$ signature
      (O) A symbol or process adopted or executed by a system entity
      with present intention to declare that a data object is genuine.
      (See: digital signature, electronic signature.)

   $ signature certificate
      (I) A public-key certificate that contains a public key that is
      intended to be used for verifying digital signatures, rather than
      for encrypting data or performing other cryptographic functions.

      Tutorial: A v3 X.509 public-key certificate may have a "keyUsage"
      extension that indicates the purpose for which the certified
      public key is intended. (See: certificate profile.)

   $ signed receipt
      (I) An S/MIME service [R2634] that (a) provides, to the originator
      of a message, proof of delivery of the message and (b) enables the
      originator to demonstrate to a third party that the recipient was
      able to verify the signature of the original message.

      Tutorial: The receipt is bound to the original message by a
      signature; consequently, the service may be requested only for a
      message that is signed. The receipt sender may optionally also
      encrypt the receipt to provide confidentiality between the receipt
      sender and the receipt recipient.

   $ signer
      (N) A human being or organization entity that uses a private key
      to sign (i.e., create a digital signature on) a data object. [DSG]

   $ SILS
      (N) See: Standards for Interoperable LAN/MAN Security.

   $ simple authentication
      1. (I) An authentication process that uses a password as the
      information needed to verify an identity claimed for an entity.
      (Compare: strong authentication.)

      2. (O) "Authentication by means of simple password arrangements."
      [X509]

   $ Simple Authentication and Security Layer (SASL)
      (I) An Internet specification [R2222, R4422] for adding
      authentication service to connection-based protocols. (Compare:
      EAP, GSS-API.)

      Tutorial: To use SASL, a protocol includes a command for
      authenticating a user to a server and for optionally negotiating
      protection of subsequent protocol interactions. The command names
      a registered security mechanism. SASL mechanisms include Kerberos,
      GSS-API, S/KEY, and others. Some protocols that use SASL are IMAP4
      and POP3.

   $ Simple Key Management for Internet Protocols (SKIP)
      (I) A key-distribution protocol that uses hybrid encryption to
      convey session keys that are used to encrypt data in IP packets.
      (See:  SKIP reference in [R2356].)

      Tutorial: SKIP was designed by Ashar Aziz and Whitfield Diffie at
      Sun Microsystems and proposed as the standard key management
      protocol for IPsec, but IKE was chosen instead. Although IKE is
      mandatory for an IPsec implementation, the use of SKIP is not
      excluded.

      SKIP uses the Diffie-Hellman-Merkle algorithm (or could use
      another key-agreement algorithm) to generate a key-encrypting key
      for use between two entities. A session key is used with a
      symmetric algorithm to encrypt data in one or more IP packets that
      are to be sent from one entity to the other. A symmetric KEK is
      established and used to encrypt the session key, and the encrypted
      session key is placed in a SKIP header that is added to each IP
      packet that is encrypted with that session key.

   $ Simple Mail Transfer Protocol (SMTP)
      (I) A TCP-based, Application-Layer, Internet Standard protocol
      (RFC 821) for moving electronic mail messages from one computer to
      another.

   $ Simple Network Management Protocol (SNMP)
      (I) A (usually) UDP-based, Application-Layer, Internet Standard
      protocol (RFCs 3410-3418) for conveying management information
      between system components that act as managers and agents.

   $ Simple Public Key Infrastructure (SPKI)
      (I) A set of experimental concepts (RFCs 2692, 2693) that were
      proposed as alternatives to the concepts standardized in PKIX.

   $ simple security property
      (N) /formal model/ Property of a system whereby a subject has read
      access to an object only if the clearance of the subject dominates
      the classification of the object. See: Bell-LaPadula model.

   $ single sign-on
      1. (I) An authentication subsystem that enables a user to access
      multiple, connected system components (such as separate hosts on a
      network) after a single login at only one of the components. (See:
      Kerberos.)

      2. (O) /Liberty Alliance/ A security subsystem that enables a user
      identity to be authenticated at an identity provider -- i.e., at a
      service that authenticates and asserts the user's identity -- and
      then have that authentication be honored by other service
      providers.

      Tutorial: A single sign-on subsystem typically requires a user to
      log in once at the beginning of a session, and then during the
      session transparently grants access by the user to multiple,
      separately protected hosts, applications, or other system
      resources, without further login action by the user (unless, of
      course, the user logs out). Such a subsystem has the advantages of
      being user friendly and enabling authentication to be managed
      consistently across an entire enterprise. Such a subsystem also
      has the disadvantage of requiring all the accessed components to
      depend on the security of the same authentication information.

   $ singular identity
      (I) See: secondary definition under "identity".

   $ site
      (I) A facility -- i.e., a physical space, room, or building
      together with its physical, personnel, administrative, and other
      safeguards -- in which system functions are performed. (See:
      node.)

   $ situation
      (I) See: security situation.

