Thread: using fgets for getting multiple strings (one time pad encryption code)

I am new to C programming. Please i need your help with the following: after compiling & running the code i wrote(one time pad encryption) on devc++ it doesn't get the two strings:it takes one string , then it shows the program is not responding. I was asked to use the fgets function, not scanf or gets, but even so i tried the gets function. It was returning answers as NULL.

Code:

#include<stdio.h> 
#include<stdlib.h> 
#include<string.h> 
#define clear_buffer while(getchar()!='\n'); 
char encrypt(char texte[100],char cle[100]){ 
char encrypted; 
int i=0; 
while(texte[i]!='\0' && cle[i]!='\0'){ 
    encrypted=(texte[i]+cle[i])%26; 
    i++; 
return encrypted;   
} 
} 
char decrypt(char encrypted[100],char cle[100]){ 
char decrypted; 
int i=0; 
while(encrypted[i]!='\0' && cle[i]!='\0'){ 
    decrypted=(encrypted[i]-cle[i])%26; 
    i++; 
return decrypted; 
} 
} 
int main() 
{ 
char reponse,texte[100],cle[100],encrypted[100]; 
int i=0; 
do{ 
printf("Voulez vous crypter ou decrypter un texte?(Ecrire C pour crypter et D pour decrypter)\n"); 
scanf("%c",&reponse); 
}while (reponse!='C'&& reponse!='D'&& reponse!='c'&& reponse!='d');//controle pour obliger l'utilisateur à donner une reponse valide 
if(reponse=='C'||reponse=='c'){ 
printf("Donner un texte a crypter\n"); 
fgets(texte,100,stdin); 
while(texte[i]!=0) 
i++; 
if (i>0 && texte[i-1]!='\n') 
clear_buffer; 
printf("Donner une cle de meme taille\n"); 
fgets(cle,100,stdin); 
i=0; 
while(cle[i]!=0) 
i++; 
if (i>0 && cle[i-1]!='\n') 
clear_buffer; 
printf("Le texte crypte est:%s\n",encrypt(texte,cle)); 

}else{ 
printf("Donner un texte (deja crypte) à decrypter\n"); 
fgets(encrypted,100,stdin); 
    i=0; 
while(encrypted[i]!=0) 
i++; 
if (i>0 && encrypted[i-1]!='\n') 
clear_buffer; 
printf("Donner la cle (deja utilisee pour crypter\n"); 
fgets(cle,100,stdin); 
    i=0; 
while(cle[i]!=0) 
i++; 
if (i>0 && cle[i-1]!='\n') 
clear_buffer; 
printf("Le texte decrypte est:%s\n",decrypt(encrypted,cle)); 
} 
system("pause"); 
return 0; 
}

Originally Posted by whiteflags

Just looking things over, your program has the problems described on the following pages:

• Question 12.18b
• Question 12.18a

Though the first problem may not show up until you try to fool the program by not entering C or D when it asks you to. The page says you should "either use scanf to read everything or nothing," which I actually agree on. One preferred suggestion would be this:

Code:

do{ 
   printf("Voulez vous crypter ou decrypter un texte?(Ecrire C pour crypter et D pour decrypter)\n"); 
   fgets(texte, sizeof texte, stdin);
   response = texte[0];
} while (reponse!='C' && reponse!='D' && reponse!='c' && reponse!='d');
// continue here

There of course may be some improvements you wish to make, like ensuring that texte[1] is '\n', and if it isn't, then rejecting the response.

This problem is also solved by using clear_buffer; after each scanf call, or writing the scanf call as:

Code:

 scanf(" %c", &response);

If you take the preferred suggestion to the first problem, the second problem may be solved automatically. Choosing not to do that, you could strategically place clear_buffer; statements in between scanf and fgets calls.

Other things: 1) What happened to the indent style... 2) Why are all the string sizes magic numbers.

And concerning indenting, i don't know why when doing copy paste here it is gone .

It looks like you have a segmentation fault. I found it with my debugger:

Code:

Program received signal SIGSEGV, Segmentation fault.
0x75e18ff0 in strlen () from C:\WINDOWS\SysWOW64\msvcrt.dll
(gdb) bt
#0  0x75e18ff0 in strlen () from C:\WINDOWS\SysWOW64\msvcrt.dll
#1  0x004037dd in __mingw_pformat ()
#2  0x00402003 in __mingw_vprintf ()
#3  0x004013df in printf (__format=0x4080f7 <__register_frame_info+4227319> "Le texte crypte est:%s\n") at c:/tdm-gcc-32/include/stdio.h:256
#4  0x00401693 in main () at techloveotp.c:56

You're probably not used to reading backtraces, but that's okay. The problem occurs on line 56, which makes sense because higher in the trace there was a problem with printf.

Now that I take a good look at your code, you have to rework some things. Let's take a look at how you call encrypt and decrypt.

Code:

printf("Le texte crypte est:%s\n", encrypt(texte, cle));
printf("Le texte decrypte est:%s\n", decrypt(encrypted, cle));

The problem with this code is that both encrypt and decrypt return a char, a single character, when printf requires a string for %s. You need to store the cipher text in one of the strings, either texte or cle, and return a pointer to the first character in the string.

