Allow using Botan for the TPM2 TSS' crypto needs

This adds an implementation of the tpm2-tss crypto callbacks. If
enabled, Botan will be used for the client-side crypto functions to
communicate with the TPM. This lets applications shed a transitive
dependency on another crypto library (like OpenSSL or mbedTLS).

The crypto callbacks are available in tpm2-tss 4.0 and later. Before
that, calling TPM2::Context::use_botan_crypto_backend() will result in
an exception.

Co-Authored-By: René Meusel <[email protected]>

src/lib/prov/tpm2/tpm2_context.cpp

@@ -21,6 +21,10 @@
 #include <tss2/tss2_tcti.h>
 #include <tss2/tss2_tctildr.h>
 
+#if defined(BOTAN_HAS_TPM2_CRYPTO_BACKEND)
+   #include <botan/internal/tpm2_crypto_backend.h>
+#endif
+
 namespace Botan::TPM2 {
 
 namespace {
@@ -32,8 +36,20 @@ constexpr TPM2_HANDLE storage_root_key_handle = TPM2_HR_PERSISTENT + 1;
 struct Context::Impl {
       TSS2_TCTI_CONTEXT* m_tcti_ctx;
       ESYS_CONTEXT* m_ctx;
+
+#if defined(BOTAN_HAS_TPM2_CRYPTO_BACKEND)
+      std::unique_ptr<CryptoCallbackState> m_crypto_callback_state;
+#endif
 };
 
+bool Context::supports_botan_crypto_backend() noexcept {
+#if defined(BOTAN_TSS2_SUPPORTS_CRYPTO_CALLBACKS) and defined(BOTAN_HAS_TPM2_CRYPTO_BACKEND)
+   return true;
+#else
+   return false;
+#endif
+}
+
 std::shared_ptr<Context> Context::create(const std::string& tcti_nameconf) {
    // We cannot std::make_shared as the constructor is private
    return std::shared_ptr<Context>(new Context(tcti_nameconf.c_str()));
@@ -61,6 +77,32 @@ Context::Context(const char* tcti_name, const char* tcti_conf) : m_impl(std::mak
    BOTAN_ASSERT_NONNULL(m_impl->m_ctx);
 }
 
+void Context::use_botan_crypto_backend(const std::shared_ptr<Botan::RandomNumberGenerator>& rng) {
+#if defined(BOTAN_HAS_TPM2_CRYPTO_BACKEND)
+   BOTAN_STATE_CHECK(!uses_botan_crypto_backend());
+   m_impl->m_crypto_callback_state = std::make_unique<CryptoCallbackState>(rng);
+   enable_crypto_callbacks(shared_from_this());
+#else
+   BOTAN_UNUSED(rng);
+   throw Not_Implemented("This build of botan does not provide the TPM2 crypto backend");
+#endif
+}
+
+bool Context::uses_botan_crypto_backend() const noexcept {
+#if defined(BOTAN_HAS_TPM2_CRYPTO_BACKEND)
+   return m_impl->m_crypto_callback_state != nullptr;
+#else
+   return false;
+#endif
+}
+
+#if defined(BOTAN_HAS_TPM2_CRYPTO_BACKEND)
+CryptoCallbackState& Context::crypto_callback_state() {
+   BOTAN_ASSERT_NONNULL(m_impl->m_crypto_callback_state);
+   return *m_impl->m_crypto_callback_state;
+}
+#endif
+
 ESYS_CONTEXT* Context::esys_context() noexcept {
    return m_impl->m_ctx;
 }

src/lib/prov/tpm2/tpm2_context.h

@@ -26,6 +26,8 @@ struct ESYS_CONTEXT;
 
 namespace Botan::TPM2 {
 
+struct CryptoCallbackState;
+
 class PrivateKey;
 class SessionBundle;
 
@@ -57,6 +59,32 @@ class BOTAN_PUBLIC_API(3, 6) Context final : public std::enable_shared_from_this
       Context& operator=(const Context&) = delete;
       Context& operator=(Context&& ctx) noexcept = default;
 
+      /**
+       * Overrides the TSS2's crypto callbacks with Botan's functionality.
+       *
+       * This replaces all cryptographic functionality required for the
+       * communication with the TPM by botan's implementations. The TSS2
+       * would otherwise use OpenSSL or mbedTLS.
+       *
+       * Note that the provided @p rng should not be dependent on the TPM.
+       *
+       * @param rng  the RNG to use for the crypto operations
+       * @throws Not_Implemented if the TPM2-TSS does not support crypto callbacks
+       * @sa supports_botan_crypto_backend()
+       */
+      void use_botan_crypto_backend(const std::shared_ptr<Botan::RandomNumberGenerator>& rng);
+
+      /**
+       * Checks if the TSS2 supports registering Botan's crypto backend at runtime.
+       * Older versions of the TSS2 do not support this feature ( 4.0.0), also
+       * Botan may be compiled without support for TSS' crypto backend.
+       * @return true if the TSS2 supports Botan's crypto backend
+       */
+      static bool supports_botan_crypto_backend() noexcept;
+
+      /// @returns true if botan is used for the TSS' crypto functions
+      bool uses_botan_crypto_backend() const noexcept;
+
       /// @return an ESYS_CONTEXT* for use in other TPM2 functions.
       ESYS_CONTEXT* esys_context() noexcept;
 
@@ -105,6 +133,11 @@ class BOTAN_PUBLIC_API(3, 6) Context final : public std::enable_shared_from_this
       Context(const char* tcti_nameconf);
       Context(const char* tcti_name, const char* tcti_conf);
 
+#if defined(BOTAN_HAS_TPM2_CRYPTO_BACKEND)
+      friend void enable_crypto_callbacks(const std::shared_ptr<Context>&);
+      CryptoCallbackState& crypto_callback_state();
+#endif
+
    private:
       struct Impl;  // PImpl to avoid TPM2-TSS includes in this header
       std::unique_ptr<Impl> m_impl;

src/lib/prov/tpm2/tpm2_crypto_backend/info.txt

@@ -0,0 +1,20 @@
+<defines>
+TPM2_CRYPTO_BACKEND -> 20240806
+</defines>
+
+<module_info>
+name -> "TPM2 Crypto Backend"
+brief -> "Implementation of the TPM2-TSS crypto callbacks"
+</module_info>
+
+<requires>
+hash
+hmac
+modes
+pk_pad
+eme_raw
+</requires>
+
+<header:internal>
+tpm2_crypto_backend.h
+</header:internal>