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8.5.
VIGENÈRE’S AUTO KEY CIPHER 151
ciphertext X Z C Y Y I N T C Q A Q X
key M L O O K
plaintext l o o k
Eventually this produces look out a comet.
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The Auto Key Cipher defeats all the types of frequency analysis we’ve seen
so far, making it the strongest cipher system yet. Its main disadvantage is one
we haven’t seen before: even simple copying misteaks can cause havoc. Since it
is easy to make a small error, and any mistake propagates through the rest of the
cipher, even a single error can make an Auto Key enciphered text unreadable.
Example: Unfortunately, DSAUB ARIVI FF contains a small error. If the key
is D, can you find the error 14 .
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How might we go about breaking an Auto Key cipher Consider the following
message, where we assume we know the ciphertext, and have somehow
guessed one plaintext letter and know that the priming key was a single letter.
· · · · · · · · s · · · · · · · · · · ·
· · · · · · · · · · · · · · · · · · · ·
Y B C M G W R U V S N Q I T F N W R U V
By working forwards and backwards we fill in some:
· · · · · · · d s · · · · · · · · · · ·
· · · · · · · · d s · · · · · · · · · ·
Y B C M G W R U V S N Q I T F N W R U V
and then more
and before long 15
· · · · · · r d s a · · · · · · · · · ·
· · · · · · · r d s a · · · · · · · · ·
Y B C M G W R U V S N Q I T F N W R U V
b a c k w a r d s a n d f o r w a r d s
x b a c k w a r d s a n d f o r w a r d
Y B C M G W R U V S N Q I T F N W R U V
If the priming key is longer, then we need to know as many consecutive letters of
plaintext as there were priming key letters, but the same forwards and backwards
stepping will then decrypt the cipher.
14 a simple error is the message. The R in the ciphertext should be a P
15 This should look quite familiar to those who completed Exercise 7.19.