man > property(7)

PROPERTY(7SSL)                                 OpenSSL                                PROPERTY(7SSL)

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>.

3.0.13                                       2026-07-29                               PROPERTY(7SSL)