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Consider protocol like QUIC or MinimaLT which are essentially layer4 replacement for TCP/UDP (riding on top of UDP currently because of fear of random firewalls dropping new L4 protocol).

From crypto point of view they are mostly the same designs (using same EC stuff djb has done). IP addresses are not relevant for identity anymore as identity is established via use of PKI.

This is great, long-lived session can move from one IP to another one, and session can be kept up, as IP address does not matter.

However I don't think either of them expose identity to MITM. I think there might be quite nice use-case in your own home FW or corporate FW to say 'Alice can connect to server X port 22'. Then regardless what IP Alice is using, as long as she offers the specified identity, she is passed by the FW.

Could it be possible for arbitrary MITM to securely inspect identity so that no other arbitrary MITM could have faked it?

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You could simply run a key-exchange between alice's long term key and the FW's key and use it to authenticate the ciphertext with a MAC based on the shared key. No need for MitM, decryption, etc. Essentially as a wrapper around these protocols. –  CodesInChaos Nov 14 '13 at 16:28
    
@CodesInChaos then Alice would need to know who is the FW. It would also introduce additional delay to end-to-end session establishment. –  ytti Nov 14 '13 at 16:30
    
I guess if Alice would encrypt with one key and sign resulting encrypted data with another key, FW would be able to check signature without being able to decrypt? Presumably without latency cost, as FW would need to have been taught identity of Alice at prior time? –  ytti Nov 14 '13 at 16:44
    
What is "FW"$\hspace{.02 in}$? $\;$ –  Ricky Demer Nov 14 '13 at 21:35
    
@RickyDemer oh sorry, firewall, in this context MITM who we want to be able to be able to identify sender, but not decrypt content. –  ytti Nov 14 '13 at 21:36

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