MD: Use no-op for context SHA-256 init() and free()
When MBEDTLS_MD_SINGLE_HASH is set, the underlying digest's
context is embedded into mbedtls_md_context_t, which is
zeroized before the underlying digest's init() function
is called. For those digests where initialization is
zeroization, the init() call can therefore be omitted.
Similarly, when free()-ing an mbedtls_md_context_t, the
entire context is zeroized in the end, hence if the
underlying digest's free() function is zeroization,
it can be omitted.
diff --git a/include/mbedtls/md_internal.h b/include/mbedtls/md_internal.h
index 2cd5185..d44c8af 100644
--- a/include/mbedtls/md_internal.h
+++ b/include/mbedtls/md_internal.h
@@ -79,9 +79,21 @@
//#define MBEDTLS_MD_INFO_SHA256
/* SHA-256 */
+static inline void mbedtls_md_sha256_init_free_dummy( void* ctx )
+{
+ /* Zero-initialization can be skipped. */
+ ((void) ctx);
+}
#define MBEDTLS_MD_INFO_SHA256_TYPE MBEDTLS_MD_SHA256
#define MBEDTLS_MD_INFO_SHA256_CTX_TYPE mbedtls_sha256_context
+#if defined(MBEDTLS_MD_SINGLE_HASH)
+/* mbedtls_md_sha256_init() only zeroizes, which is redundant
+ * because mbedtls_md_context is zeroized in mbedtls_md_init(),
+ * and the mbedtls_sha256_context is embedded in mbedtls_md_context_t. */
+#define MBEDTLS_MD_INFO_SHA256_INIT_FUNC mbedtls_md_sha256_init_free_dummy
+#else
#define MBEDTLS_MD_INFO_SHA256_INIT_FUNC mbedtls_sha256_init
+#endif /* MBEDTLS_MD_SINGLE_HASH */
#define MBEDTLS_MD_INFO_SHA256_NAME "SHA256"
#define MBEDTLS_MD_INFO_SHA256_SIZE 32
#define MBEDTLS_MD_INFO_SHA256_BLOCKSIZE 64
@@ -90,7 +102,14 @@
#define MBEDTLS_MD_INFO_SHA256_FINISH_FUNC mbedtls_sha224_finish_wrap
#define MBEDTLS_MD_INFO_SHA256_DIGEST_FUNC mbedtls_sha256_wrap
#define MBEDTLS_MD_INFO_SHA256_ALLOC_FUNC mbedtls_sha224_ctx_alloc
+#if defined(MBEDTLS_MD_SINGLE_HASH)
+/* mbedtls_md_sha256_free() only zeroizes, which is redundant
+ * because mbedtls_md_context is zeroized in mbedtls_md_init(),
+ * and the mbedtls_sha256_context is embedded in mbedtls_md_context_t. */
+#define MBEDTLS_MD_INFO_SHA256_FREE_FUNC mbedtls_md_sha256_init_free_dummy
+#else
#define MBEDTLS_MD_INFO_SHA256_FREE_FUNC mbedtls_sha224_ctx_free
+#endif /* MBEDTLS_MD_SINGLE_HASH */
#define MBEDTLS_MD_INFO_SHA256_CLONE_FUNC mbedtls_sha224_clone_wrap
#define MBEDTLS_MD_INFO_SHA256_PROCESS_FUNC mbedtls_sha224_process_wrap