ESNI -> ECHO #207
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ESNI -> ECHO #207
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the client creates a ClientHelloInner value with the true SNI, encrypts that, and puts it into an extension in ClientHelloOuter, which is what is actually sent to the server. There are a number of open issues here: - This design protects against the HRR splicing attack by having the transcript include ClientHelloInner.nonce, which relies on transcript secrecy. An alternative design would be to explicitly include it in the key schedule. However, this is a bit odd in split mode. - You have to use trial decryption on the client to determine whether the ClientHelloInner or ClientHelloOuter was accepted, and the client doesn't even know when it gets HRR whether it applies to the inner or outer CH. Should we consider having an extension indicate this? Obviously that leaks some information, however the primary thing that it leaks is whether the server knew the ESNI key, which can be independently determined by sending a new CH with the relevant config. This whole dual transcript thing is definitely a weak spot that would need some extensive analysis.
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This was referenced Mar 8, 2020
Closed
We replaced multiple "outer_extension" extensions with a single "outer_extensions" extension.
ekr
approved these changes
Mar 9, 2020
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This change implements the ESNI -> ECHO design discussed in Singapore. There a couple pending TODOs, particularly around cross-CH HRR binding, but the meat is what's currently being analyzed.
cc @davidben, @grittygrease, @martinthomson, @karthikbhargavan