The object-capability model is a
computer security model. A
capability
A capability is the ability to execute a specified course of action or to achieve certain outcomes.
As it applies to human capital, capability represents performing or achieving certain actions/outcomes in terms of the intersection of capacity an ...
describes a transferable right to perform one (or more) operations on a given object. It can be obtained by the following combination:
:* An unforgeable reference (in the sense of object references or protected pointers) that can be sent in messages.
:* A message that specifies the operation to be performed.
The security model relies on not being able to forge references.
:* Objects can interact only by sending messages on references.
:* A reference can be obtained by:
::# Initial conditions: In the initial state of the computational world being described, object A may already have a reference to object B.
::# Parenthood: If A creates B, at that moment A obtains the only reference to the newly created B.
::# Endowment: If A creates B, B is born with that subset of A's references with which A chose to endow it.
::# Introduction: If A has references to both B and C, A can send to B a message containing a reference to C. B can retain that reference for subsequent use.
In the object-capability model, ''all'' computation is performed following the above rules.
Advantages that motivate
object-oriented programming
Object-oriented programming (OOP) is a programming paradigm based on the concept of " objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of ...
, such as encapsulation or
information hiding
In computer science, information hiding is the principle of segregation of the ''design decisions'' in a computer program that are most likely to change, thus protecting other parts of the program from extensive modification if the design decisio ...
,
modularity
Broadly speaking, modularity is the degree to which a system's components may be separated and recombined, often with the benefit of flexibility and variety in use. The concept of modularity is used primarily to reduce complexity by breaking a s ...
, and
separation of concerns
In computer science, separation of concerns is a design principle for separating a computer program into distinct sections. Each section addresses a separate '' concern'', a set of information that affects the code of a computer program. A concern ...
, correspond to security goals such as
least privilege and
privilege separation in capability-based programming.
[
]
The object-capability model was first proposed by
Jack Dennis and Earl C. Van Horn in 1966.
Loopholes in object-oriented programming languages
Some object-based programming languages (e.g.
JavaScript
JavaScript (), often abbreviated as JS, is a programming language that is one of the core technologies of the World Wide Web, alongside HTML and CSS. As of 2022, 98% of Website, websites use JavaScript on the Client (computing), client side ...
,
Java
Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mo ...
, and
C#) provide ways to access resources in other ways than according to the rules above including the following:
:*Direct
assignment to the
instance variables of an object in Java and C#.
:*Direct
reflective inspection of the meta-data of an object in Java and C#.
:*The pervasive ability to import primitive modules, e.g. java.io.File that enable external effects.
Such use of
undeniable authority
Undeniable may refer to:
Albums
* ''Undeniable'' (AZ album)
* ''Undeniable'' (Chipmunks album)
* ''Undeniable'' (Raven-Symoné album)
* ''Undeniable'' (Hellyeah album) (stylized as Unden!able), released 2016
Songs
* "Undeniable" (Mat Kearney ...
violates the conditions of the object-capability model.
Caja and
Joe-E are variants of JavaScript and Java, respectively, that impose restrictions to eliminate these loopholes.
Advantages of object capabilities
Computer scientist E. Dean Tribble stated that in
smart contracts
A smart contract is a computer program or a transaction protocol that is intended to automatically execute, control or document events and actions according to the terms of a contract or an agreement. The objectives of smart contracts are the re ...
, identity-based access control did not support well dynamically changing permissions, compared to the object-capability model. He analogized the ocap model with giving a
valet
A valet or varlet is a male servant who serves as personal attendant to his employer. In the Middle Ages and Ancien Régime, valet de chambre was a role for junior courtiers and specialists such as artists in a royal court, but the term "val ...
the key to one's car, without handing over the right to car ownership.
The structural properties of object capability systems favor modularity in code design and ensure reliable encapsulation in code implementation.
These structural properties facilitate the analysis of some security properties of an object-capability program or operating system. Some of these in particular, information flow properties can be analyzed at the level of object references and connectivity, independent of any knowledge or analysis of the code that determines the behavior of the objects. As a consequence, these security properties can be established and maintained in the presence of new objects that contain unknown and possibly malicious code.
