diff --git a/boot/bootutil/include/bootutil/crypto/ecdh_p256.h b/boot/bootutil/include/bootutil/crypto/ecdh_p256.h
index a37d50a..b6d16ba 100644
--- a/boot/bootutil/include/bootutil/crypto/ecdh_p256.h
+++ b/boot/bootutil/include/bootutil/crypto/ecdh_p256.h
@@ -68,6 +68,8 @@
 #endif /* MCUBOOT_USE_TINYCRYPT */
 
 #if defined(MCUBOOT_USE_MBED_TLS)
+#define NUM_ECC_BYTES 32
+
 typedef struct bootutil_ecdh_p256_context {
     mbedtls_ecp_group grp;
     mbedtls_ecp_point P;