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TF-M Initial Attestation Service Integration Guide
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************
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
***************************
**Signing of token**: In the current implementation the token is not properly
signed. Signature is generated according to the
`COSE <https://datatracker.ietf.org/doc/rfc8152/>`__ format. But its actual
value is not a correct ECDSA P256 signature, due to the lack of support of the
ECDSA algorithm in the current implementation of the TF-M Crypto service. A fake
signature is created, which is the concatenation of the token's hash value
twice.
***************************************
Claims in the initial attestation token
***************************************
The initial attestation token is formed of claims. A claim is a data item,
which is represented in a key - value structure. The following fixed set of
claims are included in the token:
- **Challenge**: Input object from caller. Can be a single nonce from
server or hash of nonce and attested data. It is intended to provide
freshness to report and the caller has responsibility to arrange
this. Allowed length: 32, 48, 64 bytes. The claim is modeled to be
eventually represented by the EAT standard claim nonce. Until such a
time as that standard exists, the claim will be represented by a custom
claim. Value is encoded as byte string.
- **Instance ID**: It represents the unique identifier of the instance. In
the PSA definition it is a hash of the public attestation key of the
instance. The claim is modeled to be eventually represented by the EAT
standard claim UEID of type GUID. Until such a time as that standard
exists, the claim will be represented by a custom claim Value is encoded
as byte string.
- **Verification service indicator**: Optional, recommended claim. It
is used by a Relying Party to locate a validation service for the
token. The value is a text string that can be used to locate the service
or a URL specifying the address of the service. The claim is modelled to
be eventually represented by the EAT standard claim origination. Until
such a time as that standard exists, the claim will be represented by
a custom claim. Value is encoded as text string.
- **Profile definition**: Optional, recommended claim. It contains the
name of a document that describes the 'profile' of the token, being
a full description of the claims, their usage, verification and token
signing. The document name may include versioning. Custom claim with a
value encoded as text string.
- **Implementation ID**: It represents the original implementation
signer of the attestation key and identifies the contract between the
report and verification. A verification service will use this claim
to locate the details of the verification process. Custom claim with a
value encoded as byte string.
- **Security lifecycle**: It represents the current lifecycle state of
the instance. Custom claim with a value encoded as an integer.
- **Client ID**: The partition ID of that secure partition or non-secure
thread who called the initial attestation API. Custom claim with a value
encoded as a `signed` integer. Negative number represents non-secure
caller, positive numbers represents secure callers, zero is invalid.
- **HW version**: Optional claim. Globally unique number in EAN-13 format
identifying the GDSII that went to fabrication, HW and ROM. It can be
used to reference the security level of the PSA-ROT via a certification
website. Custom claim with a value is encoded as text string.
- **Boot seed**: It represents a random value created at system boot
time that will allow differentiation of reports from different system
sessions. The size is 32 bytes. Custom claim with a value is encoded as
byte string.
- **Software components**: Optional, recommended claim. It represents
the software state of the system. The value of the claim is an array
of CBOR map entries, with one entry per software component within the
device. Each map contains multiple claims that describe evidence about
the details of the software component.
- **Measurement type**: Optional claim. It represents the role of the
software component. Value is encoded as short(!) text string.
- **Measurement value**: It represents a hash of the invariant software
component in memory at start-up time. The value must be a cryptographic
hash of 256 bits or stronger. Value is encoded as byte string.
- **Security epoch**: Optional claim. It represents the security control
point of the software component. Value is encoded as unsigned integer.
- **Version**: Optional claim. It represents the issued software
version. Value is encoded as text string.
- **Signer ID**: It represents the hash of a signing authority public key.
Value is encoded as byte string.
- **Measurement description**: Optional claim. It represents the way in
which the measurement value of the software component is computed. Value
is encoded as text string containing an abbreviated description (name)
of the measurement method.
- **No software measurements**: In the event that the implementation
does not contain any software measurements then the software components
claim above can be omitted but instead it is mandatory to include this
claim to indicate this is a deliberate state. Custom claim a value is
encoded as an unsigned integer set to 1.
