mbed TLS

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mbed TLS
Developer(s) Paul Bakker
Stable release

2.2.0 (November 1, 2015; 28 days ago (2015-11-01) [1]) [±]
2.1.3 (November 1, 2015; 28 days ago (2015-11-01) [1]) [±]
1.3.15 (November 1, 2015; 28 days ago (2015-11-01) [1]) [±]

1.2.18 (November 1, 2015; 28 days ago (2015-11-01) [1]) [±]
Preview release 1.4 DTLS preview (February 16, 2015; 9 months ago (2015-02-16)[2]) [±]
Written in C
Operating system Multi-platform
Type Security library
License Apache 2.0 (GPLv2 is still available that was the default previously or proprietary; PolarSSL has a GPL linking exception for free software.[3]
Website https://tls.mbed.org/

mbed TLS (previously PolarSSL) is a dual-licensed Apache 2.0 (with GPLv2 also available) implementation of the TLS and SSL protocols and the respective cryptographic algorithms and support code required. Since version 2.0, SSL is disabled by default. Stated on the website is that mbed TLS aims to be "easy to understand, use, integrate and expand".


The PolarSSL SSL library is the official continuation fork of the XySSL SSL library. XySSL was created by the French "white hat hacker" Christophe Devine and was first released on November 1, 2006, under GPL and BSD licenses. In 2008, Christophe Devine was no longer able to support XySSL and allowed Paul Bakker to create the official fork, named PolarSSL.[4] In November 2014, PolarSSL was acquired by ARM Holdings.[5]

In 2011, the Dutch government approved an integration between OpenVPN and PolarSSL, which is named OpenVPN-NL. This version of OpenVPN has been approved for use in protecting government communications up to the level of Restricted.[6]

As of the release of version 1.3.10, PolarSSL has been rebranded to mbed TLS to better show its fit inside the mbed ecosystem.[7] It was also announced that the licence would change from GPL to Apache License later in 2015 (with a goal of before October).[8]


The core SSL library is written in the C programming language and implements the SSL module, the basic cryptographic functions and provides various utility functions. Unlike OpenSSL and other implementations of TLS, mbed TLS is designed to fit on small embedded devices, with the minimum complete TLS stack requiring under 60KB of program space and under 64 KB of RAM. It is also highly modular: each component, such as a cryptographic function, can be used independently from the rest of the framework. Versions are also available for Microsoft Windows and Linux. Because mbed TLS is written in the C programming language, without external dependencies, it works on most operating systems and architectures without any trouble.

Later versions of PolarSSL than 1.3.0, add abstraction layers for memory allocation and threading to the core "to support better integration with existing embedded operating systems".[9]

Design priorities[edit]

The mbed TLS library expresses a focus on readability of the code, documentation, automated regression tests, a loosely coupled design and portable code.[10]

Development documentation[edit]

The following documentation is available for developers:

  • High Level Design;[11] This document provides a high level description of the different modules inside the library, with UML diagrams, use cases and interactions in common scenarios.
  • API documentation;[12] The API documentation is Doxygen-generated documentation from the header files of the library.
  • Source code documentation;[13] The source code of the library is documented to clarify structures, decisions and code constructs.

Automated testing[edit]

mbed TLS provides automated testing of the code and of PolarSSL's compatibility as follows:

  • A test framework is included with the source code that contains over 5000 automated tests (based on the number of tests in version 1.3.2 of the library) to test for regressions and compatibility on different platforms.
  • A continuous integration system based on Buildbot[14]
  • A compatibility script (compat.sh[15]) that tests compatibility of SSL communication with OpenSSL.


mbed TLS is used as the SSL component in large open source projects:


mbed TLS is currently available for most Operating Systems including Linux, Microsoft Windows, OS X, OpenWrt, Android, iOS and FreeRTOS. Chipsets supported at least include ARM, x86, PowerPC, MIPS.


mbed TLS supports a number of different cryptographic algorithms:

AES, Camellia, DES, RC4, RC5, Triple DES, XTEA, Blowfish
Cryptographic hash functions
MD5, MD2, MD4, SHA-1, SHA-2
Public-key cryptography
RSA, Diffie-Hellman key exchange, Elliptic curve cryptography (ECC), Elliptic curve Diffie–Hellman (ECDH), Elliptic Curve DSA (ECDSA)

See also[edit]


  1. ^ a b c d "mbed TLS 2.2.0, 2.1.3, 1.3.15 and PolarSSL 1.2.18 released". 2015-11-01. Retrieved 2015-11-19. 
  2. ^ "Download archive - mbed TLS (Previously PolarSSL)". 2015-02-16. Retrieved 2015-03-01. 
  3. ^ https://tls.mbed.org/foss-license-exception
  4. ^ "About us". PolarSSL. Retrieved 2014-05-08. 
  5. ^ "PolarSSL is now a part of ARM". 2014-11-24. 
  6. ^ [1][dead link]
  7. ^ "mbed TLS 1.3.10 released". 2015-02-08. Retrieved 2015-02-09. 
  8. ^ "PolarSSL is dead, long live mbed TLS". ARM. Retrieved 2015-02-10. 
  9. ^ "New features in PolarSSL 1.3.0 - Tech Updates". Polarssl.org. Retrieved 2014-05-08. 
  10. ^ "PolarSSL Features: easy to use SSL library and well-documented". Polarssl.org. Retrieved 2014-05-08. 
  11. ^ "PolarSSL High Level Design". Polarssl.org. Retrieved 2014-05-08. 
  12. ^ "v1.3.6 source code documentation - API Documentation". PolarSSL. Retrieved 2014-05-08. 
  13. ^ "polarssl/polarssl ¡ GitHub". Github.com. Retrieved 2014-05-08. 
  14. ^ "PolarSSL Core Features: Check out all technical details". Polarssl.org. Retrieved 2014-05-08. 
  15. ^ executable file 1038 lines (940 sloc) 39.573 kb (2014-02-13). "polarssl/tests/compat.sh at development · polarssl/polarssl · GitHub". Github.com. Retrieved 2014-05-08. 

External links[edit]