   $ SKEME
      (I) A key-distribution protocol from which features were adapted
      for IKE. [SKEME]

   $ SKIP
      (I) See: Simple Key Management for Internet Protocols.

   $ SKIPJACK
      (N) A type 2, 64-bit block cipher [SKIP, R2773] with a key size of
      80 bits. (See: CAPSTONE, CLIPPER, FORTEZZA, Key Exchange
      Algorithm.)

      Tutorial: SKIPJACK was developed by NSA and formerly classified at
      the U.S. DoD "Secret" level. On 23 June 1998, NSA announced that
      SKIPJACK had been declassified.

   $ slot
      (O) /MISSI/ One of the FORTEZZA PC card storage areas that are
      each able to hold an X.509 certificate plus other data, including
      the private key that is associated with a public-key certificate.

   $ smart card
      (I) A credit-card sized device containing one or more integrated
      circuit chips that perform the functions of a computer's central
      processor, memory, and input/output interface. (See: PC card,
      smart token.)

      Usage: Sometimes this term is used rather strictly to mean a card
      that closely conforms to the dimensions and appearance of the kind
      of plastic credit card issued by banks and merchants. At other
      times, the term is used loosely to include cards that are larger
      than credit cards, especially cards that are thicker, such as PC
      cards.

   $ smart token
      (I) A device that conforms to the definition of "smart card"
      except that rather than having the standard dimensions of a credit
      card, the token is packaged in some other form, such as a military
      dog tag or a door key. (See: smart card, cryptographic token.)

   $ SMI
      (I) See: security management infrastructure.

   $ SMTP
      (I) See: Simple Mail Transfer Protocol.

   $ smurf attack
      (D) /slang/ A denial-of-service attack that uses IP broadcast
      addressing to send ICMP ping packets with the intent of flooding a
      system. (See: fraggle attack, ICMP flood.)

      Deprecated Term: It is likely that other cultures use different
      metaphors for this concept. Therefore, to avoid international
      misunderstanding, IDOCs SHOULD NOT use this term.

      Derivation: The Smurfs are a fictional race of small, blue
      creatures that were created by a cartoonist. Perhaps the inventor
      of this attack thought that a swarm of ping packets resembled a
      gang of smurfs. (See: Deprecated Usage under "Green Book".)

      Tutorial: The attacker sends ICMP echo request ("ping") packets
      that appear to originate not from the attacker's own IP address,
      but from the address of the host or router that is the target of
      the attack. Each packet is addressed to an IP broadcast address,
      e.g., to all IP addresses in a given network. Thus, each echo
      request that is sent by the attacker results in many echo
      responses being sent to the target address. This attack can
      disrupt service at a particular host, at the hosts that depend on
      a particular router, or in an entire network.

   $ sneaker net
      (D) /slang/ A process that transfers data between systems only
      manually, under human control; i.e., a data transfer process that
      involves an air gap.

      Deprecated Term: It is likely that other cultures use different
      metaphors for this concept. Therefore, to avoid international
      misunderstanding, IDOCs SHOULD NOT use this term.

   $ Snefru
      (N) A public-domain, cryptographic hash function (a.k.a. "The
      Xerox Secure Hash Function") designed by Ralph C. Merkle at Xerox
      Corporation. Snefru can produce either a 128-bit or 256-bit output
      (i.e., hash result). [Schn] (See: Khafre, Khufu.)

   $ sniffing
      (D) /slang/ Synonym for "passive wiretapping"; most often refers
      to capturing and examining the data packets carried on a LAN.
      (See: password sniffing.)

      Deprecated Term: IDOCs SHOULD NOT use this term; it unnecessarily
      duplicates the meaning of a term that is better established. (See:
      Deprecated Usage under "Green Book".

   $ SNMP
      (I) See: Simple Network Management Protocol.

   $ social engineering
      (D) Euphemism for non-technical or low-technology methods, often
      involving trickery or fraud, that are used to attack information
      systems. Example: phishing.

      Deprecated Term: IDOCs SHOULD NOT use this term; it is too vague.
      Instead, use a term that is specific with regard to the means of
      attack, e.g., blackmail, bribery, coercion, impersonation,
      intimidation, lying, or theft.

   $ SOCKS
      (I) An Internet protocol [R1928] that provides a generalized proxy
      server that enables client-server applications (e.g., TELNET, FTP,
      or HTTP; running over either TCP or UDP) to use the services of a
      firewall.

      Tutorial: SOCKS is layered under the IPS Application Layer and
      above the Transport Layer. When a client inside a firewall wishes
      to establish a connection to an object that is reachable only
      through the firewall, it uses TCP to connect to the SOCKS server,
      negotiates with the server for the authentication method to be
      used, authenticates with the chosen method, and then sends a relay
      request. The SOCKS server evaluates the request, typically based
      on source and destination addresses, and either establishes the
      appropriate connection or denies it.

