Key export and an opaque KeyFormat

It seems to me that the current API has little support for an origin which wants to generate a key and then use it for multiple sessions. The only way I can see in the API of doing this is to use exportKey and store the key in the clear. This is not an appealing prospect. It also means that if an origin wants to persist a key in this way for its own later use then it has no choice but to make it exportable.

So I would like to propose adding a value "opaqueLocal" to enum KeyFormat. The behavior of this format would be as follows: the result of calling exportKey with this KeyFormat would be a durable key reference that would be bound to the particular machine (and possibly UA). The contents of this reference are opaque to the caller - it may be just a key name that refers to a local key store, or it could be the entire key object wrapped with a key held by the UA. Calling importKey on this result will work on the same machine and UA, but fail on all other machines. Also, the algorithm, extractable and keyUsages parameters will be ignored and the values from the original key object will be used.

The concept is similar to BCRYPT_OPAQUE_KEY_BLOB in CNG: http://msdn.microsoft.com/library/windows/desktop/aa375434.aspx

Thoughts?