drivers: crypto: Add authenticated decryption framework

Add framework for autheticated decryption of data. Currently this
patch optionally imports mbedtls library as a backend if build option
"DECRYPTION_SUPPORT = aes_gcm" is set to perform authenticated decryption
using AES-GCM algorithm.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Change-Id: I2966f0e79033151012bf4ffc66f484cd949e7271
diff --git a/Makefile b/Makefile
index f3cb9be..f7ae5ea 100644
--- a/Makefile
+++ b/Makefile
@@ -623,7 +623,7 @@
 
 ifeq ($(MEASURED_BOOT),1)
     ifneq (${TRUSTED_BOARD_BOOT},1)
-        $(error MEASURED_BOOT requires TRUSTED_BOARD_BOOT=1")
+        $(error MEASURED_BOOT requires TRUSTED_BOARD_BOOT=1)
     else
         $(info MEASURED_BOOT is an experimental feature)
     endif
@@ -635,6 +635,14 @@
     endif
 endif
 
+ifneq (${DECRYPTION_SUPPORT},none)
+    ifeq (${TRUSTED_BOARD_BOOT}, 0)
+        $(error TRUSTED_BOARD_BOOT must be enabled for DECRYPTION_SUPPORT to be set)
+    else
+        $(info DECRYPTION_SUPPORT is an experimental feature)
+    endif
+endif
+
 ################################################################################
 # Process platform overrideable behaviour
 ################################################################################
@@ -843,6 +851,7 @@
 $(eval $(call add_define,EL3_EXCEPTION_HANDLING))
 $(eval $(call add_define,CTX_INCLUDE_MTE_REGS))
 $(eval $(call add_define,CTX_INCLUDE_EL2_REGS))
+$(eval $(call add_define,DECRYPTION_SUPPORT_${DECRYPTION_SUPPORT}))
 $(eval $(call add_define,ENABLE_AMU))
 $(eval $(call add_define,ENABLE_ASSERTIONS))
 $(eval $(call add_define,ENABLE_BTI))
diff --git a/docs/getting_started/build-options.rst b/docs/getting_started/build-options.rst
index da5dcbf..af4895e 100644
--- a/docs/getting_started/build-options.rst
+++ b/docs/getting_started/build-options.rst
@@ -160,6 +160,12 @@
 -  ``DEBUG``: Chooses between a debug and release build. It can take either 0
    (release) or 1 (debug) as values. 0 is the default.
 
+-  ``DECRYPTION_SUPPORT``: This build flag enables the user to select the
+   authenticated decryption algorithm to be used to decrypt firmware/s during
+   boot. It accepts 2 values: ``aes_gcm`` and ``none``. The default value of
+   this flag is ``none`` to disable firmware decryption which is an optional
+   feature as per TBBR. Also, it is an experimental feature.
+
 -  ``DISABLE_BIN_GENERATION``: Boolean option to disable the generation
    of the binary image. If set to 1, then only the ELF image is built.
    0 is the default.
diff --git a/drivers/auth/crypto_mod.c b/drivers/auth/crypto_mod.c
index 110c504..c63ff08 100644
--- a/drivers/auth/crypto_mod.c
+++ b/drivers/auth/crypto_mod.c
@@ -124,3 +124,35 @@
 	return crypto_lib_desc.calc_hash(alg, data_ptr, data_len, output);
 }
 #endif	/* MEASURED_BOOT */
+
+/*
+ * Authenticated decryption of data
+ *
+ * Parameters:
+ *
+ *   dec_algo: authenticated decryption algorithm
+ *   data_ptr, len: data to be decrypted (inout param)
+ *   key, key_len, key_flags: symmetric decryption key
+ *   iv, iv_len: initialization vector
+ *   tag, tag_len: authentication tag
+ */
+int crypto_mod_auth_decrypt(enum crypto_dec_algo dec_algo, void *data_ptr,
+			    size_t len, const void *key, unsigned int key_len,
+			    unsigned int key_flags, const void *iv,
+			    unsigned int iv_len, const void *tag,
+			    unsigned int tag_len)
+{
+	assert(crypto_lib_desc.auth_decrypt != NULL);
+	assert(data_ptr != NULL);
+	assert(len != 0U);
+	assert(key != NULL);
+	assert(key_len != 0U);
+	assert(iv != NULL);
+	assert((iv_len != 0U) && (iv_len <= CRYPTO_MAX_IV_SIZE));
+	assert(tag != NULL);
+	assert((tag_len != 0U) && (tag_len <= CRYPTO_MAX_TAG_SIZE));
+
+	return crypto_lib_desc.auth_decrypt(dec_algo, data_ptr, len, key,
+					    key_len, key_flags, iv, iv_len, tag,
+					    tag_len);
+}
diff --git a/drivers/auth/cryptocell/712/cryptocell_crypto.c b/drivers/auth/cryptocell/712/cryptocell_crypto.c
index 25eb6bc..cf43175 100644
--- a/drivers/auth/cryptocell/712/cryptocell_crypto.c
+++ b/drivers/auth/cryptocell/712/cryptocell_crypto.c
@@ -301,5 +301,5 @@
 /*
  * Register crypto library descriptor
  */
-REGISTER_CRYPTO_LIB(LIB_NAME, init, verify_signature, verify_hash);
+REGISTER_CRYPTO_LIB(LIB_NAME, init, verify_signature, verify_hash, NULL);
 