*********************************************
Initial attestation token (IAT) data encoding
*********************************************
The initial attestation token is planned to be aligned with future version of
`Entity Attestation Token <https://tools.ietf.org/html/draft-mandyam-eat-01>`__
format. The token is encoded according to the
`CBOR <https://tools.ietf.org/html/rfc7049>`__ format and signed according to
`COSE <https://tools.ietf.org/html/rfc8152>`__ standard.
**************
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``.
The CBOR library is located in ``lib/ext/qcbor`` folder.
Service source files
====================
- CBOR library
- ``lib/ext/qcbor`` This library is used to create a proper CBOR token.
It can be used on 32-bit and 64-bit machines. It was designed to suite
constrained devices with low memory usage and without dynamic memory
allocation.
It is a fork of this external `QCBOR library <https://github.com/laurencelundblade/QCBOR>`__.
- ``lib/ext/qcbor/inc/qcbor.h``: Public API documentation of CBOR
library.
- COSE library:
- ``lib/t_cose``: This library is used to sign a CBOR token and create
the COSE header and signature around the initial attestation token. Only
a subset of the `COSE <https://tools.ietf.org/html/rfc8152>`__ standard
is implemented. Only the cose_sign1 signature schema is supported.
- ``lib/t_cose/src/t_cose_crypto.h``: Expose an API to bind ``t_cose``
library with available crypto library in the device.
- ``lib/t_cose/src/t_cose_psa_crypto.c``: Implements the exposed API
and ports ``t_cose`` to psa_crypto library.
- Initial Attestation Service:
- ``attestation_core.c`` : Implements core functionalities such as
implementation of APIs, retrieval of claims and token creation.
- ``attest_token.c``: Implements the token creation function such as
start and finish token creation and adding claims to the token.
- ``attestation_crypto_stub.c``: Temporary file, it implements the
missing psa_crypto APIs.
- ``attestation_key.c``: Get the attestation key from platform layer
and register it to psa_crypto service for further usage.
- ``tfm_attestation.c``: Implements the SPM abstraction layer, and bind
the attestation service to the SPM implementation in TF-M project.
- ``tfm_attestation_secure_api.c``: Implements the secure API layer to
allow other services in the secure domain to request functionalities
from the attestation service using the PSA API interface.
Service interface definitions
=============================
- **Boot loader interface**: The attestation service might include data
in the token about the distinct software components in the device. This data
is provided by the boot loader and must be encoded in the TLV format,
definition is described below in the boot loader interface paragraph. Possible
claims in the boot status are describe above in the software components
paragraph.
- **Hardware abstraction layer**:
- Headers are located in ``platform/include`` folder.
- ``tfm_attest_hal.h``: Expose an API to get the following claims:
security lifecycle, verification service indicator, profile definition.
- ``tfm_plat_boot_seed.h``: Expose an API to get the boot seed claim.
- ``tfm_plat_device_id.h``: Expose an API to get the following claims:
implementation ID, hardware version, instance ID.
- **SPM interface**:
- ``attestation.h``: Expose an API to bind attestation service to an SPM
implementation.
- **PSA interface**:
- ``psa_initial_attestation.h``: Public API definition of initial
attestation service.
- **Crypto interface**:
- ``t_cose_crypto.h``: Expose an API to bind the ``t_cose`` implementation
to any cryptographic library.
- ``tfm_plat_crypto_keys.h``: Expose an API to get the attestation key from
platform layer.
PSA interface
=============
The TF-M Initial Attestation Service exposes the following PSA
interface:
.. code-block:: 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);
enum psa_attest_err_t
psa_initial_attest_get_token_size(uint32_t challenge_size,
uint32_t *token_size);
The caller must allocate a large enough buffer, where the token is going to be
created by Initial Attestation Service. The size of the created token is highly
dependent on the number of software components in the system and the provided
attributes of these. The ``psa_initial_attest_get_token_size()`` function can be
called to get the exact size of the created token.
