Crypto: Add alternatives of mbed-crypto implementations
Select MBEDTLS_AES_SETKEY_DEC_ALT and MBEDTLS_AES_DECRYPT_ALTA when
AES-CCM is selected.
Add emtpy wrappers of mbedtls_internal_aes_decrypt() and
mbedtls_aes_setkey_dec() to replace mbed-crypto implementation when
the MBEDTLS_AES_SETKEY_DEC_ALT and MBEDTLS_AES_DECRYPT_ALTA are
enabled in AES-CCM mode. It can decrease memory footprint.
Add description of tfm_mbedcrypto_alt.c in Crypto document.
Change-Id: I3b9071735bfd6bafea8189dfde153d6050aefe27
Signed-off-by: David Hu <david.hu@arm.com>
diff --git a/secure_fw/services/crypto/tfm_mbedcrypto_alt.c b/secure_fw/services/crypto/tfm_mbedcrypto_alt.c
new file mode 100644
index 0000000..9cf9277
--- /dev/null
+++ b/secure_fw/services/crypto/tfm_mbedcrypto_alt.c
@@ -0,0 +1,58 @@
+/*
+ * Copyright (c) 2020, Arm Limited. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ */
+
+/*
+ * This file collects the alternative functions to replace the
+ * implementations in mbed-crypto if the corresponding mbed-crypto
+ * MBEDTLS__FUNCTION_NAME__ALT is selected.
+ */
+
+/*
+ * A dummy include. Just add a dependency to make sure this file is compiled
+ * after all crypto header files are installed and configuration flags are set.
+ */
+#include "tfm_mbedcrypto_include.h"
+#if defined(MBEDTLS_AES_DECRYPT_ALT) || defined(MBEDTLS_AES_SETKEY_DEC_ALT)
+#include "mbedtls/aes.h"
+#endif
+
+#if defined(MBEDTLS_AES_DECRYPT_ALT) && defined(MBEDTLS_CCM_C)
+#pragma message("mbedtls_internal_aes_decrypt() is replaced by an empty wrapper to decrease memory footprint")
+/*
+ * Replace the decryption process with an empty wrapper in AES-CCM mode.
+ * The decryption process is exactly the same as encryption process. Skip
+ * the decryption implementation to decrease memory footprint.
+ */
+int mbedtls_internal_aes_decrypt(mbedtls_aes_context *ctx,
+ const unsigned char input[16],
+ unsigned char output[16])
+{
+ (void)ctx;
+ (void)input;
+ (void)output;
+
+ return MBEDTLS_ERR_AES_FEATURE_UNAVAILABLE;
+}
+#endif
+
+#if defined(MBEDTLS_AES_SETKEY_DEC_ALT) && defined(MBEDTLS_CCM_C)
+#pragma message("mbedtls_aes_setkey_dec() is replaced by an empty wrapper to decrease memory footprint")
+/*
+ * Replace the decryption process with an empty wrapper in AES-CCM mode.
+ * The decryption process is exactly the same as encryption process. Skip
+ * the decryption key setting to decrease memory footprint.
+ */
+int mbedtls_aes_setkey_dec(mbedtls_aes_context *ctx, const unsigned char *key,
+ unsigned int keybits)
+{
+ (void)ctx;
+ (void)key;
+ (void)keybits;
+
+ return MBEDTLS_ERR_AES_FEATURE_UNAVAILABLE;
+}
+#endif