Coincidence Counting

by fortenforge, Oct 11, 2009, 1:41 AM

Coincidence Counting was an algorithm conceived by William Friedman. In the most simplest sense, it involves putting two pieces of text side by side and counting the number of letters that are identical and appear in the same position in the text. You then take this number and then divide it by the total number of letters in the text or divide it by the expected value for the total to get the Index of Coincidence. Most often in cryptography we find the Index of Coincidence with relation to a ciphertext compared with itself. I'll spend a lot of time on the Index of Coincidence because it is not a simple idea to grasp.
Index of Coincidence

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What is the Index of Coincidence used for?

by dragon96, Oct 11, 2009, 5:05 AM

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It can be used for a variety of things. You can find out if two pieces of text were written in the same language, you can easily decrypt vigenere and other polyalphabetic ciphers, it can be used to easily find out what method was used to encrypt a certain piece of ciphertext.

by fortenforge, Oct 11, 2009, 5:27 PM

A guide to the science of secrecy

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fortenforge
Archives
- February 2010
A major change in Cryptography
The Cipher Contest
+ January 2010
ADFGVX Cipher
ADFGX Cipher
Double Playfair
Random Practice
A Note about the Previous Post's decryption
Finally decrypting it
+ December 2009
Decrypting the Ciphertext from the previous post
Playfair Ciphertext (For Later)
Decrypting Playfair
+ November 2009
Recognizing that a cipher is a Playfair cipher
Playfair cipher
After the Vigenere cipher was broken
Using the IC to break the Vigenere cipher
Nice Quote
The difference between a code and a cipher
Linguistics Math Jam
Finding the IC without doing the counting every single time
Finding the IC of a sample text
+ October 2009
Index of Coincidence of a ciphertext with respect to itself.
Index of Coincidence with 2 pieces of text
Coincidence Counting
A Random Cipher I Created
+ September 2009
The Kasiski Examination
Finding the Key Length
Excercise for you.
Cracking the Vigenere Cipher knowing the keyword length.
The Keyword Length
Unexplained Phenomenon
+ August 2009
Why frequency analysis does not work
Mathematics of the Vigenere Cipher
Example of a vigenere cipher encryption.
Polyalphabetic encryption
+ July 2009
Kerckhoffs' principle
Homophonic cipher
Expanding on Nomenclators
Nomenclators
Good stuff to know
Example of Decryption of a Substitution Cipher.
Frequency Analysis
Using symbols other than letters.
Number of Possible Keys for Substitution cipher.
Monoalphabetic Substitution.
+ June 2009
Exercises
Mathematics of the Caesar Cipher
Caesar Cipher
Intro
Shouts
Submit

  • Good website!

    by bluegoose101, Aug 5, 2021, 6:28 PM

  • uh-huh, a great place here

    by fenchelfen, Sep 1, 2019, 11:30 AM

  • uh, yeah he is o_O

    by SonyWii, Oct 8, 2010, 2:11 PM

  • dude i think you're my roommate from camp :O

    by themorninglighttt, Aug 29, 2010, 10:06 PM

  • what i'm still not a contrib D:

    by SonyWii, Aug 6, 2010, 2:20 PM

  • I see what you did there

    by Jongy, Aug 1, 2010, 11:52 PM

  • omg, apparently you like cryptography; and apparently I'm not a contribb D:

    by SonyWii, Jul 26, 2010, 9:48 PM

  • Thank You

    by fortenforge, Jan 17, 2010, 6:35 PM

  • Wow this is a really cool blog

    by alkjash, Jan 16, 2010, 7:04 PM

  • Hi :)

    by fortenforge, Jan 7, 2010, 12:12 AM

  • Hi :)

    by Richard_Min, Jan 5, 2010, 9:29 PM

  • Hi :) :)

    by fortenforge, Jan 3, 2010, 10:14 PM

  • HELLO FORTENFORGE I AM THE PERSON SITTING NEXT TO YOU IN IDEAMATH

    by ButteredButNotEaten, Dec 24, 2009, 4:19 AM

  • @dragon96 Not if you celebrate Christmas with neon lights
    @batteredbutnotdefeated Sure, You are now a contributer

    by fortenforge, Dec 20, 2009, 4:39 AM

  • I too share a love for cryptography and cryptanalysis, may I be a contrib?

    by batteredbutnotdefeated, Dec 20, 2009, 2:38 AM

  • The green is too bright for Christmas. :P

    by dragon96, Dec 20, 2009, 2:12 AM

  • I thought I'd change the colors for the Holidays :lol:

    by fortenforge, Dec 13, 2009, 10:53 PM

  • hi, some "simple" cryptography here: http://www.artofproblemsolving.com/Forum/weblog_entry.php?t=317795

    by phiReKaLk6781, Dec 12, 2009, 3:46 AM

  • Yeah, that is binary, for modern cryptography, most text is converted to binary first and then algorithm's for encryption are preformed on the binary rather than the English letters. The text is converted using the ASCII table or UNICODE.

    by fortenforge, Oct 13, 2009, 10:33 PM

  • Whoa, I love your background! Is that binary?

    by pianogirl, Oct 13, 2009, 8:34 PM

  • Sure, I'll add you as a contributer...

    by fortenforge, Oct 2, 2009, 4:44 AM

  • May I make a post on one cipher I made up? (It's a good code for science people! *hint hint*)

    by dragon96, Oct 2, 2009, 4:04 AM

  • Nice blog, this is interesting... :lol:

    and guess who i am :ninja:

    by Yoshi, Sep 21, 2009, 4:02 AM

  • Thanks :lol:

    by fortenforge, Sep 17, 2009, 1:33 AM

  • Very interesting blog. Nice!

    by AIME15, Sep 16, 2009, 5:21 PM

  • When you mean 'write' do you mean like programming? Much of cryptography has to do with programming and most modern cryptographers are excellent programmers because modern complex ciphers are difficult to implement by hand.

    See if you can write a program for the substitution cipher. The user should be able to enter the key and the message. I know it is possible to do it in pretty much any language because I was able to do it in c.

    by fortenforge, Aug 7, 2009, 8:17 PM

  • Hello. I don't know much about advanced cryptography but I did write a Caeser Chipher encrypter and decrypter!

    by Poincare, Jul 31, 2009, 8:55 PM

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Tags
vigenere cipher
cipher
Playfair cipher
Cryptography in general
frequency analysis
Index of Coincidence
Monoalphabetic Substitution
decryption
math
Caesar
Nomenclators
ADFGVX
cryptogram
Cryptograms
Enigma
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