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Return to RFC 4949 Internet Security Glossary Definitions, RFC 4949 Internet Security Glossary, RFC 4949 Internet Security Glossary Bibliography

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• A1 computer system

(O) /TCSEC/ See: Tutorial under “Trusted Computer System Evaluation Criteria”. (Compare: beyond A1.)

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• AA

(D) See: Deprecated Usage under “attribute authority”.

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• ABA Guidelines

(Noun | N]]) “American Bar Association (ABA) Digital Signature Guidelines” [DSG], a framework of legal principles for using digital signatures and digital certificates in electronic commerce.

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• Abstract Syntax Notation One (ASN.1)

(Noun | N]]) A standard for describing data objects. [Larm, X680] (See: CMS.)

Usage: IDOCs SHOULD use the term “ASN.1” narrowly to describe the notation or language called “Abstract Syntax Notation One”. IDOCs MAY use the term more broadly to encompass the notation, its

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associated encoding rules (see: BER), and software tools that assist in its use, when the context makes this meaning clear.

Tutorial: OSIRM defines computer network functionality in layers. Protocols and data objects at higher layers are abstractly defined to be implemented using protocols and data objects from lower layers. A higher layer may define transfers of abstract objects between computers, and a lower layer may define those transfers concretely as strings of bits. Syntax is needed to specify data formats of abstract objects, and encoding rules are needed to transform abstract objects into bit strings at lower layers. OSI standards use ASN.1 for those specifications and use various encoding rules for those transformations. (See: BER.)

In ASN.1, formal names are written without spaces, and separate words in a name are indicated by capitalizing the first letter of each word except the first word. For example, the name of a CRL is “certificateRevocationList”.

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• ACC

(I) See: access control center.

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• acceptable risk

(I) A risk that is understood and tolerated by a system's user, operator, owner, or accreditor, usually because the cost or difficulty of implementing an effective countermeasure for the associated vulnerability exceeds the expectation of loss. (See: adequate security, risk, “second law” under “Courtney's laws”.)

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• access

1a. (I) The ability and means to communicate with or otherwise interact with a system to use system resources either to handle information or to gain knowledge of the information the system contains. (Compare: handle.)

Usage: The definition is intended to include all types of communication with a system, including one-way communication in either direction. In actual practice, however, passive users might be treated as not having “access” and, therefore, be exempt from most requirements of the system's security policy. (See: “passive user” under “user”.)

1b. (O) “Opportunity to make use of an information system (IS) resource.” [C4009]

2. (O) /formal model/ “A specific type of interaction between a subject and an object that results in the flow of information from one to the other.” [NCS04]

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• Access Certificate for Electronic Services (ACES)

(O) A PKI operated by the U.S. Government's General Services Administration in cooperation with industry partners. (See: CAM.)

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• access control

1. (I) Protection of system resources against unauthorized access.

2. (I) A process by which use of system resources is regulated according to a security policy and is permitted only by authorized entities (users, programs, processes, or other systems) according to that policy. (See: access, access control service, computer security, discretionary access control, mandatory access control, role-based access control.)

3. (I) /formal model/ Limitations on interactions between subjects and objects in an information system.

4. (O) “The prevention of unauthorized use of a resource, including the prevention of use of a resource in an unauthorized manner.” [I7498-2]

5. (O) /U.S. Government/ A system using physical, electronic, or human controls to identify or admit personnel with properly authorized access to a SCIF.

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• access control center (ACC)

(I) A computer that maintains a database (possibly in the form of an access control matrix) defining the security policy for an access control service, and that acts as a server for clients requesting access control decisions.

Tutorial: An ACC is sometimes used in conjunction with a key center to implement access control in a key-distribution system for symmetric cryptography. (See: BLACKER, Kerberos.)

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• access control list (ACL)

(I) /information system/ A mechanism that implements access control for a system resource by enumerating the system entities that are permitted to access the resource and stating, either implicitly or explicitly, the access modes granted to each entity. (Compare: access control matrix, access list, access profile, capability list.)

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• access control matrix

(I) A rectangular array of cells, with one row per subject and one column per object. The entry in a cell – that is, the entry for a particular subject-object pair – indicates the access mode that the subject is permitted to exercise on the object. Each column is

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equivalent to an “access control list” for the object; and each row is equivalent to an “access profile” for the subject.

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• access control service

(I) A security service that protects against a system entity using a system resource in a way not authorized by the system's security policy. (See: access control, discretionary access control, id[[entity-based security policy, mandatory access control, rule- based security policy.)

