On the Subject of the Vigenère CM

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

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 keyword on screen 1.
  • If the resulting number is above 26, subtract 26 until the number is between 1 - 26.
  • Turn the number back into a letter to get your Nth decrypted letter.

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 keyword on screen 1.
  • If the resulting number is below 1, add 26 until the number is between 1 - 26.
  • Turn the number back into a letter to get your Nth decrypted letter.

Do this for each letter of the encrypted word and the keyword to get your decrypted word.

When you reach the end of the keyword, treat it as if it wraps around to the beginning of the keyword.

Example 1

Encrypted Word: KLEJ
Keyword: BOG
Using Encrypt Rules

K + B → 11 + 2 → 13 → M
L + O → 12 + 15 → 27 - 26 → 1 → A
E + G → 5 + 7 → 12 → L
J + B → 10 + 2 → 12 → L

Decrypted Word: MALL

Example 2

Encrypted Word: PIPOCQWS
Keyword: CHIN
Using Decrypt Rules

P - C → 16 - 3 → 13 → M
I - H → 9 - 8 → 1 → A
P - I → 16 - 9 → 7 → G
O - N → 15 - 14 → 1 → A
C - C → 3 - 3 → 0 → 26 → Z
Q - H → 17 - 8 → 9 → I
W - I → 23 - 9 → 14 → N
S - N → 19 - 14 → 5 → E

Decrypted Word: MAGAZINE

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