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c++11rsagmp

GMP Segfault on Inverse modulus

I'm trying to write a simple program in C++ to demonstrate the math behind RSA. I'm using the GMP library ( https://gmplib.org/ ) so that I can scale it up later with larger primes.

When I attempt to calculate D, the decryption exponent, as the modular inverse of e mod phi(n), it segfaults and I have am lost as to why.

Can anyone shed some light on this issue?

#include <gmp.h>    // For the GMP library 

int main()

{
mpz_t n,p,q,e,c,d,h;

mpz_init(n);        
mpz_init(h);
mpz_init_set_str(e, "65537", 10);
mpz_init_set_str(p, "1298849", 10);
mpz_init_set_str(q, "1298863", 10);
mpz_mul(n,p,q);
mpz_sub_ui(p, p, 1UL);
mpz_sub_ui(q, q, 1UL);
mpz_mul(h, p, q);
gmp_printf ("%Zd\n", h);

//This next line segfaults it.
mpz_invert(d,e,h);

return 0; 
}

Any help is appreciated, I'm pretty stumped!

To Compile: g++ -std=c++11 Example.cpp -lgmp -lgmpxx -o Example

Solution

  • You never initialized c or d, so you can't use them for calculation.