Tutorial: This service includes protecting against use of a resource in an unauthorized manner by an entity (i.e., a principal) that is authorized to use the resource in some other manner. (See: insider.) The two basic mechanisms for implementing this service are ACLs and tickets.

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• access level

1. (D) Synonym for the hierarchical “classification level” in a security level. [C4009] (See: security level.)

2. (D) Synonym for “clearance level”.

Deprecated Definitions: IDOCs SHOULD NOT use this term with these definitions because they duplicate the meaning of more specific terms. Any IDOC that uses this term SHOULD provide a specific definition for it because access control may be based on many attributes other than classification level and clearance level.

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• access list

(I) /physical security/ Roster of persons who are authorized to enter a controlled area. (Compare: access control list.)

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• access mode

(I) A distinct type of data processing operation (e.g., read, write, append, or execute, or a combination of operations) that a subject can potentially perform on an object in an information system. [Huff] (See: read, write.)

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• access policy

(I) A kind of “security policy”. (See: access, access control.)

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• access profile

(O) Synonym for “capability list”.

Usage: IDOCs that use this term SHOULD state a definition for it because the definition is not widely known.

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• access right

(I) Synonym for “authorization”; emphasizes the possession of the authorization by a system entity.

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• accountability

(I) The property of a system or system resource that ensures that the actions of a system entity may be traced uniquely to that entity, which can then be held responsible for its actions. [Huff] (See: audit service.)

Tutorial: Accountability (a.k.a. individual accountability) typically requires a system ability to positively associate the id[[entity of a user with the time, method, and mode of the user's access to the system. This ability supports detection and subsequent investigation of security breaches. Individual persons who are system users are held accountable for their actions after being notified of the rules of behavior for using the system and the penalties associated with violating those rules.

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• accounting See: COMSEC accounting.

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• accounting legend code (ALC)

(O) /U.S. Government/ Numeric system used to indicate the minimum accounting controls required for items of COMSEC material within the CMCS. [C4009] (See: COMSEC accounting.)

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• accreditation

(Noun | N]]) An administrative action by which a designated authority declares that an information system is approved to operate in a particular security configuration with a prescribed set of safeguards. [FP102, SP37] (See: certification.)

Tutorial: An accreditation is usually based on a technical certification of the system's security mechanisms. To accredit a system, the approving authority must determine that any residual risk is an acceptable risk. Although the terms “certification” and “accreditation” are used more in the U.S. DoD and other U.S. Government agencies than in commercial organizations, the concepts apply any place where managers are required to deal with and accept responsibility for security risks. For example, the American Bar Association is developing accreditation criteria for CAs.

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• accreditation boundary

(O) Synonym for “security perimeter”. [C4009]

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• accreditor

(Noun | N]]) A management official who has been designated to have the formal authority to “accredit” an information system, i.e., to authorize the operation of, and the processing of sensitive data in, the system and to accept the residual risk associated with the system. (See: accreditation, residual risk.)

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• ACES

(O) See: Access Certificate for Electronic Services.

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• ACL

(I) See: access control list.

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• acquirer

1. (O) /SET/ “The financial institution that establishes an account with a merchant and processes payment card authorizations and payments.” [SET1]

2. (O) /SET/ “The institution (or its agent) that acquires from the card acceptor the financial data relating to the transaction and initiates that data into an interchange system.” [SET2]

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• activation data

(Noun | N]]) Secret data, other than keys, that is required to access a cryptographic module. (See: CIK. Compare: initialization value.)

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• active attack

(I) See: secondary definition under “attack”.

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• active content

1a. (I) Executable software that is bound to a document or other data file and that executes automatically when a user accesses the file, without explicit initiation by the user. (Compare: mobile code.)

Tutorial: Active content can be mobile code when its associated file is transferred across a network.

1b. (O) “Electronic documents that can carry out or trigger actions automatically on a computer platform without the intervention of a user. [This technology enables] mobile code associated with a document to execute as the document is rendered.” [SP28]

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• active user

(I) See: secondary definition under “system user”.

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• active wiretapping

(I) A wiretapping attack that attempts to alter data being communicated or otherwise affect data flow. (See: wiretapping. Compare: active attack, passive wiretapping.)

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• add-on security

(Noun | N]]) The retrofitting of protection mechanisms, implemented by hardware or software, in an information system after the system has become operational. [FP039] (Compare: baked-in security.)

