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I am basing my implementation of the RSA algorithm on information in the book "A handbook of applied cryptography" (chapter 8.2) :

I am basing my implementation of the RSA algorithm from the book "A handbook of applied cryptography" (chapter 8.2) :

Here is my code; you can run it after having linked with the -lgmp parameter:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <math.h>
#include <sys/types.h>
#include "gmp.h"
void generatePrimes(mpz_t* p, mpz_t* q);
void computeNandF(mpz_t* q, mpz_t* p, mpz_t *phi, mpz_t* n);
void generateE(mpz_t* phi, mpz_t* e);
void enc(mpz_t* e, mpz_t* n, mpz_t* d, mpz_t* c, char[]);
void dec(mpz_t* m, mpz_t* c, mpz_t* d, mpz_t* n);
void makeKeys(mpz_t n, mpz_t e, mpz_t d, mpz_t p, mpz_t q);
void encrFile(mpz_t e, mpz_t n, mpz_t d, mpz_t c);
gmp_randstate_t stat;
int main() {
 mpz_t p, q, phi, e, n, d, c, dc;
 char msg[40] = "welcome to cryptoworld";
 int *mes;
 int len = strlen(msg);
 mpz_init(p);
 mpz_init(q);
 mpz_init(phi);
 mpz_init(e);
 mpz_init(n);
 mpz_init(d);
 mpz_init(c);
 mpz_init(dc);
 // RSA algorithm
 generatePrimes(&p, &q);
 computeNandF(&q, &p, &phi, &n);
 generateE(&phi, &e);
 // extended Euclidean
 mpz_invert(d, e, phi);
 printf("d = ");
 mpz_out_str(stdout, 10, d);
 printf("\n------------------------------------------------------------------------------------------\n");
 enc(&e, &n, &d, &c, msg);
 dec(&dc, &c, &d, &n);
 printf("\n------------------------------------------------------------------------------------------\n");
 printf("encrypt message = ");
 mpz_out_str(stdout, 10, c);
 printf("\n");
 printf("\n------------------------------------------------------------------------------------------\n");
 printf("message as int after decr = ");
 mpz_out_str(stdout, 10, dc);
 printf("\n");
 mpz_export(mes, (size_t*) malloc(sizeof (size_t)), 1, sizeof (mes[0]), 0, 0, dc);
 char r[40];
 printf("message as string after decr = ");
 for (int i = 0; i < len; i++) {
 r[i] = (char) mes[i];
 printf("%c", r[i]);
 }
 printf("\n");
 encrFile(e, n, d, c);
 mpz_clear(p);
 mpz_clear(q);
 mpz_clear(phi);
 mpz_clear(n);
 mpz_clear(e);
 mpz_clear(c);
 mpz_clear(d);
 mpz_clear(dc);
 return 0;
}
void generatePrimes(mpz_t* p, mpz_t* q) {
 int primetest;
 long sd = 0;
 mpz_t seed;
 gmp_randinit(stat, GMP_RAND_ALG_LC, 120);
 mpz_init(seed);
 srand((unsigned) getpid());
 sd = rand();
 mpz_set_ui(seed, sd);
 gmp_randseed(stat, seed);
 mpz_urandomb(*p, stat, 512);
 primetest = mpz_probab_prime_p(*p, 10);
 if (primetest != 0) {
 printf("p is prime\n");
 } else {
 //printf("p wasnt prime,choose next prime\n");
 mpz_nextprime(*p, *p);
 }
 mpz_urandomb(*q, stat, 512);
 primetest = mpz_probab_prime_p(*q, 10);
 if (primetest != 0) {
 // printf("q is prime\n");
 } else {
 // printf("p wasnt prime,choose next prime\n");
 mpz_nextprime(*q, *q);
 }
 printf("p and q generated!!\n");
 printf("p = ");
 mpz_out_str(stdout, 10, *p);
 printf("q = ");
 mpz_out_str(stdout, 10, *q);
 printf("\n------------------------------------------------------------------------------------------\n");
 mpz_clear(seed);
 return;
}
void computeNandF(mpz_t* q, mpz_t* p, mpz_t *phi, mpz_t* n) {
 mpz_t temp1, temp2;
 mpz_init(temp1);
 mpz_init(temp2);
 //n=p*q
 mpz_mul(*n, *q, *p);
 mpz_sub_ui(temp1, *q, 1); //temp1=q-1
 mpz_sub_ui(temp2, *p, 1); //temp2=p-1
 //φ=(p-1)(q-1)
 mpz_mul(*phi, temp1, temp2);
 printf("phi and n generated!!\n");
 printf(" n= ");
 mpz_out_str(stdout, 10, *n);
 printf("phi = ");
 mpz_out_str(stdout, 10, *phi);
 printf("\n------------------------------------------------------------------------------------------\n");
}
void generateE(mpz_t* phi, mpz_t* e) {
 mpz_t temp, seed;
 mpz_init(seed);
 mpz_init(temp);
 long sd = 0;
 gmp_randinit(stat, GMP_RAND_ALG_LC, 120);
 srand((unsigned) getpid());
 sd = rand();
 mpz_set_ui(seed, sd);
 gmp_randseed(stat, seed);
 do {
 mpz_urandomm(*e, stat, *phi + 1);
 mpz_gcd(temp, *phi, *e); //temp=gcd(e,φ)
 } while (mpz_cmp_ui(temp, 1) != 0); //while gcd!=1
 printf("e generated \n e = ");
 mpz_out_str(stdout, 10, *e);
 printf("\n------------------------------------------------------------------------------------------\n");
 }
void enc(mpz_t* e, mpz_t* n, mpz_t* d, mpz_t* c, char msg[]) {
 int r[40];
 for (int i = 0; i < strlen(msg); i++) {
 r[i] = (int) msg[i];
 }
 int *m = r;
 mpz_t M;
 mpz_init(M);
 mpz_import(M, strlen(msg), 1, sizeof (m[0]), 0, 0, m);
 printf("message as int before encryption = ");
 mpz_out_str(stdout, 10, M);
 printf("\n");
 mpz_powm(*c, M, *e, *n);
}
void dec(mpz_t* m, mpz_t* c, mpz_t* d, mpz_t* n) {
 mpz_powm(*m, *c, *d, *n);
}
void encrFile(mpz_t e, mpz_t n, mpz_t d, mpz_t c) {
 char text[80];
 FILE *file;
 file = fopen("text.txt", "r");
 int i = 0;
 if (file) {
 while ((x = getc(file)) != EOF) {
 i++;
 putchar(x);
 text[i] = (char) x;
 }
 
