ADFGVX Cipher, Which Combines a Substitution Cipher with a Transposition Cipher in a Special Way Described Below

M107: Combining Ciphers

In this handout, we work through the ADFGVX cipher, which combines a substitution cipher with a transposition cipher in a special way described below. ADFGVX Cipher

i. First, ﬁll in the following 6 × 6 grid with the alphabet and 10 extra characters.

ADFGVX A D F G V X

ii. The first step of encryption comes by finding the letter of the plaintext in the grid above and replacing it by the two-letter character obtained from the row-column identifiers for its position. Use this process to obtain a temporary substitution ciphertext.

iii. Choose a keyword, for example SEARCH (leave oﬀ repeated letters).

iv. The second step of the encryption comes by writing the keyword and then writing the temporary ciphertext ciphertext from Part ii. in a grid below the keyword (see below). Add characters at the end to ﬁll up the grid if necessary.

SEARCH CIPHER TEXTHE REXYZT

v. Transpose the columns of the previous grid until the letters of the keyword are alphabetical. For instance the result of previous table is

ACEHRS PEIRHC XHEETT XZETYR

vi. Create the ﬁnal ciphertext by writing out the columns left-to-right.

1. Fill in the ADFGVX-grid above in any fashion you want using the alphabet and the characters 0-9. Choose a short plaintext message to send. 2. Using your ADFGVX-grid on the previous page, perform the temporary substitution encryption on your plaintext message (remember that each plaintext letter gets re- placed by a pair of ciphertext letters).

3. Choose a keyword of your choice. Write it below, and then form the associated ciphertext grid using the temporary ciphertext obtained in the previous problem.

4. Arrange the columns of the resulting grid so that your keyword is alphabetical.

5. Finally, write out the columns successively to produce the ﬁnal ciphertext. Pass the ciphertext, your keyword, and your ADFGVX-grid to your partner. Then ﬁgure out how to decrypt your partner’s ciphertext using the information they give you.

6. Why will Frequency Analysis not work too well on this cipher?

7. What are some weaknesses to this cipher?

8. Would this cipher be secure if we left oﬀ the rearranging of the columns?