Re: [quicwg/base-drafts] Spoofed retry token attack on IP authentication (#2394)

[Kazuho Oku <[e-mail]>]

> So the conclusion is that you cannot do anything about it in v1. There isn't even much reason to strongly prevent token reuse since only path observers can reuse it, and they have powers anyway.

We cannot do anything in any version of QUIC to prevent a MOTS attacker from spoofing an address, because an IP address is never authenticated. From an endpoint's view, it is impossible to tell whether if it's talking with the legitimate owner of IP address X or if it's talking with somebody spoofing address X.

OTOH, as I pointed out in 
https://github.com/quicwg/base-drafts/issues/2394#issuecomment-459700403, we might want to prohibit a token from being used to create multiple connections. This is because one of the use-case of Retry tokens is to limit costly TLS handshake operations to connection establish attempts that have proved (probabilistically) to own the IP address (by responding to the Retry).

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