On 30.01.2016 14:36, Peter Lebbing wrote: > On 29/01/16 19:32, Bjarni Runar Einarsson wrote: >> Also, if I go with a), does that leak the fact that there were >> hidden recipients? Does it leak how many? > > I'd say yes and yes. Every recipient has their own Public Key Encrypted > Session Key (PKESK) packet with the (shared) session key encrypted to > their key. The only difference between a regular recipient and a hidden > one is that the regular ones identify which key the packet is meant for, > whereas each hidden recipient has a packet without that identification. > So the number of PKESK packets without identification is equal to the > number of hidden recipients. It leaks that there were and how many there > were, just like you can identify all non-hidden recipients by the > remaining PKESK packets.
Leakage of exact number of hidden recipients can be mitigated by adding random number of pseudo-recipients (e.g. generate some more keys on your localhost and add them to recipients).
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