ACTION-22: Key export

We've gone around on this a few times, including at the f2f, so here is a concrete proposal. I'm trying to find a balance between extensibility and not loading up the API with a bunch of stuff, so feedback is welcome.

I see the following use cases for key import/export:

-          Create session key object from derived key bytes (using either KDF or secret agreement): this would require raw key import

-          Create key object from public key received from peer (for asymmetric encryption or signature verification): this would require public key import, where the public key is likely ASN.1 encoded in many apps

-          Export/import (wrapped) content encryption key for data encryption: this could be just the wrapped key or something like a PKCS#7 RecipientInfo (which is ASN.1 encoded). Import/export requires a handle to the wrapping key.

-          Export/import of private keys for distribution, with formats like PKCS#8.

>From an API perspective, supporting export seems to be straightforward. The Key object needs an export (or wrap) method, which takes a target format and potentially a wrapping key as parameters.

It seems to me there are two API models to support import. Either have an ability to create an empty Key object, then invoke an import method on that object, or make it part of the construction of the Key object. I propose the latter, so that we don't complicate the state model of the Key object.

So in WebIDL,

interface Crypto {

... other stuff ...

KeyGenerator importKey(DOMString format, ArrayBuffer keyBlob, optional Key wrappingKey=null);
}

interface Key {

... other stuff ...

KeyExporter exportKey(DOMString format, optional Key wrappingKey=null);
}

Where KeyExporter is exactly like KeyGenerator but returns a value instead of a Key object.

One big issue is what key formats should be supported. For symmetric keys it makes sense to support a raw format, but for asymmetric keys things are more complex. As has been brought up on other threads, many commonly-used formats are ASN.1 based and so it seems like supporting that would really help interoperability. However, I'd like to avoid a repeat of the mandatory algorithms discussion. Any ideas here are welcome.