 int r[40];
 for (int i = 0; i < strlen(text); i++) {
 r[i] = (int) text[i];
 }
 int *m = r;
 mpz_t M;
 mpz_init(M);
 mpz_import(M, strlen(text), 1, sizeof (m[0]), 0, 0, m);
 printf("message as int before encryption = ");
 mpz_out_str(stdout, 10, M);
 printf("\n");
 
 mpz_powm(c, M, e, n);
 printf("encrypt txt = ");
 mpz_out_str(stdout, 10, c);
 printf("\n");
 
 fclose(file);
 file = fopen("text.txt", "w");
 
 mpz_out_raw(file, c);
 fclose(file);
 }
}

Any comments and improvements in my implementation will be appreciated.

Here is my code; you can run it after having linked with the -lgmp parameter:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <math.h>
#include <sys/types.h>
#include "gmp.h"
void generatePrimes(mpz_t* p, mpz_t* q);
void computeNandF(mpz_t* q, mpz_t* p, mpz_t *phi, mpz_t* n);
void generateE(mpz_t* phi, mpz_t* e);
void enc(mpz_t* e, mpz_t* n, mpz_t* d, mpz_t* c, char[]);
void dec(mpz_t* m, mpz_t* c, mpz_t* d, mpz_t* n);
void makeKeys(mpz_t n, mpz_t e, mpz_t d, mpz_t p, mpz_t q);
void encrFile(mpz_t e, mpz_t n, mpz_t d, mpz_t c);
gmp_randstate_t stat;
int main() {
 mpz_t p, q, phi, e, n, d, c, dc;
 char msg[40] = "welcome to cryptoworld";
 int *mes;
 int len = strlen(msg);
 mpz_init(p);
 mpz_init(q);
 mpz_init(phi);
 mpz_init(e);
 mpz_init(n);
 mpz_init(d);
 mpz_init(c);
 mpz_init(dc);
 // RSA algorithm
 generatePrimes(&p, &q);
 computeNandF(&q, &p, &phi, &n);
 generateE(&phi, &e);
 // extended Euclidean
 mpz_invert(d, e, phi);
 printf("d = ");
 mpz_out_str(stdout, 10, d);
 printf("\n------------------------------------------------------------------------------------------\n");
 enc(&e, &n, &d, &c, msg);
 dec(&dc, &c, &d, &n);
 printf("\n------------------------------------------------------------------------------------------\n");
 printf("encrypt message = ");
 mpz_out_str(stdout, 10, c);
 printf("\n");
 printf("\n------------------------------------------------------------------------------------------\n");
 printf("message as int after decr = ");
 mpz_out_str(stdout, 10, dc);
 printf("\n");
 mpz_export(mes, (size_t*) malloc(sizeof (size_t)), 1, sizeof (mes[0]), 0, 0, dc);
 char r[40];
 printf("message as string after decr = ");
 for (int i = 0; i < len; i++) {
 r[i] = (char) mes[i];
 printf("%c", r[i]);
 }
 printf("\n");
 encrFile(e, n, d, c);
 mpz_clear(p);
 mpz_clear(q);
 mpz_clear(phi);
 mpz_clear(n);
 mpz_clear(e);
 mpz_clear(c);
 mpz_clear(d);
 mpz_clear(dc);
 return 0;
}
void generatePrimes(mpz_t* p, mpz_t* q) {
 int primetest;
 long sd = 0;
 mpz_t seed;
 gmp_randinit(stat, GMP_RAND_ALG_LC, 120);
 mpz_init(seed);
 srand((unsigned) getpid());
 sd = rand();
 mpz_set_ui(seed, sd);
 gmp_randseed(stat, seed);
 mpz_urandomb(*p, stat, 512);
 primetest = mpz_probab_prime_p(*p, 10);
 if (primetest != 0) {
 printf("p is prime\n");
