Access control is the ability to permit or deny the use of a particular resource by a particular entity. Access control mechanisms can be used in managing physical resources (such as a movie theater, to which only ticketholders should be admitted), logical resources (a bank account, with a limited number of people authorized to make a withdrawal), or digital resources (for example, a private text document on a computer, which only certain users should be able to read).
Item Control or Electronic Key Management is an area within (and possibly integrated with)an access control system which concerns the managing of possession and location of small assets or physical (mechanical) keys.
Physical access of a person
may be allowed depending on payment, authorization, etc. Also there may be one-way traffic of people
. These can be enforced by personnel such as a border guard
, a doorman
, a ticket
checker, etc., or with a device such as a turnstile
. There may be fences
to avoid circumventing this access control. An alternative of access control in the strict sense (physically controlling access itself) is a system of checking authorized presence, see e.g. Ticket controller (transportation)
. A variant is exit control, e.g. of a shop (checkout
) or a country.
In physical security, the term access control refers to the practice of restricting entrance to a property, a building, or a room to authorized persons. Physical access control can be achieved by a human (a guard, bouncer, or receptionist), through mechanical means such as locks and keys, or through technological means such as access control systems. Within these environments, physical key management may also be employed as a means of further managing and monitoring access to mechanically keyed areas or access to certain small assets.
Physical access control is a matter of who, where, and when. An access control system determines who is allowed to enter or exit, where they are allowed to exit or enter, and when they are allowed to enter or exit. Historically this was partially accomplished through keys and locks. When a door is locked only someone with a key can enter through the door depending on how the lock is configured. Mechanical locks and keys do not allow restriction of the key holder to specific times or dates. Mechanical locks and keys do not provide records of the key used on any specific door and the keys can be easily copied or transferred to an unauthorized person. When a mechanical key is lost or the key holder is no longer authorized to use the protected area, the locks must re-keyed.
Electronic access control uses computers to solve the limitations of mechanical locks and keys. A wide range of credentials can be used to replace mechanical keys. The electronic access control system grants access based on the credential presented. When access is granted, the door is unlocked for a predetermined time and the transaction is recorded. When access is refused, the door remains locked and the attempted access is recorded. The system will also monitor the door and alarm if the door is forced open or held open too long after being unlocked.
Access Control System Operation
When a credential is presented to a reader, the reader sends the credential’s information, usually a number, to a control panel, a highly reliable processor. The control panel compares the credential's number to an access control list, grants or denies the presented request, and sends a transaction log to a database. When access is denied based on the access control list, the door remains locked. If there is a match between the credential and the access control list, the control panel operates a relay that in turn unlocks the door. The control panel also ignores a door open signal to prevent an alarm. Often the reader provides feedback, such as a flashing red LED for an access denied and a flashing green LED for an access granted.
The above description illustrates a single factor transaction. Credentials can be passed around, thus subverting the access control list. For example, Alice has access rights to the server room but Bob does not. Alice either gives Bob her credential or Bob takes it; he now has access to the server room. To prevent this, two-factor authentication can be used. In a two factor transaction, the presented credential and a second factor are needed for access to be granted. The second factor can be a PIN, a second credential, operator intervention, or a biometric input. Often the factors are characterized as
- something you have, such as a credential,
- something you know, e.g. a PIN, or
- something you are, typically a biometric input.
Access Control System Components
An access control point, which can be a door
, turnstile, parking gate, elevator, or other physical barrier where granting access can be electrically controlled. Typically the access point is a door. An electronic access control door can contain several elements. At its most basic there is an electric lock (see electronic lock
.) The lock is unlocked by an operator with a switch. To automate this, operator intervention is replaced by a reader. The reader could be a keypad where a code is entered, it could be a card reader
, or it could be a biometric reader. Readers do not usually make an access decision but send a card number to an access control panel that verifies the number against an access list. To monitor the door position a magnetic door switch is used. In concept the door switch is not unlike those on refrigerators or car doors. Generally only entry is controlled and exit is uncontrolled. In cases where exit is also controlled a second reader is used on the opposite side of the door. In cases where exit is not controlled, free exit, a device called a request-to-exit (REX) is used. Request-to-exit devices can be a pushbutton or a motion detector. When the button is pushed or the motion detector detects motion at the door, the door alarm is temporarily ignored while the door is opened. Exiting a door without having to electrically unlock the door is called mechanical free egress. This is an important safety feature. In cases where the lock must be electrically unlocked on exit, the request-to-exit device also unlocks the door.