   $ soft TEMPEST
      (O) The use of software techniques to reduce the radio frequency
      information leakage from computer displays and keyboards. [Kuhn]
      (See: TEMPEST.)

   $ soft token
      (D) A data object that is used to control access or authenticate
      authorization. (See: token.)

      Deprecated Term: IDOCs SHOULD NOT use this term as defined here;
      the definition duplicates the meaning of other, standard terms.
      Instead, use "attribute certificate" or another term that is
      specific with regard to the mechanism being used.

   $ software
      (I) Computer programs (which are stored in and executed by
      computer hardware) and associated data (which also is stored in
      the hardware) that may be dynamically written or modified during
      execution. (Compare: firmware.)

   $ software error
      (I) /threat action/ See: secondary definitions under "corruption",
      "exposure", and "incapacitation".

   $ SORA
      (O) See: SSO-PIN ORA.

   $ source authentication
      (D) Synonym for "data origin authentication" or "peer entity
      authentication". (See: data origin authentication, peer entity
      authentication).

      Deprecated Term: IDOCs SHOULD NOT use this term because it is
      ambiguous and, in either meaning, duplicates the meaning of
      internationally standardized terms. If the intent is to
      authenticate the original creator or packager of data received,
      then use "data origin authentication". If the intent is to
      authenticate the identity of the sender of data in the current
      instance, then use "peer entity authentication".

   $ source integrity
      (I) The property that data is trustworthy (i.e., worthy of
      reliance or trust), based on the trustworthiness of its sources
      and the trustworthiness of any procedures used for handling data
      in the system. Usage: a.k.a. Biba integrity. (See: integrity.
      Compare: correctness integrity, data integrity.)

      Tutorial: For this kind of integrity, there are formal models of
      unauthorized modification (see: Biba model) that logically
      complement the more familiar models of unauthorized disclosure
      (see: Bell-LaPadula model). In these models, objects are labeled
      to indicate the credibility of the data they contain, and there
      are rules for access control that depend on the labels.

   $ SP3
      (O) See: Security Protocol 3.

   $ SP4
      (O) See: Security Protocol 4.

   $ spam
      1a. (I) /slang verb/ To indiscriminately send unsolicited,
      unwanted, irrelevant, or inappropriate messages, especially
      commercial advertising in mass quantities.

      1b. (I) /slang noun/ Electronic "junk mail". [R2635]

      Deprecated Usage: IDOCs SHOULD NOT use this term in uppercase
      letters, because SPAM(trademark) is a trademark of Hormel Foods
      Corporation. Hormel says, "We do not object to use of this slang
      term [spam] to describe [unsolicited advertising email], although
      we do object to the use of our product image in association with
      that term. Also, if the term is to be used, it SHOULD be used in
      all lower-case letters to distinguish it from our trademark SPAM,
      which SHOULD be used with all uppercase letters." (See: metadata.)

      Tutorial: In sufficient volume, spam can cause denial of service.
      (See: flooding.) According to Hormel, the term was adopted as a
      result of a Monty Python skit in which a group of Vikings sang a
      chorus of 'SPAM, SPAM, SPAM ...' in an increasing crescendo,

      drowning out other conversation. This lyric became a metaphor for
      the unsolicited advertising messages that threaten to overwhelm
      other discourse on the Internet.

   $ SPD
      (I) See: Security Policy Database.

   $ special access program (SAP)
      (O) /U.S. Government/ "Sensitive program, [that is] approved in
      writing by a head of agency with [i.e., who has] original top
      secret classification authority, [and] that imposes need-to-know
      and access controls beyond those normally provided for access to
      Confidential, Secret, or Top Secret information. The level of
      controls is based on the criticality of the program and the
      assessed hostile intelligence threat. The program may be an
      acquisition program, an intelligence program, or an operations and
      support program." [C4009] (See: formal access approval, SCI.
      Compare: collateral information.)

   $ SPI
      (I) See: Security Parameters Index.

   $ SPKI
      (I) See: Simple Public Key Infrastructure.

   $ split key
      (I) A cryptographic key that is generated and distributed as two
      or more separate data items that individually convey no knowledge
      of the whole key that results from combining the items. (See: dual
      control, split knowledge.)

   $ split knowledge
      1. (I) A security technique in which two or more entities
      separately hold data items that individually do not convey
      knowledge of the information that results from combining the
      items. (See: dual control, split key.)

      2. (O) "A condition under which two or more entities separately
      have key components [that] individually convey no knowledge of the
      plaintext key [that] will be produced when the key components are
      combined in the cryptographic module." [FP140]

   $ spoof
      (I) /threat action/ See: secondary definition under "masquerade".

   $ spoofing attack
      (I) Synonym for "masquerade attack".

   $ spread spectrum
      (N) A TRANSEC technique that transmits a signal in a bandwidth
      much greater than the transmitted information needs. [F1037]
      Example: frequency hopping.