 } else {
 //printf("p wasnt prime,choose next prime\n");
 mpz_nextprime(*p, *p);
 }
 mpz_urandomb(*q, stat, 512);
 primetest = mpz_probab_prime_p(*q, 10);
 if (primetest != 0) {
 // printf("q is prime\n");
 } else {
 // printf("p wasnt prime,choose next prime\n");
 mpz_nextprime(*q, *q);
 }
 printf("p and q generated!!\n");
 printf("p = ");
 mpz_out_str(stdout, 10, *p);
 printf("q = ");
 mpz_out_str(stdout, 10, *q);
 printf("\n------------------------------------------------------------------------------------------\n");
 mpz_clear(seed);
 return;
}
void computeNandF(mpz_t* q, mpz_t* p, mpz_t *phi, mpz_t* n) {
 mpz_t temp1, temp2;
 mpz_init(temp1);
 mpz_init(temp2);
 //n=p*q
 mpz_mul(*n, *q, *p);
 mpz_sub_ui(temp1, *q, 1); //temp1=q-1
 mpz_sub_ui(temp2, *p, 1); //temp2=p-1
 //φ=(p-1)(q-1)
 mpz_mul(*phi, temp1, temp2);
 printf("phi and n generated!!\n");
 printf(" n= ");
 mpz_out_str(stdout, 10, *n);
 printf("phi = ");
 mpz_out_str(stdout, 10, *phi);
 printf("\n------------------------------------------------------------------------------------------\n");
}
void generateE(mpz_t* phi, mpz_t* e) {
 mpz_t temp, seed;
 mpz_init(seed);
 mpz_init(temp);
 long sd = 0;
 gmp_randinit(stat, GMP_RAND_ALG_LC, 120);
 srand((unsigned) getpid());
 sd = rand();
 mpz_set_ui(seed, sd);
 gmp_randseed(stat, seed);
 do {
 mpz_urandomm(*e, stat, *phi + 1);
 mpz_gcd(temp, *phi, *e); //temp=gcd(e,φ)
 } while (mpz_cmp_ui(temp, 1) != 0); //while gcd!=1
 printf("e generated \n e = ");
 mpz_out_str(stdout, 10, *e);
 printf("\n------------------------------------------------------------------------------------------\n");
 }
void enc(mpz_t* e, mpz_t* n, mpz_t* d, mpz_t* c, char msg[]) {
 int r[40];
 for (int i = 0; i < strlen(msg); i++) {
 r[i] = (int) msg[i];
 }
 int *m = r;
 mpz_t M;
 mpz_init(M);
 mpz_import(M, strlen(msg), 1, sizeof (m[0]), 0, 0, m);
 printf("message as int before encryption = ");
 mpz_out_str(stdout, 10, M);
 printf("\n");
 mpz_powm(*c, M, *e, *n);
}
void dec(mpz_t* m, mpz_t* c, mpz_t* d, mpz_t* n) {
 mpz_powm(*m, *c, *d, *n);
}
void encrFile(mpz_t e, mpz_t n, mpz_t d, mpz_t c) {
 char text[80];
 FILE *file;
 file = fopen("text.txt", "r");
 int i = 0;
 if (file) {
 while ((x = getc(file)) != EOF) {
 i++;
 putchar(x);
 text[i] = (char) x;
 }
 
 int r[40];
 for (int i = 0; i < strlen(text); i++) {
 r[i] = (int) text[i];
 }
 int *m = r;
 mpz_t M;
 mpz_init(M);
 mpz_import(M, strlen(text), 1, sizeof (m[0]), 0, 0, m);
 printf("message as int before encryption = ");
 mpz_out_str(stdout, 10, M);
 printf("\n");
 
 mpz_powm(c, M, e, n);
 printf("encrypt txt = ");
 mpz_out_str(stdout, 10, c);
 printf("\n");
 
 fclose(file);
 file = fopen("text.txt", "w");
 
 mpz_out_raw(file, c);
 fclose(file);
 }
}

Any comments and improvements in my implementation will be appreciated.