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Transport Layer Security
The Transport Layer Security family of protocols were initially proposed under the name Secure Socket Layer (SSL). The first deployments used this name and many researchers still refer to this security protocol as SSL [FKC1996]_. In this chapter, we use the official name that was standardized by the IETF: TLS for `Transport Layer Security`.
The TLS protocol was designed to be usable by a wide range of applications that use the transport layer to reliably exchange information. TLS is mainly used over the TCP protocol. There are variants of TLS that operate over SCTP :rfc:`3436` or UDP :rfc:`6347`, but these are outside the scope of this chapter.
A TLS session operates over a TCP connection. TLS is responsible for the encryption and the authentication of the SDUs exchanged by the application layer protocol while TCP provides the reliable delivery of this encrypted and authenticated bytestream. TLS is used by many different application layer protocols. The most frequent ones are HTTP (HTTP over TLS is called HTTPS), SMTP :rfc:`3207` or POP and IMAP :rfc:`2595`, but proprietary application-layer protocols also use TLS [AM2019]_.
A TLS session can be initiated in two different ways. First, the application can use a dedicated TCP port number for application layer protocol x-over-TLS. This is the solution used by many HTTP servers that reserve port :math:`443` for HTTP over TLS. This solution works, but it requires to reserve two ports for each application : one where the application-layer protocol is used directly over TCP and another one where the application-layer protocol is used over TLS. Given the limited number of TCP ports that are available, this is not a scalable solution. The table below provides some of the reserved port numbers for application layer protocols on top of TLS.
Application
TCP port
TLS port
POP3
110
995
IMAP
143
993
NNTP
119
563
HTTP
80
443
FTP
21
990
A second approach to initiate a TLS session is to use the standard TCP port number for the application layer protocol and define a special message in this protocol to trigger the start of the TLS session. This is the solution used for SMTP with the ``STARTTLS`` message. This extension to SMTP :rfc:`3207` defines the new STARTTLS command. The client can issue this command to indicate to the server that it wants to start a TLS session as shown in the example below captured during a session on port 25.
In the remaining parts of this chapter, we assume that the TLS session starts immediately after the establishment of the TCP connection. This corresponds to the deployments on web servers. We focus our presentation of TLS on this very popular use case. TLS is a complex protocol that supports other features than the one used by web servers. A more detailed presentation of TLS may be found in [KPS2002]_ and [Ristic2015]_.
A TLS session is divided in two phases: the handshake and the data transfer. During the handshake, the client and the server negotiate the security parameters and the keys that will be used to secure the data transfer. During the second phase, all the messages exchanged are encrypted and authenticated with the negotiated algorithms and keys.
The TLS handshake
When used to interact with a regular web server, the TLS handshake has three important objectives:
Securely negotiate the cryptographic algorithms that will be used by the client and the server over the TLS session
Verify that the client interacts with a valid server
Securely agree on the keys that will be used to encrypt and authenticate the messages exchanged over the TLS session

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../../protocols/tls.rst:45
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locale/pot/protocols/tls.pot, string 16