      Tutorial: Usually uses a sequential, noise-like signal structure
      to spread the normally narrowband information signal over a
      relatively wide band of frequencies. The receiver correlates the
      signals to retrieve the original information signal. This
      technique decreases potential interference to other receivers,
      while achieving data confidentiality and increasing immunity of
      spread spectrum receivers to noise and interference.

   $ spyware
      (D) /slang/ Software that an intruder has installed
      surreptitiously on a networked computer to gather data from that
      computer and send it through the network to the intruder or some
      other interested party. (See: malicious logic, Trojan horse.)

      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.

      Tutorial: Some examples of the types of data that might be
      gathered by spyware are application files, passwords, email
      addresses, usage histories, and keystrokes. Some examples of
      motivations for gathering the data are blackmail, financial fraud,
      identity theft, industrial espionage, market research, and
      voyeurism.

   $ SSH(trademark)
      (N) See: Secure Shell(trademark).

   $ SSL
      (I) See: Secure Sockets Layer.

   $ SSO
      (I) See: system security officer.

   $ SSO PIN
      (O) /MISSI/ One of two PINs that control access to the functions
      and stored data of a FORTEZZA PC card. Knowledge of the SSO PIN
      enables a card user to perform the FORTEZZA functions intended for
      use by an end user and also the functions intended for use by a
      MISSI CA. (See: user PIN.)

   $ SSO-PIN ORA (SORA)
      (O) /MISSI/ A MISSI organizational RA that operates in a mode in
      which the ORA performs all card management functions and,
      therefore, requires knowledge of the SSO PIN for FORTEZZA PC cards
      issued to end users.

   $ Standards for Interoperable LAN/MAN Security (SILS)
      1. (N) The IEEE 802.10 standards committee. (See: [FP191].)

      2. (N) A set of IEEE standards, which has eight parts: (a) Model,
      including security management, (b) Secure Data Exchange protocol,
      (c) Key Management, (d) [has been incorporated in (a)], (e) SDE
      Over Ethernet 2.0, (f) SDE Sublayer Management, (g) SDE Security
      Labels, and (h) SDE PICS Conformance. Parts b, e, f, g, and h are
      incorporated in IEEE Standard 802.10-1998.

   $ star property
      (N) See: *-property.

   $ Star Trek attack
      (D) /slang/ An attack that penetrates your system where no attack
      has ever gone before.

      Deprecated Usage: IDOCs SHOULD NOT use this term; it is a joke for
      Trekkies. (See: Deprecated Usage under "Green Book".)

   $ static
      (I) /adjective/ Refers to a cryptographic key or other parameter
      that is relatively long-lived. (Compare: ephemeral.)

   $ steganography
      (I) Methods of hiding the existence of a message or other data.
      This is different than cryptography, which hides the meaning of a
      message but does not hide the message itself. Examples: For
      classic, physical methods, see [Kahn]; for modern, digital
      methods, see [John]. (See: cryptology. Compare: concealment
      system, digital watermarking.)

   $ storage channel
      (I) See: covert storage channel.

   $ storage key
      (I) A cryptographic key used by a device for protecting
      information that is being maintained in the device, as opposed to
      protecting information that is being transmitted between devices.
      (See: cryptographic token, token copy. Compare: traffic key.)

   $ stream cipher
      (I) An encryption algorithm that breaks plain text into a stream
      of successive elements (usually, bits) and encrypts the n-th
      plaintext element with the n-th element of a parallel key stream,
      thus converting the plaintext stream into a ciphertext stream.
      [Schn] (See: block cipher.)

   $ stream integrity service
      (I) A data integrity service that preserves integrity for a
      sequence of data packets, including both (a) bit-by-bit datagram
      integrity of each individual packet in the set and (b) packet-by-
      packet sequential integrity of the set as a whole. (See: data
      integrity. Compare: datagram integrity service.)

      Tutorial: Some internetwork applications need only datagram
      integrity, but others require that an entire stream of packets be
      protected against insertion, reordering, deletion, and delay:
      -  "Insertion": The destination receives an additional packet that
         was not sent by the source.
      -  "Reordering": The destination receives packets in a different
         order than that in which they were sent by the source.
      -  "Deletion": A packet sent by the source is not ever delivered
         to the intended destination.
      -  "Delay": A packet is detained for some period of time at a
         relay, thus hampering and postponing the packet's normal timely
         delivery from source to destination.

   $ strength
      1. (I) /cryptography/ A cryptographic mechanism's level of
      resistance to attacks [R3766]. (See: entropy, strong, work
      factor.)

