451 lines
21 KiB
Plaintext
451 lines
21 KiB
Plaintext
.\" Automatically generated by Pod::Man 2.27 (Pod::Simple 3.28)
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.\" ========================================================================
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.\"
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.IX Title "OSSL-GUIDE-LIBRARIES-INTRODUCTION 7ossl"
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.TH OSSL-GUIDE-LIBRARIES-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\-libraries\-introduction
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\&\- OpenSSL Guide: An introduction to the OpenSSL libraries
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.SH "INTRODUCTION"
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.IX Header "INTRODUCTION"
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OpenSSL supplies two libraries that can be used by applications known as
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\&\f(CW\*(C`libcrypto\*(C'\fR and \f(CW\*(C`libssl\*(C'\fR.
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.PP
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The \f(CW\*(C`libcrypto\*(C'\fR library provides APIs for general purpose cryptography such as
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encryption, digital signatures, hash functions, etc. It additionally supplies
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supporting APIs for cryptography related standards, e.g. for reading and writing
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digital certificates (also known as X.509 certificates). Finally it also
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supplies various additional supporting APIs that are not directly cryptography
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related but are nonetheless useful and depended upon by other APIs. For
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example the \*(L"\s-1BIO\*(R"\s0 functions provide capabilities for abstracting I/O, e.g. via a
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file or over a network.
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.PP
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The \f(CW\*(C`libssl\*(C'\fR library provides functions to perform secure communication between
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two peers across a network. Most significantly it implements support for the
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\&\s-1SSL/TLS, DTLS\s0 and \s-1QUIC\s0 standards.
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.PP
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The \f(CW\*(C`libssl\*(C'\fR library depends on and uses many of the capabilities supplied by
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\&\f(CW\*(C`libcrypto\*(C'\fR. Any application linked against \f(CW\*(C`libssl\*(C'\fR will also link against
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\&\f(CW\*(C`libcrypto\*(C'\fR, and most applications that do this will directly use \s-1API\s0 functions
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supplied by both libraries.
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.PP
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Applications may be written that only use \f(CW\*(C`libcrypto\*(C'\fR capabilities and do not
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link against \f(CW\*(C`libssl\*(C'\fR at all.
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.SH "PROVIDERS"
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.IX Header "PROVIDERS"
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As well as the two main libraries, OpenSSL also comes with a set of providers.
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.PP
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A provider in OpenSSL is a component that collects together algorithm
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implementations (for example an implementation of the symmetric encryption
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algorithm \s-1AES\s0). In order to use an algorithm you must have at least one
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provider loaded that contains an implementation of it. OpenSSL comes with a
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number of providers and they may also be obtained from third parties.
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.PP
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Providers may either be \*(L"built-in\*(R" or in the form of a separate loadable module
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file (typically one ending in \*(L".so\*(R" or \*(L".dll\*(R" dependent on the platform). A
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built-in provider is one that is either already present in \f(CW\*(C`libcrypto\*(C'\fR or one
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that the application has supplied itself directly. Third parties can also supply
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providers in the form of loadable modules.
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.PP
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If you don't load a provider explicitly (either in program code or via config)
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then the OpenSSL built-in \*(L"default\*(R" provider will be automatically loaded.
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.PP
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See \*(L"\s-1OPENSSL PROVIDERS\*(R"\s0 below for a description of the providers that OpenSSL
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itself supplies.
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.PP
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Loading and unloading providers is quite an expensive operation. It is normally
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done once, early on in the application lifecycle and those providers are kept
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loaded for the duration of the application execution.
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.SH "LIBRARY CONTEXTS"
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.IX Header "LIBRARY CONTEXTS"
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Many OpenSSL \s-1API\s0 functions make use of a library context. A library context can
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be thought of as a \*(L"scope\*(R" within which configuration options take effect. When
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a provider is loaded, it is only loaded within the scope of a given library
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context. In this way it is possible for different components of a complex
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application to each use a different library context and have different providers
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loaded with different configuration settings.
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.PP
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If an application does not explicitly create a library context then the
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\&\*(L"default\*(R" library context will be used.
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.PP
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Library contexts are represented by the \fB\s-1OSSL_LIB_CTX\s0\fR type. Many OpenSSL \s-1API\s0
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functions take a library context as a parameter. Applications can always pass
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\&\fB\s-1NULL\s0\fR for this parameter to just use the default library context.
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.PP
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The default library context is automatically created the first time it is
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needed. This will automatically load any available configuration file and will
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initialise OpenSSL for use. Unlike in earlier versions of OpenSSL (prior to
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1.1.0) no explicit initialisation steps need to be taken.
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.PP
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Similarly when the application exits, the default library context is
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automatically destroyed. No explicit de-initialisation steps need to be taken.
