Use the windows crate for the TPM key

A failure to open the TPM fell back to a software key, which minted a new CA
and kept it for good. Fail instead, as macOS does without a Secure Enclave.

core/Cargo.lock

@@ -3111,6 +3111,7 @@ dependencies = [
  "tracing",
  "tracing-subscriber",
  "which",
+ "windows",
 ]
 
 [[package]]
@@ -3709,12 +3710,89 @@ version = "0.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "712e227841d057c1ee1cd2fb22fa7e5a5461ae8e48fa2ca79ec42cfc1931183f"
 
+[[package]]
+name = "windows"
+version = "0.62.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "527fadee13e0c05939a6a05d5bd6eec6cd2e3dbd648b9f8e447c6518133d8580"
+dependencies = [
+ "windows-collections",
+ "windows-core",
+ "windows-future",
+ "windows-numerics",
+]
+
+[[package]]
+name = "windows-collections"
+version = "0.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "23b2d95af1a8a14a3c7367e1ed4fc9c20e0a26e79551b1454d72583c97cc6610"
+dependencies = [
+ "windows-core",
+]
+
+[[package]]
+name = "windows-core"
+version = "0.62.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b8e83a14d34d0623b51dce9581199302a221863196a1dde71a7663a4c2be9deb"
+dependencies = [
+ "windows-implement",
+ "windows-interface",
+ "windows-link",
+ "windows-result",
+ "windows-strings",
+]
+
+[[package]]
+name = "windows-future"
+version = "0.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e1d6f90251fe18a279739e78025bd6ddc52a7e22f921070ccdc67dde84c605cb"
+dependencies = [
+ "windows-core",
+ "windows-link",
+ "windows-threading",
+]
+
+[[package]]
+name = "windows-implement"
+version = "0.60.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "053e2e040ab57b9dc951b72c264860db7eb3b0200ba345b4e4c3b14f67855ddf"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.119",
+]
+
+[[package]]
+name = "windows-interface"
+version = "0.59.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3f316c4a2570ba26bbec722032c4099d8c8bc095efccdc15688708623367e358"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.119",
+]
+
 [[package]]
 name = "windows-link"
 version = "0.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f0805222e57f7521d6a62e36fa9163bc891acd422f971defe97d64e70d0a4fe5"
 
+[[package]]
+name = "windows-numerics"
+version = "0.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6e2e40844ac143cdb44aead537bbf727de9b044e107a0f1220392177d15b0f26"
+dependencies = [
+ "windows-core",
+ "windows-link",
+]
+
 [[package]]
 name = "windows-result"
 version = "0.4.1"
@@ -3724,6 +3802,15 @@ dependencies = [
  "windows-link",
 ]
 
+[[package]]
+name = "windows-strings"
+version = "0.5.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7837d08f69c77cf6b07689544538e017c1bfcf57e34b4c0ff58e6c2cd3b37091"
+dependencies = [
+ "windows-link",
+]
+
 [[package]]
 name = "windows-sys"
 version = "0.52.0"
@@ -3758,6 +3845,15 @@ dependencies = [
  "windows_x86_64_msvc",
 ]
 
+[[package]]
+name = "windows-threading"
+version = "0.2.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3949bd5b99cafdf1c7ca86b43ca564028dfe27d66958f2470940f73d86d75b37"
+dependencies = [
+ "windows-link",
+]
+
 [[package]]
 name = "windows_aarch64_gnullvm"
 version = "0.52.6"

core/Cargo.toml

@@ -52,6 +52,10 @@ path = "tests/age.rs"
 name = "mitm"
 path = "tests/mitm.rs"
 
+[[test]]
+name = "tpm"
+path = "tests/tpm.rs"
+
 [[test]]
 name = "managers"
 path = "tests/managers.rs"
@@ -79,6 +83,9 @@ rama = { version = "0.4.0", features = ["net-apple-networkextension"] }
 security-framework = "3.7.0"
 security-framework-sys = "2.17.0"
 
+[target.'cfg(windows)'.dependencies]
+windows = { version = "0.62", features = ["Win32_Foundation", "Win32_Security_Cryptography"] }
+
 [dev-dependencies]
 tokio = { version = "1.53.1", features = ["macros", "rt-multi-thread"] }
 

core/src/mitm/ca.rs

@@ -8,7 +8,7 @@ use rcgen::{
 use std::sync::Arc;
 use time::{Duration, OffsetDateTime};
 
-/// Held in the Secure Enclave on macOS, in memory elsewhere.
+/// Held in the Secure Enclave on macOS, the TPM on Windows, in memory elsewhere.
 pub type CaSigner = Arc<dyn SigningKey + Send + Sync>;
 
 pub struct CertificateAuthority {

core/src/os/mod.rs

@@ -1,5 +1,7 @@
 #[cfg(target_os = "macos")]
 pub mod macos;
+#[cfg(windows)]
+pub mod windows;
 
 #[cfg(target_os = "macos")]
 pub use macos::{system_config, uninstall};

core/src/os/windows/mod.rs

@@ -0,0 +1,3 @@
+mod tpm;
+
+pub use tpm::CaKey;

core/src/os/windows/tpm.rs

@@ -0,0 +1,153 @@
+use rama::error::BoxError;
+use rama::tls::boring::core::bn::BigNum;
+use rama::tls::boring::core::ecdsa::EcdsaSig;
+use rama::tls::boring::core::sha::sha256;
+use rcgen::{Error, PublicKeyData, SignatureAlgorithm, SigningKey, PKCS_ECDSA_P256_SHA256};
+use windows::core::{Owned, Result as WinResult, HSTRING};
+use windows::Win32::Foundation::{NTE_BAD_KEY, NTE_BAD_KEYSET};
+use windows::Win32::Security::Cryptography::*;
+
+const KEY_NAME: &str = "Strix Firewall CA";
+
+/// A P-256 key held by the TPM. Signing asks the TPM to sign, so the private key never leaves it.
+pub struct CaKey {
+    // Fields drop in order, so the key handle closes before its provider's.
+    key_handle: Owned<NCRYPT_KEY_HANDLE>,
+    _provider_handle: Owned<NCRYPT_PROV_HANDLE>,
+    /// 0x04 || X || Y, the form rcgen writes into a certificate's SubjectPublicKeyInfo.
+    public_point: Vec<u8>,
+}
+
+impl CaKey {
+    /// A machine key, so the service and an elevated installer share it.
+    pub fn load_or_create() -> Result<Self, BoxError> {
+        match open_key(KEY_NAME, NCRYPT_MACHINE_KEY_FLAG)? {
+            Some(key) => Ok(key),
+            None => Ok(create_key(KEY_NAME, NCRYPT_MACHINE_KEY_FLAG)?),
+        }
+    }
+
+    /// A per-user key under a random name, which needs no elevation; `delete_from_tpm` it after.
+    pub fn create_test_key() -> WinResult<Self> {
+        let name = format!("Strix Firewall test {:016x}", rand::random::<u64>());
+        create_key(&name, NCRYPT_FLAGS(0))
+    }
+
+    /// Deletes the key from the TPM, not just this handle to it.
+    pub fn delete_from_tpm(mut self) -> WinResult<()> {
+        // NCryptDeleteKey also frees the handle, so the Owned mustn't free it again.
+        let key_handle = std::mem::take(&mut *self.key_handle);
+        unsafe { NCryptDeleteKey(key_handle, 0) }
+    }
+
+    /// Takes both handles and reads the key's public point once, for rcgen.
+    fn from_handles(
+        provider_handle: Owned<NCRYPT_PROV_HANDLE>,
+        key_handle: Owned<NCRYPT_KEY_HANDLE>,
+    ) -> WinResult<Self> {
+        // A BCRYPT_ECCKEY_BLOB header, then X and Y.
+        const HEADER_LEN: usize = size_of::<BCRYPT_ECCKEY_BLOB>();
+        let mut blob = [0u8; HEADER_LEN + 64];
+        let mut blob_len = 0;
+        unsafe {
+            NCryptExportKey(
+                *key_handle,
+                None,
+                BCRYPT_ECCPUBLIC_BLOB,
+                None,
+                Some(&mut blob),
+                &mut blob_len,
+                NCRYPT_FLAGS(0),
+            )?
+        };
+        let (header, x_and_y) = blob.split_at(HEADER_LEN);
+        let is_p256 = header[..4] == BCRYPT_ECDSA_PUBLIC_P256_MAGIC.to_le_bytes();
+        if !is_p256 || blob_len as usize != blob.len() {
+            return Err(windows::core::Error::new(NTE_BAD_KEY, "key is not an ECDSA P-256 key"));
+        }
+        let public_point = [&[0x04], x_and_y].concat();
+        Ok(CaKey { key_handle, _provider_handle: provider_handle, public_point })
+    }
+}
+
+impl PublicKeyData for CaKey {
+    fn der_bytes(&self) -> &[u8] {
+        &self.public_point
+    }
+
+    fn algorithm(&self) -> &'static SignatureAlgorithm {
+        &PKCS_ECDSA_P256_SHA256
