What is the Bazeries Cipher?

The Bazeries Cipher uses two Polybius squares. One of the squares features the alphabet written vertically in order. For the other square, choose a number less than a million, spell it out, and use it as the keyword for the other Polybius square, written horizontally. Finally, take the plaintext and split it into groups, with each group being the length of each digit in the random number. Reverse the text in each group. The normal alphabet Polybius square represents the plaintext letter, and the keyed horizontal Polybius square represents the ciphertext letter to replace it with.

Example

Plaintext: Whoever has made a voyage up the Hudson must remember the Kaatskill mountains.

Random Number: 81257

Spelled out number: Eighty one thousand two hundred fifty seven

Plaintext Polybius:

A F L Q V
B G M R W
C H N S X
D I O T Y
E K P U Z

Ciphertext Polybius:

E I G H T
Y O N U S
A D W R F
V B C K L
M P Q X Z

Grouped Text:
W H O E V E R H / A / S M / A D E A V / O Y A G E U P / T H E H U D S O / N / M U / S T R E M / E M B E R T H / E K A A T S K I / L / L M / O U N T A / I N S

Grouped Text with Reversals:
H R E V E O H W / A / M S / V A E D A / P U E G A Y O / O S D U H E H T / N / U M / M E R T S / H T R E B M E / I K S T A A K E / L / M L / A T N U O / S N I

Ciphertext:
DUMTM CDSEN RTEMV EQXMO ELCCR VXDMD KWXNN MUKRD VUMYN MBPRK EEPMG NGEKW XCRWB

Vulnerabilities of Bazeries Cipher

Being a letter-for-letter substitution, the ciphertext for a Bazeries cipher will have a very English-like frequency count and monographic IC unless the plaintext is chosen carefully. This alone could be enough to reveal the most common letters, such as E, T, A and so on.

In addition, if the solver attempts to find common English words in the ciphertext both forward and backward, the random number could be revealed.

How to Crack a Bazeries Cipher

After doing a frequency count and IC analysis to determine that the ciphertext is likely a letter-for-letter substitution, if straight substitution fails, try looking for common English words both forward and backward, being aware of the possibility of common words being split up between groups. During this process, look for likely group lengths to attempt to reveal the random number that was used. Guessing the first number correctly will allow you to solve a significant portion of the unknown ciphertext Polybius square.

A potentially quicker way to solve Bazeries is to try each decimal digit, as well as things like TEN, ELEVEN, FORTY, and so on, in the ciphertext Polybius square. Then, compare the frequency of the resulting letters to an English frequency chart. If it matches the typical English frequency chart, your guess might be the correct one.