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.PP
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See \s-1\fIOSSL_LIB_CTX\s0\fR\|(3) for more information about library contexts.
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See also \*(L"\s-1ALGORITHM FETCHING\*(R"\s0 in \fIossl\-guide\-libcrypto\-introduction\fR\|(7).
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.SH "PROPERTY QUERY STRINGS"
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.IX Header "PROPERTY QUERY STRINGS"
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In some cases the available providers may mean that more than one implementation
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of any given algorithm might be available. For example the OpenSSL \s-1FIPS\s0 provider
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supplies alternative implementations of many of the same algorithms that are
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available in the OpenSSL default provider.
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.PP
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The process of selecting an algorithm implementation is known as \*(L"fetching\*(R".
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When OpenSSL fetches an algorithm to use it is possible to specify a \*(L"property
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query string\*(R" to guide the selection process. For example a property query
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string of \*(L"provider=default\*(R" could be used to force the selection to only
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consider algorithm implementations in the default provider.
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.PP
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Property query strings can be specified explicitly as an argument to a function.
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It is also possible to specify a default property query string for the whole
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library context using the \fIEVP_set_default_properties\fR\|(3) or
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\&\fIEVP_default_properties_enable_fips\fR\|(3) functions. Where both
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default properties and function specific properties are specified then they are
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combined. Function specific properties will override default properties where
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there is a conflict.
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.PP
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See \*(L"\s-1ALGORITHM FETCHING\*(R"\s0 in \fIossl\-guide\-libcrypto\-introduction\fR\|(7) for more
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information about fetching. See \fIproperty\fR\|(7) for more information about
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properties.
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.SH "MULTI-THREADED APPLICATIONS"
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.IX Header "MULTI-THREADED APPLICATIONS"
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|
As long as OpenSSL has been built with support for threads (the default case
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on most platforms) then most OpenSSL \fIfunctions\fR are thread-safe in the sense
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that it is safe to call the same function from multiple threads at the same
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time. However most OpenSSL \fIdata structures\fR are not thread-safe. For example
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the \fIBIO_write\fR\|(3) and \fIBIO_read\fR\|(3) functions are thread safe. However it
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|
would not be thread safe to call \fIBIO_write()\fR from one thread while calling
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\&\fIBIO_read()\fR in another where both functions are passed the same \fB\s-1BIO\s0\fR object
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since both of them may attempt to make changes to the same \fB\s-1BIO\s0\fR object.
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.PP
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There are exceptions to these rules. A small number of functions are not thread
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safe at all. Where this is the case this restriction should be noted in the
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documentation for the function. Similarly some data structures may be partially
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or fully thread safe. For example it is always safe to use an \fB\s-1OSSL_LIB_CTX\s0\fR in
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multiple threads.
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.PP
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See \fIopenssl\-threads\fR\|(7) for a more detailed discussion on OpenSSL threading
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support.
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.SH "ERROR HANDLING"
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.IX Header "ERROR HANDLING"
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Most OpenSSL functions will provide a return value indicating whether the
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function has been successful or not. It is considered best practice to always
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check the return value from OpenSSL functions (where one is available).
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.PP
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Most functions that return a pointer value will return \s-1NULL\s0 in the event of a
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failure.
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.PP
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Most functions that return an integer value will return a positive integer for
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success. Some of these functions will return 0 to indicate failure. Others may
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return 0 or a negative value for failure.
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.PP
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Some functions cannot fail and have a \fBvoid\fR return type. There are also a
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small number of functions that do not conform to the above conventions (e.g.
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they may return 0 to indicate success).
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.PP
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Due to the above variations in behaviour it is important to check the
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documentation for each function for information about how to interpret the
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return value for it.
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.PP
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It is sometimes necessary to get further information about the cause of a
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failure (e.g. for debugging or logging purposes). Many (but not all) functions
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will add further information about a failure to the OpenSSL error stack. By
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using the error stack you can find out information such as a reason code/string
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for the error as well as the exact file and source line within OpenSSL that
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emitted the error.
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.PP
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OpenSSL supplies a set of error handling functions to query the error stack. See
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\&\fIERR_get_error\fR\|(3) for information about the functions available for querying
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error data. Also see \fIERR_print_errors\fR\|(3) for information on some simple
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helper functions for printing error data. Finally look at \fIERR_clear_error\fR\|(3)
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for how to clear old errors from the error stack.
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.SH "OPENSSL PROVIDERS"
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.IX Header "OPENSSL PROVIDERS"
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OpenSSL comes with a set of providers.
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.PP
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The algorithms available in each of these providers may vary due to build time
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configuration options. The \fIopenssl\-list\fR\|(1) command can be used to list the
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currently available algorithms.
