PROPERTY(7SSL)

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version 4.0.3 · 13 versions
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💖 PROPERTY(7SSL) — the OpenSSL property era, bestie edition 💖

okay so this is the property page from the OpenSSL manual — it's literally, no cap, the glow-up guide to Properties, a selection mechanism for algorithm implementations. if you've ever wondered how OpenSSL picks which of your many algorithm implementations gets to be the main character, it's this. 🎀

✨ DESCRIPTION ✨

as of OpenSSL 3.0, a whole new method got introduced to decide which of multiple implementations of an algorithm will be used. and the way this works is centered around the concept of properties. every implementation defines a number of properties, and when an algorithm is being selected, you can use filters based on these properties to choose the most appropriate implementation of that algorithm. it's giving casting director energy, fr fr. 🌈

properties are like variables — you reference them by name and they have a value assigned. cute, simple, iconic.

🏷️ Property Names

property names fall into two categories: the ones reserved by the OpenSSL project, and the user defined ones. 💅

a reserved property name is a single C-style identifier — except leading underscores are not 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 ever use lowercase letters. lowercase only, bestie. it's a whole aesthetic. 🦋

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 is basically the 'true' property name, and it's 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 (no duplicates, we don't do that here 💀). 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. every provider showing up with its own little name tag at the party. 🫶

🔍 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 (it's right down there, keep scrolling bestie 😭).

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. 🏆

and if there is more than one such optimal candidate? the result will be chosen from amongst those in an indeterminate way. yeah. a little chaotic, a little mysterious, we don't make the rules. 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. lazy girl math that is actually correct for once 💅✨

🌍 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. local stays winning. 👑

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. not me mixing those two up 😭 they are NOT the same thing.

🔮 PREDEFINED NAMES 🔮

currently known predefined names are:

  • "provider" — the conventional property value is the provider's name. this may be different from the name returned by OSSL_PROVIDER_get0_name(3).

it is a convention among OpenSSL provider implementations to define a property with this name. it is not mandatory to do this.

  • "version" — the conventional property value is the provider's version.

OpenSSL provider implementations do not define a property with this name.

  • "fips" — the conventional property value is boolean ("yes" or "no"), indication whether the implementation conforms to FIPS standards or not.

it is a convention among OpenSSL provider implementations to define a property with this name where applicable. it is not mandatory to do this, but strongly recommended. 👀

📐 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. origin story unlocked. ⭐

Copyright 2019-2026 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>.

okay that's the whole page bestie, go forth and fetch responsibly 🧚💜

mwah 💋 — this was the OpenSSL 4.0.3 edition, dated 2026-09-29, and it ate, no crumbs.

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