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178 lines
6.2 KiB
Plaintext
178 lines
6.2 KiB
Plaintext
=pod
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=head1 NAME
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property - Properties, a selection mechanism for algorithm implementations
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=head1 DESCRIPTION
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As of OpenSSL 3.0, a new method has been introduced to decide which of
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multiple implementations of an algorithm will be used.
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The method is centered around the concept of properties.
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Each implementation defines a number of properties and when an algorithm
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is being selected, filters based on these properties can be used to
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choose the most appropriate implementation of the algorithm.
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Properties are like variables, they are referenced by name and have a value
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assigned.
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=head2 Property Names
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Property names fall into two categories: those reserved by the OpenSSL
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project and user defined names.
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A I<reserved> property name consists of a single C-style identifier
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(except for leading underscores not being permitted), which begins
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with a letter and can be followed by any number of letters, numbers
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and underscores.
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Property names are case-insensitive, but OpenSSL will only use lowercase
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letters.
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A I<user defined> property name is similar, but it B<must> consist of
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two or more C-style identifiers, separated by periods.
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The last identifier in the name can be considered the 'true' property
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name, which is prefixed by some sort of 'namespace'.
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Providers for example could include their name in the prefix and use
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property names like
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<provider_name>.<property_name>
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<provider_name>.<algorithm_name>.<property_name>
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=head2 Properties
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A I<property> is a I<name=value> pair.
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A I<property definition> is a sequence of comma separated properties.
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There can be any number of properties in a definition, however each name must
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be unique.
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For example: "" defines an empty property definition (i.e., no restriction);
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"my.foo=bar" defines a property named I<my.foo> which has a string value I<bar>
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and "iteration.count=3" defines a property named I<iteration.count> which
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has a numeric value of I<3>.
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The full syntax for property definitions appears below.
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=head2 Implementations
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Each implementation of an algorithm can define any number of
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properties.
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For example, the default provider defines the property I<provider=default>
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for all of its algorithms.
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Likewise, OpenSSL's FIPS provider defines I<provider=fips> and the legacy
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provider defines I<provider=legacy> for all of their algorithms.
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=head2 Queries
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A I<property query clause> is a single conditional test.
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For example, "fips=yes", "provider!=default" or "?iteration.count=3".
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The first two represent mandatory clauses, such clauses B<must> match
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for any algorithm to even be under consideration.
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The third clause represents an optional clause.
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Matching such clauses is not a requirement, but any additional optional
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match counts in favor of the algorithm.
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More details about that in the B<Lookups> section.
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A I<property query> is a sequence of comma separated property query clauses.
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It is an error if a property name appears in more than one query clause.
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The full syntax for property queries appears below, but the available syntactic
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features are:
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=over 4
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=item *
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B<=> is an infix operator providing an equality test.
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=item *
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B<!=> is an infix operator providing an inequality test.
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=item *
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B<?> is a prefix operator that means that the following clause is optional
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but preferred.
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=item *
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B<-> is a prefix operator that means any global query clause involving the
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following property name should be ignored.
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=item *
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B<"..."> is a quoted string.
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The quotes are not included in the body of the string.
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=item *
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B<'...'> is a quoted string.
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The quotes are not included in the body of the string.
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=back
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=head2 Lookups
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When an algorithm is looked up, a property query is used to determine
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the best matching algorithm.
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All mandatory query clauses B<must> be present and the implementation
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that additionally has the largest number of matching optional query
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clauses will be used.
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If there is more than one such optimal candidate, the result will be
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chosen from amongst those in an indeterminate way.
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Ordering of optional clauses is not significant.
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=head2 Shortcut
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In order to permit a more concise expression of boolean properties, there
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is one short cut: a property name alone (e.g. "my.property") is
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exactly equivalent to "my.property=yes" in both definitions and queries.
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=head2 Global and Local
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Two levels of property query are supported.
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A context based property query that applies to all fetch operations and a local
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property query.
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Where both the context and local queries include a clause with the same name,
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the local clause overrides the context clause.
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It is possible for a local property query to remove a clause in the context
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property query by preceding the property name with a '-'.
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For example, a context property query that contains "fips=yes" would normally
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result in implementations that have "fips=yes".
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However, if the setting of the "fips" property is irrelevant to the
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operations being performed, the local property query can include the
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clause "-fips".
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Note that the local property query could not use "fips=no" because that would
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disallow any implementations with "fips=yes" rather than not caring about the
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setting.
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=head1 SYNTAX
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The lexical syntax in EBNF is given by:
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Definition ::= PropertyName ( '=' Value )?
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( ',' PropertyName ( '=' Value )? )*
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Query ::= PropertyQuery ( ',' PropertyQuery )*
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PropertyQuery ::= '-' PropertyName
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| '?'? ( PropertyName (( '=' | '!=' ) Value)?)
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Value ::= NumberLiteral | StringLiteral
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StringLiteral ::= QuotedString | UnquotedString
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QuotedString ::= '"' [^"]* '"' | "'" [^']* "'"
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UnquotedString ::= [A-Za-z] [^{space},]+
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NumberLiteral ::= '0' ( [0-7]* | 'x' [0-9A-Fa-f]+ ) | '-'? [1-9] [0-9]+
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PropertyName ::= [A-Za-z] [A-Za-z0-9_]* ( '.' [A-Za-z] [A-Za-z0-9_]* )*
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The flavour of EBNF being used is defined by:
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L<https://www.w3.org/TR/2010/REC-xquery-20101214/#EBNFNotation>.
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=head1 HISTORY
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Properties were added in OpenSSL 3.0
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=head1 COPYRIGHT
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Copyright 2019-2023 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the Apache License 2.0 (the "License"). 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 LICENSE in the source distribution or at
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L<https://www.openssl.org/source/license.html>.
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=cut
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