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• adequate security

(O) /U.S. DoD/ “Security commensurate with the risk and magnitude of harm resulting from the loss, misuse, or unauthorized access to or modification of information.” (See: acceptable risk, residual risk.)

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• administrative security

1. (I) Management procedures and constraints to prevent unauthorized access to a system. (See: “third law” under “Courtney's laws”, manager, operational security, procedural security, security architecture. Compare: technical security.)

Examples: Clear delineation and separation of duties; configuration control.

Usage: Administrative security is usually understood to consist of methods and mechanisms that are implemented and executed primarily by people, rather than by automated systems.

2. (O) “The management constraints, operational procedures, accountability procedures, and supplemental controls established to provide an acceptable level of protection for sensitive data.” [FP039]

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• administrator

1. (O) /Common Criteria/ A person that is responsible for configuring, maintaining, and administering the TOE in a correct manner for maximum security. (See: administrative security.)

2. (O) /ITSEC/ A person in contact with the TOE, who is responsible for maintaining its operational capability.

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• Advanced Encryption Standard (AES)

(Noun | N]]) A U.S. Government standard [FP197] (the successor to DES) that (a) specifies “the AES algorithm”, which is a symmetric block cipher that is based on Rijndael and uses key sizes of 128, 192, or 256 bits to operate on a 128-bit block, and (b) states policy for using that algorithm to protect unclassified, sensitive data.

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Tutorial: Rijndael was designed to handle additional block sizes and key lengths that were not adopted in the AES. Rijndael was selected by NIST through a public competition that was held to find a successor to the DEA; the other finalists were MARS, RC6, Serpent, and Twofish.

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• adversary

1. (I) An entity that attacks a system. (Compare: cracker, intruder, hacker.)

2. (I) An entity that is a threat to a system.

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• AES

(Noun | N]]) See: Advanced Encryption Standard.

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• Affirm

(O) A formal methodology, language, and integrated set of software tools developed at the University of Southern California's Information Sciences Institute for specifying, coding, and verifying software to produce correct and reliable programs. [Cheh]

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• aggregation

(I) A circumstance in which a collection of information items is required to be classified at a higher security level than any of the items is classified individually. (See: classification.)

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• AH

(I) See: Authentication Header

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• air gap

(I) An interface between two systems at which (a) they are not connected physically and (b) any logical connection is not automated (i.e., data is transferred through the interface only manually, under human control). (See: sneaker net. Compare: gateway.)

Example: Computer A and computer B are on opposite sides of a room. To move data from A to B, a person carries a disk across the room. If A and B operate in different security domains, then moving data across the air gap may involve an upgrade or downgrade operation.

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• ALC

(O) See: accounting legend code.

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• algorithm

(I) A finite set of step-by-step instructions for a problem- solving or computation procedure, especially one that can be implemented by a computer. (See: cryptographic algorithm.)

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• alias

(I) A name that an entity uses in place of its real name, usually for the purpose of either anonymity or masquerade.

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• Alice and Bob

(I) The parties that are most often called upon to illustrate the operation of bipartite security protocols. These and other dramatis personae are listed by Schneier [Schn].

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• American National Standards Institute (ANSI)

(Noun | N]]) A private, not-for-profit association that administers U.S. private-sector voluntary standards.

Tutorial: ANSI has approximately 1,000 member organizations, including equipment users, manufacturers, and others. These include commercial firms, governmental agencies, and other institutions and international entities.

ANSI is the sole U.S. representative to (a) ISO and (b) (via the U.S. National Committee) the International Electrotechnical Commission (IEC), which are the two major, non-treaty, international standards organizations.

ANSI provides a forum for ANSI-accredited standards development groups. Among those groups, the following are especially relevant to Internet security:

• International Committee for Information Technology

Standardization (INCITS) (formerly X3): Primary U.S. focus of standardization in information and communications technologies, encompassing storage, processing, transfer, display, management, organization, and retrieval of information. Example: [A3092].

• Accredited Standards Committee X9: Develops, establishes,

maintains, and promotes standards for the financial services industry. Example: [A9009].

• Alliance for Telecommunications Industry Solutions (ATIS):

Develops standards, specifications, guidelines, requirements, technical reports, industry processes, and verification tests for interoperability and reliability of telecommunications networks, equipment, and software. Example: (A1523).

