[kryptiskt]

> So I'm wondering: Is there any way to do this a) with encryption, b) without entrusting the keys to the operator of the service, and c) without informing the recipients in advance?

Give half the key to the service and half to your friends, then you're only vulnerable to a conspiracy between them. Or do some more elaborate m of n thing.

[jarrett]

What about my third desire:

> c) without informing the recipients in advance

If you're willing to arrange the protocols with your recipients in advance, there are multiple viable approaches. But what if telling them in advance is itself a security risk?