WARNING!:UNEDITED!
The Running Key cipher has the same internal workings as the Vigenere cipher. The difference lies in how the key is chosen; the Vigenere cipher uses a short key that repeats, whereas the running key cipher uses a long key such as an excerpt from a book. This means the key does not repeat, making cryptanalysis more difficult. The cipher can still be broken though, as there are statistical patterns in both the key and the plaintext which can be exploited.
If the key for the running key cipher comes from a statistically random source, then it becomes a 'one time pad' cipher. One time pads are theoretically unbreakable ciphers, because every possible decryption is equally likely.
The Algorithm
The 'key' for a running key cipher is a long piece of text, e.g. an excerpt from a book. The running key cipher uses the following tableau (the 'tabula recta') to encipher the plaintext:
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
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A | A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
B | B C D E F G H I J K L M N O P Q R S T U V W X Y Z A
C | C D E F G H I J K L M N O P Q R S T U V W X Y Z A B
D | D E F G H I J K L M N O P Q R S T U V W X Y Z A B C
E | E F G H I J K L M N O P Q R S T U V W X Y Z A B C D
F | F G H I J K L M N O P Q R S T U V W X Y Z A B C D E
G | G H I J K L M N O P Q R S T U V W X Y Z A B C D E F
H | H I J K L M N O P Q R S T U V W X Y Z A B C D E F G
I | I J K L M N O P Q R S T U V W X Y Z A B C D E F G H
J | J K L M N O P Q R S T U V W X Y Z A B C D E F G H I
K | K L M N O P Q R S T U V W X Y Z A B C D E F G H I J
L | L M N O P Q R S T U V W X Y Z A B C D E F G H I J K
M | M N O P Q R S T U V W X Y Z A B C D E F G H I J K L
N | N O P Q R S T U V W X Y Z A B C D E F G H I J K L M
O | O P Q R S T U V W X Y Z A B C D E F G H I J K L M N
P | P Q R S T U V W X Y Z A B C D E F G H I J K L M N O
Q | Q R S T U V W X Y Z A B C D E F G H I J K L M N O P
R | R S T U V W X Y Z A B C D E F G H I J K L M N O P Q
S | S T U V W X Y Z A B C D E F G H I J K L M N O P Q R
T | T U V W X Y Z A B C D E F G H I J K L M N O P Q R S
U | U V W X Y Z A B C D E F G H I J K L M N O P Q R S T
V | V W X Y Z A B C D E F G H I J K L M N O P Q R S T U
W | W X Y Z A B C D E F G H I J K L M N O P Q R S T U V
X | X Y Z A B C D E F G H I J K L M N O P Q R S T U V W
Y | Y Z A B C D E F G H I J K L M N O P Q R S T U V W X
Z | Z A B C D E F G H I J K L M N O P Q R S T U V W X Y
To encipher a message, write the key stream above the plaintext, in this case our key is from a Terry Pratchett book: 'How does the duck know that? said Victor'. If we needed to encipher a longer plaintext, we could just continue reading from the book.
HOWDOESTHEDUCKKNOWTHATSAIDVI
DEFENDTHEEASTWALLOFTHECASTLE
Now we take the letter we will be encoding, 'D', and find it on the first column on the tableau. Then, we move along the 'D' row of the tableau until we come to the column with the 'H' at the top (The 'H' is the keyword letter for the first 'D'), the intersection is our ciphertext character, 'K'.
So, the ciphertext for the above plaintext is:
HOWDOESTHEDUCKKNOWTHATSAIDVI
DEFENDTHEEASTWALLOFTHECASTLE
KSBHBHLALIDMVGKYZKYAHXUAAWGM
PINKY_DUST
