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/pkwrite.h b/library/pkwrite.h
index 8aebd0c..537bd0f 100644
--- a/library/pkwrite.h
+++ b/library/pkwrite.h
@@ -105,4 +105,27 @@
 
 #endif /* MBEDTLS_ECP_C */
 
+#if defined(MBEDTLS_ECP_LIGHT)
+#include "mbedtls/ecp.h"
+
+#if defined(MBEDTLS_ECP_DP_CURVE25519_ENABLED) || defined(MBEDTLS_ECP_DP_CURVE448_ENABLED)
+#define MBEDTLS_PK_HAVE_RFC8410_CURVES
+
+static inline int mbedtls_pk_is_rfc8410_curve(mbedtls_ecp_group_id id)
+{
+#if defined(MBEDTLS_ECP_DP_CURVE25519_ENABLED)
+    if (id == MBEDTLS_ECP_DP_CURVE25519) {
+        return 1;
+    }
+#endif
+#if defined(MBEDTLS_ECP_DP_CURVE448_ENABLED)
+    if (id == MBEDTLS_ECP_DP_CURVE448) {
+        return 1;
+    }
+#endif
+    return 0;
+}
+#endif /* MBEDTLS_ECP_DP_CURVE25519_ENABLED || MBEDTLS_ECP_DP_CURVE448_ENABLED */
+#endif /* MBEDTLS_ECP_LIGHT */
+
 #endif /* MBEDTLS_PK_WRITE_H */