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the ``ServerHello`` message that contains the protocol version chosen by the server (assumed to be the same as the client version in this chapter), the 32 random bytes chosen by the server, the `Cipher Suite` selected by the server from the list advertised by the client and a `Session Id`. This `Session Id` is an identifier which is chosen by the server. It identifies the TLS session and the security parameters (algorithms and keys) negotiated for this session. It is used to support session resumption.
the ``Certificate`` message provides the certificate (or usually a chain of certificates) that binds a domain name to the public key used by the server. TLS uses the server certificates to authenticate the server. It relies on a Public Key Infrastructure that is composed of a set of root certification authorities that issue certificates to certification authorities that in the end issue certificates to servers. TLS clients are usually configured with the public keys of several root certification authorities and use this information to validate the certificates that they receive from servers. For historical reasons, the TLS certificates are encoded in ASN.1 format. The details of the ASN.1 syntax [Dubuisson2000]_ are outside the scope of this book.
the ``ServerKeyExchange`` message is used by the server to transmit the information that is required to perform the key exchange. The content of this message is function of the selected key exchange algorithm.
the ``ServerHelloDone`` indicates that the server has sent all the messages for the first phase of the handshake.
At this point, it is time to describe the TLS key exchange. TLS supports different key exchange mechanisms that can be negotiated as part of the selection of the cipher suite. We focus on two of them to highlight their differences:
``RSA``. This key exchange algorithm uses the encryption capabilities of the RSA public-key algorithm. The client has validated the server's public key thanks to the ``Certificate`` message. It then generates a (48 bytes) random number, encrypts it with the server public key and sends the encrypted number to the server in the ``ClientKeyExchange`` message. The server uses its private key to decrypt the random number. At this point, the client and the server share the same (48 bytes long) secret and use it to derive the secret keys required to encrypt and authenticate data in the second phase. With this key exchange algorithm, the server does not need to send a ``ServerKeyExchange`` message.
``DHE_RSA``. This key exchange algorithm is the Ephemeral Diffie Hellman key exchange with RSA signatures to authenticate the key exchange. It operates as a classical authenticated Diffie Hellman key exchange. If this key exchange has been selected by the server, it sends its Diffie Hellman parameters in the ``ServerKeyExchange`` message and signs them with its private key. The client then continues the key exchange and sends the results of its own computation in the ``ClientKeyExchange`` message. ``DHE_RSA`` is thus an authenticated Diffie Hellman key exchange where the initial message is sent by the server (instead of the client as in our first example but since the protocol is symmetric, this does not matter).
An important difference between ``DHE_RSA`` and ``RSA`` is their reaction against attacks. ``DHE_RSA`` is considered by many to be stronger than ``RSA`` because it supports `Perfect Forward Secrecy`. This property is important against attackers that are able to eavesdrop all the (encrypted) data sent and received by a server. Consider that Terrence is such an attacker that has stored all the packets exchanged by Bob's server during the last six months. If he manages, by any means, to obtain Bob's private key, he will be able to decrypt all the keys used to secure the TLS sessions with Bob's server during this period. With ``DHE_RSA``, a similar attack is less devastating. If Terrence knows Bob's private key, he will be able to launch a man-in-the-middle attack against future TLS sessions with Bob's server. However, he will not be able to recover the keys used for all the past sessions that he captured.
Perfect Forward Secrecy
Perfect Forward Secrecy (PFS) is an important property for key exchange protocols. A protocol provides PFS if its design guarantees that the keys used for former sessions will not be compromised even if the private key of the server is compromised. This is a very important property. ``DHE_RSA`` provides Perfect Forward Secrecy, but the ``RSA`` key exchange does not provide this property. In practice, ``DHE_RSA`` is costly from a computational viewpoint. Recent implementations of TLS thus prefer ``ECDHE_RSA`` or ``ECDHE_ECDSA`` when Perfect Forward Secrecy is required.
All the information required for the key exchange has now been transmitted. There are two important messages that will be sent by the client and the server to conclude the handshake and start the data transfer phase.
The client sends the ``ChangeCipherSpec`` message followed by the ``Finished`` message. The ``ChangeCipherSpec`` message indicates that the client has received all the information required to generate the security keys for this TLS session. This messages can also appear later in the session to indicate a change in the encryption algorithms that are used, but this usage is outside the scope of this book. The ``Finished`` message is more important. It confirms to the server that the TLS handshake has been performed correctly and that no attacker has been able to modify the data sent by the client or the server. This is the first message that is encrypted with the selected security keys. It contains a hash of all the messages that were exchanged during the handshake.
The server also sends a ``ChangeCipherSpec`` message followed by a ``Finished`` message.
