Name: Phillips-C
Type: Substitution
Ease of use: Easy / Intermediate / Difficult
Breaking the code: Easy / Intermediate / Difficult
The Phillips-C cipher is a rather complex polyalphabetic substitution cipher based on the Phillips cipher. I couldn't find any background information on the cipher or its inventor, so any insight will be greatly appreciated. This page describes the Phillips-C cipher, which is basically the Phillips cipher but this variant shifts columns instead of rows.
The cipher consists of using a basic 5 by 5 Polybius square which is altered each step of the way. The letters 'J' and 'I' are combined into the 'I' to get a total of 25 letters. The Polybius square is first constructed using a keyed alphabet. Five plaintext letters are then encoded by taking the letter one down and one right from the plaintext letter (diagonally). If a plaintext letter is in the 5th column, the letter one row down in the 1st column is used. If a plaintext letter is on the bottom row, the letter to the right on the top row is used.
After the first 5 letters are encoded, the left column of the Polybius square is shifted right one position, effectively trading places with the second column. This goes on until square number 5, where the left column will now be at the right. Then follow three more steps, where the new left column is shifted to the right one column at a time. This totals to 8 different Polybius squares and is best illustrated by the following example where the keyword 'POLYALPHABETIC' was used to create a keyed alphabet:
#1 #2 #3 #4
P O L Y A O P L Y A O L P Y A O L Y P A
H B E T I B H E T I B E H T I B E T H I
C D F G K D C F G K D F C G K D F G C K
M N Q R S N M Q R S N Q M R S N Q R M S
U V W X Z V U W X Z V W U X Z V W X U Z
#5 #6 #7 #8
O L Y A P L O Y A P L Y O A P L Y A O P
B E T I H E B T I H E T B I H E T I B H
D F G K C F D G K C F G D K C F G K D C
N Q R S M Q N R S M Q R N S M Q R S N M
V W X Z U W V X Z U W X V Z U W X Z V U
If you look closely with the method of substitution in mind, you may notice that squares #1 and #5 are actually the same. This is also true for squares #2 and #8.
Let's use the above example squares to encode a short sentence, 'The Phillips cipher is polyalphabetic'.
Plaintext: THE PHILLIPS CIPHER IS POLYALPHABETIC
As stated before, we take the first 5 characters and encode them using the first Polybius square. Take the plaintext letter and look it up in the square. The letter one to the right and one down is its substitution. So in this case the 'T' will be substituted by the 'K'. Similarly the 'H' becomes a 'D', the 'E' a 'G' and so on. This results in the first 5 letters of our ciphertext:
Plaintext: THE PH
Ciphertext: KDG BD
The next 5 letters are encoded using the second square, continuing to the eight square and then back to the first if necessary. Eventually we will end up with our final ciphertext:
Plaintext: THE PHILLIPS CIPHER IS POLYALPHABETIC
Ciphertext: KDG BDDTTDEV RDTGGU DV IETIHTEFHGDDCQ
Formatted: KDGBD DTTDE VRDTG GUDVI ETIHT EFHGD DCQ
I think this cipher is hard to crack, yet at the same time it shows a lot of weaknesses. For instance, every block of 5 letters is essentially nothing more than a monoalphabetic substitution. The period is also rather short with 40 letters. But then again, if the method is unknown it might be a true challenge to solve it.
Continue to the tool for this cipher or go back to the list of ciphers.