English Rot13 Cipher
The English language consists of 26 letters (a-z) which is exactly divisible by 13. This allows for the equal association of the first half of the alphabet with the second half. Because of this fact, the English rot13 cipher is special.
Notice that plaintext e is associated with r and ciphertext e is also associated with r. The association of plaintext to ciphertext is the same both ways, making for an identical encoding and decoding process. This describes what is called a symmetric cipher. Because of this, the rot13 key can be simplified by writing the initial 13 letters in the first row and the next 13 letters in the second row.
Now we can use this simplified rot13 key to encode and decode messages very quickly.
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meet me for lunch in the field
zrrg zr sbe yhapu va gur svryq
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In general, rotation ciphers are not very secure. However, they are good to learn and will, at minimum, delay your adversaries from discovering your immediate plans. By the time they intercept and decode this message, lunch will be over!
Rot13 is commonly used in online forums to hide punchlines, spoilers, and solutions to puzzles.
You will find a rot-n template in the back of this book. Cut out the two pieces and make your own rot-n cipher disk like the one pictured below. Rotate the inner disk until n = 13 appears in the window.
The ciphertext is found on the outer edge and the plaintext is found on the inner edge. Use the disk to verify the following:
plaintext: cipher disk
ciphertext: pvcure qvfx
ciphertext: pvcure qvfx
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Mission 3: Decode the message using the rot13 cipher.
plaintext:
ciphertext: hfr gur sbepr
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Mission 4: Encode the message using the rot13 cipher.
plaintext: resist the dark side
ciphertext:
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