TLS Cipher suites
A TLS cipher suite is usually represented as an ASCII string that starts with TLS and contains the acronym of the key exchange algorithm, the encryption scheme with the key size and its mode of operation and the authentication algorithm. For example, ``TLS_DHE_RSA_WITH_AES_128_GCM_SHA256`` is a TLS cipher suite that uses the ``DHE_RSA`` key exchange algorithm with 128 bits AES in GCM mode for encryption and SHA-256 for authentication. The official list of TLS cipher suites is maintained by IANA [#fianaTLS]_. The NULL acronym indicates that no algorithm has been specified. For example, ``TLS_ECDH_RSA_WITH_NULL_SHA`` is a cipher suite that does not use any encryption but still uses the ``ECDH_RSA`` key exchange and ``SHA`` for authentication.
The TLS record protocol
The handshake is now finished. The client and the server will exchange authenticated and encrypted records. TLS defines different formats for the records depending on the cryptographic algorithms that have been negotiated for the session. A detailed discussion of these different types of records is outside the scope of this introduction. For illustration, we briefly describe one record format.
As other security protocols, TLS uses different keys to encrypt and authenticate records. These keys are derived from the MasterSecret that is either randomly generated by the client after the ``RSA`` key exchange or derived from the Diffie Hellman parameters after the ``DH_RSA`` key exchange. The exact algorithm used to derive the keys is defined in :rfc:`5246`.
A TLS record is always composed of four different fields :
a `Type` that indicates the type of record. The most frequent type is `application data` which corresponds to a record containing encrypted data. The other types are `handshake`, `change_cipher_spec` and `alert`.
a `Protocol Version` field that indicates the version of the TLS protocol used. This version is composed of two sub fields : a major and a minor version number.
a `Length` field. A TLS record cannot be longer than 16,384 bytes.
a `TLSPlainText` that contains the encrypted data
TLS supports several methods to encrypted records. The selected method depends on the cryptographic algorithms that have been negotiated for the TLS session. A detailed presentation of the different methods that can be used to produce the `TLSPlainText` from the user data is outside the scope of this book. As an example, we study one method: Stream Encryption. This method is used with cryptographic algorithms which can operate on a stream of bytes. The method starts with a sequence of bytes provided by the user application: the plain text. The first step is to compute the authentication code to verify the integrity of the data. For this, TLS computes :math:`MAC(SeqNum, Header, PlainText)` using HMAC where `SeqNum` is a sequence number which is incremented by one for each new TLS record transmitted. The `Header` is the header of the TLS record described above and `PlainText` is the information that needs to be encrypted. Note that the sequence number is maintained at the two endpoints of the TLS session, but it is not transmitted inside the TLS record. This sequence number is used to prevent replay attacks.
MAC-then-encrypt or Encrypt-then-MAC
When secure protocols use Message Authentication and Encryption, they need to specify how these two algorithms are combined. A first solution, which is used by the current version of TLS, is to compute the authentication code and then encrypt both the data and the authentication code. A drawback of this approach is that the receiver of an encrypted TLS record must first attempt to decrypt data that has potentially been modified by an attacker before being able to verify the authenticity of the record. A better approach is for the sender to first encrypt the data and then compute the authentication code over the encrypted data. This is the encrypt-then-MAC approach proposed in :rfc:`7366`. With encrypt-then-MAC, the receiver first checks the authentication code before attempting to decrypt the record.
Improving TLS
During the last two decades, the deployment of TLS has continued to grow. The early TLS servers were only used for critical services such as e-commerce websites or online banks. As CPU performance improved, it became much more cost-effective to use TLS to secure non-critical parts of web servers, including the delivery of HTML pages and even video services. There is now a growing number of applications that rely on TLS [AM2019]_.
In 2013, the statistics collected by the Firefox Telemetry project [#ftelemetry]_ revealed that 30% of the web pages loaded by Firefox users were done over HTTPS. In October 2019, 80% of the web pages are loaded over HTTPS. In six years, HTTPS became the dominant protocol to access web services. Another look at the deployment of HTTPS on web sites may be found in [Helme2019]_.
Measurement studies that analyzed the evolution of TLS over the years have identified several important changes in the TLS ecosystem [KRA2018]_. First, the preferred cryptographic algorithms have changed. While RC4 was used by 60% of the connections in 2012, its usage has dropped since 2015. AEA started to be deployed in 2013 and is now used for more than 90% of the connections. The deployed versions of TLS have also changed. TLS 1.0 and TLS 1.1 are now rarely used. The deployment of TLS 1.2 started in 2013 and reached 70% of the connections in 2015. Version 1.3 of TLS, that is described below, is also widely deployed.
Another interesting fact is are the key exchange schemes. In 2012, RSA was the dominant solution, used by more than 80% of the observed connections [KRA2018]_. In 2013, Edward Snowden revealed the surveillance activities of several governments. These revelations had a huge impact on the Internet community. The IETF, which standardizes Internet protocols, considered in :rfc:`7258` that such pervasive monitoring was an attack. Since then, several IETF working groups have developed solutions to counter pervasive monitoring. One of these solutions is to encourage `Perfect Forward Security`. Within TLS, this implies replacing RSA by an authenticated Diffie Hellman key exchange such as ECDHE. Measurements indicate that since summer 2014, ECDHE is more popular than RSA. In 2018, more than 90% of the observed TLS connections used ECDHE.

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locale/pot/protocols/tls.pot, string 58