NAME
    property - Properties, a selection mechanism for algorithm
    implementations

DESCRIPTION
    As of OpenSSL 3.0, a new method has been introduced to decide which of
    multiple implementations of an algorithm will be used. The method is
    centered around the concept of properties. Each implementation defines a
    number of properties and when an algorithm is being selected, filters
    based on these properties can be used to choose the most appropriate
    implementation of the algorithm.

    Properties are like variables, they are referenced by name and have a
    value assigned.

  Property Names
    Property names fall into two categories: those reserved by the OpenSSL
    project and user defined names. A *reserved* property name consists of a
    single C-style identifier (except for leading underscores not being
    permitted), which begins with a letter and can be followed by any number
    of letters, numbers and underscores. Property names are
    case-insensitive, but OpenSSL will only use lowercase letters.

    A *user defined* property name is similar, but it must consist of two or
    more C-style identifiers, separated by periods. The last identifier in
    the name can be considered the 'true' property name, which is prefixed
    by some sort of 'namespace'. Providers for example could include their
    name in the prefix and use property names like

      <provider_name>.<property_name>
      <provider_name>.<algorithm_name>.<property_name>

  Properties
    A *property* is a *name=value* pair. A *property definition* is a
    sequence of comma separated properties. There can be any number of
    properties in a definition, however each name must be unique. For
    example: "" defines an empty property definition (i.e., no restriction);
    "my.foo=bar" defines a property named *my.foo* which has a string value
    *bar* and "iteration.count=3" defines a property named *iteration.count*
    which has a numeric value of *3*. The full syntax for property
    definitions appears below.

  Implementations
    Each implementation of an algorithm can define any number of properties.
    For example, the default provider defines the property
    *provider=default* for all of its algorithms. Likewise, OpenSSL's FIPS
    provider defines *provider=fips* and the legacy provider defines
    *provider=legacy* for all of their algorithms.

  Queries
    A *property query clause* is a single conditional test. For example,
    "fips=yes", "provider!=default" or "?iteration.count=3". The first two
    represent mandatory clauses, such clauses must match for any algorithm
    to even be under consideration. The third clause represents an optional
    clause. Matching such clauses is not a requirement, but any additional
    optional match counts in favor of the algorithm. More details about that
    in the Lookups section. A *property query* is a sequence of comma
    separated property query clauses. It is an error if a property name
    appears in more than one query clause. The full syntax for property
    queries appears below, but the available syntactic features are:

    *   = is an infix operator providing an equality test.

    *   != is an infix operator providing an inequality test.

    *   ? is a prefix operator that means that the following clause is
        optional but preferred.

    *   - is a prefix operator that means any global query clause involving
        the following property name should be ignored.

    *   "..." is a quoted string. The quotes are not included in the body of
        the string.

    *   '...' is a quoted string. The quotes are not included in the body of
        the string.

  Lookups
    When an algorithm is looked up, a property query is used to determine
    the best matching algorithm. All mandatory query clauses must be present
    and the implementation that additionally has the largest number of
    matching optional query clauses will be used. If there is more than one
    such optimal candidate, the result will be chosen from amongst those in
    an indeterminate way. Ordering of optional clauses is not significant.

  Shortcut
    In order to permit a more concise expression of boolean properties,
    there is one short cut: a property name alone (e.g. "my.property") is
    exactly equivalent to "my.property=yes" in both definitions and queries.

  Global and Local
    Two levels of property query are supported. A context based property
    query that applies to all fetch operations and a local property query.
    Where both the context and local queries include a clause with the same
    name, the local clause overrides the context clause.

    It is possible for a local property query to remove a clause in the
    context property query by preceding the property name with a '-'. For
    example, a context property query that contains "fips=yes" would
    normally result in implementations that have "fips=yes".

    However, if the setting of the "fips" property is irrelevant to the
    operations being performed, the local property query can include the
    clause "-fips". Note that the local property query could not use
    "fips=no" because that would disallow any implementations with
    "fips=yes" rather than not caring about the setting.

SYNTAX
    The lexical syntax in EBNF is given by:

     Definition     ::= PropertyName ( '=' Value )?
                            ( ',' PropertyName ( '=' Value )? )*
     Query          ::= PropertyQuery ( ',' PropertyQuery )*
     PropertyQuery  ::= '-' PropertyName
                      | '?'? ( PropertyName (( '=' | '!=' ) Value)?)
     Value          ::= NumberLiteral | StringLiteral
     StringLiteral  ::= QuotedString | UnquotedString
     QuotedString   ::= '"' [^"]* '"' | "'" [^']* "'"
     UnquotedString ::= [A-Za-z] [^{space},]+
     NumberLiteral  ::= '0' ( [0-7]* | 'x' [0-9A-Fa-f]+ ) | '-'? [1-9] [0-9]+
     PropertyName   ::= [A-Za-z] [A-Za-z0-9_]* ( '.' [A-Za-z] [A-Za-z0-9_]* )*

    The flavour of EBNF being used is defined by:
    <https://www.w3.org/TR/2010/REC-xquery-20101214/#EBNFNotation>.

HISTORY
    Properties were added in OpenSSL 3.0

COPYRIGHT
    Copyright 2019-2023 The OpenSSL Project Authors. All Rights Reserved.

    Licensed under the Apache License 2.0 (the "License"). You may not use
    this file except in compliance with the License. You can obtain a copy
    in the file LICENSE in the source distribution or at
    <https://www.openssl.org/source/license.html>.

