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Cross-implementation decisions may be differentiators #561

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Oct 13, 2023
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Cross-implementation decisions may be differentiators #561

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Cross-implementation decisions may be differentiators
chris-wood Oct 9, 2023
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Update draft-ietf-tls-esni.md
chris-wood Oct 13, 2023
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Update draft-ietf-tls-esni.md
chris-wood Oct 13, 2023
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15 changes: 10 additions & 5 deletions draft-ietf-tls-esni.md
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Expand Up @@ -1445,9 +1445,13 @@ following capabilities:
1. The attacker does not know the ECHConfigList used by the server.
1. The attacker keeps per-connection state only. In particular, it does not
track endpoints across connections.
1. ECH and GREASE ECH are designed so that the following features do not vary:
the code points of extensions negotiated in the clear; the length of
messages; and the values of plaintext alert messages.

Moreover, real ECH and GREASE ECH are designed so that the following features
do not noticeably vary to the attacker, i.e., they are not distinguishers:

1. the code points of extensions negotiated in the clear, and their order;
1. the length of messages; and
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1. the values of plaintext alert messages.

This leaves a variety of practical differentiators out-of-scope. including,
though not limited to, the following:
Expand All @@ -1459,8 +1463,9 @@ though not limited to, the following:
1. HRR issuance, which may depend on ECH acceptance.

These can be addressed with more sophisticated implementations, but some
mitigations require coordination between the client and server. These
mitigations are out-of-scope for this specification.
mitigations require coordination between the client and server, and even
across different client and server implementations. These mitigations are
out-of-scope for this specification.

### Maintain Forward Secrecy

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