pw_tls_client: Add time facade
Add a time facade that defines a time() and gettimeofday() API layer.
The two APIs are commonly used by TLS libraries for expiration check.
Change-Id: I49f26814f381d190e43174a06636fdcd7b4118b6
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/50282
Reviewed-by: Ali Zhang <alizhang@google.com>
Commit-Queue: Yecheng Zhao <zyecheng@google.com>
diff --git a/pw_tls_client/docs.rst b/pw_tls_client/docs.rst
index 5adfdba..ab1bf19 100644
--- a/pw_tls_client/docs.rst
+++ b/pw_tls_client/docs.rst
@@ -54,6 +54,19 @@
Then follow instruction for setting ``dir_pw_third_party_chromium_verifier`` to
the path of the downloaded repo.
+3. Date time
+-------------
+TLS needs a trust-worthy source of wall clock time in order to check
+expiration. Provisioning of time source for TLS communication is very specific
+to the TLS library in use. However, common TLS libraires, such as BoringSSL
+and MbedTLS, support the use of C APIs ``time()`` and ``getimtofday()`` for
+obtaining date time. To accomodate the use of these libraries, a facade target
+``pw_tls_client:time`` is added that wraps these APIs. For gn build,
+specify the backend target with variable ``pw_tls_client_C_TIME_BACKEND``.
+
+If downstream project chooses to use other TLS libraires that handle time source
+differently, then it needs to be investigated separately.
+
Setup
=====
This module requires the following setup: