Improve version negotiation #368
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Adds requirement from RFC 8446 section 4.2.1: If this extension is not present, servers which are compliant with this specification and which also support TLS 1.2 MUST negotiate TLS 1.2 or prior as specified in [RFC5246], even if ClientHello.legacy_version is 0x0304 or later.
Adds the MUST NOT requirement from RFC 8446 section 4.1.2: If a server established a TLS connection with a previous version of TLS and receives a TLS 1.3 ClientHello in a renegotiation, it MUST retain the previous protocol version. In particular, it MUST NOT negotiate TLS 1.3. and the following requirement from RFC 8446 section 4.1.3: A legacy TLS client performing renegotiation with TLS 1.2 or prior and which receives a TLS 1.3 ServerHello during renegotiation MUST abort the handshake with a "protocol_version" alert. Note that renegotiation is not possible when TLS 1.3 has been negotiated.
Adds requirement from RFC 8446 section 4.2.1: If this extension is present in the ClientHello, servers MUST NOT use the ClientHello.legacy_version value for version negotiation and MUST use only the "supported_versions" extension to determine client preferences.
This is a protocol failure and not just a refusal. From RFC 8446 section 4.1.2: Because TLS 1.3 forbids renegotiation, if a server has negotiated TLS 1.3 and receives a ClientHello at any other time, it MUST terminate the connection with an "unexpected_message" alert.
Makes compliant with RFC 8446 section 4.2.1: Implementations of this specification MUST send this extension in the ClientHello containing all versions of TLS which they are prepared to negotiate (for this specification, that means minimally 0x0304, but if previous versions of TLS are allowed to be negotiated, they MUST be present as well).
kazu-yamamoto
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Jul 1, 2019
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Rebased and merged. |
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While comparing to the RFC I see some items missing regarding extension "supported_versions" and how version is negotiated and verified.