Add RFC: libobs keychain APIs

Author: derrod

text/0000-libobs-keychain-api.md

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+# Summary
+
+This RFC proposes the addition of OS keychain APIs to the libobs platform utilities to faciliate secure storage of credentials.
+
+# Motivation
+
+While the credentials stored by OBS in profiles are not of severe sensitivity due to limited scope, as a modern desktop application we should still do our best to store them securely.
+
+# Detailed Changes
+
+This RFC proposes the addition of the following libobs API methods:
+
+- `bool os_keychain_available(void)`
+- `bool os_keychain_save(const char *namespace, const char *key, const char *data)`
+- `bool os_keychain_load(const char *namespace, const char *key, char **data)`
+- `bool os_keychain_delete(const char *namespace, const char *key)`
+
+As the API requires OS-specific implementations it shall become part of the `util/platform.h` header.
+
+**Implementation notes:**
+
+Operating-system backends:
+- Windows: `wincred.h` / `Cred*` Win32 API functions
+- macOS: Keychain Services API
+- Linux: `libsecret`
+
+The `namespace` is a user-visible component that may be displayed in keychain management applications as the label or name of an entry. It must be specified, ASCII only, and must be human-readable.  
+In most cases it should identify the component and contents, e.g. for OAuth credentials stored by the OBS Studio UI this could be "OBS Studio OAuth Credentials".  
+An entry can only be loaded or deleted if the namespace and key match the ones specified when saving.
+
+`os_keychain_available()` will always return `true` on Windows and macOS, on Linux it will return `true` if libobs was compiled with libsecret support, but does not indicate whether a keychain backend is available.
+
+**Usage examples:**
+Saving:
+```c
+const char *namespace = "OBS Studio Stream Keys";
+const char *key = "StreamKey_abcef";
+const char *value = "mystreamkey";
+bool success = os_keychain_save(namespace, key, value);
+```
+
+Loading:
+```c
+const char *namespace = "OBS Studio Stream Keys";
+const char *key = "StreamKey_abcef";
+char *value = NULL;
+bool success = os_keychain_load(namespace, key, &value);
+
+... do stuff ...
+
+if (value)
+    bfree(value);
+```
+
+Deletion:
+```c
+const char *namespace = "OBS Studio Stream Keys";
+const char *key = "StreamKey_abcef";
+bool success = os_keychain_delete(namespace, key);
+```
+
+# Drawbacks
+
+Accessing keychain information can reduce portability of profiles by not storing information in config files. OBS Studio may opt out of using a keychain if it is running in portable mode.
+
+In some cases accessing the keychain may require user confirmation, the API may block in such cases, which has to be accounted for by API users.
+
+If no keychain is available API users will have to fall back to storing data otherwise (e.g. obs data or config files).
+
+If keychain operations fail API users may have to also fall back to an insecure storage method or warn the user of faiure.
+
+# Additional Information
+
+- Windows credential store documentation: https://learn.microsoft.com/en-us/windows/win32/api/wincred/
+- macOS keychain documentation: https://developer.apple.com/documentation/security/keychain_services?language=objc
+- Linux/GNOME libsecret documentation: https://gnome.pages.gitlab.gnome.org/libsecret/
+- Flatpak "Secret" Portal: https://docs.flatpak.org/en/latest/portal-api-reference.html#gdbus-org.freedesktop.portal.Secret
+- Pull request: https://github.com/obsproject/obs-studio/pull/9122