feat: HMAC support in Crypto APIs

3 files changed, 71 insertions, 0 deletions

diff --git a/core/crypto/crypto.cpp b/core/crypto/crypto.cpp
index d12108bca0..33ba0ba704 100644
--- a/core/crypto/crypto.cpp
+++ b/core/crypto/crypto.cpp
@@ -65,6 +65,22 @@ void X509Certificate::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("load", "path"), &X509Certificate::load);
 }
 
+/// HMACContext
+
+void HMACContext::_bind_methods() {
+	ClassDB::bind_method(D_METHOD("start", "hash_type", "key"), &HMACContext::start);
+	ClassDB::bind_method(D_METHOD("update", "data"), &HMACContext::update);
+	ClassDB::bind_method(D_METHOD("finish"), &HMACContext::finish);
+}
+
+HMACContext *(*HMACContext::_create)() = nullptr;
+HMACContext *HMACContext::create() {
+	if (_create) {
+		return _create();
+	}
+	ERR_FAIL_V_MSG(nullptr, "HMACContext is not available when the mbedtls module is disabled.");
+}
+
 /// Crypto
 
 void (*Crypto::_load_default_certificates)(String p_path) = nullptr;
@@ -82,6 +98,35 @@ void Crypto::load_default_certificates(String p_path) {
 	}
 }
 
+PackedByteArray Crypto::hmac_digest(HashingContext::HashType p_hash_type, PackedByteArray p_key, PackedByteArray p_msg) {
+	Ref<HMACContext> ctx = Ref<HMACContext>(HMACContext::create());
+	ERR_FAIL_COND_V_MSG(ctx.is_null(), PackedByteArray(), "HMAC is not available witout mbedtls module.");
+	Error err = ctx->start(p_hash_type, p_key);
+	ERR_FAIL_COND_V(err != OK, PackedByteArray());
+	err = ctx->update(p_msg);
+	ERR_FAIL_COND_V(err != OK, PackedByteArray());
+	return ctx->finish();
+}
+
+// Compares two HMACS for equality without leaking timing information in order to prevent timing attakcs.
+// @see: https://paragonie.com/blog/2015/11/preventing-timing-attacks-on-string-comparison-with-double-hmac-strategy
+bool Crypto::constant_time_compare(PackedByteArray p_trusted, PackedByteArray p_received) {
+	const uint8_t *t = p_trusted.ptr();
+	const uint8_t *r = p_received.ptr();
+	int tlen = p_trusted.size();
+	int rlen = p_received.size();
+	// If the lengths are different then nothing else matters.
+	if (tlen != rlen) {
+		return false;
+	}
+
+	uint8_t v = 0;
+	for (int i = 0; i < tlen; i++) {
+		v |= t[i] ^ r[i];
+	}
+	return v == 0;
+}
+
 void Crypto::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("generate_random_bytes", "size"), &Crypto::generate_random_bytes);
 	ClassDB::bind_method(D_METHOD("generate_rsa", "size"), &Crypto::generate_rsa);
@@ -90,6 +135,8 @@ void Crypto::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("verify", "hash_type", "hash", "signature", "key"), &Crypto::verify);
 	ClassDB::bind_method(D_METHOD("encrypt", "key", "plaintext"), &Crypto::encrypt);
 	ClassDB::bind_method(D_METHOD("decrypt", "key", "ciphertext"), &Crypto::decrypt);
+	ClassDB::bind_method(D_METHOD("hmac_digest", "hash_type", "key", "msg"), &Crypto::hmac_digest);
+	ClassDB::bind_method(D_METHOD("constant_time_compare", "trusted", "received"), &Crypto::constant_time_compare);
 }
 
 /// Resource loader/saver
diff --git a/core/crypto/crypto.h b/core/crypto/crypto.h
index 8325f043bf..5cacddaea0 100644
--- a/core/crypto/crypto.h
+++ b/core/crypto/crypto.h
@@ -67,6 +67,23 @@ public:
 	virtual Error save(String p_path) = 0;
 };
 
+class HMACContext : public Reference {
+	GDCLASS(HMACContext, Reference);
+
+protected:
+	static void _bind_methods();
+	static HMACContext *(*_create)();
+
+public:
+	static HMACContext *create();
+
+	virtual Error start(HashingContext::HashType p_hash_type, PackedByteArray p_key) = 0;
+	virtual Error update(PackedByteArray p_data) = 0;
+	virtual PackedByteArray finish() = 0;
+
+	HMACContext() {}
+};
+
 class Crypto : public Reference {
 	GDCLASS(Crypto, Reference);
 
@@ -88,6 +105,12 @@ public:
 	virtual Vector<uint8_t> encrypt(Ref<CryptoKey> p_key, Vector<uint8_t> p_plaintext) = 0;
 	virtual Vector<uint8_t> decrypt(Ref<CryptoKey> p_key, Vector<uint8_t> p_ciphertext) = 0;
 
+	PackedByteArray hmac_digest(HashingContext::HashType p_hash_type, PackedByteArray p_key, PackedByteArray p_msg);
+
+	// Compares two PackedByteArrays for equality without leaking timing information in order to prevent timing attacks.
+	// @see: https://paragonie.com/blog/2015/11/preventing-timing-attacks-on-string-comparison-with-double-hmac-strategy
+	bool constant_time_compare(PackedByteArray p_trusted, PackedByteArray p_received);
+
 	Crypto() {}
 };
 
diff --git a/core/register_core_types.cpp b/core/register_core_types.cpp
index 7e32f215e7..9883ce12a0 100644
--- a/core/register_core_types.cpp
+++ b/core/register_core_types.cpp
@@ -168,6 +168,7 @@ void register_core_types() {
 	ClassDB::register_class<AESContext>();
 	ClassDB::register_custom_instance_class<X509Certificate>();
 	ClassDB::register_custom_instance_class<CryptoKey>();
+	ClassDB::register_custom_instance_class<HMACContext>();
 	ClassDB::register_custom_instance_class<Crypto>();
 	ClassDB::register_custom_instance_class<StreamPeerSSL>();