diff --git a/drivers/auth/mbedtls/mbedtls_common.mk b/drivers/auth/mbedtls/mbedtls_common.mk
index 4b83015..044b368 100644
--- a/drivers/auth/mbedtls/mbedtls_common.mk
+++ b/drivers/auth/mbedtls/mbedtls_common.mk
@@ -23,13 +23,17 @@
 
 
 LIBMBEDTLS_SRCS		:= $(addprefix ${MBEDTLS_DIR}/library/,	\
+					aes.c 					\
 					asn1parse.c 				\
 					asn1write.c 				\
+					cipher.c 				\
+					cipher_wrap.c 				\
 					memory_buffer_alloc.c			\
 					oid.c 					\
 					platform.c 				\
 					platform_util.c				\
 					bignum.c				\
+					gcm.c 					\
 					md.c					\
 					md_wrap.c				\
 					pk.c 					\
@@ -87,11 +91,17 @@
     $(error "TF_MBEDTLS_KEY_ALG=${TF_MBEDTLS_KEY_ALG} not supported on mbed TLS")
 endif
 
+ifeq (${DECRYPTION_SUPPORT}, aes_gcm)
+    TF_MBEDTLS_USE_AES_GCM	:=	1
+else
+    TF_MBEDTLS_USE_AES_GCM	:=	0
+endif
+
 # Needs to be set to drive mbed TLS configuration correctly
 $(eval $(call add_define,TF_MBEDTLS_KEY_ALG_ID))
 $(eval $(call add_define,TF_MBEDTLS_KEY_SIZE))
 $(eval $(call add_define,TF_MBEDTLS_HASH_ALG_ID))
-
+$(eval $(call add_define,TF_MBEDTLS_USE_AES_GCM))
 
 $(eval $(call MAKE_LIB,mbedtls))
 
diff --git a/drivers/auth/mbedtls/mbedtls_crypto.c b/drivers/auth/mbedtls/mbedtls_crypto.c
index 04fbc64..2a98014 100644
--- a/drivers/auth/mbedtls/mbedtls_crypto.c
+++ b/drivers/auth/mbedtls/mbedtls_crypto.c
@@ -4,10 +4,12 @@
  * SPDX-License-Identifier: BSD-3-Clause
  */
 
+#include <assert.h>
 #include <stddef.h>
 #include <string.h>
 
 /* mbed TLS headers */
+#include <mbedtls/gcm.h>
 #include <mbedtls/md.h>
 #include <mbedtls/memory_buffer_alloc.h>
 #include <mbedtls/oid.h>
@@ -17,6 +19,7 @@
 #include <drivers/auth/crypto_mod.h>
 #include <drivers/auth/mbedtls/mbedtls_common.h>
 #include <drivers/auth/mbedtls/mbedtls_config.h>
+#include <plat/common/platform.h>
 
 #define LIB_NAME		"mbed TLS"
 
