Tool to decrypt/encrypt with the Bazeries cipher automatically (ciphering with 2 Polybius grids and a key)
Bazeries Cipher  dCode
Tag(s) : Substitution Cipher, PolyAlphabetic Cipher
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The Bazeries Cipher is a ciphering system created by Etienne Bazeries combining two grids (Polybius), and one key creating superencryption.
Bazeries encryption uses a number N, and two identical grids (usually square grids of 25 distinct characters). Bazeries suggested generating the second grid from the number N, number written in letters, but any keyword is fine too.
Example: To crypt DCODE with N=23, use a first grid, generated with the alphabet (one letter should be removed) and written in columns and the second grid generated with the keyword TWENTYTHREE (one could have took TWOTHREE)
Grid 1  Grid 2  



The message is segmented by groups of letters with cardinality equals to each digit of N (repeated if necessary).
Example: The number 23 is made of the digits 2 and 3, so split in 2 then 3 letters: DC then ODE.
If the groups of letters are larger than 10, indicate the successive sizes, separating them with commas if necessary.
The groups are then written backward
Example: DC becomes CD and ODE becomes EDO
The letters are located in the grid 1 and replaced by the letter in the same position in grid 2. The encrypted message is the result obtained.
Example: C (line 3, column 1, grid 1) is replaced by D (line 3, column 1, grid 1) and so on. The final Bazeries ciphered message is DLSLO.
Bazeries decryption requires a number N and two grids (or the keys to generate them).
Example: The cipher message is DLSLO, the number N=23, grid 1 transposed (without key) is:
\  1  2  3  4  5 

1  A  F  L  Q  V 
2  B  G  M  R  W 
3  C  H  N  S  X 
4  D  I  O  T  Y 
5  E  K  P  U  Z 
\  1  2  3  4  5 

1  T  W  E  N  Y 
2  H  R  A  B  C 
3  D  F  G  I  K 
4  L  M  O  P  Q 
5  S  U  V  X  Z 
The message is segmented by groups of letters with cardinality equals to each digit of N (repeated if necessary).
Example: 23 is made of the digits 2 and 3, let's split the message by 2 then 3 letters: DL and SLO.
Groups of letters are written backward
Example: DL becomes LD and SLO becomes OLS
Each letter is located in the second grid, and replaced by the letter with the same coordinate in the first grid.
Example: L (line 4, column 1, grid 2) is replaced by D (line 4, column 1, grid 1) and so on. The original plain text is DCODE.
A Bazeries ciphered message has an index of coincidence close to the language of the plain text.
The presence of a number (usually at least 2 digits) is a clue.
One can crack Bazeries using frequency analysis, as it is a substitution, but a manual analysis is then needed to find the key used and reverse segments of the message.
Grids can be written in rows or in columns, they also can be switched.
Bazeries is already considered a variant of the Polybius cipher.
Etienne Bazeries would have created this cipher near 1890.
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