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• American Standard Code for Information Interchange (ASCII)

(Noun | N]]) A scheme that encodes 128 specified characters – the numbers 0-9, the letters a-z and A-Z, some basic punctuation symbols, some control codes that originated with Teletype machines, and a blank space – into the 7-bit binary integers. Forms the basis of the character set representations used in most computers and many Internet standards. [FP001] (See: code.)

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• Anderson report

(O) A 1972 study of computer security that was written by James P. Anderson for the U.S. Air Force [Ande].

Tutorial: Anderson collaborated with a panel of experts to study Air Force requirements for multilevel security. The study recommended research and development that was urgently needed to provide secure information processing for command and control systems and support systems. The report introduced the reference monitor concept and provided development impetus for computer and network security technology. However, many of the security problems that the 1972 report called “current” still plague information systems today.

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• anomaly detection

(I) An intrusion detection method that searches for activity that is different from the normal behavior of system entities and system resources. (See: IDS. Compare: misuse detection.)

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• anonymity

(I) The condition of an id[[entity being unknown or concealed. (See: alias, anonymizer, anonymous credential, anonymous login, id[[entity, onion routing, persona certificate. Compare: privacy.)

Tutorial: An application may require security services that maintain anonymity of users or other system entities, perhaps to preserve their privacy or hide them from attack. To hide an entity's real name, an alias may be used; for example, a financial institution may assign account numbers. Parties to transactions can thus remain relatively anonymous, but can also accept the transactions as legitimate. Real names of the parties cannot be easily determined by observers of the transactions, but an authorized third party may be able to map an alias to a real name, such as by presenting the institution with a court order. In other applications, anonymous entities may be completely untraceable.

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• anonymizer

(I) An internetwork service, usually provided via a proxy server, that provides anonymity and privacy for clients. That is, the service enables a client to access servers (a) without allowing

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anyone to gather information about which servers the client accesses and (b) without allowing the accessed servers to gather information about the client, such as its IP address.

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• anonymous credential

(D) /U.S. Government/ A credential that (a) can be used to authenticate a person as having a specific attribute or being a member of a specific group (e.g., military veterans or U.S. citizens) but (b) does not reveal the individual id[[entity of the person that presents the credential. [M0404] (See: anonymity.)

Deprecated Term: IDOCs SHOULD NOT use this term; it mixes concepts in a potentially misleading way. For example, when the credential is an X.509 certificate, the term could be misunderstood to mean that the certificate was signed by a CA that has a persona certificate. Instead, use “attribute certificate”, “organizational certificate”, or “persona certificate” depending on what is meant, and provide additional explanations as needed.

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• anonymous login

(I) An access control feature (actually, an access control vulnerability) in many Internet hosts that enables users to gain access to general-purpose or public services and resources of a host (such as allowing any user to transfer data using FTP) without having a pre-established, id[[entity-specific account (i.e., user name and password). (See: anonymity.)

Tutorial: This feature exposes a system to more threats than when all the users are known, pre-registered entities that are individually accountable for their actions. A user logs in using a special, publicly known user name (e.g., “anonymous”, “guest”, or “ftp”). To use the public login name, the user is not required to know a secret password and may not be required to input anything at all except the name. In other cases, to complete the normal sequence of steps in a login protocol, the system may require the user to input a matching, publicly known password (such as “anonymous”) or may ask the user for an e-mail address or some other arbitrary character string.

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• ANSI

(Noun | N]]) See: American National Standards Institute.

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• anti-jam

(Noun | N]]) “Measures ensuring that transmitted information can be received despite deliberate jamming attempts.” [C4009] (See: electronic security, frequency hopping, jam, spread spectrum.)

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• apex trust anchor

(Noun | N]]) The trust anchor that is superior to all other trust anchors in a particular system or context. (See: trust anchor, top CA.)

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• API

(I) See: application programming interface.

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• APOP

(I) See: POP3 APOP.

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• Application Layer

See: Internet Protocol Suite, OSIRM.

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• application program

(I) A computer program that performs a specific function directly for a user (as opposed to a program that is part of a computer operating system and exists to perform functions in support of application programs).

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• architecture

(I) See: security architecture, system architecture.

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• archive

1a. (I) /noun/ A collection of data that is stored for a relatively long period of time for historical and other purposes, such as to support audit service, availability service, or system integrity service. (Compare: backup, repository.)

1b. (I) /verb/ To store data in such a way as to create an archive. (Compare: back up.)

Tutorial: A digital signature may need to be verified many years after the signing occurs. The CA – the one that issued the certificate containing the public key needed to verify that signature – may not stay in operation that long. So every CA needs to provide for long-term storage of the information needed to verify the signatures of those to whom it issues certificates.

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• ARPANET

(I) Advanced Research Projects Agency (ARPA) Network, a pioneer packet-switched network that (a) was designed, implemented, operated, and maintained by BBN from January 1969 until July 1975 under contract to the U.S. Government; (b) led to the development of today's Internet; and © was decommissioned in June 1990. [B4799, Hafn]

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• ASCII

(Noun | N]]) See: American Standard Code for Information Interchange.

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• ASN.1

(Noun | N]]) See: Abstract Syntax Notation One.

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• asset

(I) A system resource that is (a) required to be protected by an information system's security policy, (b) intended to be protected by a countermeasure, or © required for a system's mission.

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• association

(I) A cooperative relationship between system entities, usually for the purpose of transferring information between them. (See: security association.)

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• assurance See: security assurance.

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• assurance level

(Noun | N]]) A rank on a hierarchical scale that judges the confidence someone can have that a TOE adequately fulfills stated security requirements. (See: assurance, certificate policy, EAL, TCSEC.)

Example: U.S. Government guidance [M0404] describes four assurance levels for id[[entity authentication, where each level “describes the U.S. Federal Government agency's degree of certainty that the user has presented [a credential] that refers to [the user's] id[[entity.” In that guidance, assurance is defined as (a) “the degree of confidence in the vetting process used to establish the id[[entity of the individual to whom the credential was issued” and (b) “the degree of confidence that the individual who uses the credential is the individual to whom the credential was issued.”

The four levels are described as follows:

• Level 1: Little or no confidence in the asserted id[[entity.
• Level 2: Some confidence in the asserted id[[entity.
• Level 3: High confidence in the asserted id[[entity.
• Level 4: Very high confidence in the asserted id[[entity.

Standards for determining these levels are provided in a NIST publication [SP12]. However, as noted there, an assurance level is “a degree of confidence, not a true measure of how secure the system actually is. This distinction is necessary because it is extremely difficult – and in many cases, virtually impossible – to know exactly how secure a system is.”

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• asymmetric cryptography

(I) A modern branch of cryptography (popularly known as “public- key cryptography”) in which the algorithms use a pair of keys (a public key and a private key) and use a different component of the pair for each of two counterpart cryptographic operations (e.g.,

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encryption and decryption, or signature creation and signature verification). (See: key pair, symmetric cryptography.)

Tutorial: Asymmetric algorithms have key management advantages over equivalently strong symmetric ones. First, one key of the pair need not be known by anyone but its owner; so it can more easily be kept secret. Second, although the other key is shared by all entities that use the algorithm, that key need not be kept secret from other, non-using entities; thus, the key-distribution part of key management can be done more easily.

Asymmetric cryptography can be used to create algorithms for encryption, digital signature, and key agreement:

• In an asymmetric encryption algorithm (e.g., “RSA”), when Alice

wants to ensure confidentiality for data she sends to Bob, she encrypts the data with a public key provided by Bob. Only Bob has the matching private key that is needed to decrypt the data. (Compare: seal.)

• In an asymmetric digital signature algorithm (e.g., “DSA”),

when Alice wants to ensure data integrity or provide authentication for data she sends to Bob, she uses her private key to sign the data (i.e., create a digital signature based on the data). To verify the signature, Bob uses the matching public key that Alice has provided.

• In an asymmetric key-agreement algorithm (e.g., “Diffie-

Hellman-Merkle”), Alice and Bob each send their own public key to the other party. Then each uses their own private key and the other's public key to compute the new key value.

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• asymmetric key

(I) A cryptographic key that is used in an asymmetric cryptographic algorithm. (See: asymmetric cryptography, private key, public key.)

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• ATIS

(Noun | N]]) See: “Alliance for Telecommunications Industry Solutions” under “ANSI”.

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• attack

1. (I) An intentional act by which an entity attempts to evade security services and violate the security policy of a system. That is, an actual assault on system security that derives from an intelligent threat. (See: penetration, violation, vulnerability.)

2. (I) A method or technique used in an assault (e.g., masquerade). (See: blind attack, distributed attack.)

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Tutorial: Attacks can be characterized according to intent:

• An “active attack” attempts to alter system resources or affect

their operation.