A credential is something you know, such as number or PIN, something you have, such as an access badge
, something you are, such as a biometric feature, or some combination of these. The typical credential is an access card, key fob, or other key. There are many card technologies including magnetic stripe, bar code, Wiegand, 125 kHz proximity, contact smart cards, and contactless smart cards. Typical biometric technologies include fingerprint, facial recognition, iris recognition, retinal scan, voice, and hand geometry.
Bar Code Technology
A bar code is a series of alternating dark and light stripes that are read by an optical scanner. The organization and width of the lines is determined by the bar code protocol selected. There are many different protocols but code 39 is the most popular in the security industry. Sometimes the digits represented by the dark and light bars are also printed to allow people to read the number without an optical reader. The advantage of using bar code technology is that it is cheap and easy to generate the credential and it can easily be applied to cards or other items. The disadvantage of this technology is that it is cheap and easy to generate a credential making the technology susceptible to fraud and the optical reader can have reliability problems with dirty or smudged credentials. One attempt to reduce fraud is to print the bar code using carbon-based ink and then cover the bar code with a dark red overlay. The bar code can then be read with an optical reader tuned to the infrared spectrum, but can not easily be copied by a copy machine. This does not address the ease with which bar code numbers can be generated from a computer using almost any printer.
Magnetic Stripe Technology
Magnetic stripe technology, usually called mag-stripe, is so named because of the stripe of magnetic oxide tape that is laminated on a card. There are three tracks of data on the magnetic stripe. Typically the data on each of the tracks follows a specific encoding standard, but it is possible to encode any format on any track. A mag-stripe card is cheap compared to other card technologies and is easy to program. The magnetic stripe holds more data than a bar code can in the same space. While a mag-stripe is more difficult to generate than a bar code, the technology for reading and encoding data on a mag-stripe is widespread and easy to acquire. Magnetic stripe technology is also susceptible to misreads, card wear, and data corruption.
Wiegand Card Technology
card technology is a patented technology using embedded ferromagnetic
wires strategically positioned to create a unique pattern that generates the identification number. Like magnetic stripe
or bar code
, this card must be swiped through a reader to be read. Unlike those other technologies the identification media is embedded in the card and not susceptible to wear. This technology once gained popularity because of the difficulty in duplicating the technology creating a high perception of security. This technology is being replaced by proximity cards because of the limited source of supply, the relatively better tamper resistance of proximity readers, and the convenience of the touch-less functionality in proximity readers.
Proximity Card Technology
The Wiegand effect
was used in early access cards. This method was abandoned in favor of other technologies. The new technologies retained the Wiegand upstream data so that the new readers were compatible with old systems. Readers are still called Wiegand but no longer use the Wiegand effect. A Wiegand reader radiates a 1" to 5" electrical field around itself. Cards use a simple LC circuit
. When a card is presented to the reader, the reader's electrical field excites a coil in the card. The coil charges a capacitor
and in turn powers a integrated circuit
. The integrated circuit outputs the card number to the coil which transmits it to the reader.
A common proximity format is 26 bit Wiegand. This format uses a facility code, sometimes also called a site code. The facility code is a unique number common to all of the cards in a particular set. The idea is that an organization will have their own facility code and a set of numbered cards incrementing from 1. Another organization has a different facility code and their card set also increments from 1. Thus different organizations can have card sets with the same card numbers but since the facility codes differ, the cards only work at one organization. This idea worked fine for a while but there is no governing body controlling card numbers, and different manufacturers can supply cards with identical facility codes and identical card numbers to different organizations. Thus there is a problem of duplicate cards. To counteract this problem some manufacturers have created formats beyond 26 bit Wiegand that they control and issue to organizations.
In the 26 bit Wiegand format, bit 1 is an even parity bit. Bits 2-9 are a facility code. Bits 10-25 are the card number. Bit 26 is an odd parity bit. Other formats have a similar structure of a leading facility code followed by the card number and including parity bits for error checking.
There are two types of smart cards: contact and contactless. Both have an embedded microprocessor and memory. The smart card differs from the card typically called a proximity card in that the microchip in the proximity card has only one function: to provide the reader with the card’s identification number. The processor on the smart card has an operating system and can handle multiple applications such as a cash card, a pre-paid membership card, and even an access control card. The difference between the two types of smart cards is found in the manner with which the microprocessor on the card communicates with the outside world. A contact smart card has eight contacts, which must physically touch contacts on the reader to convey information between them. A contactless smart card uses the same radio-based technology as the proximity card with the exception of the frequency band used. Smart cards allow the access control system to save user information on a credential carried by the user rather than requiring more memory on each controller.
