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PROPERTY(7ossl) OpenSSL PROPERTY(7ossl)
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:
o = is an infix operator providing an equality test.
o != is an infix operator providing an inequality test.
o ? is a prefix operator that means that the following clause is
optional but preferred.
o - is a prefix operator that means any global query clause involving
the following property name should be ignored.
o "..." is a quoted string. The quotes are not included in the body
of the string.
o '...' 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>.
3.0.14 2024-06-04 PROPERTY(7ossl)
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