Hacking Secret Ciphers with Python

Chapter 1 – Making Paper Cryptography Tools 9

Figure 1- 10. A message encrypted with the cipher wheel.

Each letter on the outer wheel will always be encrypted to the same letter on the inner wheel. To
save time, after you look up the first T in “The secret...” and see that it encrypts to B, you can
replace every T in the message with B. This way you only need to look up a letter once.

How to Decrypt with the Cipher Wheel

To decrypt a ciphertext, go from the inner circle to the outer circle. Let’s say you receive this
ciphertext from a friend, “Iwt ctl ephhldgs xh Hldgsuxhw.” You and everyone else won’t be able
to decrypt it unless you know the key (or unless you are a clever hacker). But your friend has
decided to use the key 15 for each message she sends you.

Line up the letter A on the outer circle (the one with the dot below it) over the letter on the inner
circle that has the number 15 (which is the letter P). The first letter in the secret message is I, so
we find I on the inner circle and look at the letter next to it on the outer circle, which is T. The W
in the ciphertext will decrypt to the letter H. One by one, we can decrypt each letter in the
ciphertext back to the plaintext, “The new password is Swordfish.”

I W T C T L E P H H L D G S

↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓

T H E N E W P A S S W O R D

X H H L D G S U X H W.

↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓

I S S W O R D F I S H.

If we use an incorrect key like 16 instead of the correct key 15, the decrypted message is “Sgd
mdv ozrrvnqc hr Rvnqcehrg.” This plaintext doesn’t look plain at all. Unless the correct key is
used, the decrypted message will never be understandable English.