CODEWITHSUNDEEP
ccryptography
Lorenzo Toscano
Lorenzo Toscano

Reputation: 55

Coding Enigma algorithm, can encrypt but not decrypt

I have to code the enigma algorithm used by Nazi-Germany to encrypt messages sent over radio, you can read how it works right here

As of right now i am only coding the rotors, i didn't code the Plugboard or the rotor initial position yet, since i seem to have problems with this already.

This is what i wrote:

    for (int i=0; i<strlen(sentence);i++){

        sentence[i]=toupper(sentence[i]);

        temp=rs1[0];
        for(int s=0;s<26-1;s++)
        {
            rs1[s]=rs1[s+1];
        }
        rs1[26-1]=temp;
        j++;

        if (j==26){

            j=0;
            temp=rs2[0];
            for(int s=0;s<26-1;s++)
            {
                rs2[s]=rs2[s+1];
            }
            rs2[26-1]=temp;
            z++;

        }

        if (z==26){

            z=0;
            temp=rs2[0];
            for(int s=0;s<26-1;s++)
            {
                rs2[s]=rs2[s+1];
            }
            rs2[26-1]=temp;

        }

        q=(sentence[i]-65);

        //this cant decrypt

        sentence[i]=encrypt(sentence[i],rs1);
        //printf("%s\n",sentence);
        sentence[i]=encrypt(sentence[i],rs2);
        //printf("%s\n",sentence);
        sentence[i]=encrypt(sentence[i],rs3);
        //printf("%s\n",sentence);
        sentence[i]=reflect(sentence[i]);
        //printf("%s\n",sentence);
        sentence[i]=encrypt(sentence[i],rs3);
        //printf("%s\n",sentence);
        sentence[i]=encrypt(sentence[i],rs2);
        //printf("%s\n",sentence);
        sentence[i]=encrypt(sentence[i],rs1);
        //printf("%s\n",sentence);

    }

the functions are:

char encrypt(char c, char r[]){

int num=(int)c-65;
char newc;

newc=r[num];

return newc;
}

char relfect(char c){

char reflector[27]={'A','Q','W','S','E','D','R','F','T','G','Y','H','U','J','O','K','I','L','P','Z','V','C','X','B','N','M'};
int num=(int)c-65;
char newc;

newc=reflector[num];

return newc;
}

When I compile and execute my code I manage to encrypt the word i input, but when I reset the program and insert the encrypted word it doesnt decrypt it, which seems odd.

EXAMPLE: If I input Hello I get QOBAJ but in I restart the program and enter QOBAJ I get MWCJC instead of Hello and i can't figure out why.

The rotors i use are:

1- EKMFLGDQVZNTOWYHXUSPAIBRCJ

2- AJDKSIRUXBLHWTMCQGZNPYFVOE

3- BDFHJLCPRTXVZNYEIWGAKMUSQO

Thank you for helping me.

Lorenzo.

Upvotes: 3

Views: 241

Answers (1)

Jim Rhodes
Jim Rhodes

Reputation: 5095

First of all, I don't see where you initialize the variables j or z and you never use the variable q after assigning a value to it. Assuming that you do initialize everything, that you don't need the variable q and that your code is otherwise correct then the problem is here:

if (z==26){
    z=0;
    temp=rs2[0];
    for(int s=0;s<26-1;s++)
    {
        rs2[s]=rs2[s+1];
    }
    rs2[26-1]=temp;
}

In this block of code you should be working with rs3 not rs2. It looks like you did a copy and paste but did not change all of the names.

EDIT:

You must reverse the lookup mechanism for decryption and you do not need a reflect function. Replace

sentence[i]=reflect(sentence[i]);

with:

sentence[i]=encrypt(sentence[i],reflector);

The decrypt function will look like this:

char decrypt(char c, char r[])
{
    for ( int num = 0; num < 26; ++num )
    {
        if ( r[num] == c )
        {
            return((char)(65 + num));
        }
    }
    return(c); // Should never get here
}

Then to perform decryption, call decrypt instead of encrypt.

Upvotes: 3

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