adding elements to array of structs in C

Question

So for a crypto class, we are implementing Mental Poker with the SRA protocol. We are using the openSSL library for BIGNUM http://www.openssl.org/docs/crypto/bn.html and I am having an issue when adding encrypted cards to an array of BIGNUM structs.

#include <openssl/bn.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

BIGNUM* encryptedDeck[52];
BIGNUM* bobHand[5];
BIGNUM* aliceHand[5];

int main(int argc, char* argv[]){
    int lcv = 0;
    BIGNUM *P,*Q,*N,*alpha,*alphaPrime,*beta,*betaPrime,*temp,*Pminus;
    BN_CTX *ctx = BN_CTX_new();
    P = BN_new(); //same for all BIGNUM pointers

    BN_generate_prime(P,512,1,NULL,NULL,NULL,NULL);
    BN_generate_prime(Q,512,1,NULL,NULL,NULL,NULL);
    BN_generate_prime(alpha,128,1,NULL,NULL,NULL,NULL); //Alice's key [ gcd(alpha,P) = 1 ]
    BN_generate_prime(beta,128,1,NULL,NULL,NULL,NULL);  //Bob's key. Same rule as alice's

    BN_one(temp);  //set temp to be a BIGNUM equivalent to integer 1
    BN_sub(Pminus,P,temp);

    BN_mul(N,P,Q,ctx);

    temp = BN_new();
    BIGNUM *encryptedCard = BN_new();
    for(lcv; lcv < 52; lcv++){
        BN_bin2bn(deck[lcv],strlen(deck[lcv]),temp);
        BN_mod_exp(encryptedCard,temp,beta,P,ctx);    //encrypt temp, store in encryptedCard

        printf("ec: %s\n\n",BN_bn2dec(encryptedCard));  //prints *correct numbers

        encryptedDeck[lcv] = encryptedCard;  //store cards in the array of encrypted cards
    }

    printf("00: %s\n\n",BN_bn2dec(encryptedDeck[0]));
    printf("01: %s\n\n",BN_bn2dec(encryptedDeck[1]));
    //...
    printf("40: %s\n\n",BN_bn2dec(encryptedDeck[40]));
}

The print statement in the forloop prints 52 different values (1 for each card.)

I intended for each of those values to simply be added to the array encryptedDeck, but when I check the values after exiting the loop, they are all equivalent to the 52nd card. So for some reason, each index in the array is being overwritten for every new card? Or something weird like that.

Is there some obvious thing that I am missing when dealing with arrays of structs? The only thing that I would think of right off the bat is something about the array not being initialized with enough space or something.

I think that it is possible to convert the BIGNUM values into char* and store them in an array that way, but I tried to avoid this because I don't know how big the pointers need to be, or even have a guess of their range. something like

char encryptedDeck[52][something ridiculous];

I omitted some code, but I believe this should still compile (provided you have the openssl library, and link it when compiling) and fill in the BN_new() for all the other BIGNUM pointers that I didn't initialize.

+1 for a clear question and well-formatted code. Nice to see it from a newcomer to SO. — user529758
– user529758, Commented Oct 18, 2012 at 5:41
+1 likewise reason. Even if the code didn't compile, I could still follow it, and the description was spot-on with what his goal, and observed problem, were all about. Totally rare for a new SO poster. — WhozCraig
– WhozCraig, Commented Oct 18, 2012 at 6:06

WhozCraig · Accepted Answer · 2012-10-18 05:59:10Z

2

Just above your for-loop:

BIGNUM *encryptedCard = BN_new();

You never change what this points to. You just keep dropping new data into the same card, then save that card into the current slot of the for-loop.

My surmise is to move the card inside the loop.

for(lcv; lcv < 52; lcv++){
    BIGNUM *encryptedCard = BN_new();
    BN_bin2bn(deck[lcv],strlen(deck[lcv]),temp);
    BN_mod_exp(encryptedCard,temp,beta,P,ctx);
    printf("ec: %p: %s\n\n",encryptedCard, BN_bn2dec(encryptedCard));
    encryptedDeck[lcv] = encryptedCard;
}

Noted: I don't use this crypto library, so I can't tell you if temp needs the same locality in the loop, but seems like where your problem may be.

edited Oct 18, 2012 at 5:59

answered Oct 18, 2012 at 5:45

WhozCraig

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7 Comments

AChrapko Over a year ago

sorry, I should be more explicit. The two functions from the openssl library are the first 2 in the for-loop. They handle taking a card from the plain-text deck, making it a BIGNUM, encrypting, and storing into another BIGNUM. BN_bin2bn(char* str, strlen(str), BIGNUM whereToStore) and BN_mod_exp(BIGNUM result, BIGNUM* a, BIGNUM* p, BIGNUM* m, ctx) so the first parameter of mod_exp is what changes the Card, which I then drop into the array.

WhozCraig Over a year ago

@AChrapko Sure, so it is intended to have that pointer (the same pointer) stored in the table at each slot in the deck? I only bring this up because you mentioned they are all the same card after the loop runs out. Check the deck and see if they're all the same pointer, and if so, its prolly not correct. (Or, I really need to get out more and use openssl rather than BSAFE =) ).

AChrapko Over a year ago

okay, okay I know what you're talking about. So the thing that really throws me off is having to use BIGNUM structs as my array datatype. When I first looked at the writeup I thought "ah yes, an array of 52 char*, and memcpy will be my best friend". So yes, I think they are all the same pointer. What would be my memcpy equivalent here?

unwind Over a year ago

You can use BN_copy() to copy BIGNUMs around, but it seems easier to just move the creation call inside the loop, as WhozCraig said.

WhozCraig Over a year ago

@AChrapko I updated the loop above to print the pointer as well as the text dump in the printf stmt. If the deck isn't holding cards, then you can do what you were before, but you need to pull the data out of the encryptedCard bignum and save it off prior to the next loop iteration or it will be lost. Anyway you look at it, you're generating new data, but then slamming over the old data without saving it. If you want it in a form besides a BIGNUM-ptr it needs to be done before you loop-around. If a BIGNUM-ptr is good, the above loop should work. I think you understand now.

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