      2. (N) /Common Criteria/ "Strength of function" is a
      "qualification of a TOE security function expressing the minimum
      efforts assumed necessary to defeat its expected security behavior
      by directly attacking its underlying security mechanisms": (See:
      strong.)
      -  Basic: "A level of the TOE strength of function where analysis
         shows that the function provides adequate protection against
         casual breach of TOE security by attackers possessing a low
         attack potential."
      -  Medium: "... against straightforward or intentional breach ...
         by attackers possessing a moderate attack potential."
      -  High: "... against deliberately planned or organized breach ...
         by attackers possessing a high attack potential."

   $ strong
      1. (I) /cryptography/ Used to describe a cryptographic algorithm
      that would require a large amount of computational power to defeat
      it. (See: strength, work factor, weak key.)

      2. (I) /COMPUSEC/ Used to describe a security mechanism that would
      be difficult to defeat. (See: strength, work factor.)

   $ strong authentication
      1. (I) An authentication process that uses a cryptographic
      security mechanism -- particularly public-key certificates -- to
      verify the identity claimed for an entity. (Compare: simple
      authentication.)

      2. (O) "Authentication by means of cryptographically derived
      credentials." [X509]

   $ subject
      1a. (I) A process in a computer system that represents a principal
      and that executes with the privileges that have been granted to
      that principal. (Compare: principal, user.)

      1b. (I) /formal model/ A system entity that causes information to
      flow among objects or changes the system state; technically, a
      process-domain pair. A subject may itself be an object relative to
      some other subject; thus, the set of subjects in a system is a
      subset of the set of objects. (See: Bell-LaPadula model, object.)

      2. (I) /digital certificate/ The name (of a system entity) that is
      bound to the data items in a digital certificate; e.g., a DN that
      is bound to a key in a public-key certificate. (See: X.509.)

   $ subject CA
      (D) The CA that is the subject of a cross-certificate issued by
      another CA. [X509] (See: cross-certification.)

      Deprecated Term: IDOCs SHOULD NOT use this term because it is not
      widely known and could be misunderstood. Instead, say "the CA that
      is the subject of the cross-certificate".

   $ subnetwork
      (N) An OSI term for a system of packet relays and connecting links
      that implement OSIRM layer 2 or 3 to provide a communication
      service that interconnects attached end systems. Usually, the
      relays are all of the same type (e.g., X.25 packet switches, or
      interface units in an IEEE 802.3 LAN). (See: gateway, internet,
      router.)

   $ subordinate CA (SCA)
      1. (I) A CA whose public-key certificate is issued by another
      (superior) CA. (See: certification hierarchy. Compare: cross-
      certification.)

      2. (O) /MISSI/ The fourth-highest (i.e., bottom) level of a MISSI
      certification hierarchy; a MISSI CA whose public-key certificate
      is signed by a MISSI CA rather than by a MISSI PCA. A MISSI SCA is
      the administrative authority for a subunit of an organization,
      established when it is desirable to organizationally distribute or
      decentralize the CA service. The term refers both to that
      authoritative office or role, and to the person who fills that
      office. A MISSI SCA registers end users and issues their
      certificates and may also register ORAs, but may not register
      other CAs. An SCA periodically issues a CRL.

   $ subordinate DN
      (I) An X.500 DN is subordinate to another X.500 DN if it begins
      with a set of attributes that is the same as the entire second DN
      except for the terminal attribute of the second DN (which is
      usually the name of a CA). For example, the DN <C=FooLand, O=Gov,
      OU=Treasurer, CN=DukePinchpenny> is subordinate to the DN
      <C=FooLand, O=Gov, CN=KingFooCA>.

   $ subscriber
      (I) /PKI/ A user that is registered in a PKI and, therefore, can
      be named in the "subject" field of a certificate issued by a CA in
      that PKI. (See: registration, user.)

      Usage: This term is needed to distinguish registered users from
      two other kinds of PKI users:
      -  Users that access the PKI but are not identified to it: For
         example, a relying party may access a PKI repository to obtain
         the certificate of some other party. (See: access.)
      -  Users that do not access the PKI: For example, a relying party
         (see: certificate user) may use a digital certificate that was
         obtained from a database that is not part of the PKI that
         issued the certificate.

   $ substitution
      1. (I) /cryptography/ A method of encryption in which elements of
      the plain text retain their sequential position but are replaced
      by elements of cipher text. (Compare: transposition.)

      2. (I) /threat action/ See: secondary definition under
      "falsification".

   $ subsystem
      (I) A collection of related system components that together
      perform a system function or deliver a system service.

   $ superencryption
      (I) An encryption operation for which the plaintext input to be
      transformed is the ciphertext output of a previous encryption
      operation. (Compare: hybrid encryption.)

   $ superuser
      (I) /UNIX/ Synonym for "root".

   $ survivability
      (I) The ability of a system to remain in operation or existence
      despite adverse conditions, including natural occurrences,
      accidental actions, and attacks. (Compare: availability,
      reliability.)

