osnma-longan-nano: add Merkle tree root

osnma-longan-nano/Cargo.toml

@@ -22,6 +22,7 @@ panic-halt = "0.2"
 riscv-rt = "0.11"
 
 [build-dependencies]
+hex = "0.4"
 ecdsa = { version = "0.16", features = ["pkcs8"] }
 p256 = { version = "0.13", features = ["ecdsa"] }
 spki = { version = "0.7", features = ["pem"] }

osnma-longan-nano/README.md

@@ -18,19 +18,23 @@ The instructions to set up the Rust riscv32imac toolchain can be found in the
 documentation for the [longan-nano](https://github.com/riscv-rust/longan-nano)
 crate.
 
-The OSNMA ECDSA P-256 public key is embedded in the binary during the build
-process. The public key is taken from the `pubkey.pem` file found in the root
-folder of this crate. A "fake" public key is provided so that osnma-longan-demo
-can be built without access to the authentic public key. A binary built with
-this fake public key will not work with the Galileo signal in space, since
-it will not be able to validate the TESLA root key.
+The OSNMA ECDSA P-256 public key and the Merkle tree root are embedded in the
+binary during the build process. The public key is taken from the `pubkey.pem`
+file found in the root folder of this crate, and its Public Key ID is taken from
+the `pubkey_id.txt` file. The Merkle tree root is taken from the
+`merkle_tree_root.txt` file. "Fake" files are provided so that osnma-longan-demo
+can be built without access to the real cryptographic material. A binary built with
+this fake cryptographic material will not work with the Galileo signal-in-space.
 
 The fake `pubkey.pem` needs to be replaced with the authentic key, using the
 same PEM file format. Instructions about how to obtain the authentic public key
-can be found in the
-[galileo-osnma README](https://github.com/daniestevez/galileo-osnma#quick-start-using-galmon).
+can be found in the [galileo-osnma
+README](https://github.com/daniestevez/galileo-osnma#quick-start-using-galmon).
+Likewise the Public Key ID in `pubkey_id.txt` needs to be replaced by the
+correct one, and the Merkle tree root needs to be written to the file
+`merkle_tree_root.txt`.
 
-Once the file `pubkey.pem` contains the authentic public key, the crate can be
+Once these files contain the real cryptographic material, the firmware can be
 built using
 ```
 cargo build --release

osnma-longan-nano/build.rs

@@ -25,6 +25,24 @@ fn pubkey_to_rust(pubkey: &[u8], pubkey_id: u8, path: &Path) -> std::io::Result<
     )
 }
 
+fn read_merkle_tree_root(path: &str) -> std::io::Result<[u8; 32]> {
+    let hex = fs::read_to_string(path)?;
+    Ok(hex::decode(hex.trim())
+        .expect("invalid Merkle tree root")
+        .try_into()
+        .unwrap())
+}
+
+fn merkle_tree_to_rust(merkle_tree_root: &[u8; 32], path: &Path) -> std::io::Result<()> {
+    fs::write(
+        path,
+        format!(
+            "const OSNMA_MERKLE_TREE_ROOT: [u8; 32] = {:?};\n",
+            merkle_tree_root,
+        ),
+    )
+}
+
 fn main() {
     // Put the memory definition somewhere the linker can find it
     let out_dir = PathBuf::from(env::var("OUT_DIR").unwrap());
@@ -37,4 +55,8 @@ fn main() {
     println!("cargo:rerun-if-changed=build.rs");
     println!("cargo:rerun-if-changed=pubkey.pem");
     println!("cargo:rerun-if-changed=pubkey_id.txt");
+
+    let merkle_tree_root = read_merkle_tree_root("merkle_tree_root.txt").unwrap();
+    merkle_tree_to_rust(&merkle_tree_root, &out_dir.join("osnma_merkle_tree.rs")).unwrap();
+    println!("cargo:rerun-if-changed=merkle_tree_root.txt");
 }

osnma-longan-nano/merkle_tree_root.txt

@@ -0,0 +1 @@
+0000000000000000000000000000000000000000000000000000000000000000

osnma-longan-nano/src/main.rs

@@ -15,8 +15,12 @@ use panic_halt as _;
 use riscv_rt::entry;
 
 // The OSNMA public key, as a [u8; N] constant, is generated from pubkey.pem in
-// the build script and included here.
+// the build script and included here, together with is public key ID (generated
+// from pubkey_id.txt).
 include!(concat!(env!("OUT_DIR"), "/osnma_pubkey.rs"));
+// The OSNMA Merkle tree root, as a [u8; 32] constant, is generated from
+// merkle_tree_root.txt in the build script and included here.
+include!(concat!(env!("OUT_DIR"), "/osnma_merkle_tree.rs"));
 
 struct Board {
     tx: serial::Tx<USART0>,
@@ -78,7 +82,8 @@ impl OsnmaInterface {
     fn new(board: Board) -> OsnmaInterface {
         let pubkey = VerifyingKey::from_sec1_bytes(&OSNMA_PUBKEY).unwrap();
         let pubkey = PublicKey::from_p256(pubkey, OSNMA_PUBKEY_ID).force_valid();
-        let osnma = Osnma::<SmallStorage>::from_pubkey(pubkey, false);
+        let osnma =
+            Osnma::<SmallStorage>::from_merkle_tree(OSNMA_MERKLE_TREE_ROOT, Some(pubkey), false);
         OsnmaInterface { osnma, board }
     }