+    }
+}
+
+impl SigningKey for CaKey {
+    fn sign(&self, msg: &[u8]) -> Result<Vec<u8>, Error> {
+        let mut r_and_s = [0u8; 64];
+        let mut signature_len = 0;
+        let signed = unsafe {
+            NCryptSignHash(
+                *self.key_handle,
+                None,
+                &sha256(msg),
+                Some(&mut r_and_s),
+                &mut signature_len,
+                NCRYPT_SILENT_FLAG,
+            )
+        };
+        signed.map_err(|_| Error::RemoteKeyError)?;
+        // NCrypt returns r || s, each half the length; X.509 wants DER SEQUENCE { r, s }.
+        let (r, s) = r_and_s[..signature_len as usize].split_at(signature_len as usize / 2);
+        let der = BigNum::from_slice(r)
+            .and_then(|r| EcdsaSig::from_private_components(r, BigNum::from_slice(s)?))
+            .and_then(|signature| signature.to_der());
+        der.map_err(|_| Error::RemoteKeyError)
+    }
+}
+
+/// `None` if the TPM holds no key by that name.
+/// `scope` is `NCRYPT_MACHINE_KEY_FLAG`, or empty for the current user's keys.
+fn open_key(name: &str, scope: NCRYPT_FLAGS) -> WinResult<Option<CaKey>> {
+    let provider_handle = open_tpm_provider()?;
+    let mut key_handle = NCRYPT_KEY_HANDLE::default();
+    let opened = unsafe {
+        NCryptOpenKey(
+            *provider_handle,
+            &mut key_handle,
+            &HSTRING::from(name),
+            CERT_KEY_SPEC::default(), // legacy, unused by the TPM provider
+            scope | NCRYPT_SILENT_FLAG,
+        )
+    };
+    match opened {
+        Err(error) if error.code() == NTE_BAD_KEYSET => Ok(None),
+        Err(error) => Err(error),
+        Ok(()) => CaKey::from_handles(provider_handle, unsafe { Owned::new(key_handle) }).map(Some),
+    }
+}
+
+/// Generates the key inside the TPM, which never lets it out.
+fn create_key(name: &str, scope: NCRYPT_FLAGS) -> WinResult<CaKey> {
+    let provider_handle = open_tpm_provider()?;
+    let mut key_handle = NCRYPT_KEY_HANDLE::default();
+    unsafe {
+        NCryptCreatePersistedKey(
+            *provider_handle,
+            &mut key_handle,
+            NCRYPT_ECDSA_P256_ALGORITHM,
+            &HSTRING::from(name),
+            CERT_KEY_SPEC::default(), // legacy, unused by the TPM provider
+            scope,
+        )?
+    };
+    let key_handle = unsafe { Owned::new(key_handle) };
+    // Until finalized the key is only a handle; this generates and stores it.
+    unsafe { NCryptFinalizeKey(*key_handle, NCRYPT_SILENT_FLAG)? };
+    CaKey::from_handles(provider_handle, key_handle)
+}
+
+/// The Microsoft Platform Crypto Provider, which keeps its keys in the TPM.
+fn open_tpm_provider() -> WinResult<Owned<NCRYPT_PROV_HANDLE>> {
+    let mut provider_handle = NCRYPT_PROV_HANDLE::default();
+    unsafe { NCryptOpenStorageProvider(&mut provider_handle, MS_PLATFORM_CRYPTO_PROVIDER, 0)? };
+    Ok(unsafe { Owned::new(provider_handle) })
+}

core/tests/tpm.rs

@@ -0,0 +1,21 @@
+#![cfg(windows)]
+
+use firewall::mitm::CertificateAuthority;
+use firewall::os::windows::CaKey;
+use rcgen::PublicKeyData;
+
+/// Skipped on machines without a TPM.
+#[test]
+fn key_signs_a_verifiable_ca() {
+    let key = match CaKey::create_test_key() {
+        Ok(key) => key,
+        Err(error) => return eprintln!("skipping, no TPM key: {error}"),
+    };
+    let ca = CertificateAuthority::generate(&["registry.npmjs.org"], &key);
+    let spki = key.subject_public_key_info();
+    key.delete_from_tpm().expect("delete the test key");
+    let ca = ca.expect("mint a ca");
+    let public = ca.crt.public_key().expect("ca public key");
+    assert!(ca.crt.verify(&public).expect("verify the ca"));
+    assert_eq!(public.public_key_to_der().unwrap(), spki);
+}