   $ swIPe
      (I) An encryption protocol for IP that provides confidentiality,
      integrity, and authentication and can be used for both end-to-end
      and intermediate-hop security. [Ioan] (Compare: IPsec.)

      Tutorial: The swIPe protocol is an IP predecessor that is
      concerned only with encryption mechanisms; policy and key
      management are handled outside the protocol.

   $ syllabary
      (N) /encryption/ A list of individual letters, combinations of
      letters, or syllables, with their equivalent code groups, used for
      spelling out proper names or other unusual words that are not
      present in the basic vocabulary (i.e., are not in the codebook) of
      a code used for encryption.

   $ symmetric cryptography
      (I) A branch of cryptography in which the algorithms use the same
      key for both of two counterpart cryptographic operations (e.g.,
      encryption and decryption). (See: asymmetric cryptography.
      Compare: secret-key cryptography.)

      Tutorial: Symmetric cryptography has been used for thousands of
      years [Kahn]. A modern example is AES.

      Symmetric cryptography has a disadvantage compared to asymmetric
      cryptography with regard to key distribution. For example, when
      Alice wants to ensure confidentiality for data she sends to Bob,
      she encrypts the data with a key, and Bob uses the same key to
      decrypt. However, keeping the shared key secret entails both cost

      and risk when the key is distributed to both Alice and Bob. (See:
      key distribution, key management.)

   $ symmetric key
      (I) A cryptographic key that is used in a symmetric cryptographic
      algorithm. (See: symmetric cryptography.)

   $ SYN flood
      (I) A denial-of-service attack that sends a large number of TCP
      SYN (synchronize) packets to a host with the intent of disrupting
      the operation of that host. (See: blind attack, flooding.)

      Tutorial: This attack seeks to exploit a vulnerability in the TCP
      specification or in a TCP implementation. Normally, two hosts use
      a three-way exchange of packets to establish a TCP connection: (a)
      host 1 requests a connection by sending a SYN packet to host 2;
      (b) host 2 replies by sending a SYN-ACK (acknowledgement) packet
      to host 1; and (c) host 1 completes the connection by sending an
      ACK packet to host 2. To attack host 2, host 1 can send a series
      of TCP SYNs, each with a different phony source address. ([R2827]
      discusses how to use packet filtering to prevent such attacks from
      being launched from behind an Internet service provider's
      aggregation point.) Host 2 treats each SYN as a request from a
      separate host, replies to each with a SYN-ACK, and waits to
      receive the matching ACKs. (The attacker can use random or
      unreachable sources addresses in the SYN packets, or can use
      source addresses that belong to third parties, that then become
      secondary victims.)

      For each SYN-ACK that is sent, the TCP process in host 2 needs
      some memory space to store state information while waiting for the
      matching ACK to be returned. If the matching ACK never arrives at
      host 2, a timer associated with the pending SYN-ACK will
      eventually expire and release the space. But if host 1 (or a
      cooperating group of hosts) can rapidly send many SYNs to host 2,
      host 2 will need to store state information for many pending SYN-
      ACKs and may run out of space. This can prevent host 2 from
      responding to legitimate connection requests from other hosts or
      even, if there are flaws in host 2's TCP implementation, crash
      when the available space is exhausted.

   $ synchronization
      (I) Any technique by which a receiving (decrypting) cryptographic
      process attains an internal state that matches the transmitting
      (encrypting) process, i.e., has the appropriate keying material to
      process the cipher text and is correctly initialized to do so.

   $ system
      (I) Synonym for "information system".

      Usage: This is a generic definition, and is the one with which the
      term is used in this Glossary. However, IDOCs that use the term,
      especially IDOCs that are protocol specifications, SHOULD state a
      more specific definition. Also, IDOCs that specify security
      features, services, and assurances need to define which system
      components and system resources are inside the applicable security
      perimeter and which are outside. (See: security architecture.)

   $ system architecture
      (N) The structure of system components, their relationships, and
      the principles and guidelines governing their design and evolution
      over time. [DoD10] (Compare: security architecture.)

   $ system component
      1. (I) A collection of system resources that (a) forms a physical
      or logical part of the system, (b) has specified functions and
      interfaces, and (c) is treated (e.g., by policies or
      specifications) as existing independently of other parts of the
      system. (See: subsystem.)

      2. (O) /ITSEC/ An identifiable and self-contained part of a TOE.

      Usage: Component is a relative term because components may be
      nested; i.e., one component of a system may be a part of another
      component of that system.

      Tutorial: Components can be characterized as follows:
      -  A "physical component" has mass and takes up space.
      -  A "logical component" is an abstraction used to manage and
         coordinate aspects of the physical environment, and typically
         represents a set of states or capabilities of the system.

   $ system entity
      (I) An active part of a system -- a person, a set of persons
      (e.g., some kind of organization), an automated process, or a set
      of processes (see: subsystem) -- that has a specific set of
      capabilities. (Compare: subject, user.)