A personal identification number (PIN) falls in the category of what you know rather than what you have. The PIN is usually a number consisting of four to eight digits. Less and the number is too easy to guess. More and the number is too difficult to remember. The advantage to using a PIN as an access credential is that once the number is memorized, the credential cannot be lost or left somewhere. The disadvantage is the difficulty some people have in remembering numbers that are not frequently used and the ease with which a PIN can be observed and therefore used by unauthorized people. The PIN is even less secure than a bar code or magnetic stripe card.
In computer security
, access control includes authentication
. It also includes measures such as physical devices, including biometric scans and metal locks
, hidden paths, digital signatures
, social barriers, and monitoring by humans and automated systems.
In any access control model, the entities that can perform actions in the system are called subjects, and the entities representing resources to which access may need to be controlled are called objects (see also Access Control Matrix). Subjects and objects should both be considered as software entities, rather than as human users: any human user can only have an effect on the system via the software entities that they control. Although some systems equate subjects with user IDs, so that all processes started by a user by default have the same authority, this level of control is not fine-grained enough to satisfy the Principle of least privilege, and arguably is responsible for the prevalence of malware in such systems (see computer insecurity).
In some models, for example the object-capability model, any software entity can potentially act as both a subject and object.
Access control models used by current systems tend to fall into one of two classes: those based on capabilities and those based on access control lists (ACLs). In a capability-based model, holding an unforgeable reference or capability to an object provides access to the object (roughly analogous to how possession of your house key grants you access to your house); access is conveyed to another party by transmitting such a capability over a secure channel. In an ACL-based model, a subject's access to an object depends on whether its identity is on a list associated with the object (roughly analogous to how a bouncer at a private party would check your ID to see if your name is on the guest list); access is conveyed by editing the list. (Different ACL systems have a variety of different conventions regarding who or what is responsible for editing the list and how it is edited.)
Both capability-based and ACL-based models have mechanisms to allow access rights to be granted to all members of a group of subjects (often the group is itself modeled as a subject).
Access control systems provide the essential services of identification and authentication (I&A), authorization, and accountability where:
- identification and authentication determine who can log on to a system, and the association of users with the software subjects that they are able to control as a result of logging in;
- authorization determines what a subject can do;
- accountability identifies what a subject (or all subjects associated with a user) did.
Identification and authentication (I&A)
Identification and authentication (I&A) is the process of verifying that an identity is bound to the entity that asserts it. The I&A process assumes that there was an initial vetting of the identity, during which an authenticator was established. Subsequently, the entity asserts an identity together with an authenticator as a means for validation. The only requirements for the identifier is that it must be unique within its security domain.
Authenticators are commonly based on at least one of these four factors:
- Something you know, such as a password or a personal identification number (PIN). This assumes that only the owner of the account knows the password or PIN needed to access the account.
- Something you have, such as a smart card or token. This assumes that only the owner of the account has the necessary smart card or token needed to unlock the account.
- Something you are, such as fingerprint, voice, retina, or iris characteristics.
- Where you are, for example inside or outside a company firewall, or proximity of login location to a personal GPS device.
Authorization applies to subjects rather than to users (the association between a user and the subjects initially controlled by that user having been determined by I&A). Authorization determines what a subject can do on the system.
Most modern operating systems define sets of permissions that are variations or extensions of three basic types of access:
- Read (R): The subject can
- Read file contents
- List directory contents
- Write (W): The subject can change the contents of a file or directory with the following tasks:
- Execute (X): If the file is a program, the subject can cause the program to be run. (In Unix systems, the 'execute' permission doubles as a 'traverse directory' permission when granted for a directory.)
These rights and permissions are implemented differently in systems based on discretionary access control (DAC) and mandatory access control (MAC).
Accountability uses such system components as audit trails (records) and logs to associate a subject with its actions. The information recorded should be sufficient to map the subject to a controlling user. Audit trails and logs are important for
- Detecting security violations
- Re-creating security incidents
If no one is regularly reviewing your logs and they are not maintained in a secure and consistent manner, they may not be admissible as evidence.
