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How do you agree a protocol to encrypt the message to agree the protocol?

This is the message that returns a list of supported ciphers and key exchange protocols. There’s no data in this first packet.

Alice: I’d like to connect Bob: Sure here is a list of protocols we could use:

You modify bob’s message so that bob only suggests insecure protocols.

You might be proposing that Alice asks Trent for Bob’s public key … But that’s not how TLS works.

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Bob's list of supported protocols is an input into the (authenticated) final handshake message, and that authentication failing will prevent the connection from being considered successfully established.

If the "negotiated" cipher suite is weak enough to allow real-time impersonation of Bob, though, pre-1.3 versions are still vulnerable; that's another reason not to keep insecure cipher suites around in a TLS config.

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