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+Testing strategy for `MBEDTLS_USE_PSA_CRYPTO`
+=============================================
+
+This document records the testing strategy used so far in implementing
+`MBEDTLS_USE_PSA_CRYPTO`.
+
+
+General considerations
+----------------------
+
+There needs to be at least one build in `all.sh` that enables
+`MBEDTLS_USE_PSA_CRYPTO` and runs the full battery of tests; currently that's
+ensured by the fact that `scripts/config.py full` enables
+`MBEDTLS_USE_PSA_CRYPTO`. There needs to be at least one build with
+`MBEDTLS_USE_PSA_CRYPTO` disabled (as long as it's optional); currently that's
+ensured by the fact that it's disabled in the default config.
+
+Generally, code review is enough to ensure that PSA APIs are indeed used where
+they should be when `MBEDTLS_USE_PSA_CRYPTO` is enabled.
+
+However, when it comes to TLS, we also have the option of using debug messages
+to confirm which code path is taken. This is generally unnecessary, except when
+a decision is made at run-time about whether to use the PSA or legacy code
+path. (For example, for record protection, previously (until 3.1), some ciphers were supported
+via PSA while some others weren't, with a run-time fallback. In this case, it's
+good to have a debug message checked by the test case to confirm that the
+right decision was made at run-time, i. e. that we didn't use the fallback for
+ciphers that are supposed to be supported.)
+
+
+New APIs meant for application use
+----------------------------------
+
+For example, `mbedtls_pk_setup_opaque()` is meant to be used by applications
+in order to create PK contexts that can then be passed to existing TLS and
+X.509 APIs (which remain unchanged).
+
+In that case, we want:
+
+- unit testing of the new API and directly-related APIs - for example:
+  - in `test_suite_pk` we have a new test function `pk_psa_utils` that exercises
+    `mbedtls_pk_setup_opaque()` and checks that various utility functions
+  (`mbedtls_pk_get_type()` etc.) work and the functions that are expected to
+  fail (`mbedtls_pk_verify()` etc) return the expected error.
+  - in `test_suite_pk` we modified the existing `pk_psa_sign` test function to
+    check that signature generation works as expected
+  - in `test_suite_pkwrite` we should have a new test function checking that
+    exporting (writing out) the public part of the key works as expected and
+    that exporting the private key fails as expected.
+- integration testing of the new API with each existing API which should
+  accepts a context created this way - for example:
+  - in `programs/ssl/ssl_client2` a new option `key_opaque` that causes the
+    new API to be used, and one or more tests in `ssl-opt.sh` using that.
+    (We should have the same server-side.)
+  - in `test_suite_x509write` we have a new test function
+    `x509_csr_check_opaque()` checking integration of the new API with the
+    existing `mbedtls_x509write_csr_set_key()`. (And also
+    `mbedtls_x509write_crt_set_issuer_key()` since #5710.)
+
+For some APIs, for example with `mbedtls_ssl_conf_psk_opaque()`, testing in
+`test_suite_ssl` was historically not possible, so we only have testing in
+`ssl-opt.sh`.
+
+New APIs meant for internal use
+-------------------------------
+
+For example, `mbedtls_cipher_setup_psa()` (no longer used, soon to be
+deprecated - #5261) was meant to be used by the TLS layer, but probably not
+directly by applications.
+
+In that case, we want:
+
+- unit testing of the new API and directly-related APIs - for example:
+  - in `test_suite_cipher`, the existing test functions `auth_crypt_tv` and
+    `test_vec_crypt` gained a new parameter `use_psa` and corresponding test
+    cases
+- integration testing:
+  - usually already covered by existing tests for higher-level modules:
+    - for example simple use of `mbedtls_cipher_setup_psa()` in TLS is already
+      covered by running the existing TLS tests in a build with
+      `MBEDTLS_USA_PSA_CRYPTO` enabled
+  - however if use of the new API in higher layers involves more logic that
+    use of the old API, specific integrations test may be required
+    - for example, the logic to fall back from `mbedtls_cipher_setup_psa()` to
+      `mbedtls_cipher_setup()` in TLS is tested by `run_test_psa` in
+      `ssl-opt.sh`.
+
+Internal changes
+----------------
+
+For example, use of PSA to compute the TLS 1.2 PRF.
+
+Changes in this category rarely require specific testing, as everything should
+be already be covered by running the existing tests in a build with
+`MBEDTLS_USE_PSA_CRYPTO` enabled; however we need to make sure the existing
+test have sufficient coverage, and improve them if necessary.
+
+However, if additional logic is involved, or there are run-time decisions about
+whether to use the PSA or legacy code paths, specific tests might be in order.