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.PP
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The names of the algorithms shown from \fIopenssl\-list\fR\|(1) can be used as an
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algorithm identifier to the appropriate fetching function. Also see the provider
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specific manual pages linked below for further details about using the
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algorithms available in each of the providers.
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.PP
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As well as the OpenSSL providers third parties can also implement providers.
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For information on writing a provider see \fIprovider\fR\|(7).
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.SS "Default provider"
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.IX Subsection "Default provider"
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The default provider is built-in as part of the \fIlibcrypto\fR library and
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contains all of the most commonly used algorithm implementations. Should it be
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needed (if other providers are loaded and offer implementations of the same
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algorithms), the property query string \*(L"provider=default\*(R" can be used as a
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search criterion for these implementations. The default provider includes all
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of the functionality in the base provider below.
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.PP
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If you don't load any providers at all then the \*(L"default\*(R" provider will be
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automatically loaded. If you explicitly load any provider then the \*(L"default\*(R"
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provider would also need to be explicitly loaded if it is required.
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.PP
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See \fIOSSL_PROVIDER\-default\fR\|(7).
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.SS "Base provider"
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.IX Subsection "Base provider"
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The base provider is built in as part of the \fIlibcrypto\fR library and contains
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algorithm implementations for encoding and decoding of OpenSSL keys.
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Should it be needed (if other providers are loaded and offer
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implementations of the same algorithms), the property query string
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\&\*(L"provider=base\*(R" can be used as a search criterion for these implementations.
|
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Some encoding and decoding algorithm implementations are not \s-1FIPS\s0 algorithm
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implementations in themselves but support algorithms from the \s-1FIPS\s0 provider and
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are allowed for use in \*(L"\s-1FIPS\s0 mode\*(R". The property query string \*(L"fips=yes\*(R" can be
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used to select such algorithms.
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.PP
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See \fIOSSL_PROVIDER\-base\fR\|(7).
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.SS "\s-1FIPS\s0 provider"
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.IX Subsection "FIPS provider"
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The \s-1FIPS\s0 provider is a dynamically loadable module, and must therefore
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be loaded explicitly, either in code or through OpenSSL configuration
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(see \fIconfig\fR\|(5)). It contains algorithm implementations that have been
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validated according to \s-1FIPS\s0 standards. Should it be needed (if other
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providers are loaded and offer implementations of the same algorithms), the
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property query string \*(L"provider=fips\*(R" can be used as a search criterion for
|
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these implementations. All approved algorithm implementations in the \s-1FIPS\s0
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provider can also be selected with the property \*(L"fips=yes\*(R". The \s-1FIPS\s0 provider
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may also contain non-approved algorithm implementations and these can be
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selected with the property \*(L"fips=no\*(R".
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.PP
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Typically the \*(L"Base provider\*(R" will also need to be loaded because the \s-1FIPS\s0
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provider does not support the encoding or decoding of keys.
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.PP
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See \s-1\fIOSSL_PROVIDER\-FIPS\s0\fR\|(7) and \fIfips_module\fR\|(7).
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.SS "Legacy provider"
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|
.IX Subsection "Legacy provider"
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|
The legacy provider is a dynamically loadable module, and must therefore
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be loaded explicitly, either in code or through OpenSSL configuration
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(see \fIconfig\fR\|(5)). It contains algorithm implementations that are considered
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insecure, or are no longer in common use such as \s-1MD2\s0 or \s-1RC4.\s0 Should it be needed
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(if other providers are loaded and offer implementations of the same algorithms),
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the property \*(L"provider=legacy\*(R" can be used as a search criterion for these
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implementations.
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.PP
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See \fIOSSL_PROVIDER\-legacy\fR\|(7).
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.SS "Null provider"
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.IX Subsection "Null provider"
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|
The null provider is built in as part of the \fIlibcrypto\fR library. It contains
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no algorithms in it at all. When fetching algorithms the default provider will
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be automatically loaded if no other provider has been explicitly loaded. To
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prevent that from happening you can explicitly load the null provider.
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.PP
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You can use this if you create your own library context and want to ensure that
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all \s-1API\s0 calls have correctly passed the created library context and are not
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accidentally using the default library context. Load the null provider into the
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default library context so that the default library context has no algorithm
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implementations available.
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.PP
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See \fIOSSL_PROVIDER\-null\fR\|(7).
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.SH "CONFIGURATION"
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.IX Header "CONFIGURATION"
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By default OpenSSL will load a configuration file when it is first used. This
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will set up various configuration settings within the default library context.
