error in assigning a const character to an unsigned char array in C++

Question

#include <iostream>
#include <fstream>
#include <cstring>
using namespace std;

typedef unsigned long int WORD;  /* Should be 32-bit = 4 bytes        */
#define w         32             /* word size in bits                 */
#define r         12             /* number of rounds                  */
#define b         16             /* number of bytes in key            */
#define c          4             /* number words in key               */
                                 /* c = max(1,ceil(8*b/w))            */
#define t         26             /* size of table S = 2*(r+1) words   */
WORD S [t],L[c];                        /* expanded key table         */
WORD P = 0xb7e15163, Q = 0x9e3779b9;    /* magic constants            */
/* Rotation operators. x must be unsigned, to get logical right shift*/
#define ROTL(x,y) (((x)<<(y&(w-1))) | ((x)>>(w-(y&(w-1)))))
#define ROTR(x,y) (((x)>>(y&(w-1))) | ((x)<<(w-(y&(w-1)))))    

void RC5_DECRYPT(WORD *ct, WORD *pt) /* 2 WORD input ct/output pt     */
{
    WORD i, B = ct[1], A = ct[0];
    for (i = r; i > 0; i--)
    {
        B = ROTR(B - S[2 * i + 1], A)^A;
        A = ROTR(A - S[2 * i], B)^B;
    }
    pt[1] = B - S[1]; pt[0] = A - S[0];
}

void RC5_SETUP(unsigned char *K) /* secret input key K 0...b-1]       */
{
    WORD i, j, k, u = w/8, A, B, L[c];

    /* Initialize L, then S, then mix key into S */
    for (i = b - 1, L[c - 1] = 0; i != -1; i--) 
        L[i/u] = (L[i/u] << 8) + K[i];

    for (S[0] = P, i = 1; i < t; i++) 
        S[i] = S[i - 1] + Q;

    for (A=B=i=j=k=0; k<3*t; k++,i=(i+1)%t,j=(j+1)%c)      /* 3*t > 3*c */
    {
        A = S[i] = ROTL(S [i]+(A+B),3);
        B = L[j] = ROTL(L[j]+(A+B),(A+B));
    }
}

void printword(WORD A)
{
    WORD k;
    for (k=0 ;k<w; k+=8) 
        printf("%02.2lX",(A>>k)&0xFF);
}

int main()
{ 
    WORD i, j, k, pt [2], pt2 [2], ct [2] = {0,0};
    unsigned char key[b];
    ofstream out("cpt.txt");
    ifstream in("key.txt");

    if (!in)
    { 
        cout << "Cannot open file.\n"; 
        return 1; 
    }  

    if (!out)
    { 
        cout << "Cannot open file.\n"; 
        return 1; 
    }  

    key="111111000001111";

    RC5_SETUP(key);
    ct[0]=2185970173;
    ct[1]=3384368406;

    for (i=1; i<2; i++)
    {
         RC5_DECRYPT(ct,pt2);
         printf("\n   plaintext "); 
         printword(pt[0]); 
         printword(pt[1]);
    }

    return 0;
}

When I compile this code i get two warnings and also an error saying that I cant assign a char value to my character array. Why is that?

Answer 1

Change:

key="111111000001111";

to

strncpy((char *)key, "111111000001111", sizeof(key));

Answer 2

Because key = "111111000001111"; is not valid in any way. You cannot assign values to an array this way in either C or C++. Try using strcpy or memcpy with the appropriate casts.

Answer 3

You can't set the value of the key array using a statement like:

key = "111111000001111";

You'll need to use strcpy(), or memcpy(), or something like (the non-standard, but arguably less error prone) strlcpy().

However, you can initialize it like so:

unsigned char key[b] = "111111000001111";

But note that as long as the string is one character less than the dimension you'll get a null terminated string, but if the number of characters is equal to the dimension you'll get an array of characters that is not null terminated, without an error (though you might get a warning). In C++ that scenario should generate an error. See http://stackoverflow.com/questions/1675477/why-doesnt-the-compiler-detect-out-of-bounds-in-string-constant-initialization/1676325#1676325 for details.