5 property - Properties, a selection mechanism for algorithm implementations
9 As of OpenSSL 3.0, a new method has been introduced to decide which of
10 multiple implementations of an algorithm will be used.
11 The method is centered around the concept of properties.
12 Each implementation defines a number of properties and when an algorithm
13 is being selected, filters based on these properties can be used to
14 choose the most appropriate implementation of the algorithm.
16 Properties are like variables, they are referenced by name and have a value
21 Property names fall into two categories: those reserved by the OpenSSL
22 project and user defined names.
23 A I<reserved> property name consists of a single C-style identifier
24 (except for leading underscores not being permitted), which begins
25 with a letter and can be followed by any number of letters, numbers
27 Property names are case-insensitive, but OpenSSL will only use lowercase
30 A I<user defined> property name is similar, but it B<must> consist of
31 two or more C-style identifiers, separated by periods.
32 The last identifier in the name can be considered the 'true' property
33 name, which is prefixed by some sort of 'namespace'.
34 Providers for example could include their name in the prefix and use
37 <provider_name>.<property_name>
38 <provider_name>.<algorithm_name>.<property_name>
42 A I<property> is a I<name=value> pair.
43 A I<property definition> is a sequence of comma separated properties.
44 There can be any number of properties in a definition.
45 For example: "" defines a null property definition; "my.foo=bar"
46 defines a property named I<my.foo> which has a string value I<bar> and
47 "iteration.count=3" defines a property named I<iteration.count> which
48 has a numeric value of I<3>.
49 The full syntax for property definitions appears below.
51 =head2 Implementations
53 Each implementation of an algorithm can define any number of
55 For example, the default provider defines the property I<default=yes>
56 for all of its algorithms.
57 Likewise, the FIPS provider defines I<fips=yes> and the legacy provider
58 defines I<legacy=yes> for all of their algorithms.
62 A I<property query clause> is a single conditional test.
63 For example, "fips=yes", "default!=yes" or "?iteration.count!=3".
64 The first two represent mandatory clauses, such clauses B<must> match
65 for any algorithm to even be under consideration.
66 The third clause represents an optional clause.
67 Matching such clauses is not a requirement, but any additional optional
68 match counts in favor of the algorithm.
69 More details about that in the B<Lookups> section.
70 A I<property query> is a sequence of comma separated property query clauses.
71 The full syntax for property queries appears below.
75 When an algorithm is looked up, a property query is used to determine
76 the best matching algorithm.
77 All mandatory query clauses B<must> be present and the implementation
78 that additionally has the largest number of matching optional query
80 If there is more than one such optimal candidate, the result will be
81 chosen from amongst those in an indeterminate way.
82 Ordering of optional clauses is not significant.
86 In order to permit a more concise expression of boolean properties, there
87 is one short cut: a property name alone (e.g. "default") is
88 exactly equivalent to "default=yes" in both definitions and queries.
92 The lexical syntax in EBNF is given by:
94 Definition ::= PropertyName ( '=' Value )?
95 ( ',' PropertyName ( '=' Value )? )*
96 Query ::= PropertyQuery ( ',' PropertyQuery )*
97 PropertyQuery ::= '-' PropertyName
98 | '?'? ( PropertyName (( '=' | '!=' ) Value)?)
99 Value ::= NumberLiteral | StringLiteral
100 StringLiteral ::= QuotedString | UnquotedString
101 QuotedString ::= '"' [^"]* '"' | "'" [^']* "'"
102 UnquotedString ::= [^{space},]+
103 NumberLiteral ::= '0' ( [0-7]* | 'x' [0-9A-Fa-f]+ ) | '-'? [1-9] [0-9]+
104 PropertyName ::= [A-Z] [A-Z0-9_]* ( '.' [A-Z] [A-Z0-9_]* )*
106 Railroad diagrams for this grammar can be found in the
107 F<crypto/property/properties.xhtml> file.
111 Properties were added in OpenSSL 3.0
115 Copyright 2019 The OpenSSL Project Authors. All Rights Reserved.
117 Licensed under the Apache License 2.0 (the "License"). You may not use
118 this file except in compliance with the License. You can obtain a copy
119 in the file LICENSE in the source distribution or at
120 L<https://www.openssl.org/source/license.html>.