These structural properties stem from the two rules governing access to existing objects:
:1) An object ''A'' can send a message to ''B'' only if object ''A'' holds a reference to ''B''.
:2) An object ''A'' can obtain a reference to ''C'' only if object ''A'' receives a message containing a reference to ''C''.
As a consequence of these two rules, an object can obtain a reference to another object only through a preexisting chain of references. In short, "Only connectivity begets connectivity."
Glossary of related terms
;object-capability system
: A computational system that implements principles described in this article.
;object
: An object has local state and behavior. An object in this sense is both a ''subject'' and an ''object'' in the sense used in the access control literature.
;reference
: An unforgeable communications channel (protected pointer, opaque address) that unambiguously designates a single object, and provides permission to send messages to that object.
;message
: What is sent on a reference. Depending on the system, messages may or may not themselves be first-class objects.
;request
: An operation in which a message is sent on a reference. When the message is received, the receiver will have access to any references included in the message.
;attenuation
: A common
design pattern
A design pattern is the re-usable form of a solution to a design problem. The idea was introduced by the architect Christopher Alexander and has been adapted for various other disciplines, particularly software engineering. The " Gang of Four" b ...
in object-capability systems: given one reference of an object, create another reference for a proxy object with certain security restrictions, such as only permitting read-only access or allowing revocation. The proxy object performs security checks on messages that it receives and passes on any that are allowed. Deep attenuation refers to the case where the same attenuation is applied transitively to any objects obtained via the original attenuated object, typically by use of a "membrane".
Implementations
Almost all historical systems that have been described as "capability systems" can be modeled as object-capability systems. (Note, however, that some uses of the term "capability" are not consistent with the model, such as POSIX "capabilities".)
KeyKOS
KeyKOS is a persistent, pure capability-based operating system for the IBM S/370 mainframe computers. It allows emulating the environments of VM, MVS, and Portable Operating System Interface (POSIX). It is a predecessor of the Extremely Rel ...
,
EROS
In Greek mythology, Eros (, ; grc, Ἔρως, Érōs, Love, Desire) is the Greek god of love and sex. His Roman counterpart was Cupid ("desire").''Larousse Desk Reference Encyclopedia'', The Book People, Haydock, 1995, p. 215. In the e ...
,
Integrity (operating system),
CapROS
Capability-based Reliable Operating System (CapROS) is an operating system incorporating pure capability-based security. It features automatic persistence of data and processes, even across system reboots. Capability systems naturally support th ...
,
Coyotos,
seL4,
OKL4 and
Fiasco.OC are secure operating systems that implement the object-capability model.
Languages that implement object capabilities
* Act 1 (1981)
*Eden (1985),
*
Emerald
Emerald is a gemstone and a variety of the mineral beryl (Be3Al2(SiO3)6) colored green by trace amounts of chromium or sometimes vanadium.Hurlbut, Cornelius S. Jr. and Kammerling, Robert C. (1991) ''Gemology'', John Wiley & Sons, New York, p ...
(1987),
*Trusty Scheme (1992),
*W7 (1995),
*
Joule
The joule ( , ; symbol: J) is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force applie ...
(1996),
*Original-E (1997),
Oz-E(2005),
*
Joe-E (2005),
CaPerl(2006),
Emily(2006)
*
Caja (2007–2021)
Monte(2008–present)
Pony(2014–present)
[
*]Wyvern
A wyvern ( , sometimes spelled wivern) is a legendary winged dragon that has two legs.
The wyvern in its various forms is important in heraldry, frequently appearing as a mascot of schools and athletic teams (chiefly in the United States, Unit ...
(2012–present)
*Newspeak
Newspeak is the fictional language of Oceania, a totalitarian superstate that is the setting of the 1949 dystopian novel ''Nineteen Eighty-Four'', by George Orwell. In the novel, the Party created Newspeak to meet the ideological requirements ...
(2007–present)
* Hacklang (2021-present)
Rholang
(2018-present)
See also
*Capability-based security
Capability-based security is a concept in the design of secure computing systems, one of the existing security models. A capability (known in some systems as a key) is a communicable, unforgeable token of authority. It refers to a value that refe ...
* Capability-based addressing
*Actor model
References
{{DEFAULTSORT:Object-Capability Model
Computer security models