Originally Posted by whiteflags

It looks like you have a segmentation fault. I found it with my debugger:

Code:

Program received signal SIGSEGV, Segmentation fault.
0x75e18ff0 in strlen () from C:\WINDOWS\SysWOW64\msvcrt.dll
(gdb) bt
#0  0x75e18ff0 in strlen () from C:\WINDOWS\SysWOW64\msvcrt.dll
#1  0x004037dd in __mingw_pformat ()
#2  0x00402003 in __mingw_vprintf ()
#3  0x004013df in printf (__format=0x4080f7 <__register_frame_info+4227319> "Le texte crypte est:%s\n") at c:/tdm-gcc-32/include/stdio.h:256
#4  0x00401693 in main () at techloveotp.c:56

You're probably not used to reading backtraces, but that's okay. The problem occurs on line 56, which makes sense because higher in the trace there was a problem with printf.

Now that I take a good look at your code, you have to rework some things. Let's take a look at how you call encrypt and decrypt.

Code:

printf("Le texte crypte est:%s\n", encrypt(texte, cle));
printf("Le texte decrypte est:%s\n", decrypt(encrypted, cle));

The problem with this code is that both encrypt and decrypt return a char, a single character, when printf requires a string for %s. You need to store the cipher text in one of the strings, either texte or cle, and return a pointer to the first character in the string.

Here is my new code:

Code:

#include<stdio.h>#include<stdlib.h>
#include<string.h>
#define clear_buffer while(getchar()!='\n');
char* encrypt(char texte[],char cle[]){
	char *encrypted;
	int i=0;
	while(texte[i]!='\0' && cle[i]!='\0'){
		encrypted[i]=(texte[i]-65+cle[texte[i]-65])%26;
		i++;
	}
	encrypted[i+1]='\0';
	return encrypted;	
	
	}
char* decrypt(char encrypted[],char cle[]){
	char *decrypted;
	int i=0;
	
	while(encrypted[i]!='\0' && cle[i]!='\0'){
		decrypted[i]=(encrypted[i]-cle[i])%26;
		i++;
	}
	decrypted[i+1]=0;
	return decrypted;
	
}
int main()
{
	char reponse,texte[100],cle[100],encrypted[100];
	int i=0;
	
	do{
		printf("Voulez vous crypter ou decrypter un texte?(Ecrire C pour crypter et D pour decrypter)\n");
		scanf("%c",&reponse);
	}while (reponse!='C'&& reponse!='D'&& reponse!='c'&& reponse!='d');//controle pour obliger l'utilisateur à donner c ou d
	if(reponse=='C'||reponse=='c'){
			clear_buffer;//vider le buffer apres le scanf de la reponse
		printf("Donner un texte a crypter\n");
		fgets(texte,100,stdin);
		while(texte[i]!=0)
			i++;
		if (i>0 && texte[i-1]!='\n')
			clear_buffer;
		printf("Donner une cle de meme taille\n");
		fgets(cle,100,stdin);
		i=0;
		while(cle[i]!=0)
			i++;
		if (i>0 && cle[i-1]!='\n')
			clear_buffer;
		printf("Le texte crypte est:%s\n",encrypt(texte,cle));
	
	}else{
			clear_buffer;//vider le buffer apres le scanf de la reponse
			//do{
		printf("Donner un texte (deja crypte) à decrypter\n");
		fgets(encrypted,100,stdin);	
		i=0;
		while(encrypted[i]!=0)
			i++;
		if (i>0 && encrypted[i-1]!='\n')
			clear_buffer;
		printf("Donner la cle (deja utilisee pour crypter\n");
		fgets(cle,100,stdin);
		i=0;
		while(cle[i]!=0)
			i++;
		if (i>0 && cle[i-1]!='\n')
			clear_buffer;
	//	}while(sizeof encrypted!=sizeof cle);
		printf("Le texte decrypte est:%s\n",decrypt(encrypted,cle));
	}
	system("pause");
	return 0;
}

The problem now is with the final output: after i enter the word & key to be encrypted, i get some weird question marks instead of the ciphertext.

Any ideas?
thank you in advance

The problem now is with the final output: after i enter the word & key to be encrypted, i get some weird question marks instead of the ciphertext.

You still aren't following my advice. I said you need to use texte or cle.

Originally Posted by whiteflags

You still aren't following my advice. I said you need to use texte or cle.

i am sorry. i haven't understanded well. Could you clarify more?
& by the way here is a screenshot of the input & output.

Originally Posted by techlover1

i am sorry. i haven't understanded well. Could you clarify more?

You need to rewrite both functions using these two strings only:

Code:

char* encrypt(char texte[],char cle[])
char* decrypt(char encrypted[],char cle[])

If you cannot, then you must pass in a third string, to be the memory space for the result, and return that string.

Other ways to do this are needlessly complicated for you right now.

& by the way here is a screenshot of the input & output.

Well, thank you. It would have been additionally helpful to know what you wanted to see, if the program were correct. This way, if I or someone else works on the problem, we know we have a working solution before we post more advice.

Now the problem of unreadable password was solved.
The other problem is when decrypting a ciphertext already encrypted with my program it gives a word with a letter different than the original one ;so word->encrypted ... then encrypted->decrypted gives decrypted!=word,and even on paper the ciphertext should be different than the one i'm getting