1# Maintained branches 2 3At any point in time, we have a number of maintained branches, currently consisting of: 4 5- The [`main`](https://github.com/Mbed-TLS/mbedtls/tree/main) branch: 6 this always contains the latest release, including all publicly available 7 security fixes. 8- The [`development`](https://github.com/Mbed-TLS/mbedtls/tree/development) branch: 9 this is where the next major version of Mbed TLS (version 4.0) is being 10 prepared. It has API changes that make it incompatible with Mbed TLS 3.x, 11 as well as all the new features and bug fixes and security fixes. 12- One or more long-time support (LTS) branches: these only get bug fixes and 13 security fixes. Currently, the supported LTS branches are: 14- [`mbedtls-2.28`](https://github.com/Mbed-TLS/mbedtls/tree/mbedtls-2.28). 15- [`mbedtls-3.6`](https://github.com/Mbed-TLS/mbedtls/tree/mbedtls-3.6). 16 17We retain a number of historical branches, whose names are prefixed by `archive/`, 18such as [`archive/mbedtls-2.7`](https://github.com/Mbed-TLS/mbedtls/tree/archive/mbedtls-2.7). 19These branches will not receive any changes or updates. 20 21We use [Semantic Versioning](https://semver.org/). In particular, we maintain 22API compatibility in the `main` branch across minor version changes (e.g. 23the API of 3.(x+1) is backward compatible with 3.x). We only break API 24compatibility on major version changes (e.g. from 3.x to 4.0). We also maintain 25ABI compatibility within LTS branches; see the next section for details. 26 27We will make regular LTS releases on an 18-month cycle, each of which will have 28a 3 year support lifetime. On this basis, 3.6 LTS (released March 2024) will be 29supported until March 2027. The next LTS release will be a 4.x release, which is 30planned for September 2025. 31 32## Backwards Compatibility for application code 33 34We maintain API compatibility in released versions of Mbed TLS. If you have 35code that's working and secure with Mbed TLS x.y.z and does not rely on 36undocumented features, then you should be able to re-compile it without 37modification with any later release x.y'.z' with the same major version 38number, and your code will still build, be secure, and work. 39 40Note that this guarantee only applies if you either use the default 41compile-time configuration (`mbedtls/mbedtls_config.h`) or the same modified 42compile-time configuration. Changing compile-time configuration options can 43result in an incompatible API or ABI, although features will generally not 44affect unrelated features (for example, enabling or disabling a 45cryptographic algorithm does not break code that does not use that 46algorithm). 47 48Note that new releases of Mbed TLS may extend the API. Here are some 49examples of changes that are common in minor releases of Mbed TLS, and are 50not considered API compatibility breaks: 51 52* Adding or reordering fields in a structure or union. 53* Removing a field from a structure, unless the field is documented as public. 54* Adding items to an enum. 55* Returning an error code that was not previously documented for a function 56 when a new error condition arises. 57* Changing which error code is returned in a case where multiple error 58 conditions apply. 59* Changing the behavior of a function from failing to succeeding, when the 60 change is a reasonable extension of the current behavior, i.e. the 61 addition of a new feature. 62 63There are rare exceptions where we break API compatibility: code that was 64relying on something that became insecure in the meantime (for example, 65crypto that was found to be weak) may need to be changed. In case security 66comes in conflict with backwards compatibility, we will put security first, 67but always attempt to provide a compatibility option. 68 69## Backward compatibility for the key store 70 71We maintain backward compatibility with previous versions of the 72PSA Crypto persistent storage since Mbed TLS 2.25.0, provided that the 73storage backend (PSA ITS implementation) is configured in a compatible way. 74We intend to maintain this backward compatibility throughout a major version 75of Mbed TLS (for example, all Mbed TLS 3.y versions will be able to read 76keys written under any Mbed TLS 3.x with x <= y). 77 78Mbed TLS 3.x can also read keys written by Mbed TLS 2.25.0 through 2.28.x 79LTS, but future major version upgrades (for example from 2.28.x/3.x to 4.y) 80may require the use of an upgrade tool. 81 82Note that this guarantee does not currently fully extend to drivers, which 83are an experimental feature. We intend to maintain compatibility with the 84basic use of drivers from Mbed TLS 2.28.0 onwards, even if driver APIs 85change. However, for more experimental parts of the driver interface, such 86as the use of driver state, we do not yet guarantee backward compatibility. 87 88## Long-time support branches 89 90For the LTS branches, additionally we try very hard to also maintain ABI 91compatibility (same definition as API except with re-linking instead of 92re-compiling) and to avoid any increase in code size or RAM usage, or in the 93minimum version of tools needed to build the code. The only exception, as 94before, is in case those goals would conflict with fixing a security issue, we 95will put security first but provide a compatibility option. (So far we never 96had to break ABI compatibility in an LTS branch, but we occasionally had to 97increase code size for a security fix.) 98 99For contributors, see the [Backwards Compatibility section of 100CONTRIBUTING](CONTRIBUTING.md#backwards-compatibility). 101 102## Current Branches 103 104The following branches are currently maintained: 105 106- [main](https://github.com/Mbed-TLS/mbedtls/tree/main) 107- [`development`](https://github.com/Mbed-TLS/mbedtls/) 108- [`mbedtls-3.6`](https://github.com/Mbed-TLS/mbedtls/tree/mbedtls-3.6) 109 maintained until March 2027, see 110 <https://github.com/Mbed-TLS/mbedtls/releases/tag/v3.6.0>. 111- [`mbedtls-2.28`](https://github.com/Mbed-TLS/mbedtls/tree/mbedtls-2.28) 112 maintained until the end of 2024, see 113 <https://github.com/Mbed-TLS/mbedtls/releases/tag/v2.28.8>. 114 115Users are urged to always use the latest version of a maintained branch. 116
