[noise] Regarding Static Key Authentication

[Marian Beermann]

Hi Nadim,

yes, if the intention is to have an AKE (authenticated key exchange),
then the peer's static key needs to be authenticated in one way or
another. Noise does not provide an out-of-the-box way to do that.

-Marian

On 01.05.2018 14:31, Nadim Kobeissi wrote:
> Dear David,
> So, the conclusion is that any `s` appearing in either a pre-message or
> message pattern, is assumed to be authenticated out-of-band, as in
> independently of the Noise handshake, by the recipient party?
> 
> Thank you,
> 
> Nadim Kobeissi
> Symbolic Software • https://symbolic.software
> Sent from office
> 
> 
> On Tue, May 1, 2018 at 2:10 PM David Wong <davidwong.crypto at gmail.com
> <mailto:davidwong.crypto at gmail.com>> wrote:
> 
>     > If a token 's' appears in a Noise handshake pattern pre-message
>     flight, it
>     > is reasonable for us to assume that this key represented by 's' was
>     > pre-authenticated by the parties. That is, if the initiator sent
>     's' in a
>     > pre-message, then the responder is assumed to have authenticated
>     's' already
>     > out of band, using for example a QR code as is the current
>     use-case, for
>     > example, in the Signal secure messenger.
> 
>     I don't think this is a good comparison. Signal allows you to
>     post-handshake authenticate the session whereas a pre-message `s`
>     means that you have pinned `s` and thus you trust the session from the
>     start.
> 
>     `s` in a message pattern implies that you have a way to ensure that
>     you know that `s`. This can be done in different ways:
> 
>     * out of band post-handshake (like Signal)
>     * by having the sender also send a signature from some authority that
>     you trust (PKI)
>     * by recognizing the cert from a trust store
>     * ...?
> 
>     Hope that helps,
>     David
> 
> 
> 
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