Minimal ring-lwe encryption

ringlew.c

#include <math.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#define M 256
#define P 2048
/* Possible 256 values from 0 to 255*/
#define V 256
typedef struct vec3D_t vec3D_t;
struct vec3D_t {
  int x, y, z;
};

typedef struct pkey_t pkey_t;
struct pkey_t {
  int x, y, z, s;
};

int mod(int a, int b){
  int ret = 0;
  if (a > 0) return a%b;
  ret = a;
  while (ret < 0)
    ret += b;
  return ret;
}
int stepfunc(int x) {
  //  if (x > ((P/V)-1))
  //    return 1;
  //  return 0;
  return x / ((P / V) - 1);
}

pkey_t encrypt(int msg, pkey_t *pkey, int n) {
  pkey_t ret;

  /* randomly pick three vectors */
  pkey_t v0 = pkey[rand() % n], v1 = pkey[rand() % n], v2 = pkey[rand() % n];
  // pkey_t v0 = pkey[0], v1 = {0}, v2 = {0}, v3 = {0};
  ret.x = (v0.x + v1.x + v2.x) % P;
  ret.y = (v0.y + v1.y + v2.y) % P;
  ret.z = (v0.z + v1.z + v2.z) % P;
  ret.s = ((v0.s + v1.s + v2.s) + msg * ((P / V) - 1)) % P;
  printf("encrypted %d as u=%d v=%d,%d,%d\n", msg, ret.s, ret.x, ret.y, ret.z);
  return ret;
}

int decrypt(pkey_t msg, vec3D_t skey) {
  int sol = (msg.x * skey.x + msg.y * skey.y + msg.z * skey.z) % P;
  int ret = stepfunc(mod(msg.s - sol, P));
  printf("decrypted %d from u=%d v=%d,%d,%d\n", ret, msg.s, msg.x, msg.y,
         msg.z);
  return ret;
};

int main() {
  srand(time(NULL));
  vec3D_t skey = (vec3D_t){1091, 1203, 792};
  pkey_t pkey[M] = {0};

  printf("skey: %d %d %d\n", skey.x, skey.y, skey.z);
  for (int i = 0; i < M; i++) {
    pkey[i].x = rand() % P;
    pkey[i].y = rand() % P;
    pkey[i].z = rand() % P;
    pkey[i].s =
        ((pkey[i].x * skey.x + pkey[i].y * skey.y + pkey[i].z * skey.z));
    /* introduce cute noises */
    pkey[i].s += ((rand() % 3));
    pkey[i].s %=  P;

    printf("%dx %dy %dz = %d\n", pkey[i].x, pkey[i].y, pkey[i].z, pkey[i].s);
  }
  for (int i = 0; i < V; i++){
    int msg = i;
    int testv = decrypt(encrypt(msg, pkey, M), skey);
    printf("-----------\n");
  }
  return 0;
}