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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 thePSA Cryptography API. This is currently a preview for evaluation purposes only.

Mbed TLS should build out of the box on most systems. Some platform specific options are available in the fully documented configuration fileinclude/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 scriptscripts/config.py (use--help for usage instructions).

Compiler options can be set using conventional environment variables such asCC andCFLAGS when using the Make and CMake build system (see below).

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

The main Mbed TLS documentation is available viaReadTheDocs.

Documentation for the PSA Cryptography API is availableon GitHub.

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

1. Make sure thatDoxygen is installed.
2. Runmake apidoc.
3. Browseapidoc/index.html orapidoc/modules.html.

For other sources of documentation, see theSUPPORT document.

There are currently three active build systems used within Mbed TLS releases:

• GNU Make
• CMake
• Microsoft Visual Studio

The main systems used for development are CMake and GNU Make. Those systems are always complete and up-to-date. The others should reflect all changes present in the CMake and Make build system, although features may not be ported there automatically.

The Make and CMake build systems create three libraries: libmbedcrypto/libtfpsacrypto, libmbedx509, and libmbedtls. Note that libmbedtls depends on libmbedx509 and libmbedcrypto/libtfpsacrypto, and libmbedx509 depends on libmbedcrypto/libtfpsacrypto. As a result, some linkers will expect flags to be in a specific order, for example the GNU linker wants-lmbedtls -lmbedx509 -lmbedcrypto.

You need the following tools to build the library with the provided makefiles:

• GNU Make 3.82 or a build tool 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.
• CMake 3.10.2 or later (if using CMake).
• Microsoft Visual Studio 2017 or later (if using Visual Studio).
• Doxygen 1.8.11 or later (if building the documentation; slightly older versions should work).

Thedevelopment branch and thembedtls-3.6 long-term support branch of Mbed TLS use aGit submodule (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).

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 invokepython instead ofpython3. To install the packages system-wide, omit the--user option.
• A C compiler for the host platform, for some test data.

If you are cross-compiling, you must set theCC environment variable to a C compiler for the host platform when generating the configuration-independent files.

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

• If not cross-compiling, runningmake with any target, or justmake, will automatically generate required files.
• On non-Windows systems, when not cross-compiling, CMake will generate the required files automatically.
• Runmake generated_files to generate all the configuration-independent files.
• On Unix/POSIX systems, runtests/scripts/check-generated-files.sh -u to generate all the configuration-independent files.
• On Windows, runscripts\make_generated_files.bat to generate all the configuration-independent files.

We require GNU Make. To build the library and the sample programs, GNU Make and a C compiler are sufficient. Some of the more advanced build targets require some Unix/Linux tools.

We intentionally only use a minimum of functionality in the makefiles in order to keep them as simple and independent of different toolchains as possible, to allow users to more easily move between different platforms. Users who need more features are recommended to use CMake.

In order to build from the source code using GNU Make, just enter at the command line:

make

In order to run the tests, enter:

make check

The tests need Python to be built and Perl to be run. If you don't have one of them installed, you can skip building the tests with:

make no_test

You'll still be able to run a much smaller set of tests with:

programs/test/selftest

In order to build for a Windows platform, you should useWINDOWS_BUILD=1 if the target is Windows but the build environment is Unix-like (for instance when cross-compiling, or compiling from an MSYS shell), andWINDOWS=1 if the build environment is a Windows shell (for instance using mingw32-make) (in that case some targets will not be available).

Setting the variableSHARED in your environment will build shared libraries in addition to the static libraries. SettingDEBUG gives you a debug build. You can overrideCFLAGS andLDFLAGS by setting them in your environment or on the make command line; compiler warning options may be overridden separately usingWARNING_CFLAGS. Some directory-specific options (for example,-I directives) are still preserved.

Please note that settingCFLAGS overrides its default value of-O2 and settingWARNING_CFLAGS overrides its default value (starting with-Wall -Wextra), so if you just want to add some warning options to the default ones, you can do so by settingCFLAGS=-O2 -Werror for example. SettingWARNING_CFLAGS is useful when you want to get rid of its default content (for example because your compiler doesn't accept-Wall as an option). Directory-specific options cannot be overridden from the command line.

Depending on your platform, you might run into some issues. Please check the Makefiles inlibrary/,programs/ andtests/ for options to manually add or remove for specific platforms. You can also checkthe Mbed TLS Knowledge Base for articles on your platform or issue.

In case you find that you need to do something else as well, please let us know what, so we can add it to theMbed TLS Knowledge Base.

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

mkdir /path/to/build_dir && cd /path/to/build_dircmake /path/to/mbedtls_sourcecmake --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 modes available within the CMake buildsystem. 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 modes 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 theinitial invocation of cmake. This means thatCC=your_cc make andmake CC=your_cc willnot work (similarly withCFLAGS 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 toremove the build directory and create it again.

Note that it is possible to build in-place; this will however overwrite theprovided Makefiles (seescripts/tmp_ignore_makefiles.sh if you want topreventgit status from showing them as modified). In order to do so, fromthe Mbed TLS source directory, use:

cmake .make

If you want to changeCC orCFLAGS afterwards, you will need to remove theCMake 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 (dependingon the build mode as seen above), it's merely prepended to it.

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

find_package(MbedTLS)

If prompted, setMbedTLS_DIR to${YOUR_MBEDTLS_INSTALL_DIR}/cmake. Thiscreates the following targets:

• MbedTLS::tfpsacrypto (Crypto library)
• MbedTLS::mbedtls (TLS library)
• MbedTLS::mbedx509 (X509 library)

You can then use these directly throughtarget_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 addits include directories to your target (transitively, in the case ofPUBLIC orINTERFACE link libraries).

Mbed TLS supports being built as a CMake subproject. One canuseadd_subdirectory() from a parent CMake project to include Mbed TLS as asubproject.

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

The solution filembedTLS.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 inprograms/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”.

We've included example programs for a lot of different features and uses inprograms/.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.

Mbed TLS includes an elaborate test suite intests/ that initially requires Python to generate the tests files (e.g.test\_suite\_ssl.c). These files are generated from afunction file (e.g.suites/test\_suite\_ssl.function) and adata file (e.g.suites/test\_suite\_ssl.data). Thefunction file contains the test functions. Thedata 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, fullmbedtls_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 inour testing infrastructure repository.

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
• What external dependencies does Mbed TLS rely on?
• How do I configure Mbed TLS

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 andsize_t must be at least 32 bits wide.
• The typesuint8_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.

Arm'sPlatform Security Architecture (PSA) 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.

ThePSA cryptography API 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 atmbed-crypto@arm.com. All feedback received by email is treated confidentially.

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.

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 thePSA driver example and guide for information on writing a driver.

Unless specifically indicated otherwise in a file, Mbed TLS files are provided under a dualApache-2.0 ORGPL-2.0-or-later license. See theLICENSE file for the full text of these licenses, andthe 'License and Copyright' section in the contributing guidelines for more information.

This project contains code from other projects. This code is located within thetf-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 fromProject Everest and are distributed under the Apache 2.0 license.
• drivers/p256-m/p256-m/: Files have been taken from thep256-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.

We gratefully accept bug reports and contributions from the community. Please see thecontributing guidelines for details on how to do this.

• To report a security vulnerability in Mbed TLS, please emailmbed-tls-security@lists.trustedfirmware.org. For more information, seeSECURITY.md.
• To report a bug or request a feature in Mbed TLS, pleasefile an issue on GitHub.
• Please seeSUPPORT.md for other channels for discussion and support about Mbed TLS.