# Re: How do I apply the index of coincidence?

Hmmm. Ok. So what is the first step in determining the plaintext from
the ciphertext "OPKWWECIYOPKWIRG"? If I understand the process
correctly, the key length must be determined.

On slide 35 it looks like the IC must be calculated to find likely
values of key length. The IC I get for "OPKWWECIYOPKWIRG" is
0.0583333333333. From slide 35 it appears that the likely key length is
2. Am I on the right track?

If I have this right so far, the ciphertext must be split into two
alphabets.
Alphabet 0:OKWCYPWR
Alphabet 1:PWEIOKIG

Now I'm stuck on determining the key. Any suggestions on how to
determine the key? Thanks!

.

## Relevant Pages

• Possible trapdoor in DES and AES
... DES-like block ciphers are equivalent to mixed alphabet monoalphabetic ... One would simply analyse the frequency of the ciphertext, ... plaintext message and determine the alphabet on that basis. ... The thing about a brute force attack on DES type ciphers is that it only ...
(sci.crypt)
• Re: A secure hand cipher?
... below that to spell out the ciphertext alphabet. ... 13th and 14th letters to divide the alphabet in half. ... ZMGQPOLKJIHFD BANYXWVUSECRT ...
(sci.crypt)
• Re: Long Vigenere Keys
... > Whenever I look at a crypto book, they always describe the Vigenere method ... > ciphertext. ... straight alphabet is a weakness that overwhelms second-order effects ...
(sci.crypt)
• Re: Making a Science of Invincible Ignorance
... obvious that any sized alphabet will work. ... principle* of the OTP as a platform for other alpahabets. ... some suitable form of combining plaintext and key to yield ciphertext, ...
(sci.crypt)
• Re: New unbreakable encryption method
... The basic idea is that if you have one ciphertext and your system ... is the permutation that swaps x and y and leaves ... The cipher state contains of ten permutations p_0,...,p_9 on some alphabet ... encryption of a large number of identical letters or a random message. ...
(sci.math.research)