Read and write X25519 and X448 private keys

Signed-off-by: Jethro Beekman <jethro@fortanix.com>
Co-authored-by: Gijs Kwakkel <gijs.kwakkel@fortanix.com>
Signed-off-by: Gijs Kwakkel <gijs.kwakkel@fortanix.com>
diff --git a/library/asn1write.c b/library/asn1write.c
index b9d586a..c65d937 100644
--- a/library/asn1write.c
+++ b/library/asn1write.c
@@ -195,13 +195,22 @@
                                             const char *oid, size_t oid_len,
                                             size_t par_len)
 {
+    return mbedtls_asn1_write_algorithm_identifier_ext(p, start, oid, oid_len, par_len, 1);
+}
+
+int mbedtls_asn1_write_algorithm_identifier_ext(unsigned char **p, const unsigned char *start,
+                                                const char *oid, size_t oid_len,
+                                                size_t par_len, int has_par)
+{
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     size_t len = 0;
 
-    if (par_len == 0) {
-        MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_null(p, start));
-    } else {
-        len += par_len;
+    if (has_par) {
+        if (par_len == 0) {
+            MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_null(p, start));
+        } else {
+            len += par_len;
+        }
     }
 
     MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_oid(p, start, oid, oid_len));