Aaveg Mittal
                        [email protected]
                        Homework4

. Design . There are total 7 files 6 source file(main.cc, crypt.cc, keygen.cc, invkeygen.cc solve.cc histo.cc) and 1 header file(main.h).

    main.cc: 
            . mainly contains the general code, such as userinput parsing & then calling the appropriote function. 

    function.cc: 
            .unsigned char *RC4_Init() 
        mainly contains the code for key generation.
                this function is called when the 1st commandline argument to the function is: keygen.
                its main responsibily is to take period as argument & then produce the final key using the rc4 & md5.

        .Crypt(char *keyfile, char *inpF)
              mainly contains the code for file encryption.
               It is called when the 1st commandline argument to the function is: crypt.
               This function encrypt all lowercase letters using the key file.

        . void Histo(int period, int which, char *inpFile)
               mainly contains the code for producing frequency of letters in cipher text that can be used for cryptanalysis.
               this function takes file and print the frequency of letters at index, index+T, index_2T..and so on

    . InvKeygen(char *inpF)
        takes the encrytion key file & produce the decyrption key.

        .void Solve(int max_t, char *inpF)
                    prints information that can be used to analyze cipertext to perform cryptanalysis.
                    it implemets three method kasiski, average index of coincidence & autocorralation method to produce all the data.

2. Testing

   1. Please follow the instruction mentioned on homework 2 page.

3. Cryptanalysis Report

   1. mystery1.ct T=7 key is in mystery1.invkeys file.

   2. mystery2.ct T=19 only able to find T, not able to find the correct key to decrypt the file.

      method use of cryptanalysis: started with searching of single letter words, then letter with appostrophs, then used frequenct of letters to guess some letters , also used diagram & tiagram statisicks to finally develop the key for mystery1.ct file.