System integrators might need to port these interfaces to a custom secure
partition manager implementation (SPM). Implementation in TF-M project can be
found here:
- ``interface/src/tfm_initial_attestation_api.c``: non-secure interface
implementation
- ``secure_fw/services/initial_attestation/tfm_attestation_secure_api.c``:
secure interface implementation
Secure Partition Manager (SPM) interface
========================================
The Initial Attestation Service defines the following interface towards the
secure partition manager (SPM). System integrators **must** port this interface
according to their SPM implementation.
.. code:: c
enum psa_attest_err_t
attest_get_boot_data(uint8_t major_type, void *ptr, uint32_t len);
enum psa_attest_err_t
attest_get_caller_client_id(int32_t *caller_id);
enum psa_attest_err_t
attest_check_memory_access(void *addr,
uint32_t size,
enum attest_memory_access access);
- ``attest_get_boot_data()``: Service can retrieve the relevant data from shared
memory area between boot loader and runtime software. It might be the case
that only SPM has direct access to the shared memory area, therefore this
function can be used to copy the service related data from shared memory to
a local memory buffer. In TF-M implementation this function must be called
during service initialization phase, because the shared memory region is
deliberately overlapping with secure main stack to spare some memory and reuse
this area during execution. If boot loader is not available in the system to
provide attributes of software components then this function must be
implemented in a way that just initialize service's memory buffer to:
.. code-block:: c
struct shared_data_tlv_header *tlv_header = (struct shared_data_tlv_header *)ptr;
tlv_header->tlv_magic = 2016;
tlv_header->tlv_tot_len = sizeof(struct shared_data_tlv_header *tlv_header);
- ``attest_get_caller_client_id()``: Retrieves the ID of the caller thread.
- ``attest_check_memory_access()``: Validates the availability and access rights
of memory regions received as input data: challenge object, token buffer, etc.
- ``tfm_client.h``: Service relies on the following external definitions, which
must be present or included in this header file:
.. code-block:: c
typedef struct psa_invec {
const void *base;
size_t len;
} psa_invec;
typedef struct psa_outvec {
void *base;
size_t len;
} psa_outvec;
Hardware abstraction layer
==========================
The following API definitions are intended to retrieve the platform specific
claims. System integrators **must** implement these interface according to their
SoC and software design. Detailed definition of the claims are above
in the claims in the initial attestation token paragraph.
- ``tfm_attest_hal_get_security_lifecycle()``: Get the security lifecycle of the
device.
- ``tfm_attest_hal_get_verification_service()``: Get the verification
service indicator for initial attestation.
- ``tfm_attest_hal_get_profile_definition()``: Get the name of the profile
definition document for initial attestation.
- ``tfm_plat_get_boot_seed()``: Get the boot seed, which is a constant random
number during a boot cycle.
- ``tfm_plat_get_instance_id()``: Get the UEID of the device.
- ``tfm_plat_get_implementation_id``: Get the implementation ID of the
device.
- ``tfm_plat_get_hw_version``: Get the hardware version of the device.
Boot loader interface
=====================
It is **recommended** to have a secure boot loader in the boot chain, which is
capable of measuring the runtime firmware components (calculates the hash value
of firmware images) and provide other attributes of these (version, type, etc).
The shared data between boot loader and runtime software is TLV encoded. The
definition of TLV structure is described in ``bl2/include/tfm_boot_status.h``.
The shared data is stored in a well known location in secure internal memory
and this is a contract between boot loader and runtime SW.
The structure of shared data must be the following:
- At the beginning there must be a header: ``struct shared_data_tlv_header``
This contains a magic number and a size field which covers the entire size
of the shared data area including this header.
.. code-block:: c
struct shared_data_tlv_header {
uint16_t tlv_magic;
uint16_t tlv_tot_len;
};
- After the header there come the entries which are composed from an
entry header structure: ``struct shared_data_tlv_entry`` and the data. In
the entry header is a type field ``tlv_type`` which identify the consumer of
the entry in the runtime software and specify the subtype of that data item.