@@ -226,11 +229,121 @@
 }
 #endif /* MEASURED_BOOT */
 
+#if TF_MBEDTLS_USE_AES_GCM
+/*
+ * Stack based buffer allocation for decryption operation. It could
+ * be configured to balance stack usage vs execution speed.
+ */
+#define DEC_OP_BUF_SIZE		128
+
+static int aes_gcm_decrypt(void *data_ptr, size_t len, const void *key,
+			   unsigned int key_len, const void *iv,
+			   unsigned int iv_len, const void *tag,
+			   unsigned int tag_len)
+{
+	mbedtls_gcm_context ctx;
+	mbedtls_cipher_id_t cipher = MBEDTLS_CIPHER_ID_AES;
+	unsigned char buf[DEC_OP_BUF_SIZE];
+	unsigned char tag_buf[CRYPTO_MAX_TAG_SIZE];
+	unsigned char *pt = data_ptr;
+	size_t dec_len;
+	int diff, i, rc;
+
+	mbedtls_gcm_init(&ctx);
+
+	rc = mbedtls_gcm_setkey(&ctx, cipher, key, key_len * 8);
+	if (rc != 0) {
+		rc = CRYPTO_ERR_DECRYPTION;
+		goto exit_gcm;
+	}
+
+	rc = mbedtls_gcm_starts(&ctx, MBEDTLS_GCM_DECRYPT, iv, iv_len, NULL, 0);
+	if (rc != 0) {
+		rc = CRYPTO_ERR_DECRYPTION;
+		goto exit_gcm;
+	}
+
+	while (len > 0) {
+		dec_len = MIN(sizeof(buf), len);
+
+		rc = mbedtls_gcm_update(&ctx, dec_len, pt, buf);
+		if (rc != 0) {
+			rc = CRYPTO_ERR_DECRYPTION;
+			goto exit_gcm;
+		}
+
+		memcpy(pt, buf, dec_len);
+		pt += dec_len;
+		len -= dec_len;
+	}
+
+	rc = mbedtls_gcm_finish(&ctx, tag_buf, sizeof(tag_buf));
+	if (rc != 0) {
+		rc = CRYPTO_ERR_DECRYPTION;
+		goto exit_gcm;
+	}
+
+	/* Check tag in "constant-time" */
+	for (diff = 0, i = 0; i < tag_len; i++)
+		diff |= ((const unsigned char *)tag)[i] ^ tag_buf[i];
+
+	if (diff != 0) {
+		rc = CRYPTO_ERR_DECRYPTION;
+		goto exit_gcm;
+	}
+
+	/* GCM decryption success */
+	rc = CRYPTO_SUCCESS;
+
+exit_gcm:
+	mbedtls_gcm_free(&ctx);
+	return rc;
+}
+
+/*
+ * Authenticated decryption of an image
+ */
+static int auth_decrypt(enum crypto_dec_algo dec_algo, void *data_ptr,
+			size_t len, const void *key, unsigned int key_len,
+			unsigned int key_flags, const void *iv,
+			unsigned int iv_len, const void *tag,
+			unsigned int tag_len)
+{
+	int rc;
+
+	assert((key_flags & ENC_KEY_IS_IDENTIFIER) == 0);
+
+	switch (dec_algo) {
+	case CRYPTO_GCM_DECRYPT:
+		rc = aes_gcm_decrypt(data_ptr, len, key, key_len, iv, iv_len,
+				     tag, tag_len);
+		if (rc != 0)
+			return rc;
+		break;
+	default:
+		return CRYPTO_ERR_DECRYPTION;
+	}
+
+	return CRYPTO_SUCCESS;
+}
+#endif /* TF_MBEDTLS_USE_AES_GCM */
+
 /*
  * Register crypto library descriptor
  */
 #if MEASURED_BOOT
-REGISTER_CRYPTO_LIB(LIB_NAME, init, verify_signature, verify_hash, calc_hash);
+#if TF_MBEDTLS_USE_AES_GCM
+REGISTER_CRYPTO_LIB(LIB_NAME, init, verify_signature, verify_hash, calc_hash,
+		    auth_decrypt);
 #else
-REGISTER_CRYPTO_LIB(LIB_NAME, init, verify_signature, verify_hash);
+REGISTER_CRYPTO_LIB(LIB_NAME, init, verify_signature, verify_hash, calc_hash,
+		    NULL);
+#endif
+#else /* MEASURED_BOOT */
+#if TF_MBEDTLS_USE_AES_GCM
+REGISTER_CRYPTO_LIB(LIB_NAME, init, verify_signature, verify_hash,
+		    auth_decrypt);
+#else
+REGISTER_CRYPTO_LIB(LIB_NAME, init, verify_signature, verify_hash, NULL);
+#endif
 #endif /* MEASURED_BOOT */
diff --git a/include/drivers/auth/crypto_mod.h b/include/drivers/auth/crypto_mod.h
index f211035..71cf673 100644
--- a/include/drivers/auth/crypto_mod.h
+++ b/include/drivers/auth/crypto_mod.h
@@ -13,9 +13,18 @@
 	CRYPTO_ERR_INIT,
 	CRYPTO_ERR_HASH,
 	CRYPTO_ERR_SIGNATURE,
+	CRYPTO_ERR_DECRYPTION,
 	CRYPTO_ERR_UNKNOWN
 };
 
+#define CRYPTO_MAX_IV_SIZE		16U
+#define CRYPTO_MAX_TAG_SIZE		16U
+
+/* Decryption algorithm */
+enum crypto_dec_algo {
+	CRYPTO_GCM_DECRYPT = 0
+};
+
 /*
  * Cryptographic library descriptor
  */
@@ -44,6 +53,15 @@
 			 unsigned int data_len, unsigned char *output);
 #endif /* MEASURED_BOOT */
 
