Program in C++ or C that can encrypt and decrypt using a 2 x 2 hill cipher

Question

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C++

#include<stdio.h>
#include<conio.h>
#include<string.h>
void main()
{
char aa[26]="abcdefghijklmnopqrstuvwxyz";
char pt[10];
int m,d,q=0,i,j,k[2][2],p[4],pp[4],t[5];
int k1[2][2],k2[2][2],det;
//clrscr();
printf("enter the plaintext:" );
scanf("%s",pt);
m=strlen(pt);
printf("enter the numbers:");
for(i=0;i<2;i++)
{
for(j=0;j<2;j++)
{
scanf("%d",&k[i][j]);
}
}

for(i=0;i<m;i++)
{
for(j=i;j<26;j++)

{
if(pt[i]==aa[j])

{
t[q]=j%26;
++q;
}
}
}
p[0]=(k[0][0]*t[0])+(k[0][1]*t[1]);
p[1]=(k[1][0]*t[0])+(k[1][1]*t[1]);
p[2]=(k[0][0]*t[2])+(k[0][1]*t[3]);
p[3]=(k[1][0]*t[2])+(k[1][1]*t[3]);
k1[0][0]=k[1][1];
k1[0][1]=-(k[0][1]);
k1[1][0]=-(k[1][0]);
k1[1][1]=k[0][0];

det=(k1[0][0]*k1[1][1])-(k1[0][1]*k1[1][0]);
for(i=0;i<26;i++)
{
if((det*i)%26==1)
{
d=i;
}
}
for(i=0;i<2;i++)
{
for(j=0;j<2;j++)
{
k2[i][j]=(d*k1[i][j])%26;


}
}
for(i=0;i<2;i++)
{
for(j=0;j<2;j++)
{
if(k2[i][j]<0)
k2[i][j]+=26;
}
}
pp[0]=((k2[0][0]*p[0])+(k2[0][1]*p[1]))%26;
pp[1]=((k2[1][0]*p[0])+(k2[1][1]*p[1]))%26;
pp[2]=((k2[0][0]*p[2])+(k2[0][1]*p[3]))%26;
pp[3]=((k2[1][0]*p[2])+(k2[1][1]*p[3]))%26;
for(i=0;i<m;i++)
{
printf("\nThe decrypted plain text :%c",aa[pp[i]]);
}
getch();
}

What I have tried:

problem in coding for inverse the key matrix used for encryption and decryption in Hill cipher

Posted 30-Sep-17 9:54am

Member 13438777

Updated 1-Oct-17 11:37am

CPallini

v2

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3 solutions

Add a Solution

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

OriginalGriff · Answer 1 · 2017-09-30T10:53:00

Compiling does not mean your code is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:
<pre lang="text">Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16

Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:

C#

private int Double(int value)
   {
   return value * value;
   }

Once you have an idea what might be going wrong, start using teh debugger to find out why. Put a breakpoint on your line:

C#

for(i=0;i<m;i++)

and run your app. Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?

This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!

Yes, I could probably tell you what "the problem" is - but it's not difficult to do this yourself, and you will learn something really worthwhile at the same time!

Patrice T · Answer 2 · 2017-09-30T11:54:00

Quote:
problem in coding for inverse the key matrix used for encryption and decryption in Hill cipher

This tell us absolutely nothing because every question here is about something that don't work.
A description of how things go wrong would be a good start.

Learn to indent properly your code, it show its structure and it helps reading and understanding.

C++

#include<stdio.h>
#include<conio.h>
#include<string.h>
void main()
{
    char aa[26]="abcdefghijklmnopqrstuvwxyz";
    char pt[10];
    int m,d,q=0,i,j,k[2][2],p[4],pp[4],t[5];
    int k1[2][2],k2[2][2],det;
    //clrscr();
    printf("enter the plaintext:" );
    scanf("%s",pt);
    m=strlen(pt);
    printf("enter the numbers:");
    for(i=0;i<2;i++)
    {
        for(j=0;j<2;j++)
        {
            scanf("%d",&k[i][j]);
        }
    }

    for(i=0;i<m;i++)
    {
        for(j=i;j<26;j++)
        {
            if(pt[i]==aa[j])
            {
                t[q]=j%26;
                ++q;
            }
        }
    }
    p[0]=(k[0][0]*t[0])+(k[0][1]*t[1]);
    p[1]=(k[1][0]*t[0])+(k[1][1]*t[1]);
    p[2]=(k[0][0]*t[2])+(k[0][1]*t[3]);
    p[3]=(k[1][0]*t[2])+(k[1][1]*t[3]);
    k1[0][0]=k[1][1];
    k1[0][1]=-(k[0][1]);
    k1[1][0]=-(k[1][0]);
    k1[1][1]=k[0][0];

    det=(k1[0][0]*k1[1][1])-(k1[0][1]*k1[1][0]);
    for(i=0;i<26;i++)
    {
        if((det*i)%26==1)
        {
            d=i;
        }
    }
    for(i=0;i<2;i++)
    {
        for(j=0;j<2;j++)
        {
            k2[i][j]=(d*k1[i][j])%26;
        }
    }
    for(i=0;i<2;i++)
    {
        for(j=0;j<2;j++)
        {
            if(k2[i][j]<0)
                k2[i][j]+=26;
        }
    }
    pp[0]=((k2[0][0]*p[0])+(k2[0][1]*p[1]))%26;
    pp[1]=((k2[1][0]*p[0])+(k2[1][1]*p[1]))%26;
    pp[2]=((k2[0][0]*p[2])+(k2[0][1]*p[3]))%26;
    pp[3]=((k2[1][0]*p[2])+(k2[1][1]*p[3]))%26;
    for(i=0;i<m;i++)
    {
        printf("\nThe decrypted plain text :%c",aa[pp[i]]);
    }
    getch();
}

Professional programmer's editors have this feature and others such as parenthesis matching and syntax highlighting.
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Wheb you don't understand what go wrong in your code, the answer is debugger.
There is a tool that allow you to see what your code is doing, its name is debugger. It is also a great learning tool because it show you reality and you can see which expectation match reality.
When you don't understand what your code is doing or why it does what it does, the answer is debugger.
Use the debugger to see what your code is doing. Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.

Debugger - Wikipedia, the free encyclopedia[^]

Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[^]
Basic Debugging with Visual Studio 2010 - YouTube[^]
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't find bugs, it just help you to. When the code don't do what is expected, you are close to a bug.

CPallini · Answer 3 · 2017-10-01T11:37:00

Solution 3

I would have implemented it using C++.
Well, actually I did, you may find it here: [^].

Posted 1-Oct-17 11:37am

CPallini

Program in C++ or C that can encrypt and decrypt using a 2 x 2 hill cipher

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Solution 2

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Program in C++ or C that can encrypt and decrypt using a 2 x 2 hill cipher

3 solutions

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Solution 3

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