✘ - - - DECIPHER is a book compilation that tackles different, new and old codes/ciphers. In addition, this book begins with the definition of cryptography along with it's different terminology for begginers, and better understanding.
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‣ Running key cipher is a type of polyalphabetic substitution and works just like Vigenère cipher. However, their major difference is the keyword used. Vigenère cipher repeats its keyword to encrypt the plaintext while running key cipher uses a very long keyword, with which most of the time is an excerpt from a book or a quote from a famous novel.
The cipher uses the same 26x26 square - tabula recta - with the Vigenère cipher. The keyword, on the other hand, coming from a text from a book must be agreed by both the encoder and decoder ahead of time. The page of the book, the number of the row and the first letter of the starting keyword must be determined.
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ENCRYPTION: ↳ The plaintext will be "Please cook our breakfast." Let's have the page 63, line 1 as the running key: "errors can occur in several places. A label has ..." The plaintext is written first and the running key is written under it.
PLAINTEXT: ↳ P L E A S E C O O K O U R B R E A K F A S T
RUNNING KEY: ↳ E R R O R S C A N O C C U R I N S E V E R A
To encrypt, you need to look at the meeting point of the letters P and E in the Vigenère cipher (see the attched photo above) - letter T, which serves as the first letter of the ciphertext.
The second letter of the ciphertext would be the intersection of plaintext L and running key R which is C. Continue the process until the plaintext is fully encrypted.
PLAINTEXT: ↳ P L E A S E C O O K O U R B R E A K F A S T
RUNNING KEY: ↳ E R R O R S C A N O C C U R I N S E V E R A
CIPHERTEXT: ↳ T C V O J W E O B Y Q W L S Z R S O A E J T
DECRYPTION: ↳ The process of decrypting Vigenère is the same as Running Key's. Since the decoder knows the running key used, the decoder must only look at the intersection of the ciphertext and the running key to come up with the plaintext. Let's take our example above.
CIPHERTEXT: ↳ T C V O J W E O B Y Q W L S Z R S O A E J T
RUNNING KEY: ↳ E R R O R S C A N O C C U R I N S E V E R A
Using the Tabula Recta, you will see that the intersection of ciphertext T and running key E is P, which is the first letter of the plaintext. The letter which ciphertext C and running key R is L which becomes the second letter of the plaintext.
The process continues until you have finally finished decoding the ciphertext.
CIPHERTEXT: ↳ T C V O J W E O B Y Q W L S Z R S O A E J T
RUNNING KEY: ↳ E R R O R S C A N O C C U R I N S E V E R A
PLAINTEXT: ↳ P L E A S E C O O K O U R B R E A K F A S T