   $ system high
      (I) The highest security level at which a system operates, or is
      capable of operating, at a particular time or in a particular
      environment. (See: system-high security mode.)

   $ system-high security mode
      (I) A mode of system operation wherein all users having access to
      the system possess all necessary authorizations (both security
      clearance and formal access approval) for all data handled by the
      system, but some users might not have need-to-know for all the
      data. (See: /system operation/ under "mode", formal access
      approval, protection level, security clearance.)

      Usage: Usually abbreviated as "system-high mode". This mode was
      defined in U.S. DoD policy that applied to system accreditation,
      but the term is widely used outside the Government.

   $ system integrity
      1. (I) An attribute or quality "that a system has when it can
      perform its intended function in a unimpaired manner, free from
      deliberate or inadvertent unauthorized manipulation." [C4009,
      NCS04] (See: recovery, system integrity service.)

      2. (D) "Quality of an [information system] reflecting the logical
      correctness and reliability of the operating system; the logical
      completeness of the hardware and software implementing the
      protection mechanisms; and the consistency of the data structures
      and occurrence of the stored data." [from an earlier version of
      C4009]

      Deprecated Definition: IDOCs SHOULD NOT use definition 2 because
      it mixes several concepts in a potentially misleading way.
      Instead, IDOCs should use the term with definition 1 and,
      depending on what is meant, couple the term with additional, more
      specifically descriptive and informative terms, such as
      "correctness", "reliability", and "data integrity".

   $ system integrity service
      (I) A security service that protects system resources in a
      verifiable manner against unauthorized or accidental change, loss,
      or destruction. (See: system integrity.)

   $ system low
      (I) The lowest security level supported by a system at a
      particular time or in a particular environment. (Compare: system
      high.)

   $ system resource
      (I) Data contained in an information system; or a service provided
      by a system; or a system capacity, such as processing power or
      communication bandwidth; or an item of system equipment (i.e.,

      hardware, firmware, software, or documentation); or a facility
      that houses system operations and equipment. (See: system
      component.)

   $ system security officer (SSO)
      (I) A person responsible for enforcement or administration of the
      security policy that applies to a system. (Compare: manager,
      operator.)

   $ system user
      (I) A system entity that consumes a product or service provided by
      the system, or that accesses and employs system resources to
      produce a product or service of the system. (See: access, [R2504].
      Compare: authorized user, manager, operator, principal, privileged
      user, subject, subscriber, system entity, unauthorized user.)

      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:
      -  This term usually refers to an entity that has been authorized
         to access the system, but the term sometimes is used without
         regard for whether access is authorized.
      -  This term usually refers to a living human being acting either
         personally or in an organizational role. However, the term also
         may refer to an automated process in the form of hardware,
         software, or firmware; to a set of persons; or to a set of
         processes.
      -  IDOCs SHOULD NOT use the term to refer to a mixed set
         containing both persons and processes. This exclusion is
         intended to prevent situations that might cause a security
         policy to be interpreted in two different and conflicting ways.

      A system user can be characterized as direct or indirect:
      -  "Passive user": A system entity that is (a) outside the
         system's security perimeter *and* (b) can receive output from
         the system but cannot provide input or otherwise interact with
         the system.
      -  "Active user": A system entity that is (a) inside the system's
         security perimeter *or* (b) can provide input or otherwise
         interact with the system.

   $ TACACS
      (I) See: Terminal Access Controller (TAC) Access Control System.

   $ TACACS+
      (I) A TCP-based protocol that improves on TACACS by separating the
      functions of authentication, authorization, and accounting and by
      encrypting all traffic between the network access server and

      authentication server. TACACS+ is extensible to allow any
      authentication mechanism to be used with TACACS+ clients.

   $ tamper
      (I) Make an unauthorized modification in a system that alters the
      system's functioning in a way that degrades the security services
      that the system was intended to provide. (See: QUADRANT. Compare:
      secondary definitions under "corruption" and "misuse".)

   $ tamper-evident
      (I) A characteristic of a system component that provides evidence
      that an attack has been attempted on that component or system.

      Usage: Usually involves physical evidence. (See: tamper.)

   $ tamper-resistant
      (I) A characteristic of a system component that provides passive
      protection against an attack. (See: tamper.)

      Usage: Usually involves physical means of protection.

   $ tampering
      (I) /threat action/ See: secondary definitions under "corruption"
      and "misuse".

   $ target of evaluation (TOE)
      (N) /Common Criteria/ An information technology product or system
      that is the subject of a security evaluation, together with the
      product's associated administrator and user documentation.
      (Compare: protection profile.)

