Add man 7 page about properties.

      ::= PropertyName ( '=' Value )? ( ',' PropertyName ( '=' Value )? )*

Query ::= PropertyQuery ( ',' PropertyQuery )*

PropertyQuery ::= '-'? PropertyName

        | '?' ( PropertyName (( '=' | '!=' ) Value)?)

        | '?'? ( PropertyName (( '=' | '!=' ) Value)?)

Value    ::= NumberLiteral

        | StringLiteral

StringLiteral ::= QuotedString | UnquotedString

=pod

=head1 NAME

property - Properties, a selection mechanism for algorithm implementations

=head1 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.

=head2 Property Names

Property names fall into two categories: those reserved by the OpenSSL

project and user defined names.

A I<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 I<user defined> property name is similar, but it B<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>

=head2 Properties

A I<property> is a I<name=value> pair.

A I<property definition> is a sequence of comma separated properties.

There can be any number of properties in a definition.

For example: "" defines a null property definition; "my.foo=bar"

defines a property named I<my.foo> which has a string value I<bar> and

"iteration.count=3" defines a property named I<iteration.count> which

has a numeric value of I<3>.

The full syntax for property definitions appears below.

=head2 Implementations

Each implementation of an algorithm can define any number of

properties.

For example, the default provider defines the property I<default=yes>

for all of its algorithms.

Likewise, the FIPS provider defines I<fips=yes> and the legacy provider

defines I<legacy=yes> for all of their algorithms.

=head2 Queries

A I<property query clause> is a single conditional test.

For example, "fips=yes", "default!=yes" or "?iteration.count!=3".

The first two represent mandatory clauses, such clauses B<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 B<Lookups> section.

A I<property query> is a sequence of comma separated property query clauses.

The full syntax for property queries appears below.

=head2 Lookups

When an algorithm is looked up, a property query is used to determine

the best matching algorithm.

All mandatory query clauses B<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.

=head2 Shortcut

In order to permit a more concise expression of boolean properties, there

is one short cut: a property name alone (e.g. "default") is

exactly equivalent to "default=yes" in both definitions and queries.

=head1 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 ::= [^{space},]+

 NumberLiteral  ::= '0' ( [0-7]* | 'x' [0-9A-Fa-f]+ ) | '-'? [1-9] [0-9]+

 PropertyName   ::= [A-Z] [A-Z0-9_]* ( '.' [A-Z] [A-Z0-9_]* )*

Railroad diagrams for this grammar can be found in the

F<crypto/property/properties.xhtml> file.

=head1 HISTORY

Properties were added in OpenSSL 3.0

=head1 COPYRIGHT

Copyright 2019 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

L<https://www.openssl.org/source/license.html>.

=cut