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authorTamas Ban <tamas.ban@arm.com>2018-10-12 09:28:36 +0100
committerTamas Ban <tamas.ban@arm.com>2018-12-05 16:05:19 +0100
commit788bbc17465f7b97f11617b09d5fadecf8796b2a (patch)
tree5c9fad082b7dd292eeba2f8d2e2d3c6c3255c3ca
parent229f9db2a46d7d02c596b9b8f7d8fcd181fe8bff (diff)
downloadtrusted-firmware-m-788bbc17465f7b97f11617b09d5fadecf8796b2a.tar.gz
Attest: Add a service integration guide
This patch introduces an integration guide to the Initial Attestation Service which provides a high level overview of the service and provides guidance on its usage from applications. Change-Id: I256db731173ddd1ecfbe4d2a8c3a0eda58eafe61 Signed-off-by: Tamas Ban <tamas.ban@arm.com>
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+# TF-M Initial Attestation Service Integration Guide
+
+## Introduction
+TF-M Initial Attestation Service allows the application to prove the device
+identity during an authentication process to a verification entity. The initial
+attestation service can create a token on request, which contains a fix set of
+device specific data. Device must contain an attestation key pair, which is
+unique per device. The token is signed with the private part of attestation key
+pair. The public part of the key pair is known by the verification entity. The
+public key is used to verify the token authenticity. The data items in the token
+used to verify the device integrity and assess its trustworthiness. Attestation
+key provisioning is out of scope for the attestation service and is expected to
+take part during manufacturing of the device.
+
+## Current service limitations
+
+**CBOR encoding** - The final data encoding of initial attestation token
+is going to follow the Concise Binary Object Representation, aka.
+[CBOR](http://cbor.io/).
+Currently TLV (Type-Length-Value) encoding is used.
+
+**Signing of token** - In the current implementation the token is not signed.
+Claims are concatenated in raw format and there is no signature applied. The
+final implementation is going to use signed token. Signatures are going to be
+generated according to the [COSE](https://datatracker.ietf.org/doc/rfc8152/)
+format.
+
+## Claims in the initial attestation token
+The initial attestation token is formed of claims. A claim is a data entry,
+which is represented in a key - value structure. The following fixed set of
+claims are included in the token:
+ - **Challenge** - Input object from caller. It can be a nonce value from a
+ server, or a hash value of attested data.
+ - **Client ID** - The ID of that secure partition or non-secure thread who
+ called the Initial Attestation Service.
+ - **Boot status** - Measurements(hash) of firmware components by secure
+ bootloader.
+ - **Boot seed** - Random number, which is non-volatile during the same boot
+ cycle.
+ - **Device ID** - Universally and globally unique ID of the device.
+
+## Claims encoding
+Currently claims are encoded in TLV format. The definition of TLV structure is
+described in `bl2\include\tfm_boot_status.h`. The Initial Attestation Service
+related type identifiers are described in
+`secure_fw\services\initial_attestation\attestation.h`
+
+Claim structure:
+```
+---------------------------------------------------------------
+| Major_type(uint8_t) | Minor_type(uint8_t) | Length(uit16_t) |
+---------------------------------------------------------------
+| Raw data |
+---------------------------------------------------------------
+```
+
+## Code structure
+The PSA interface for the Initial Attestation Service is located in
+`interface\include`.
+The only header to be included by applications that want to use functions from
+the PSA API is `psa_initial_attestation.h`.
+The TF-M Initial Attestation Service source files are located in
+`secure_fw\services\initial_attestation`.
+
+### PSA interface
+
+The TF-M Initial Attestation Service exposes the following PSA interface:
+
+ ``` c
+ enum psa_attest_err_t
+ psa_initial_attest_get_token(const uint8_t *challenge_obj,
+ uint32_t challenge_size,
+ uint8_t *token,
+ uint32_t *token_size);
+```
+
+### Service source files
+
+ - `attestation_core.c` : This file implements core functionalities such as
+ retrieval of claims and token creation.
+
+### SPM interface
+The Secure Partition Manager provides an interface to retrieve data from secure
+bootloader which is relevant to runtime services. The attestation token must
+contain the measurements of firmware components. These measurements are done by
+the bootloader and stored in a shared data region for runtime firmware. Only SPM
+has direct access to the shared data region. Services can retrieve relevant data
+with the following interface:
+ - `int32_t tfm_core_get_boot_data(uint8_t major_type, void *ptr,
+ uint32_t len)`
+
+This function must be called during service initialization phase, because the
+shared data region is deliberately overlapping with secure main stack to spare
+some memory and reuse this area during execution. After calling the function SPM
+copies the service relevant data, identified by `major_type`, to service's
+memory space.
+
+### Shared data structure
+The format of shared data is TLV encoded. The definition of TLV structure is
+described in `bl2\include\tfm_boot_status.h`.
+
+Structure of shared data:
+```
+---------------------------------------------------------------
+| Magic number(uint16_t) | Shared data total length(uint16_t) |
+---------------------------------------------------------------
+| Major_type(uint8_t) | Minor_type(uint8_t) | Length(uit16_t) |
+---------------------------------------------------------------
+| Raw data |
+---------------------------------------------------------------
+| . |
+| . |
+| . |
+---------------------------------------------------------------
+| Major_type(uint8_t) | Minor_type(uint8_t) | Length(uit16_t) |
+---------------------------------------------------------------
+| Raw data |
+---------------------------------------------------------------
+```
+
+Structure of TLV entry's header:
+ - **Major_type** - Identifies the secure service which the data is sent to.
+ - **Minor_type** - Identifies the type of the data.
+ - **Length** - The length of one TLV entry including the header.
+
+## Initial Attestation Service integration guide
+In order to interact with Initial Attestation Service and retrieve attestation
+token, the described PSA interface in `psa_initial_attestation.h` must be
+called. It can be called from secure and non-secure context as well.
+
+The caller must allocate a big enough buffer, where the token is going to be
+created by Initial Attestation Service. Detailed description of the interface
+can be found in the above mentioned header file.
+
+### Measured boot
+It **must** have a secure bootloader in the boot chain, which is capable to
+measure the runtime firmware components (calculates hash value of theirs).
+The data must be stored in the above described TLV format in a shared data
+region in secure internal RAM. The address and format of shared data region is
+a contract between the bootloader and the secure firmware.
+
+ --------------
+
+*Copyright (c) 2018, Arm Limited. All rights reserved.*