Vignère ciphertext can be difficult to decrypt. It is a combination
of a Caesar shift combined with a keyword. The length of the keyword determines
the number of different encryptions that are applied to the plaintext. For example, if the keyword is 4 chars in length then the plaintext is divided into 4 subtexts and a seperate Caesar shift is applied to each subtext depending on the value of the corresponding letter in the keyword. The
cipher is polyalphabetic which means that a character can be enciphered in
different ways. A frequency analysis of Vignère often shows a fairly balanced
set of frequencies - because the keyword is used to switch between different
Charles Babbage worked out a method to decrypt this cipher.
Look for sequences of letters that appear more than once in the ciphertext.
These are likely to be the same sequences of plaintext letters that have
been enciphered using the same parts of the key. However, a small number
the matching sequences may be purely random.
Find the spacing between the repeated sequences. Try to find the common
factor. This may suggest the length of the keyword.
Create a subsets of characters based on the length of the keyword and do
a separate frequency analysis on each of them. Compare each to a standard
distribution and try to match them up using the distribution of peaks and
than an exact pattern matching. Find out the character shift which should
give the keyword character eg. if shifted 4 places to the left then the letter
This program uses a slightly shortened approach. If you don't enter in a
keyword then it can break down the text into subtexts so that you can see whether
a pattern is present. It will suggest a keyword. It assumes that the most
in the subtext is "e" which is not always the case.
Subdivide the ciphertext into a number of subtexts which are then frequency
Assuming that the most common letter is e then it suggests a possible keyword
and presents a decryption based on this
You can then go back and try out different keywords and see the result
Here is an example ciphertext which uses a 3 letter key. You can probably
work out the keyword from the suggestion - (hint: computer company). Conversely,
you can paste into this box any ciphertext of your own that you want to decipher.