dpp-util: add dpp_point_from_asn1()

Given an ASN1 blob of the right form, parse and create
an l_ecc_point object. The form used is specific to DPP
hence why this isn't general purpose and put into dpp-util.

src/dpp-util.c

@@ -796,3 +796,72 @@ uint8_t *dpp_point_to_asn1(const struct l_ecc_point *p, size_t *len_out)
 
 	return asn1;
 }
+
+/*
+ * Only checking for the ASN.1 form:
+ *
+ * SEQUENCE {
+ * 	SEQUENCE {
+ * 		OBJECT IDENTIFIER ecPublicKey
+ * 		OBJECT IDENTIFIER key type (p256/p384)
+ * 	}
+ * 	BITSTRING (key data)
+ * }
+ */
+struct l_ecc_point *dpp_point_from_asn1(const uint8_t *asn1, size_t len)
+{
+
+	const uint8_t *outer_seq;
+	size_t outer_len;
+	const uint8_t *inner_seq;
+	size_t inner_len;
+	const uint8_t *elem;
+	const uint8_t *key_data;
+	size_t elen = 0;
+	uint8_t tag;
+	unsigned int curve_num;
+	const struct l_ecc_curve *curve;
+
+	/* SEQUENCE */
+	outer_seq = asn1_der_find_elem(asn1, len, 0, &tag, &outer_len);
+	if (!outer_seq || tag != ASN1_ID_SEQUENCE)
+		return NULL;
+
+	/* SEQUENCE */
+	inner_seq = asn1_der_find_elem(outer_seq, outer_len, 0, &tag, &inner_len);
+	if (!inner_seq || tag != ASN1_ID_SEQUENCE)
+		return NULL;
+
+	/* OBJECT IDENTIFIER (ecPublicKey) */
+	elem = asn1_der_find_elem(inner_seq, inner_len, 0, &tag, &elen);
+	if (!elem || tag != ASN1_ID_OID)
+		return NULL;
+
+	/* Check that this OID is ecPublicKey */
+	if (!asn1_oid_eq(&ec_oid, elen, elem))
+		return NULL;
+
+	elem = asn1_der_find_elem(inner_seq, inner_len, 1, &tag, &elen);
+	if (!elem || tag != ASN1_ID_OID)
+		return NULL;
+
+	/* Check if ELL supports this curve */
+	if (asn1_oid_eq(&ec_p256_oid, elen, elem))
+		curve_num = 19;
+	else if (asn1_oid_eq(&ec_p384_oid, elen, elem))
+		curve_num = 20;
+	else
+		return NULL;
+
+	curve = l_ecc_curve_from_ike_group(curve_num);
+	if (!curve)
+		return NULL;
+
+	/* BITSTRING */
+	key_data = asn1_der_find_elem(outer_seq, outer_len, 1, &tag, &elen);
+	if (!key_data || tag != ASN1_ID_BIT_STRING || elen > 2)
+		return NULL;
+
+	return l_ecc_point_from_data(curve, key_data[1],
+					key_data + 2, elen - 2);
+}

src/dpp-util.h

@@ -167,3 +167,4 @@ bool dpp_derive_ke(const uint8_t *i_nonce, const uint8_t *r_nonce,
 				void *ke);
 
 uint8_t *dpp_point_to_asn1(const struct l_ecc_point *p, size_t *len_out);
+struct l_ecc_point *dpp_point_from_asn1(const uint8_t *asn1, size_t len);