README.md

README for Mbed TLS
===================

Mbed TLS is a C library that implements cryptographic primitives, X.509 certificate manipulation and the SSL/TLS and DTLS protocols. Its small code footprint makes it suitable for embedded systems.

Mbed TLS includes a reference implementation of the [PSA Cryptography API](#psa-cryptography-api). This is currently a preview for evaluation purposes only.

Configuration
-------------

Mbed TLS should build out of the box on most systems. Some platform specific options are available in the fully documented configuration file `include/mbedtls/mbedtls_config.h`, which is also the place where features can be selected. This file can be edited manually, or in a more programmatic way using the Python 3 script `scripts/config.py` (use `--help` for usage instructions).

Compiler options can be set using conventional environment variables such as `CC` and `CFLAGS`.

We provide some non-standard configurations focused on specific use cases in the `configs/` directory. You can read more about those in `configs/README.txt`

Documentation
-------------

The main Mbed TLS documentation is available via [ReadTheDocs](https://mbed-tls.readthedocs.io/).

Documentation for the PSA Cryptography API is available [on GitHub](https://arm-software.github.io/psa-api/crypto/).

To generate a local copy of the library documentation in HTML format, tailored to your compile-time configuration:

1. Make sure that [Doxygen](http://www.doxygen.nl/) is installed.
1. Run `mkdir /path/to/build_dir && cd /path/to/build_dir`
1. Run `cmake /path/to/mbedtls/source`
1. Run `make apidoc`
1. Browse `apidoc/index.html` or `apidoc/modules.html`.

For other sources of documentation, see the [SUPPORT](SUPPORT.md) document.

Compiling
---------

We use CMake to configure and drive our build process. Three libraries are built: libtfpsacrypto, libmbedx509, and libmbedtls. Note that libmbedtls depends on libmbedx509 and libtfpsacrypto, and libmbedx509 depends on libtfpsacrypto. As a result, some linkers will expect flags to be in a specific order, for example the GNU linker wants `-lmbedtls -lmbedx509 -ltfpsacrypto`.

### Tool versions

You need the following tools to build the library:

* CMake 3.10.2 or later.
* A build system that CMake supports.
* A C99 toolchain (compiler, linker, archiver). We actively test with GCC 5.4, Clang 3.8, Arm Compiler 6, IAR 8 and Visual Studio 2017. More recent versions should work. Slightly older versions may work.
* Python 3.8 to generate the test code. Python is also needed to integrate PSA drivers and to build the development branch (see next section).
* Perl to run the tests, and to generate some source files in the development branch.
* Doxygen 1.8.11 or later (if building the documentation; slightly older versions should work).

### Git usage

The `development` branch and the `mbedtls-3.6` long-term support branch of Mbed TLS use a [Git submodule](https://git-scm.com/book/en/v2/Git-Tools-Submodules#_cloning_submodules) ([framework](https://github.com/Mbed-TLS/mbedtls-framework)). This is not needed to merely compile the library at a release tag. This is not needed to consume a release archive (zip or tar).

### Generated source files in the development branch

The source code of Mbed TLS includes some files that are automatically generated by scripts and whose content depends only on the Mbed TLS source, not on the platform or on the library configuration. These files are not included in the development branch of Mbed TLS, but the generated files are included in official releases. This section explains how to generate the missing files in the development branch.

The following tools are required:

* Perl, for some library source files and for Visual Studio build files.
* Python 3.8 and some Python packages, for some library source files, sample programs and test data. To install the necessary packages, run:
    ```
    python3 -m pip install --user -r scripts/basic.requirements.txt
    ```
    Depending on your Python installation, you may need to invoke `python` instead of `python3`. To install the packages system-wide, omit the `--user` option.
* A C compiler for the host platform, for some test data.

The scripts that generate the configuration-independent files will look for a host C compiler in the following places (in order of preference):

1. The `HOSTCC` environment variable. This can be used if `CC` is pointing to a cross-compiler.
2. The `CC` environment variable.
3. An executable called `cc` in the current path.

