239 lines
9.5 KiB
Plaintext
239 lines
9.5 KiB
Plaintext
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.\" ========================================================================
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.\"
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.IX Title "OSSL-GUIDE-LIBSSL-INTRODUCTION 7ossl"
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.TH OSSL-GUIDE-LIBSSL-INTRODUCTION 7ossl "2024-01-30" "3.2.1" "OpenSSL"
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.\" For nroff, turn off justification. Always turn off hyphenation; it makes
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.\" way too many mistakes in technical documents.
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.if n .ad l
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.nh
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.SH "NAME"
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ossl\-guide\-libssl\-introduction, ssl
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\&\- OpenSSL Guide: An introduction to libssl
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.SH "INTRODUCTION"
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.IX Header "INTRODUCTION"
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The OpenSSL \f(CW\*(C`libssl\*(C'\fR library provides implementations of several secure network
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communications protocols. Specifically it provides \s-1SSL/TLS \s0(SSLv3, TLSv1,
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TLSv1.1, TLSv1.2 and TLSv1.3), \s-1DTLS \s0(DTLSv1 and DTLSv1.2) and \s-1QUIC \s0(client side
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only). The library depends on \f(CW\*(C`libcrypto\*(C'\fR for its underlying cryptographic
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operations (see \fIossl\-guide\-libcrypto\-introduction\fR\|(7)).
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.PP
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The set of APIs supplied by \f(CW\*(C`libssl\*(C'\fR is common across all of these different
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network protocols, so a developer familiar with writing applications using one
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of these protocols should be able to transition to using another with relative
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ease.
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.PP
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An application written to use \f(CW\*(C`libssl\*(C'\fR will include the \fI<openssl/ssl.h>\fR
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header file and will typically use two main data structures, i.e. \fB\s-1SSL\s0\fR and
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\&\fB\s-1SSL_CTX\s0\fR.
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.PP
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An \fB\s-1SSL\s0\fR object is used to represent a connection to a remote peer. Once a
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connection with a remote peer has been established data can be exchanged with
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that peer.
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.PP
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When using \s-1DTLS\s0 any data that is exchanged uses \*(L"datagram\*(R" semantics, i.e.
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the packets of data can be delivered in any order, and they are not guaranteed
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to arrive at all. In this case the \fB\s-1SSL\s0\fR object used for the connection is also
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used for exchanging data with the peer.
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.PP
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Both \s-1TLS\s0 and \s-1QUIC\s0 support the concept of a \*(L"stream\*(R" of data. Data sent via a
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stream is guaranteed to be delivered in order without any data loss. A stream
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can be uni\- or bi-directional.
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.PP
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\&\s-1SSL/TLS\s0 only supports one stream of data per connection and it is always
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bi-directional. In this case the \fB\s-1SSL\s0\fR object used for the connection also
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represents that stream. See \fIossl\-guide\-tls\-introduction\fR\|(7) for more
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information.
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.PP
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The \s-1QUIC\s0 protocol can support multiple streams per connection and they can be
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uni\- or bi-directional. In this case an \fB\s-1SSL\s0\fR object can represent the
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underlying connection, or a stream, or both. Where multiple streams are in use
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a separate \fB\s-1SSL\s0\fR object is used for each one. See
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\&\fIossl\-guide\-quic\-introduction\fR\|(7) for more information.
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.PP
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An \fB\s-1SSL_CTX\s0\fR object is used to create the \fB\s-1SSL\s0\fR object for the underlying
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connection. A single \fB\s-1SSL_CTX\s0\fR object can be used to create many connections
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(each represented by a separate \fB\s-1SSL\s0\fR object). Many \s-1API\s0 functions in libssl
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exist in two forms: one that takes an \fB\s-1SSL_CTX\s0\fR and one that takes an \fB\s-1SSL\s0\fR.
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Typically settings that you apply to the \fB\s-1SSL_CTX\s0\fR will then be inherited by
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any \fB\s-1SSL\s0\fR object that you create from it. Alternatively you can apply settings
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directly to the \fB\s-1SSL\s0\fR object without affecting other \fB\s-1SSL\s0\fR objects. Note that
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you should not normally make changes to an \fB\s-1SSL_CTX\s0\fR after the first \fB\s-1SSL\s0\fR
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object has been created from it.
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.SH "DATA STRUCTURES"
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.IX Header "DATA STRUCTURES"
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As well as \fB\s-1SSL_CTX\s0\fR and \fB\s-1SSL\s0\fR there are a number of other data structures
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that an application may need to use. They are summarised below.
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.IP "\fB\s-1SSL_METHOD\s0\fR (\s-1SSL\s0 Method)" 4
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.IX Item "SSL_METHOD (SSL Method)"
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This structure is used to indicate the kind of connection you want to make, e.g.
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whether it is to represent the client or the server, and whether it is to use
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\&\s-1SSL/TLS, DTLS\s0 or \s-1QUIC \s0(client only). It is passed as a parameter when creating
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the \fB\s-1SSL_CTX\s0\fR.
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.IP "\fB\s-1SSL_SESSION\s0\fR (\s-1SSL\s0 Session)" 4
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.IX Item "SSL_SESSION (SSL Session)"
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After establishing a connection with a peer the agreed cryptographic material
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can be reused to create future connections with the same peer more rapidly. The
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set of data used for such a future connection establishment attempt is collected
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together into an \fB\s-1SSL_SESSION\s0\fR object. A single successful connection with a
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peer may generate zero or more such \fB\s-1SSL_SESSION\s0\fR objects for use in future
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connection attempts.
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.IP "\fB\s-1SSL_CIPHER\s0\fR (\s-1SSL\s0 Cipher)" 4
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.IX Item "SSL_CIPHER (SSL Cipher)"
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During connection establishment the client and server agree upon cryptographic
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algorithms they are going to use for encryption and other uses. A single set
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of cryptographic algorithms that are to be used together is known as a
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ciphersuite. Such a set is represented by an \fB\s-1SSL_CIPHER\s0\fR object.
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.Sp
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The set of available ciphersuites that can be used are configured in the
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\&\fB\s-1SSL_CTX\s0\fR or \fB\s-1SSL\s0\fR.
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.SH "FURTHER READING"
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.IX Header "FURTHER READING"
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See \fIossl\-guide\-tls\-introduction\fR\|(7) for an introduction to the \s-1SSL/TLS\s0
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protocol and \fIossl\-guide\-quic\-introduction\fR\|(7) for an introduction to \s-1QUIC.\s0
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.PP
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See \fIossl\-guide\-libcrypto\-introduction\fR\|(7) for an introduction to \f(CW\*(C`libcrypto\*(C'\fR.
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.SH "SEE ALSO"
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.IX Header "SEE ALSO"
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\&\fIossl\-guide\-libcrypto\-introduction\fR\|(7), \fIossl\-guide\-tls\-introduction\fR\|(7),
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\&\fIossl\-guide\-quic\-introduction\fR\|(7)
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.SH "COPYRIGHT"
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.IX Header "COPYRIGHT"
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Copyright 2000\-2023 The OpenSSL Project Authors. All Rights Reserved.
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.PP
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Licensed under the Apache License 2.0 (the \*(L"License\*(R"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file \s-1LICENSE\s0 in the source distribution or at
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<https://www.openssl.org/source/license.html>.
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