Provide the E2E context discussed on the call

Author: ekr

explainer.md

@@ -30,10 +30,9 @@ components.
 
 ## Use cases
 
-The first use case to be supported by the API is the processing of encoded media, with
-end-to-end encryption intended as the motivating application. As such, the first version
-of the API will focus on this use case. However, the same approach can be used in future
-iterations to support additional use cases such as:
+The first use case to be supported by the API is the processing of encoded media.
+However, the same approach can be used in future
+This includes use cases such as:
 
 * Funny Hats (processing inserted before encoding or after decoding)
 * Background removal
@@ -42,6 +41,16 @@ iterations to support additional use cases such as:
 * App-defined bandwidth distribution between tracks
 * Custom codecs for special purposes (in combination with WebCodecs)
 
+This API can also be used for experimenting with end-to-end media
+encryption (e.g.,
+[SFrame](https://datatracker.ietf.org/doc/draft-omara-sframe/). However,
+having both the key and media available to the JavaScript is
+unsuitable for production end-to-end use. This API will also define a
+mechanism for browsers to provide built-in transformers which do not
+require JavaScript access to the key or media. Separately, this or
+some other WG may work on a mechanism for end-to-end keying.
+
+
 ## Code Examples
 0. Feature detection can be done as follows: