O

Object

A passive entity such as a system or file.

Object reuse

The process of protecting the confidentiality of objects that are reassigned after initial use. See also TCSEC.

Objectives

Specific milestones that an organization wants to perform in order to meet its goals. See also Goals.

OFB (Output Feedback)

One of four operating modes for DES (see also CBC, CFB, DES, and OFB). OFB is a stream cipher often used to encrypt satellite communications. In this mode, previous plaintext is used as feedback for key generation in the next key stream; however, the resulting ciphertext isn’t chained together (unlike CFB).

One-time Pad

A cryptographic keystream that can only be used once.

One-time Password

A password that is valid for one logon session only.

One-way Function

A problem that is easy to compute in one direction but not in the reverse direction.

Open message format

A message encrypted in an asymmetric key system by using the sender’s private key. The sender’s public key, which is available to anyone, is used to decrypt the message. This format guarantees the message’s authenticity. See also Secure message format, Secure and signed message format.

Open system

A vendor-independent system that complies with an accepted standard. This promotes interoperability between systems and components made by different vendors. See also Closed system.

Operating System

Software that controls computer hardware and resources, and facilitates the operation of application software. See also Application Software.

Orange book

See TCSEC.

OSI model

The seven-layer reference model for networks. The layers are physical, data link, network, transport, session, presentation, and application.

Owner

An individual in an organization who is responsible for management of an asset, including classification, handling, and access policy.