• A “passive attack” attempts to learn or make use of information

from a system but does not affect system resources of that system. (See: wiretapping.)

The object of a passive attack might be to obtain data that is needed for an off-line attack.

• An “off-line attack” is one in which the attacker obtains data

from the target system and then analyzes the data on a different system of the attacker's own choosing, possibly in preparation for a second stage of attack on the target.

Attacks can be characterized according to point of initiation:

• An “inside attack” is one that is initiated by an entity inside

the security perimeter (an “insider”), i.e., an entity that is authorized to access system resources but uses them in a way not approved by the party that granted the authorization.

• An “outside attack” is initiated from outside the security

perimeter, by an unauthorized or illegitimate user of the system (an “outsider”). In the Internet, potential outside attackers range from amateur pranksters to organized criminals, international terrorists, and hostile governments. Attacks can be characterized according to method of delivery:

• In a “direct attack”, the attacker addresses attacking packets

to the intended victim(s).

• In an “indirect attack”, the attacker addresses packets to a

third party, and the packets either have the address(es) of the intended victim(s) as their source address(es) or indicate the intended victim(s) in some other way. The third party responds by sending one or more attacking packets to the intended victims. The attacker can use third parties as attack amplifiers by providing a broadcast address as the victim address (e.g., “smurf attack”). (See: reflector attack. Compare: reflection attack, replay attack.)

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The term “attack” relates to some other basic security terms as shown in the following diagram:

+ - - - - - - - - - - - - + + - - - - + + - - - - - - - - - - -+

+ - - - - - - - - - - - - + + - - - - + + - - - - - - - - - - -+

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• attack potential

(I) The perceived likelihood of success should an attack be launched, expressed in terms of the attacker's ability (i.e., expertise and resources) and motivation. (Compare: threat, risk.)

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• attack sensing, warning, and response

(I) A set of security services that cooperate with audit service to detect and react to indications of threat actions, including both inside and outside attacks. (See: indicator.)

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• attack tree

(I) A branching, hierarchical data structure that represents a set of potential approaches to achieving an event in which system security is penetrated or compromised in a specified way. [Moor]

Tutorial: Attack trees are special cases of fault trees. The security incident that is the goal of the attack is represented as the root node of the tree, and the ways that an attacker could reach that goal are iteratively and incrementally represented as branches and subnodes of the tree. Each subnode defines a subgoal, and each subgoal may have its own set of further subgoals, etc. The final nodes on the paths outward from the root, i.e., the leaf nodes, represent different ways to initiate an attack. Each node other than a leaf is either an AND-node or an OR-node. To achieve the goal represented by an AND-node, the subgoals represented by all of that node's subnodes must be achieved; and for an OR-node, at least one of the subgoals must be achieved. Branches can be labeled with values representing difficulty, cost, or other attack attributes, so that alternative attacks can be compared.

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• attribute

(Noun | N]]) Information of a particular type concerning an identifiable system entity or object. An “attribute type” is the component of an attribute that indicates the class of information given by the attribute; and an “attribute value” is a particular instance of the class of information indicated by an attribute type. (See: attribute certificate.)

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• attribute authority (AA)

1. (Noun | N]]) A CA that issues attribute certificates.

2. (O) “An authority [that] assigns privileges by issuing attribute certificates.” [X509]

Deprecated Usage: The abbreviation “AA” SHOULD NOT be used in an IDOC unless it is first defined in the IDOC.

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• attribute certificate

1. (I) A digital certificate that binds a set of descriptive data items, other than a public key, either directly to a subject name or to the identifier of another certificate that is a public-key certificate. (See: capability token.)

2. (O) “A data structure, digitally signed by an [a]ttribute [a]uthority, that binds some attribute values with identification information about its holder.” [X509]

Tutorial: A public-key certificate binds a subject name to a public key value, along with information needed to perform certain cryptographic functions using that key. Other attributes of a subject, such as a security clearance, may be certified in a separate kind of digital certificate, called an attribute certificate. A subject may have multiple attribute certificates associated with its name or with each of its public-key certificates.

An attribute certificate might be issued to a subject in the following situations:

• Different lifetimes: When the lifetime of an attribute binding

is shorter than that of the related public-key certificate, or when it is desirable not to need to revoke a subject's public key just to revoke an attribute.

• Different authorities: When the authority responsible for the

attributes is different than the one that issues the public-key certificate for the subject. (There is no requirement that an attribute certificate be issued by the same CA that issued the associated public-key certificate.)