There is a size field ``tlv_len`` which covers the size of the entry header
and the data. After this structure comes the actual data.
.. code-block:: c
struct shared_data_tlv_entry {
uint16_t tlv_type;
uint16_t tlv_len;
};
- Arbitrary number and size of data entry can be in the shared memory
area.
The figure below gives of overview about the ``tlv_type`` field in the entry
header. The ``tlv_type`` always composed from a major and minorbnumber. Major
number identifies the addressee in runtime software, which the databentry is
sent to. Minor number used to encode more info about the data entry. The actual
definition of minor number could change per major number. In case of boot
status data, which is going to be processed by initial attestation service
the minor number is split further to two part: ``sw_module`` and ``claim``. The
``sw_module`` identifies the SW component in the system which the data item
belongs to and the ``claim`` part identifies the exact type of the data.
``tlv_type`` description::
|------------------------------------------------ |
| tlv_type (16 bits) |
|-------------------------------------------------|
| tlv_major(4 bits) | tlv_minor(12 bits) |
|-------------------------------------------------|
| MAJOR_IAS | sw_module(6 bits) | claim(6 bits) |
|-------------------------------------------------|
| MAJOR_CORE | TBD |
|-------------------------------------------------|
Overall structure of shared data::
---------------------------------------------------------------
| Magic number(uint16_t) | Shared data total length(uint16_t) |
---------------------------------------------------------------
| Major_type(4 bits) | Minor_type(12 bits) | Length(uint16_t) |
---------------------------------------------------------------
| Raw data |
---------------------------------------------------------------
| . |
| . |
| . |
---------------------------------------------------------------
| Major_type(4 bits) | Minor_type(12 bits) | Length(uint16_t) |
---------------------------------------------------------------
| Raw data |
---------------------------------------------------------------
Crypto interface
================
Device **must** contain an asymmetric key pair. The private part of it is used
to sign the initial attestation token. Current implementation supports only the
ECDSA P256 signature over SHA256. The public part of the key pair is used to
create the key identifier (kid) in the unprotected part of the COSE header. The
kid is used by verification entity to look up the corresponding public key to
verify the signature in the token. The `t_cose` part of the initial attestation
service implements the signature generation and kid creation. But the actual
calculation of token's hash and signature is done by the Crypto service in the
device. System integrators might need to re-implement the following functions
if they want to use initial attestation service with a different cryptographic
library than Crypto service:
- ``t_cose_crypto_pub_key_sign()``: Calculates the signature over a hash value.
- ``t_cose_crypto_get_ec_pub_key()``: Get the public key to create the key
identifier.
- ``t_cose_crypto_hash_start()``: Start a multipart hash operation.
- ``t_cose_crypto_hash_update()``: Add a message fragment to a multipart hash
operation.
- ``t_cose_crypto_hash_finish()``:Finish the calculation of the hash of a
message.
Interface needed by verification code:
- ``t_cose_crypto_pub_key_verify()``: Verify the signature over a hash value.
Key handling
------------
The provisioning of the initial attestation key is out of scope of the service
and this document. It is assumed that device maker provisions the unique
asymmetric key pair during the manufacturing process. The following API is
defined to retrieve the attestation key pair from platform layer. Software
integrators **must** port this interface according to their SoC design and make
sure that key pair is available by Crypto service:
- ``tfm_plat_get_initial_attest_key()``: Retrieve the initial attestation key
pair from platform layer.
In TF-M project the attestation key is retrieved by initial attestation service.
The key is registered and unregistered to the Crypto service by attestation
service with ``psa_import_key()`` and ``psa_destroy_key()`` API calls for
further usage. See in ``attestation_key.c``. In other implementation if the
attestation key is directly retrieved by the Crypto service then this key
handling is not necessary.
--------------
*Copyright (c) 2018-2019, Arm Limited. All rights reserved.*