Note: If you have multiple toolchains installed, it is recommended to set `CC` or `HOSTCC` to the intended host compiler before generating the files.

Any of the following methods are available to generate the configuration-independent files:

* On non-Windows systems, when not cross-compiling, CMake generates the required files automatically.
* Run `framework/scripts/make_generated_files.py` to generate all the configuration-independent files.

### CMake

In order to build the libraries using CMake in a separate directory (recommended), just enter at the command line:

    mkdir /path/to/build_dir && cd /path/to/build_dir
    cmake /path/to/mbedtls_source
    cmake --build .

In order to run the tests, enter:

    ctest

The test suites need Python to be built and Perl to be executed. If you don't have one of these installed, you'll want to disable the test suites with:

    cmake -DENABLE_TESTING=Off /path/to/mbedtls_source

If you disabled the test suites, but kept the programs enabled, you can still run a much smaller set of tests with:

    programs/test/selftest

To configure CMake for building shared libraries, use:

    cmake -DUSE_SHARED_MBEDTLS_LIBRARY=On /path/to/mbedtls_source

There are many different build types available with CMake. Most of them are available for gcc and clang, though some are compiler-specific:

-   `Release`. This generates the default code without any unnecessary information in the binary files.
-   `Debug`. This generates debug information and disables optimization of the code.
-   `Coverage`. This generates code coverage information in addition to debug information.
-   `ASan`. This instruments the code with AddressSanitizer to check for memory errors. (This includes LeakSanitizer, with recent version of gcc and clang.) (With recent version of clang, this mode also instruments the code with UndefinedSanitizer to check for undefined behaviour.)
-   `ASanDbg`. Same as ASan but slower, with debug information and better stack traces.
-   `MemSan`. This instruments the code with MemorySanitizer to check for uninitialised memory reads. Experimental, needs recent clang on Linux/x86\_64.
-   `MemSanDbg`. Same as MemSan but slower, with debug information, better stack traces and origin tracking.
-   `Check`. This activates the compiler warnings that depend on optimization and treats all warnings as errors.

Switching build types in CMake is simple. For debug mode, enter at the command line:

    cmake -D CMAKE_BUILD_TYPE=Debug /path/to/mbedtls_source

To list other available CMake options, use:

    cmake -LH

Note that, with CMake, you can't adjust the compiler or its flags after the
initial invocation of cmake. This means that `CC=your_cc make` and `make
CC=your_cc` will *not* work (similarly with `CFLAGS` and other variables).
These variables need to be adjusted when invoking cmake for the first time,
for example:

    CC=your_cc cmake /path/to/mbedtls_source

If you already invoked cmake and want to change those settings, you need to
remove the build directory and create it again.

Note that it is possible to build in-place; this will however overwrite the
legacy Makefiles still used for testing purposes (see
`scripts/tmp_ignore_makefiles.sh` if you want to prevent `git status` from
showing them as modified). In order to do so, from the Mbed TLS source
directory, use:

    cmake .
    make

If you want to change `CC` or `CFLAGS` afterwards, you will need to remove the
CMake cache. This can be done with the following command using GNU find:

    find . -iname '*cmake*' -not -name CMakeLists.txt -exec rm -rf {} +

You can now make the desired change:

    CC=your_cc cmake .
    make

Regarding variables, also note that if you set CFLAGS when invoking cmake,
your value of CFLAGS doesn't override the content provided by cmake (depending
on the build mode as seen above), it's merely prepended to it.

#### Consuming Mbed TLS

Mbed TLS provides a package config file for consumption as a dependency in other
CMake projects. You can include Mbed TLS's CMake targets yourself with:

    find_package(MbedTLS)

If prompted, set `MbedTLS_DIR` to `${YOUR_MBEDTLS_INSTALL_DIR}/cmake`. This
creates the following targets:

- `MbedTLS::tfpsacrypto` (Crypto library)
- `MbedTLS::mbedtls` (TLS library)
- `MbedTLS::mbedx509` (X509 library)

You can then use these directly through `target_link_libraries()`:

    add_executable(xyz)

    target_link_libraries(xyz
        PUBLIC MbedTLS::mbedtls
               MbedTLS::tfpsacrypto
               MbedTLS::mbedx509)

This will link the Mbed TLS libraries to your library or application, and add
its include directories to your target (transitively, in the case of `PUBLIC` or
`INTERFACE` link libraries).