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• audit

See: security audit.

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• audit log

(I) Synonym for “security audit trail”.

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• audit service

(I) A security service that records information needed to establish accountability for system events and for the actions of system entities that cause them. (See: security audit.)

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• audit trail

(I) See: security audit trail.

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• AUTH

(I) See: POP3 AUTH.

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• authenticate

(I) Verify (i.e., establish the truth of) an attribute value claimed by or for a system entity or system resource. (See: authentication, validate vs. verify, “relationship between data integrity service and authentication services” under “data integrity service”.)

Deprecated Usage: In general English usage, this term is used with the meaning “to prove genuine” (e.g., an art expert authenticates a Michelangelo painting); but IDOCs should restrict usage as follows:

• IDOCs SHOULD NOT use this term to refer to proving or checking

that data has not been changed, destroyed, or lost in an unauthorized or accidental manner. Instead, use “verify”.

• IDOCs SHOULD NOT use this term to refer to proving the truth or

accuracy of a fact or value such as a digital signature. Instead, use “verify”.

• IDOCs SHOULD NOT use this term to refer to establishing the

soundness or correctness of a construct, such as a digital certificate. Instead, use “validate”.

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• authentication

(I) The process of verifying a claim that a system entity or system resource has a certain attribute value. (See: attribute, authenticate, authentication exchange, authentication information, credential, data origin authentication, peer entity authentication, “relationship between data integrity service and authentication services” under “data integrity service”, simple authentication, strong authentication, verification, X.509.)

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Tutorial: Security services frequently depend on authentication of the id[[entity of users, but authentication may involve any type of attribute that is recognized by a system. A claim may be made by a subject about itself (e.g., at login, a user typically asserts its id[[entity) or a claim may be made on behalf of a subject or object by some other system entity (e.g., a user may claim that a data object originates from a specific source, or that a data object is classified at a specific security level).

An authentication process consists of two basic steps:

• Identification step: Presenting the claimed attribute value

(e.g., a user identifier) to the authentication subsystem.

• Verification step: Presenting or generating authentication

information (e.g., a value signed with a private key) that acts as evidence to prove the binding between the attribute and that for which it is claimed. (See: verification.)

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• authentication code

(D) Synonym for a checksum based on cryptography. (Compare: Data Authentication Code, Message Authentication Code.)

Deprecated Term: IDOCs SHOULD NOT use this uncapitalized term as a synonym for any kind of checksum, regardless of whether or not the checksum is cryptographic. Instead, use “checksum”, “Data Authentication Code”, “error detection code”, “hash”, “keyed hash”, “Message Authentication Code”, “protected checksum”, or some other recommended term, depending on what is meant.

The term mixes concepts in a potentially misleading way. The word “authentication” is misleading because the checksum may be used to perform a data integrity function rather than a data origin authentication function.

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• authentication exchange

1. (I) A mechanism to verify the id[[entity of an entity by means of information exchange.

2. (O) “A mechanism intended to ensure the id[[entity of an entity by means of information exchange.” [I7498-2]

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• Authentication Header (AH)

(I) An Internet protocol [R2402, R4302] designed to provide connectionless data integrity service and connectionless data origin authentication service for IP datagrams, and (optionally) to provide partial sequence integrity and protection against replay attacks. (See: IPsec. Compare: ESP.)

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Tutorial: Replay protection may be selected by the receiver when a security association is established. AH authenticates the upper- layer PDU that is carried as an IP SDU, and also authenticates as much of the IP PCI (i.e., the IP header) as possible. However, some IP header fields may change in transit, and the value of these fields, when the packet arrives at the receiver, may not be predictable by the sender. Thus, the values of such fields cannot be protected end-to-end by AH; protection of the IP header by AH is only partial when such fields are present.

AH may be used alone, or in combination with the ESP, or in a nested fashion with tunneling. Security services can be provided between a pair of communicating hosts, between a pair of communicating security gateways, or between a host and a gateway. ESP can provide nearly the same security services as AH, and ESP can also provide data confidentiality service. The main difference between authentication services provided by ESP and AH is the extent of the coverage; ESP does not protect IP header fields unless they are encapsulated by AH.

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• authentication information

(I) Information used to verify an id[[entity claimed by or for an entity. (See: authentication, credential, user. Compare: identification information.)

