mbedtls/BRANCHES.md

Maintained branches

At any point in time, we have a number of maintained branches, currently consisting of:

• The main branch: this always contains the latest release, including all publicly available security fixes.
• The development branch: this is where the next major version of Mbed TLS (version 4.0) is being prepared. It has API changes that make it incompatible with Mbed TLS 3.x, as well as all the new features and bug fixes and security fixes.
• One or more long-time support (LTS) branches: these only get bug fixes and security fixes. Currently, the supported LTS branches are:
• mbedtls-2.28.
• mbedtls-3.6.

We retain a number of historical branches, whose names are prefixed by archive/, such as archive/mbedtls-2.7. These branches will not receive any changes or updates.

We use Semantic Versioning. In particular, we maintain API compatibility in the main branch across minor version changes (e.g. the API of 3.(x+1) is backward compatible with 3.x). We only break API compatibility on major version changes (e.g. from 3.x to 4.0). We also maintain ABI compatibility within LTS branches; see the next section for details.

We will make regular LTS releases on an 18-month cycle, each of which will have a 3 year support lifetime. On this basis, 3.6 LTS (released March 2024) will be supported until March 2027. The next LTS release will be a 4.x release, which is planned for September 2025.

Backwards Compatibility for application code

We maintain API compatibility in released versions of Mbed TLS. If you have code that's working and secure with Mbed TLS x.y.z and does not rely on undocumented features, then you should be able to re-compile it without modification with any later release x.y'.z' with the same major version number, and your code will still build, be secure, and work.

Note that this guarantee only applies if you either use the default compile-time configuration (mbedtls/mbedtls_config.h) or the same modified compile-time configuration. Changing compile-time configuration options can result in an incompatible API or ABI, although features will generally not affect unrelated features (for example, enabling or disabling a cryptographic algorithm does not break code that does not use that algorithm).

Note that new releases of Mbed TLS may extend the API. Here are some examples of changes that are common in minor releases of Mbed TLS, and are not considered API compatibility breaks:

• Adding or reordering fields in a structure or union.
• Removing a field from a structure, unless the field is documented as public.
• Adding items to an enum.
• Returning an error code that was not previously documented for a function when a new error condition arises.
• Changing which error code is returned in a case where multiple error conditions apply.
• Changing the behavior of a function from failing to succeeding, when the change is a reasonable extension of the current behavior, i.e. the addition of a new feature.

There are rare exceptions where we break API compatibility: code that was relying on something that became insecure in the meantime (for example, crypto that was found to be weak) may need to be changed. In case security comes in conflict with backwards compatibility, we will put security first, but always attempt to provide a compatibility option.

Backward compatibility for the key store

We maintain backward compatibility with previous versions of the PSA Crypto persistent storage since Mbed TLS 2.25.0, provided that the storage backend (PSA ITS implementation) is configured in a compatible way. We intend to maintain this backward compatibility throughout a major version of Mbed TLS (for example, all Mbed TLS 3.y versions will be able to read keys written under any Mbed TLS 3.x with x <= y).

Mbed TLS 3.x can also read keys written by Mbed TLS 2.25.0 through 2.28.x LTS, but future major version upgrades (for example from 2.28.x/3.x to 4.y) may require the use of an upgrade tool.

Note that this guarantee does not currently fully extend to drivers, which are an experimental feature. We intend to maintain compatibility with the basic use of drivers from Mbed TLS 2.28.0 onwards, even if driver APIs change. However, for more experimental parts of the driver interface, such as the use of driver state, we do not yet guarantee backward compatibility.

Long-time support branches

For the LTS branches, additionally we try very hard to also maintain ABI compatibility (same definition as API except with re-linking instead of re-compiling) and to avoid any increase in code size or RAM usage, or in the minimum version of tools needed to build the code. The only exception, as before, is in case those goals would conflict with fixing a security issue, we will put security first but provide a compatibility option. (So far we never had to break ABI compatibility in an LTS branch, but we occasionally had to increase code size for a security fix.)

For contributors, see the Backwards Compatibility section of CONTRIBUTING.

Current Branches

The following branches are currently maintained:

• main
• development
• mbedtls-3.6 maintained until March 2027, see https://github.com/Mbed-TLS/mbedtls/releases/tag/v3.6.0.
• mbedtls-2.28 maintained until the end of 2024, see https://github.com/Mbed-TLS/mbedtls/releases/tag/v2.28.8.

Users are urged to always use the latest version of a maintained branch.