diff --git a/src/poly.c b/src/poly.c
index 7cb10f9..d813310 100644
--- a/src/poly.c
+++ b/src/poly.c
@@ -409,6 +409,152 @@ OUT_OF_LOOP:
 
 	return true;
 }
+
+/**
+ * Invert the polynomial a modulo p.
+ *
+ * @param a polynomial to invert
+ * @param Fq polynomial [out]
+ * @param ctx NTRU context
+ */
+bool pb_inverse_poly_p(pb_poly *a,
+		pb_poly *Fp,
+		ntru_context *ctx)
+{
+	int k = 0,
+		j = 0;
+	pb_poly *a_tmp, *b, *c, *f, *g;
+	mp_int mp_modulus, mp_minus;
+
+	/* general initialization of temp variables */
+	init_integer(&mp_modulus);
+	init_integer(&mp_minus);
+	MP_SET_INT(&mp_modulus, (unsigned long)(ctx->p));
+	MP_SET_INT(&mp_minus, 1);
+	mp_neg(&mp_minus, &mp_minus);
+	b = build_polynom(NULL, ctx->N + 1, ctx);
+	MP_SET(&(b->terms[0]), 1);
+	c = build_polynom(NULL, ctx->N + 1, ctx);
+	f = build_polynom(NULL, ctx->N + 1, ctx);
+	PB_COPY(a, f);
+
+	/* set g(x) = x^N − 1 */
+	g = build_polynom(NULL, ctx->N + 1, ctx);
+	MP_SET(&(g->terms[0]), 1);
+	mp_neg(&(g->terms[0]), &(g->terms[0]));
+	MP_SET(&(g->terms[ctx->N]), 1);
+
+	/* avoid side effects */
+	a_tmp = build_polynom(NULL, ctx->N, ctx);
+	PB_COPY(a, a_tmp);
+	erase_polynom(Fp, ctx->N);
+
+	printf("f: "); draw_polynom(f);
+	printf("g: "); draw_polynom(g);
+
+	while (1) {
+		while (mp_cmp_d(&(f->terms[0]), 0) == MP_EQ) {
+			printf("blah\n");
+			for (unsigned int i = 1; i <= ctx->N; i++) {
+				/* f(x) = f(x) / x */
+				MP_COPY(&(f->terms[i]), &(f->terms[i - 1]));
+				/* c(x) = c(x) * x */
+				MP_COPY(&(c->terms[ctx->N - i]), &(c->terms[ctx->N + 1 - i]));
+			}
+			MP_SET(&(f->terms[ctx->N]), 0);
+			MP_SET(&(c->terms[0]), 0);
+			k++;
+		}
+
+		if (get_degree(f) == 0)
+			goto OUT_OF_LOOP2;
+
+		if (get_degree(f) < get_degree(g)) {
+			pb_exch(f, g);
+			pb_exch(b, c);
+		}
+
+		{
+			pb_poly *u, *c_tmp, *g_tmp;
+			mp_int mp_tmp;
+
+			init_integer(&mp_tmp);
+			u = build_polynom(NULL, ctx->N, ctx);
+			g_tmp = build_polynom(NULL, ctx->N + 1, ctx);
+			PB_COPY(g, g_tmp);
+			c_tmp = build_polynom(NULL, ctx->N + 1, ctx);
+			PB_COPY(c, c_tmp);
+
+			/* u = ((f[0] mod p) * (g[0] inverse mod p) mod p) */
+			printf("u before: "); draw_polynom(u);
+			MP_COPY(&(f->terms[0]), &mp_tmp); /*  don't change f[0] */
+			MP_INVMOD(&(g->terms[0]), &mp_modulus, &(u->terms[0]));
+			MP_MOD(&mp_tmp, &mp_modulus, &mp_tmp);
+			MP_MUL(&(u->terms[0]), &mp_tmp, &(u->terms[0]));
+			MP_MOD(&(u->terms[0]), &mp_modulus, &(u->terms[0]));
+
+			/* f = f - u * g mod p */
+			printf("f before: "); draw_polynom(f);
+			PB_MUL(g_tmp, u, g_tmp);
+			PB_SUB(f, g_tmp, f);
+			PB_MOD(f, &mp_modulus, f, ctx->N + 1);
+
+			/* b = b - u * c mod p */
+			printf("b before: "); draw_polynom(b);
+			PB_MUL(c_tmp, u, c_tmp);
+			PB_SUB(b, c_tmp, b);
+			PB_MOD(b, &mp_modulus, b, ctx->N + 1);
+			printf("u after: "); draw_polynom(u);
+			printf("f after: "); draw_polynom(f);
+			printf("g after: "); draw_polynom(g);
+			printf("b after: "); draw_polynom(b);
+
+			mp_clear(&mp_tmp);
+			delete_polynom_multi(u, c_tmp, g_tmp, NULL);
+		}
+	}
+
+OUT_OF_LOOP2:
+	k = k % ctx->N;
+
+	/* Fp(x) = x^(N-k) * b(x) */
+	for (int i = ctx->N - 1; i >= 0; i--) {
+
+		/* b(X) = f[0]^(-1) * b(X) (mod p) */
+		{
+			pb_poly *poly_tmp;
+
+			poly_tmp = build_polynom(NULL, ctx->N + 1, ctx);
+
+			MP_INVMOD(&(f->terms[0]), &mp_modulus, &(poly_tmp->terms[0]));
+			MP_MOD(&(b->terms[i]), &mp_modulus, &(b->terms[i]));
+			MP_MUL(&(b->terms[i]), &(poly_tmp->terms[0]), &(b->terms[i]));
+
+			delete_polynom(poly_tmp);
+		}
+
+		j = i - k;
+		if (j < 0)
+			j = j + ctx->N;
+		MP_COPY(&(b->terms[i]), &(Fp->terms[j]));
+
+		/* delete_polynom(f_tmp); */
+	}
+
+	/* pull into positive space */
+	for (int i = ctx->N - 1; i >= 0; i--)
+		if (mp_cmp_d(&(Fp->terms[i]), 0) == MP_LT)
+			MP_ADD(&(Fp->terms[i]), &mp_modulus, &(Fp->terms[i]));
+
+	mp_clear(&mp_modulus);
+	delete_polynom_multi(a_tmp, b, c, f, g, NULL);
+
+	/* TODO: check if the f * Fq = 1 (mod p) condition holds true */
+
+	return true;
+}
+
+/**
  * Print the polynomial in a human readable format to stdout.
  *
  * @param poly to draw
diff --git a/src/poly.h b/src/poly.h
index 77a9b54..4f3beea 100644
--- a/src/poly.h
+++ b/src/poly.h
@@ -105,6 +105,14 @@
 				mp_error_to_string(result)); \
 }
 
+#define MP_INVMOD(...) \
+{ \
+	int result; \
+	if ((result = mp_invmod(__VA_ARGS__)) != MP_OKAY) \
+			NTRU_ABORT("Error computing modular inverse. %s", \
+				mp_error_to_string(result)); \
+}
+
 #define MP_EXPT_D(...) \
 { \
 	int result; \
@@ -182,6 +190,10 @@ bool pb_inverse_poly_q(pb_poly *a,
 		pb_poly *Fq,
 		ntru_context *ctx);
 
+bool pb_inverse_poly_p(pb_poly *a,
+		pb_poly *Fp,
+		ntru_context *ctx);
+
 void draw_polynom(pb_poly * const poly);
 
 #endif /* NTRU_POLY_H */