXYZAB
CDEFGHIJKLMNOPQRSTUVWXYZABC
DEFGHIJKLMNOPQRSTUVWXYZABCD
EFGHIJKLMNOPQRSTUVWXYZABCDE
FGHIJKLMNOPQRSTUVWXYZABCDEF
GHIJKLMNOPQRSTUVWXYZABCDEFG
HIJKLMNOPQRSTUVWXYZABCDEFGH
IJKLMNOPQRSTUVWXYZABCDEFGHI
JKLMNOPQRSTUVWXYZABCDEFGHIJ
KLMNOPQRSTUVWXYZABCDEFGHIJK
LMNOPQRSTUVWXYZABCDEFGHIJKL
MNOPQRSTUVWXYZABCDEFGHIJKLM
NOPQRSTUVWXYZABCDEFGHIJKLMN
OPQRSTUVWXYZABCDEFGHIJKLMNO
PQRSTUVWXYZABCDEFGHIJKLMNOP
QRSTUVWXYZABCDEFGHIJKLMNOPQ
RSTUVWXYZABCDEFGHIJKLMNOPQR
STUVWXYZABCDEFGHIJKLMNOPQRS
TUVWXYZABCDEFGHIJKLMNOPQRST
UVWXYZABCDEFGHIJKLMNOPQRSTU
VWXYZABCDEFGHIJKLMNOPQRSTUV
WXYZABCDEFGHIJKLMNOPQRSTUVW
XYZABCDEFGHIJKLMNOPQRSTUVWX
YZABCDEFGHIJKLMNOPQRSTUVWXY
ZABCDEFGHIJKLMNOPQRSTUVWXYZ