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|
Applications that create their own library contexts may optionally configure
|
|
them with a config file using the \fIOSSL_LIB_CTX_load_config\fR\|(3) function.
|
|
.PP
|
|
The configuration file can be used to automatically load providers and set up
|
|
default property query strings.
|
|
.PP
|
|
For information on the OpenSSL configuration file format see \fIconfig\fR\|(5).
|
|
.SH "LIBRARY CONVENTIONS"
|
|
.IX Header "LIBRARY CONVENTIONS"
|
|
Many OpenSSL functions that \*(L"get\*(R" or \*(L"set\*(R" a value follow a naming convention
|
|
using the numbers \fB0\fR and \fB1\fR, i.e. \*(L"get0\*(R", \*(L"get1\*(R", \*(L"set0\*(R" and \*(L"set1\*(R". This
|
|
can also apply to some functions that \*(L"add\*(R" a value to an existing set, i.e.
|
|
\&\*(L"add0\*(R" and \*(L"add1\*(R".
|
|
.PP
|
|
For example the functions:
|
|
.PP
|
|
.Vb 2
|
|
\& int X509_CRL_add0_revoked(X509_CRL *crl, X509_REVOKED *rev);
|
|
\& int X509_add1_trust_object(X509 *x, const ASN1_OBJECT *obj);
|
|
.Ve
|
|
.PP
|
|
In the \fB0\fR version the ownership of the object is passed to (for an add or set)
|
|
or retained by (for a get) the parent object. For example after calling the
|
|
\&\fIX509_CRL_add0_revoked()\fR function above, ownership of the \fIrev\fR object is passed
|
|
to the \fIcrl\fR object. Therefore, after calling this function \fIrev\fR should not
|
|
be freed directly. It will be freed implicitly when \fIcrl\fR is freed.
|
|
.PP
|
|
In the \fB1\fR version the ownership of the object is not passed to or retained by
|
|
the parent object. Instead a copy or \*(L"up ref\*(R" of the object is performed. So
|
|
after calling the \fIX509_add1_trust_object()\fR function above the application will
|
|
still be responsible for freeing the \fIobj\fR value where appropriate.
|
|
.PP
|
|
Many OpenSSL functions conform to a naming convention of the form
|
|
\&\fB\f(BICLASSNAME_func_name()\fB\fR. In this naming convention the \fB\s-1CLASSNAME\s0\fR is the name
|
|
of an OpenSSL data structure (given in capital letters) that the function is
|
|
primarily operating on. The \fBfunc_name\fR portion of the name is usually in
|
|
lowercase letters and indicates the purpose of the function.
|
|
.SH "DEMO APPLICATIONS"
|
|
.IX Header "DEMO APPLICATIONS"
|
|
OpenSSL is distributed with a set of demo applications which provide some
|
|
examples of how to use the various \s-1API\s0 functions. To look at them download the
|
|
OpenSSL source code from the OpenSSL website
|
|
(<https://www.openssl.org/source/>). Extract the downloaded \fB.tar.gz\fR file for
|
|
the version of OpenSSL that you are using and look at the various files in the
|
|
\&\fBdemos\fR sub-directory.
|
|
.PP
|
|
The Makefiles in the subdirectories give instructions on how to build and run
|
|
the demo applications.
|
|
.SH "FURTHER READING"
|
|
.IX Header "FURTHER READING"
|
|
See \fIossl\-guide\-libcrypto\-introduction\fR\|(7) for a more detailed introduction to
|
|
using \f(CW\*(C`libcrypto\*(C'\fR and \fIossl\-guide\-libssl\-introduction\fR\|(7) for more information
|
|
on \f(CW\*(C`libssl\*(C'\fR.
|
|
.SH "SEE ALSO"
|
|
.IX Header "SEE ALSO"
|
|
\&\fIopenssl\fR\|(1), \fIssl\fR\|(7), \fIevp\fR\|(7), \s-1\fIOSSL_LIB_CTX\s0\fR\|(3), \fIopenssl\-threads\fR\|(7),
|
|
\&\fIproperty\fR\|(7), \fIOSSL_PROVIDER\-default\fR\|(7), \fIOSSL_PROVIDER\-base\fR\|(7),
|
|
\&\s-1\fIOSSL_PROVIDER\-FIPS\s0\fR\|(7), \fIOSSL_PROVIDER\-legacy\fR\|(7), \fIOSSL_PROVIDER\-null\fR\|(7),
|
|
\&\fIopenssl\-glossary\fR\|(7), \fIprovider\fR\|(7)
|
|
.SH "COPYRIGHT"
|
|
.IX Header "COPYRIGHT"
|
|
Copyright 2000\-2023 The OpenSSL Project Authors. All Rights Reserved.
|
|
.PP
|
|
Licensed under the Apache License 2.0 (the \*(L"License\*(R"). You may not use
|
|
this file except in compliance with the License. You can obtain a copy
|
|
in the file \s-1LICENSE\s0 in the source distribution or at
|
|
<https://www.openssl.org/source/license.html>.
|