splitting up cipher text into substrings (vigenere cipher)

Question

I'm trying to implement an algorithm which will guess at a likely key length of a keyword of a vigenere cipher.

I'm going about the steps of finding the index of coincidence for each possible key length, but I can't figure out a way to split the cipher text into the substrings.

That is, I'm trying to take a certain cipher text like this

ERTEQSDFPQKCJAORIJARTARTAAFIHGNAPROEOHAGJEOIHJA (this is random text, there's no coded message here)

and split it up into different strings like this:

key length 2: ETQDP... (every second letter starting from position 0)
              RESFQ... (every second letter starting from position 1)

key length 3: EEDQ.... (every third letter starting from position 0)

and so on.

Any ideas?

UPDATE

I've tried implementing my own code now, and here's what I've done:

void findKeyLength(string textToTest)
{
size_t length = textToTest.length();
vector<char> vectorTextChar;
    //keeping key length to half the size of ciphertext; should be reasonable
for (size_t keylength = 1; keylength < length / 2; keylength++)
{
    for (size_t i = keylength; i < keylength ; i++)
    {   
        string subString = "";
        for (size_t k = i; k < length; k+=i)
        {
            vectorTextChar.push_back(textToTest[k]);
        }

        for (vector<char>::iterator it= vectorTextChar.begin(); it!=vectorTextChar.end(); ++it)
        {
            subString += *it;
        }
        cout << subString << endl; //just to see what it looks like
        cout << "Key Length : " << keylength << "IC: " << indexOfCoincidence(subString) << endl;
        vectorTextChar.clear();
    }
}
}

Like I've mentioned below, I'll have output which only reflects the substring that is based on the first characters (i.e. 1, 3, 5, 7, 9 if the keylength is 2, but not 2, 4, 6, 8, 10...)

MrSmith42 · Answer 1 · 2013-08-13T09:47:20.500

Not tested, but you could do something like this:

int len = cipherText.length;
char[] text2inspect = char[(len/keyLength) + 1]
for (int startIndex = 0; keyLength > startIndex; startIndex++){
  int destIndex = 0;
  for (int index = startIndex; len > index; index += keyLength){
    text2inspect[destIndex++] = cipherText[index];
  }
  text2inspect[destIndex] = 0; // String termination

  // add your inspection code here

}

splitting up cipher text into substrings (vigenere cipher)

1 Answers1