programming-examples/c++/11_Sets_&_Strings/C++ Program to Perform Cryptography Using Transposition Technique.cpp

/*This is a C++ Program to implement transposition technique. In cryptography, a transposition cipher is a method of encryption by which the positions held by units of plaintext (which are commonly characters or groups of characters) are shifted according to a regular system, so that the ciphertext constitutes a permutation of the plaintext. That is, the order of the units is changed (the plaintext is reordered). Mathematically a bijective function is used on the characters’ positions to encrypt and an inverse function to decrypt.*/

#include<stdio.h>
#include<string.h>

void cipher(int i, int c);
int findMin();
void makeArray(int, int);

char arr[22][22], darr[22][22], emessage[111], retmessage[111], key[55];
char temp[55], temp2[55];
int k = 0;

int main()
{
    char *message;
    int i, j, klen, emlen, flag = 0;
    int r, c, index, rows;
    printf("Enter the key\n");
    fflush(stdin);
    gets(key);
    printf("\nEnter message to be ciphered\n");
    fflush(stdin);
    gets(message);
    strcpy(temp, key);
    klen = strlen(key);
    k = 0;
    for (i = 0;; i++)
        {
            if (flag == 1)
                break;
            for (j = 0; key[j] != NULL; j++)
                {
                    if (message[k] == NULL)
                        {
                            flag = 1;
                            arr[i][j] = '-';
                        }
                    else
                        {
                            arr[i][j] = message[k++];
                        }
                }
        }
    r = i;
    c = j;
    for (i = 0; i < r; i++)
        {
            for (j = 0; j < c; j++)
                {
                    printf("%c ", arr[i][j]);
                }
            printf("\n");
        }
    k = 0;
    for (i = 0; i < klen; i++)
        {
            index = findMin();
            cipher(index, r);
        }
    emessage[k] = '\0';
    printf("\nEncrypted message is\n");
    for (i = 0; emessage[i] != NULL; i++)
        printf("%c", emessage[i]);
    printf("\n\n");
    //deciphering
    emlen = strlen(emessage);
    //emlen is length of encrypted message
    strcpy(temp, key);
    rows = emlen / klen;
    //rows is no of row of the array to made from ciphered message
    j = 0;
    for (i = 0, k = 1; emessage[i] != NULL; i++, k++)
        {
            //printf("\nEmlen=%d",emlen);
            temp2[j++] = emessage[i];
            if ((k % rows) == 0)
                {
                    temp2[j] = '\0';
                    index = findMin();
                    makeArray(index, rows);
                    j = 0;
                }
        }
    printf("\nArray Retrieved is\n");
    k = 0;
    for (i = 0; i < r; i++)
        {
            for (j = 0; j < c; j++)
                {
                    printf("%c ", darr[i][j]);
                    //retrieving message
                    retmessage[k++] = darr[i][j];
                }
            printf("\n");
        }
    retmessage[k] = '\0';
    printf("\nMessage retrieved is\n");
    for (i = 0; retmessage[i] != NULL; i++)
        printf("%c", retmessage[i]);
    return (0);
}

void cipher(int i, int r)
{
    int j;
    for (j = 0; j < r; j++)
        {
            {
                emessage[k++] = arr[j][i];
            }
        }
    // emessage[k]='\0';
}

void makeArray(int col, int row)
{
    int i, j;
    for (i = 0; i < row; i++)
        {
            darr[i][col] = temp2[i];
        }
}

int findMin()
{
    int i, j, min, index;
    min = temp[0];
    index = 0;
    for (j = 0; temp[j] != NULL; j++)
        {
            if (temp[j] < min)
                {
                    min = temp[j];
                    index = j;
                }
        }
    temp[index] = 123;
    return (index);
}

/*

Enter the key
hello

Enter the message to be ciphered
how are you

h o w   a
r e   y o
u - - - -

Encrypted message is
oe-hruw - y-ao-

Array Retrieved is
h o w   a
r e   y o
u - - - -

Message retrieved is
how are you----