On the Subject of the Porta CM

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

Do the following instructions:

  • Use the Nth letter of the keyword as the row in the table below.
  • If the Nth letter of the encrypted word is between A-M, use the letter as the column for the chart. Then the intersection of the row and column of the 2 letters will give you a decrypted letter.
  • Otherwise, find the Nth letter of the encrypted word in the same row as the Nth letter of your keyword. The column of that letter will give you a decrypted letter via the A-M row of the chart.

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

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

ABCDEFGHIJKLM
ABNOPQRSTUVWXYZ
CDOPQRSTUVWXYZN
EFPQRSTUVWXYZNO
GHQRSTUVWXYZNOP
IJRSTUVWXYZNOPQ
KLSTUVWXYZNOPQR
MNTUVWXYZNOPQRS
OPUVWXYZNOPQRST
QRVWXYZNOPQRSTU
STWXYZNOPQRSTUV
UVXYZNOPQRSTUVW
WXYZNOPQRSTUVWX
YZZNOPQRSTUVWXY

Example

Encrypted Word: CBNRRF
Keyword: AMEN

C + A → P
B + M → U
N + E → L
R + N → L
R + A → E
F + M → Y

Decrypted Word: PULLEY