Tutorial: Authentication information may exist as, or be derived from, one of the following: (a) Something the entity knows (see: password); (b) something the entity possesses (see: token); © something the entity is (see: biometric authentication).

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• authentication service

(I) A security service that verifies an id[[entity claimed by or for an entity. (See: authentication.)

Tutorial: In a network, there are two general forms of authentication service: data origin authentication service and peer entity authentication service.

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• authenticity

(I) The property of being genuine and able to be verified and be trusted. (See: authenticate, authentication, validate vs. verify.)

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• authority

(D) /PKI/ “An entity [that is] responsible for the issuance of certificates.” [X509]

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Deprecated Usage: IDOCs SHOULD NOT use this term as a synonym for attribute authority, certification authority, registration authority, or similar terms; the shortened form may cause confusion. Instead, use the full term at the first instance of usage and then, if it is necessary to shorten text, use AA, CA, RA, and other abbreviations defined in this Glossary.

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• authority certificate

(D) “A certificate issued to an authority (e.g. either to a certification authority or to an attribute authority).” [X509] (See: authority.)

Deprecated Term: IDOCs SHOULD NOT use this term because it is ambiguous. Instead, use the full term “certification authority certificate”, “attribute authority certificate”, “registration authority certificate”, etc. at the first instance of usage and then, if it is necessary to shorten text, use AA, CA, RA, and other abbreviations defined in this Glossary.

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• Authority Information Access extension

(I) The private extension defined by PKIX for X.509 certificates to indicate “how to access CA information and services for the issuer of the certificate in which the extension appears. Information and services may include on-line validation services and CA policy data.” [R3280] (See: private extension.)

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• authorization

1a. (I) An approval that is granted to a system entity to access a system resource. (Compare: permission, privilege.)

Usage: Some synonyms are “permission” and “privilege”. Specific terms are preferred in certain contexts:

• /PKI/ “Authorization” SHOULD be used, to align with

“certification authority” in the standard [X509].

• /role-based access control/ “Permission” SHOULD be used, to

align with the standard ANSI.

• /computer operating systems/ “Privilege” SHOULD be used, to

align with the literature. (See: privileged process, privileged user.)

Tutorial: The semantic]s and granularity of [[authorizations depend on the application and implementation (see: “first law” under “Courtney's laws”). An authorization may specify a particular access mode – such as read, write, or execute – for one or more system resources.

1b. (I) A process for granting approval to a system entity to access a system resource.

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2. (O) /SET/ “The process by which a properly appointed person or persons grants permission to perform some action on behalf of an organization. This process assesses transaction risk, confirms that a given transaction does not raise the account holder's debt above the account's credit limit, and reserves the specified amount of credit. (When a merchant obtains authorization, payment for the authorized amount is guaranteed – provided, of course, that the merchant followed the rules associated with the authorization process.)” [SET2]

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• authorization credential

(I) See: /access control/ under “credential”.

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• authorize

(I) Grant an authorization to a system entity.

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• authorized user

(I) /access control/ A system entity that accesses a system resource for which the entity has received an authorization. (Compare: insider, outsider, unauthorized user.)

Deprecated Usage: IDOCs that use this term SHOULD state a definition for it because the term is used in many ways and could easily be misunderstood.

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• automated information system

See: information system.

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• availability

1. (I) The property of a system or a system resource being accessible, or usable or operational upon demand, by an authorized system entity, according to performance specifications for the system; i.e., a system is available if it provides services according to the system design whenever users request them. (See: critical, denial of service. Compare: precedence, reliability, survivability.)

2. (O) “The property of being accessible and usable upon demand by an authorized entity.” [I7498-2]

3. (D) “Timely, reliable access to data and information services for authorized users.” [C4009]

Deprecated Definition: IDOCs SHOULD NOT use the term with definition 3; the definition mixes “availability” with “reliability”, which is a different property. (See: reliability.)

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Tutorial: Availability requirements can be specified by quantitative metrics, but sometimes are stated qualitatively, such as in the following:

• “Flexible tolerance for delay” may mean that brief system

outages do not endanger mission accomplishment, but extended outages may endanger the mission.

• “Minimum tolerance for delay” may mean that mission

accomplishment requires the system to provide requested services in a short time.

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• availability service

(I) A security service that protects a system to ensure its availability.

Tutorial: This service addresses the security concerns raised by denial-of-service attacks. It depends on proper management and control of system resources, and thus depends on access control service and other security services.

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• avoidance

(I) See: secondary definition under “security”.