dpp-util: add dpp_point_to_asn1

Converts an l_ecc_point to the DPP ASN.1 structure.

src/dpp-util.c

@@ -361,3 +361,93 @@ bool dpp_derive_ke(const uint8_t *i_nonce, const uint8_t *r_nonce,
 	/* ke = HKDF-Expand(bk, "DPP Key", length) */
 	return hkdf_expand(sha, bk, key_len, "DPP Key", ke, key_len);
 }
+
+#define ASN1_ID(class, pc, tag)	(((class) << 6) | ((pc) << 5) | (tag))
+
+#define ASN1_ID_SEQUENCE	ASN1_ID(0, 1, 0x10)
+#define ASN1_ID_BIT_STRING	ASN1_ID(0, 0, 0x03)
+#define ASN1_ID_OID		ASN1_ID(0, 0, 0x06)
+
+/*
+ * Values derived from OID definitions in https://www.secg.org/sec2-v2.pdf
+ * Appendix A.2.1
+ *
+ * 1.2.840.10045.2.1 (ecPublicKey)
+ */
+static uint8_t ec_oid[] = { 0x2a, 0x86, 0x48, 0xce, 0x3d, 0x02, 0x01 };
+
+/* 1.2.840.10045.3.1.7 (prime256v1) */
+static uint8_t ec_p256_oid[] = { 0x2a, 0x86, 0x48, 0xce,
+				0x3d, 0x03, 0x01, 0x07 };
+/* 1.3.132.0.34 (secp384r1) */
+static uint8_t ec_p384_oid[] = { 0x2B, 0x81, 0x04, 0x00, 0x22 };
+
+uint8_t *dpp_point_to_asn1(const struct l_ecc_point *p, size_t *len_out)
+{
+	uint8_t *asn1;
+	uint8_t *ptr;
+	uint8_t *type_oid;
+	size_t type_oid_len;
+	const struct l_ecc_curve *curve = l_ecc_point_get_curve(p);
+	ssize_t key_size = l_ecc_curve_get_scalar_bytes(curve);
+	uint64_t x[L_ECC_MAX_DIGITS];
+	ssize_t ret;
+	size_t len;
+
+	switch (key_size) {
+	case 32:
+		type_oid = ec_p256_oid;
+		type_oid_len = sizeof(ec_p256_oid);
+		break;
+	case 48:
+		type_oid = ec_p384_oid;
+		type_oid_len = sizeof(ec_p384_oid);
+		break;
+	default:
+		return NULL;
+	}
+
+	ret = l_ecc_point_get_x(p, x, sizeof(x));
+	if (ret < 0 || ret != key_size)
+		return NULL;
+
+	len = 2 + sizeof(ec_oid) + 2 + type_oid_len + 2 + key_size + 4;
+
+	if (L_WARN_ON(len > 128))
+		return NULL;
+
+	asn1 = l_malloc(len + 2);
+	ptr = asn1;
+
+	*ptr++ = ASN1_ID_SEQUENCE;
+	/* Length of both OIDs and key, plus tag/len bytes */
+	*ptr++ = len;
+
+	*ptr++ = ASN1_ID_SEQUENCE;
+
+	len = sizeof(ec_oid) + type_oid_len + 4;
+
+	*ptr++ = len;
+
+	*ptr++ = ASN1_ID_OID;
+	*ptr++ = sizeof(ec_oid);
+	memcpy(ptr, ec_oid, sizeof(ec_oid));
+	ptr += sizeof(ec_oid);
+
+	*ptr++ = ASN1_ID_OID;
+	*ptr++ = type_oid_len;
+	memcpy(ptr, type_oid, type_oid_len);
+	ptr += type_oid_len;
+
+	*ptr++ = ASN1_ID_BIT_STRING;
+	*ptr++ = key_size + 2;
+	*ptr++ = 0x00;
+	*ptr++ = 0x03;
+	memcpy(ptr, x, key_size);
+	ptr += key_size;
+
+	if (len_out)
+		*len_out = ptr - asn1;
+
+	return asn1;
+}

src/dpp-util.h

@@ -137,3 +137,5 @@ struct l_ecc_scalar *dpp_derive_k2(const struct l_ecc_point *i_proto_public,
 bool dpp_derive_ke(const uint8_t *i_nonce, const uint8_t *r_nonce,
 				struct l_ecc_scalar *m, struct l_ecc_scalar *n,
 				void *ke);
+
+uint8_t *dpp_point_to_asn1(const struct l_ecc_point *p, size_t *len_out);