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MCUXpresso_LPC55S69/middleware/mbedtls/BRANCHES.md

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Maintained branches

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

  • The master branch: this always contains the latest release, including all publicly available security fixes.
  • The development branch: this is where new features land, as well as bug fixes and security fixes.
  • One or more long-time support (LTS) branches: these only get bug fixes and security fixes.

We use Semantic Versioning. In particular, we maintain API compatibility in the master branch between major version changes. We also maintain ABI compatibility within LTS branches; see the next section for details.

Every major version will become an LTS branch when the next major version is released. We may occasionally create LTS branches from other releases at our discretion. When a new LTS branch is created, it usually remains supported for three years.

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/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).

There are rare exceptions: 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.

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.

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.

Current Branches

The following branches are currently maintained:

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