diff --git a/sim/mcuboot-sys/build.rs b/sim/mcuboot-sys/build.rs
index d181e34..aadfb68 100644
--- a/sim/mcuboot-sys/build.rs
+++ b/sim/mcuboot-sys/build.rs
@@ -13,7 +13,7 @@
     let sig_ecdsa = env::var("CARGO_FEATURE_SIG_ECDSA").is_ok();
     let overwrite_only = env::var("CARGO_FEATURE_OVERWRITE_ONLY").is_ok();
 
-    let mut conf = gcc::Config::new();
+    let mut conf = gcc::Build::new();
     conf.define("__BOOTSIM__", None);
     conf.define("MCUBOOT_USE_FLASH_AREA_GET_SECTORS", None);
     conf.define("MCUBOOT_VALIDATE_SLOT0", None);
@@ -37,10 +37,23 @@
         conf.file("mbedtls/library/bignum.c");
         conf.file("mbedtls/library/asn1parse.c");
     } else if sig_ecdsa {
-        conf.define("MCUBOOT_SIGN_ECDSA", None);
+        conf.define("MCUBOOT_SIGN_EC256", None);
         conf.define("MCUBOOT_USE_TINYCRYPT", None);
-        // TODO: Compile files + tinycrypt.
-        panic!("ECDSA not yet implemented in sim");
+
+        conf.define("MBEDTLS_CONFIG_FILE", Some("<config-boot.h>"));
+        conf.include("mbedtls/include");
+        conf.include("../../ext/tinycrypt/lib/include");
+
+        conf.file("../../boot/zephyr/keys.c");
+
+        conf.file("../../ext/tinycrypt/lib/source/utils.c");
+        conf.file("../../ext/tinycrypt/lib/source/sha256.c");
+        conf.file("../../ext/tinycrypt/lib/source/ecc.c");
+        conf.file("../../ext/tinycrypt/lib/source/ecc_dsa.c");
+        conf.file("../../ext/tinycrypt/lib/source/ecc_platform_specific.c");
+
+        conf.file("mbedtls/library/bignum.c");
+        conf.file("mbedtls/library/asn1parse.c");
     } else {
         // Neither signature type, only verify sha256.
         conf.define("MCUBOOT_USE_MBED_TLS", None);
@@ -55,7 +68,11 @@
     }
 
     conf.file("../../boot/bootutil/src/image_validate.c");
-    conf.file("../../boot/bootutil/src/image_rsa.c");
+    if sig_rsa {
+        conf.file("../../boot/bootutil/src/image_rsa.c");
+    } else if sig_ecdsa {
+        conf.file("../../boot/bootutil/src/image_ec256.c");
+    }
     conf.file("../../boot/bootutil/src/loader.c");
     conf.file("../../boot/bootutil/src/caps.c");
     conf.file("../../boot/bootutil/src/bootutil_misc.c");
@@ -69,6 +86,7 @@
     conf.compile("libbootutil.a");
 
     walk_dir("../../boot").unwrap();
+    walk_dir("../../ext/tinycrypt/lib/source").unwrap();
     walk_dir("csupport").unwrap();
     walk_dir("mbedtls/include").unwrap();
     walk_dir("mbedtls/library").unwrap();