Many systems can generate automated reports based on certain predefined criteria or thresholds, known as clipping levels. For example, a clipping level may be set to generate a report for the following:
- More than three failed logon attempts in a given period
- Any attempt to use a disabled user account
These reports help a system administrator or security administrator to more easily identify possible break-in attempts.
Access Control Techniques
Access control techniques are sometimes categorized as either discretionary or non-discretionary. The three most widely recognized models are Discretionary Access Control (DAC), Mandatory Access Control (MAC), and Role Based Access Control (RBAC). MAC and RBAC are both non-discretionary.
Discretionary Access Control
Discretionary access control
(DAC) is an access policy determined by the owner of an object. The owner decides who is allowed to access the object and what privileges they have.
Two important concepts in DAC are
- File and data ownership: Every object in the system has an owner. In most DAC systems, each object's initial owner is the subject that caused it to be created. The access policy for an object is determined by its owner.
- Access rights and permissions: These are the controls that an owner can assign to other subjects for specific resources.
Access controls may be discretionary in ACL-based or capability-based access control systems. (In capability-based systems, there is usually no explicit concept of 'owner', but the creator of an object has a similar degree of control over its access policy.)
Mandatory Access Control
Mandatory access control (MAC) is an access policy determined by the system, not the owner. MAC is used in
multilevel systems that process highly sensitive data, such as classified government and military information. A
multilevel system is a single computer system that handles multiple classification levels between subjects and
- Sensitivity labels: In a MAC-based system, all subjects and objects must have labels assigned to them. A subject's sensitivity label specifies its level of trust. An object's sensitivity label specifies the level of trust required for access. In order to access a given object, the subject must have a sensitivity level equal to or higher than the requested object.
- Data import and export: Controlling the import of information from other systems and export to other systems (including printers) is a critical function of MAC-based systems, which must ensure that sensitivity labels are properly maintained and implemented so that sensitive information is appropriately protected at all times.
Two methods are commonly used for applying mandatory access control:
- Rule-based access controls: This type of control further defines specific conditions for access to a requested object. All MAC-based systems implement a simple form of rule-based access control to determine whether access should be granted or denied by matching:
- An object's sensitivity label
- A subject's sensitivity label
- Lattice-based access controls: These can be used for complex access control decisions involving multiple objects and/or subjects. A lattice model is a mathematical structure that defines greatest lower-bound and least upper-bound values for a pair of elements, such as a subject and an object.
Few systems implement MAC. XTS-400 is an example of one that does.
Role Based Access Control
Role-based access control (RBAC) is an access policy determined by the system, not the owner. RBAC is used in
commercial applications and also in military systems, where multi-level security requirements may also exist.
RBAC differs from DAC in that DAC allows users to control access to their resources, while in RBAC, access is controlled
at the system level, outside of the user's control. Although RBAC is non-discretionary, it can be distinguished from
MAC primarily in the way permissions are handled. MAC controls read and write permissions based on a user's clearance level and additional labels. RBAC controls collections of permissions that may include complex operations
such as an e-commerce transaction, or may be as simple as read or write. A role in RBAC can be viewed as a set of
Three primary rules are defined for RBAC:
1. Role assignment: A subject can execute a transaction only if the subject has selected or been assigned a role.
2. Role authorization: A subject's active role must be authorized for the subject.
With rule 1 above, this rule ensures that users can take on only roles for which they are authorized.
3. Transaction authorization: A subject can execute a transaction only if the transaction is authorized
for the subject's active role. With rules 1 and 2, this rule ensures that users can execute only transactions
for which they are authorized.
Additional constraints may be applied as well, and roles can be combined in a hierarchy where higher-level
roles subsume permissions owned by sub-roles.
Most IT vendors offer RBAC in one or more products.
, the term access control
is defined in U.S. Federal Standard 1037C
with the following meanings:
- A service feature or technique used to permit or deny use of the components of a communication system.
- A technique used to define or restrict the rights of individuals or application programs to obtain data from, or place data onto, a storage device.
- The definition or restriction of the rights of individuals or application programs to obtain data from, or place data into, a storage device.
- The process of limiting access to the resources of an AIS to authorized users, programs, processes, or other systems.
- That function performed by the resource controller that allocates system resources to satisfy user requests.
Notice that this definition depends on several other technical terms from Federal Standard 1037C.
In public policy, access control to restrict access to systems ("authorization") or to track or monitor behavior within systems ("accountability") is an implementation feature of using trusted systems for security or social control.