#### Mbed TLS as a subproject

Mbed TLS supports being built as a CMake subproject. One can
use `add_subdirectory()` from a parent CMake project to include Mbed TLS as a
subproject.

### Microsoft Visual Studio

The build files for Microsoft Visual Studio are generated for Visual Studio 2017.

The solution file `mbedTLS.sln` contains all the basic projects needed to build the library and all the programs. The files in tests are not generated and compiled, as these need Python and perl environments as well. However, the selftest program in `programs/test/` is still available.

In the development branch of Mbed TLS, the Visual Studio solution files need to be generated first as described in [“Generated source files in the development branch”](#generated-source-files-in-the-development-branch).

Example programs
----------------

We've included example programs for a lot of different features and uses in [`programs/`](programs/README.md).
Please note that the goal of these sample programs is to demonstrate specific features of the library, and the code may need to be adapted to build a real-world application.

Tests
-----

Mbed TLS includes an elaborate test suite in `tests/` that initially requires Python to generate the tests files (e.g. `test\_suite\_ssl.c`). These files are generated from a `function file` (e.g. `suites/test\_suite\_ssl.function`) and a `data file` (e.g. `suites/test\_suite\_ssl.data`). The `function file` contains the test functions. The `data file` contains the test cases, specified as parameters that will be passed to the test function.

For machines with a Unix shell and OpenSSL (and optionally GnuTLS) installed, additional test scripts are available:

-   `tests/ssl-opt.sh` runs integration tests for various TLS options (renegotiation, resumption, etc.) and tests interoperability of these options with other implementations.
-   `tests/compat.sh` tests interoperability of every ciphersuite with other implementations.
-   `tests/scripts/test-ref-configs.pl` test builds in various reduced configurations.
-   `tests/scripts/depends.py` test builds in configurations with a single curve, key exchange, hash, cipher, or pkalg on.
-   `tests/scripts/all.sh` runs a combination of the above tests, plus some more, with various build options (such as ASan, full `mbedtls_config.h`, etc).

Instead of manually installing the required versions of all tools required for testing, it is possible to use the Docker images from our CI systems, as explained in [our testing infrastructure repository](https://github.com/Mbed-TLS/mbedtls-test/blob/main/README.md#quick-start).

Porting Mbed TLS
----------------

Mbed TLS can be ported to many different architectures, OS's and platforms. Before starting a port, you may find the following Knowledge Base articles useful:

-   [Porting Mbed TLS to a new environment or OS](https://mbed-tls.readthedocs.io/en/latest/kb/how-to/how-do-i-port-mbed-tls-to-a-new-environment-OS/)
-   [What external dependencies does Mbed TLS rely on?](https://mbed-tls.readthedocs.io/en/latest/kb/development/what-external-dependencies-does-mbedtls-rely-on/)
-   [How do I configure Mbed TLS](https://mbed-tls.readthedocs.io/en/latest/kb/compiling-and-building/how-do-i-configure-mbedtls/)

Mbed TLS is mostly written in portable C99; however, it has a few platform requirements that go beyond the standard, but are met by most modern architectures:

- Bytes must be 8 bits.
- All-bits-zero must be a valid representation of a null pointer.
- Signed integers must be represented using two's complement.
- `int` and `size_t` must be at least 32 bits wide.
- The types `uint8_t`, `uint16_t`, `uint32_t` and their signed equivalents must be available.
- Mixed-endian platforms are not supported.
- SIZE_MAX must be at least as big as INT_MAX and UINT_MAX.

