diff --git a/sim/Cargo.toml b/sim/Cargo.toml
index 24b186d..13d2727 100644
--- a/sim/Cargo.toml
+++ b/sim/Cargo.toml
@@ -3,6 +3,11 @@
 version = "0.1.0"
 authors = ["David Brown <davidb@davidb.org>"]
 
+[features]
+default = []
+
+sig-rsa = ["mcuboot-sys/sig-rsa"]
+
 [build-dependencies]
 gcc = "0.3.38"
 
@@ -14,7 +19,7 @@
 log = "0.3"
 env_logger = "0.3"
 simflash = { path = "simflash" }
-mcuboot-sys = { path = "mcuboot-sys", features = ["sig-rsa"] }
+mcuboot-sys = { path = "mcuboot-sys" }
 bitflags = "0.9"
 ring = { version = "0.11", features = ["rsa_signing"] }
 untrusted = "0.5"
diff --git a/sim/mcuboot-sys/build.rs b/sim/mcuboot-sys/build.rs
index 1b3c7d4..c4eb61d 100644
--- a/sim/mcuboot-sys/build.rs
+++ b/sim/mcuboot-sys/build.rs
@@ -20,38 +20,43 @@
     conf.define("MCUBOOT_USE_FLASH_AREA_GET_SECTORS", None);
     conf.define("MCUBOOT_VALIDATE_SLOT0", None);
 
-    if sig_rsa {
-        if sig_ecdsa {
-            panic!("mcuboot does not support RSA and ECDSA at the same time");
-        }
+    // Currently, mbed TLS cannot build with both RSA and ECDSA.
+    if sig_rsa && sig_ecdsa {
+        panic!("mcuboot does not support RSA and ECDSA at the same time");
+    }
 
+    if sig_rsa {
         conf.define("MCUBOOT_SIGN_RSA", None);
         conf.define("MCUBOOT_USE_MBED_TLS", None);
 
-        conf.file("../../boot/bootutil/src/image_validate.c");
-        conf.file("../../boot/bootutil/src/image_rsa.c");
-        conf.file("../../boot/zephyr/keys.c");
-
         conf.define("MCUBOOT_USE_MBED_TLS", None);
         conf.define("MBEDTLS_CONFIG_FILE", Some("<config-boot.h>"));
         conf.include("mbedtls/include");
         conf.file("mbedtls/library/sha256.c");
+        conf.file("../../boot/zephyr/keys.c");
 
         conf.file("mbedtls/library/rsa.c");
         conf.file("mbedtls/library/bignum.c");
         conf.file("mbedtls/library/asn1parse.c");
-    }
-    if sig_ecdsa {
+    } else if sig_ecdsa {
         conf.define("MCUBOOT_SIGN_ECDSA", None);
         conf.define("MCUBOOT_USE_TINYCRYPT", None);
         // TODO: Compile files + tinycrypt.
         panic!("ECDSA not yet implemented in sim");
+    } else {
+        // Neither signature type, only verify sha256.
+        conf.define("MCUBOOT_USE_MBED_TLS", None);
+        conf.define("MBEDTLS_CONFIG_FILE", Some("<config-boot.h>"));
+        conf.include("mbedtls/include");
+        conf.file("mbedtls/library/sha256.c");
     }
 
     if overwrite_only {
         conf.define("MCUBOOT_OVERWRITE_ONLY", None);
     }
 
+    conf.file("../../boot/bootutil/src/image_validate.c");
+    conf.file("../../boot/bootutil/src/image_rsa.c");
     conf.file("../../boot/bootutil/src/loader.c");
     conf.file("../../boot/bootutil/src/caps.c");
     conf.file("../../boot/bootutil/src/bootutil_misc.c");
diff --git a/sim/src/main.rs b/sim/src/main.rs
index 3156b9c..1bf6e88 100644
--- a/sim/src/main.rs
+++ b/sim/src/main.rs
@@ -601,7 +601,7 @@
 fn install_image(flash: &mut Flash, offset: usize, len: usize) -> Vec<u8> {
     let offset0 = offset;
 
-    let mut tlv = TlvGen::new_rsa_pss();
+    let mut tlv = make_tlv();
 
     // Generate a boot header.  Note that the size doesn't include the header.
     let header = ImageHeader {
@@ -655,6 +655,17 @@
     copy
 }
 
+// The TLV in use depends on what kind of signature we are verifying.
+#[cfg(feature = "sig-rsa")]
+fn make_tlv() -> TlvGen {
+    TlvGen::new_rsa_pss()
+}
+
+#[cfg(not(feature = "sig-rsa"))]
+fn make_tlv() -> TlvGen {
+    TlvGen::new_hash_only()
+}
+
 /// Verify that given image is present in the flash at the given offset.
 fn verify_image(flash: &Flash, offset: usize, buf: &[u8]) -> bool {
     let mut copy = vec![0u8; buf.len()];