Remove MBEDTLS_TEST_NULL_ENTROPY

[chris-jones-arm]

Context

The config option MBEDTLS_TEST_NULL_ENTROPY builds the library with no configured entropy sources. This completely negates any and all security provided by the library and as such represents a large security hole in the library.

This option was originally requested a relatively long time ago and we no longer find the reasoning behind keeping the option acceptable as it does not provide any tangible benefit for users any more.

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Rationale

To reduce testing burden from too many config options as well as remove obsolete options which no longer bear relevance to the current library direction we want to remove this option in the 3.0 release.

This is a large security issue as it could be misconfigured and must not be used in production, therefore we do not feel it has a place in the library.

There are also suitable alternatives for replicating this option such as using MBEDTLS_ENTROPY_NV_SEED or making a fake entropy function.

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Work items for 3.0

• Remove MBEDTLS_TEST_NULL_ENTROPY from config.h
• Remove all references to MBEDTLS_TEST_NULL_ENTROPY so that git grep MBEDTLS_TEST_NULL_ENTROPY finds no occurrences in the library or test code.

[gilles-peskine-arm]

In 3.0, it's enough to remove MBEDTLS_TEST_NULL_ENTROPY from config.h and ensure that defining it either results in a compilation error or does nothing. We can take care of removing it from the code and build scripts later in 3.x.

[Regis-Caelum]

@chris-jones-arm I want to work on this issue, can you guide me please.

[gilles-peskine-arm]

Hi @Regis-Caelum,

Thanks for volunteering to help! You can easily find all the places to change with git grep MBEDTLS_TEST_NULL_ENTROPY. I'm not sure what parts would be difficult. I would suggest the following order:

1. tests/scripts/all.sh
2. include/mbedtls/config.h
3. Places where tests/scripts/check-names.sh complains about
4. Tests
5. Build scripts

Please don't hesitate to ask if you need help! Either here or on a draft pull request.