PSA cryptography API
--------------------

### PSA API

Arm's [Platform Security Architecture (PSA)](https://developer.arm.com/architectures/security-architectures/platform-security-architecture) is a holistic set of threat models, security analyses, hardware and firmware architecture specifications, and an open source firmware reference implementation. PSA provides a recipe, based on industry best practice, that allows security to be consistently designed in, at both a hardware and firmware level.

The [PSA cryptography API](https://arm-software.github.io/psa-api/crypto/) provides access to a set of cryptographic primitives. It has a dual purpose. First, it can be used in a PSA-compliant platform to build services, such as secure boot, secure storage and secure communication. Second, it can also be used independently of other PSA components on any platform.

The design goals of the PSA cryptography API include:

* The API distinguishes caller memory from internal memory, which allows the library to be implemented in an isolated space for additional security. Library calls can be implemented as direct function calls if isolation is not desired, and as remote procedure calls if isolation is desired.
* The structure of internal data is hidden to the application, which allows substituting alternative implementations at build time or run time, for example, in order to take advantage of hardware accelerators.
* All access to the keys happens through key identifiers, which allows support for external cryptoprocessors that is transparent to applications.
* The interface to algorithms is generic, favoring algorithm agility.
* The interface is designed to be easy to use and hard to accidentally misuse.

Arm welcomes feedback on the design of the API. If you think something could be improved, please open an issue on our Github repository. Alternatively, if you prefer to provide your feedback privately, please email us at [`mbed-crypto@arm.com`](mailto:mbed-crypto@arm.com). All feedback received by email is treated confidentially.

### PSA implementation in Mbed TLS

Mbed TLS includes a reference implementation of the PSA Cryptography API.
However, it does not aim to implement the whole specification; in particular it does not implement all the algorithms.

### PSA drivers

Mbed TLS supports drivers for cryptographic accelerators, secure elements and random generators. This is work in progress. Please note that the driver interfaces are not fully stable yet and may change without notice. We intend to preserve backward compatibility for application code (using the PSA Crypto API), but the code of the drivers may have to change in future minor releases of Mbed TLS.

Please see the [PSA driver example and guide](https://github.com/Mbed-TLS/TF-PSA-Crypto/blob/development/docs/psa-driver-example-and-guide.md) for information on writing a driver.

License
-------

Unless specifically indicated otherwise in a file, Mbed TLS files are provided under a dual [Apache-2.0](https://spdx.org/licenses/Apache-2.0.html) OR [GPL-2.0-or-later](https://spdx.org/licenses/GPL-2.0-or-later.html) license. See the [LICENSE](LICENSE) file for the full text of these licenses, and [the 'License and Copyright' section in the contributing guidelines](CONTRIBUTING.md#License-and-Copyright) for more information.

### Third-party code included in Mbed TLS

This project contains code from other projects. This code is located within the `tf-psa-crypto/drivers/` directory. The original license text is included within project subdirectories, where it differs from the normal Mbed TLS license, and/or in source files. The projects are listed below:

* `drivers/everest/`: Files stem from [Project Everest](https://project-everest.github.io/) and are distributed under the Apache 2.0 license.
* `drivers/p256-m/p256-m/`: Files have been taken from the [p256-m](https://github.com/mpg/p256-m) repository. The code in the original repository is distributed under the Apache 2.0 license. It is distributed in Mbed TLS under a dual Apache-2.0 OR GPL-2.0-or-later license with permission from the author.

Contributing
------------

We gratefully accept bug reports and contributions from the community. Please see the [contributing guidelines](CONTRIBUTING.md) for details on how to do this.

Contact
-------

* To report a security vulnerability in Mbed TLS, please email <mbed-tls-security@lists.trustedfirmware.org>. For more information, see [`SECURITY.md`](SECURITY.md).
* To report a bug or request a feature in Mbed TLS, please [file an issue on GitHub](https://github.com/Mbed-TLS/mbedtls/issues/new/choose).
* Please see [`SUPPORT.md`](SUPPORT.md) for other channels for discussion and support about Mbed TLS.