static void XYcZ_addC(uECC_word_t * X1,
                      uECC_word_t * Y1,
                      uECC_word_t * X2,
                      uECC_word_t * Y2,
                      uECC_Curve curve) {
    /* t1 = X1, t2 = Y1, t3 = X2, t4 = Y2 */
    uECC_word_t t5[uECC_MAX_WORDS];
    uECC_word_t t6[uECC_MAX_WORDS];
    uECC_word_t t7[uECC_MAX_WORDS];
    wordcount_t num_words = curve->num_words;

    uECC_vli_modSub(t5, X2, X1, curve->p, num_words); /* t5 = x2 - x1 */
    uECC_vli_modSquare_fast(t5, t5, curve);                  /* t5 = (x2 - x1)^2 = A */
    uECC_vli_modMult_fast(X1, X1, t5, curve);                /* t1 = x1*A = B */
    uECC_vli_modMult_fast(X2, X2, t5, curve);                /* t3 = x2*A = C */
    uECC_vli_modAdd(t5, Y2, Y1, curve->p, num_words); /* t5 = y2 + y1 */
    uECC_vli_modSub(Y2, Y2, Y1, curve->p, num_words); /* t4 = y2 - y1 */

    uECC_vli_modSub(t6, X2, X1, curve->p, num_words); /* t6 = C - B */
    uECC_vli_modMult_fast(Y1, Y1, t6, curve);                /* t2 = y1 * (C - B) = E */
    uECC_vli_modAdd(t6, X1, X2, curve->p, num_words); /* t6 = B + C */
    uECC_vli_modSquare_fast(X2, Y2, curve);                  /* t3 = (y2 - y1)^2 = D */
    uECC_vli_modSub(X2, X2, t6, curve->p, num_words); /* t3 = D - (B + C) = x3 */

    uECC_vli_modSub(t7, X1, X2, curve->p, num_words); /* t7 = B - x3 */
    uECC_vli_modMult_fast(Y2, Y2, t7, curve);                /* t4 = (y2 - y1)*(B - x3) */
    uECC_vli_modSub(Y2, Y2, Y1, curve->p, num_words); /* t4 = (y2 - y1)*(B - x3) - E = y3 */

    uECC_vli_modSquare_fast(t7, t5, curve);                  /* t7 = (y2 + y1)^2 = F */
    uECC_vli_modSub(t7, t7, t6, curve->p, num_words); /* t7 = F - (B + C) = x3' */
    uECC_vli_modSub(t6, t7, X1, curve->p, num_words); /* t6 = x3' - B */
    uECC_vli_modMult_fast(t6, t6, t5, curve);                /* t6 = (y2+y1)*(x3' - B) */
    uECC_vli_modSub(Y1, t6, Y1, curve->p, num_words); /* t2 = (y2+y1)*(x3' - B) - E = y3' */

    uECC_vli_set(X1, t7, num_words);
}