Parallel RSA encryption in OpenMP

Question

Sometimes my encrypt code is going well, but sometimes it got buffer overflow or did'nt get the right answer when I try to parallel it. I don't know why. This is my code:

#ifdef _OPENMP #pragma omp parallel shared(result) //#pragma omp parallel for shared(M,C) private(x) schedule(static) default(shared) #endif do { mpz_powm(MN, M, N, pub.n); /* MN = M^N mod n */ mpz_mul(result, result, MN); /* result = result * MN */ mpz_mod(result, result, pub.n); /* result = result mod n */ } while(!tmp); unsigned int i, j; unsigned int core = (unsigned int) omp_get_num_procs(); for(i = 1; i < core; i++) { if(mpz_get_ui(pub.e) % core == i) { for(j = 1; j <= i; j++) { mpz_mul(result, result, M); /* result = result * M */ mpz_mod(result, result, pub.n); /* result = result mod n */ } } } mpz_mod(C, result, pub.n); /* C = result mod n */

You have can run mpz_powm in parallel, but the following mpz_mul writes result in which case you have to synchronize the processes on write. — Artjom B.
– Artjom B., Commented Dec 3, 2015 at 15:15
Well, it would be better to have a temporary variable, so you can compute the multiplication in parallel and then simply copy the multiplication result into the shared variable. I think you will still need to synchronize the processes a second time after the modulo operation. If you have fully working code, please don't forget to answer your own question with it. — Artjom B.
– Artjom B., Commented Dec 4, 2015 at 10:51
there are a few problems with the posted code: 1) tmp not defined and certainly not updated in the do...while(!tmp); loop 2) in a (for instance) 4 core CPU, this line: for(i = 1; i < core; i++) will only cover 3 of the cores — user3629249
– user3629249, Commented Dec 4, 2015 at 20:39

An Nisa Perlina · Accepted Answer · 2015-12-07 09:08:49Z

It's okay with modify that code to be like this. I can't get the error again. Thank you.

#pragma omp parallel shared(result) do { #pragma omp critical { mpz_powm(MN, M, N, pub.n); mpz_mul(result, result, MN); mpz_mod(result, result, pub.n); } } while(!tmp);

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