There are a few "marker" extensions here. Pretty much the same as you would see in a TLS 1.2 handshake, although x25519 and x448 aren't always supported in older TLS 1.2 Key exchange/signature method, and if they do, the client and server need to agree of point format and supported groups. However, there is ongoing work in the form of the ESNI (Encyrpted Server Name Interface) effort to make thisĠ0 0c length of supported groups (12 bytes)Īlthough DHE and RSA are presented as supported key exchange methods, it's pretty likely that the server will selected an ECC-based Although this is desirable from a privacy perspective, it turned out to be difficult to implement in practice, so was One of the original goals of the TLS working group for 1.3 was to encrypt the SNI part so that eavesdroppers can't also tell which hosts a target The client requested which would otherwise be unavailable to the server which is only listening to a particular IP address. The first is the required server name indicator which hasīeen around for quite a while and is used by multi-homed hosts to select the correct certificate to present it's the exact DNS name that The client hello preamble is followed by a variable-length list of extensions, shown in example 4. Technically, this could be left empty with a 0 length indicator, but OpenSSL negotiates in "compatibility mode".ħ7 77 77 2e 67 6f 6f 67 6c 65 2e 63 6f 6d Example 4: Start of extensions The session ID is included for backward compatibility with TLS 1.2, but is ignored if TLS 1.3 is negotiated,Īs it is in this case. If you were to use Chrome to negotiate a connection, you'd see a much smaller list (17 at my lastĬount). There are a lot of cipher suites listed here, but that's mostly because openssl's command. Methods (which are required by the protocol to be listed but are always turned off since compressed TLS data is susceptible to theĬRIME and BREACH attacks). So far, this continues to look like a TLS 1.2 client hello with the standard client random, session ID, cipher suites and compression But if that's the case, howĭoes the client advertise to the server that it actually does understand TLS 1.3? Well, read on for the answer (if you really can't If the server only understands TLS 1.2, it will just negotiate a TLS 1.2 handshake as before. TLS 1.3 is so radically different from its predecessors, and TLS implementations haveīeen shown to be so version intolerant, that a TLS 1.3 client hello looks superficially exactly like a TLS 1.2 handshake, right down to If the server doesn't understand TLS 1.3, anyway. This seems to suggest that the client is requesting a TLS 1.2 handshake. Now this bit is a little more unexpected. Shocking since the record protocol just includes the version and the length of the data contained within it). This is the TLS record protocol (not the handshake protocol), which itself hasn't changed since TLS 1.0. So far, this isn't too surprising, if you're familiar with older TLS protocols - I said this was TLS 1.3, but the second and third bytes I'll step through the whole message in parts I'll indent to show which parts are subordinate to which other parts Example 1 illustrates the header of the client hello, which is sent in plaintext to the server to kick off Use openssl's helpful s_client command with the -msg option to see the actual exchange that occurredĪs always, TLS begins with a client hello. This time, then, instead of capturing packets with tcpdump, I'll A lot more of the handshake is encrypted in this revision than in previous ones, so it's not as helpful to use tcpdump to examine the protocolĮxchange as it was when I went through TLS 1.2 - in 1.3, everything after the server hello is now encrypted, including the certificate exchange. Since the latest revision of TLS, 1.3, is now almost a year old,Īnd since it's a radical change from the TLS versions that came before it, this is probably a good time to go through the same exerciseįor it. A while back, I wrote up a walkthrough of a real TLS 1.2 handshake,ĭetailing what each byte contributed to the SSL connection establishment process.
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