+	/*
+	 * Authenticated decryption. Return one of the
+	 * 'enum crypto_ret_value' options.
+	 */
+	int (*auth_decrypt)(enum crypto_dec_algo dec_algo, void *data_ptr,
+			    size_t len, const void *key, unsigned int key_len,
+			    unsigned int key_flags, const void *iv,
+			    unsigned int iv_len, const void *tag,
+			    unsigned int tag_len);
 } crypto_lib_desc_t;
 
 /* Public functions */
@@ -54,6 +72,11 @@
 				void *pk_ptr, unsigned int pk_len);
 int crypto_mod_verify_hash(void *data_ptr, unsigned int data_len,
 			   void *digest_info_ptr, unsigned int digest_info_len);
+int crypto_mod_auth_decrypt(enum crypto_dec_algo dec_algo, void *data_ptr,
+			    size_t len, const void *key, unsigned int key_len,
+			    unsigned int key_flags, const void *iv,
+			    unsigned int iv_len, const void *tag,
+			    unsigned int tag_len);
 
 #if MEASURED_BOOT
 int crypto_mod_calc_hash(unsigned int alg, void *data_ptr,
@@ -61,21 +84,24 @@
 
 /* Macro to register a cryptographic library */
 #define REGISTER_CRYPTO_LIB(_name, _init, _verify_signature, _verify_hash, \
-							     _calc_hash) \
+			    _calc_hash, _auth_decrypt) \
 	const crypto_lib_desc_t crypto_lib_desc = { \
 		.name = _name, \
 		.init = _init, \
 		.verify_signature = _verify_signature, \
 		.verify_hash = _verify_hash, \
-		.calc_hash = _calc_hash \
+		.calc_hash = _calc_hash, \
+		.auth_decrypt = _auth_decrypt \
 	}
 #else
-#define REGISTER_CRYPTO_LIB(_name, _init, _verify_signature, _verify_hash) \
+#define REGISTER_CRYPTO_LIB(_name, _init, _verify_signature, _verify_hash, \
+			    _auth_decrypt) \
 	const crypto_lib_desc_t crypto_lib_desc = { \
 		.name = _name, \
 		.init = _init, \
 		.verify_signature = _verify_signature, \
-		.verify_hash = _verify_hash \
+		.verify_hash = _verify_hash, \
+		.auth_decrypt = _auth_decrypt \
 	}
 #endif	/* MEASURED_BOOT */
 
diff --git a/include/drivers/auth/mbedtls/mbedtls_config.h b/include/drivers/auth/mbedtls/mbedtls_config.h
index 6e179bb..dc00da7 100644
--- a/include/drivers/auth/mbedtls/mbedtls_config.h
+++ b/include/drivers/auth/mbedtls/mbedtls_config.h
@@ -79,6 +79,12 @@
 #define MBEDTLS_X509_USE_C
 #define MBEDTLS_X509_CRT_PARSE_C
 
+#if TF_MBEDTLS_USE_AES_GCM
+#define MBEDTLS_AES_C
+#define MBEDTLS_CIPHER_C
+#define MBEDTLS_GCM_C
+#endif
+
 /* MPI / BIGNUM options */
 #define MBEDTLS_MPI_WINDOW_SIZE			2
 
diff --git a/include/plat/common/platform.h b/include/plat/common/platform.h
index f5bd298..06b334d 100644
--- a/include/plat/common/platform.h
+++ b/include/plat/common/platform.h
@@ -37,6 +37,15 @@
 #define ROTPK_NOT_DEPLOYED		(1 << 1)
 
 /*******************************************************************************
+ * plat_get_enc_key_info() flags
+ ******************************************************************************/
+/*
+ * Flag used to notify caller that information provided in key buffer is an
+ * identifier rather than an actual key.
+ */
+#define ENC_KEY_IS_IDENTIFIER		(1 << 0)
+
+/*******************************************************************************
  * Function declarations
  ******************************************************************************/
 /*******************************************************************************
diff --git a/make_helpers/defaults.mk b/make_helpers/defaults.mk
index 9273469..0127607 100644
--- a/make_helpers/defaults.mk
+++ b/make_helpers/defaults.mk
@@ -65,6 +65,9 @@
 # Debug build
 DEBUG				:= 0
 
+# By default disable authenticated decryption support.
+DECRYPTION_SUPPORT		:= none
+
 # Build platform
 DEFAULT_PLAT			:= fvp