      Tutorial: The security characteristics of the target of evaluation
      (TOE) are described in specific terms by a corresponding security
      target, or in more general terms by a protection profile. In
      Common Criteria philosophy, it is important that a TOE be
      evaluated against the specific set of criteria expressed in the
      target. This evaluation consists of rigorous analysis and testing
      performed by an accredited, independent laboratory. The scope of a
      TOE evaluation is set by the EAL and other requirements specified
      in the target. Part of this process is an evaluation of the target
      itself, to ensure that it is correct, complete, and internally
      consistent and can be used as the baseline for the TOE evaluation.

   $ TCB
      (N) See: trusted computing base.

   $ TCC field
      (I) See: Transmission Control Code field.

   $ TCG
      (N) See: Trusted Computing Group.

   $ TCP
      (I) See: Transmission Control Protocol.

   $ TCP/IP
      (I) Synonym for "Internet Protocol Suite".

   $ TCSEC
      (N) See: Trusted Computer System Evaluation Criteria. (Compare:
      TSEC.)

   $ TDEA
      (I) See: Triple Data Encryption Algorithm.

   $ teardrop attack
      (D) /slang/ A denial-of-service attack that sends improperly
      formed IP packet fragments with the intent of causing the
      destination system to fail.

      Deprecated Term: IDOCs that use this term SHOULD state a
      definition for it because the term is often used imprecisely and
      could easily be misunderstood. (See: Deprecated Usage under "Green
      Book".)

   $ technical non-repudiation
      (I) See: (secondary definition under) non-repudiation.

   $ technical security
      (I) Security mechanisms and procedures that are implemented in and
      executed by computer hardware, firmware, or software to provide
      automated protection for a system. (See: security architecture.
      Compare: administrative security.)

   $ Telecommunications Security Word System (TSEC)
      (O) /U.S. Government/ A terminology for designating
      telecommunication security equipment. (Compare: TCSEC.)

      Tutorial: A TSEC designator has the following parts:
      -  Prefix "TSEC/" for items and systems, or suffix "/TSEC" for
         assemblies. (Often omitted when the context is clear.)
      -  First letter, for function: "C" COMSEC equipment system, "G"
         general purpose, "K" cryptographic, "H" crypto-ancillary, "M"
         manufacturing, "N" noncryptographic, "S" special purpose.
      -  Second letter, for type or purpose: "G" key generation, "I"
         data transmission, "L" literal conversion, "N" signal
         conversion, "O" multipurpose, "P" materials production, "S"

         special purpose, "T" testing or checking, "U" television, "W"
         teletypewriter, "X" facsimile, "Y" speech.
      -  Optional third letter, used only in designations of assemblies,
         for type or purpose: "A" advancing, "B" base or cabinet, "C"
         combining, "D" drawer or panel, "E" strip or chassis, "F" frame
         or rack, "G" key generator, "H" keyboard, "I" translator or
         reader, "J" speech processing, "K" keying or permuting, "L"
         repeater, "M" memory or storage, "O" observation, "P" power
         supply or converter, "R" receiver, "S" synchronizing, "T"
         transmitter, "U" printer, "V" removable COMSEC component, "W"
         logic programmer/programming, "X" special purpose.
      -  Model number, usually two or three digits, assigned
         sequentially within each letter combination (e.g., KG-34, KG-
         84).
      -  Optional suffix letter, used to designate a version. First
         version has no letter, next version has "A" (e.g., KG-84, KG-
         84A), etc.

   $ TELNET
      (I) A TCP-based, Application-Layer, Internet Standard protocol
      (RFC 854) for remote login from one host to another.

   $ TEMPEST
      1. (N) Short name for technology and methods for protecting
      against data compromise due to electromagnetic emanations from
      electrical and electronic equipment. [Army, Russ] (See:
      inspectable space, soft TEMPEST, TEMPEST zone. Compare: QUADRANT)

      2. (O) /U.S. Government/ "Short name referring to investigation,
      study, and control of compromising emanations from IS equipment."
      [C4009]

      Deprecated Usage: IDOCs SHOULD NOT use this term as a synonym for
      "electromagnetic emanations security"; instead, use EMSEC. Also,
      the term is NOT an acronym for Transient Electromagnetic Pulse
      Surveillance Technology.

      Tutorial: The U.S. Federal Government issues security policies
      that (a) state specifications and standards for techniques to
      reduce the strength of emanations from systems and reduce the
      ability of unauthorized parties to receive and make use of
      emanations and (b) state rules for applying those techniques.
      Other nations presumably do the same.

   $ TEMPEST zone
      (O) "Designated area [i.e., a physical volume] within a facility
      where equipment with appropriate TEMPEST characteristics ... may

      be operated." [C4009] (See: emanation security, TEMPEST. Compare:
      control zone, inspectable space.)

      Tutorial: The strength of an electromagnetic signal decreases in
      proportion to the square of the distance between the source and
      the receiver. Therefore, EMSEC for electromagnetic signals can be
      achieved by a combination of (a) reducing the strength of
      emanations to a defined level and (b) establishing around that
      equipment an appropriately sized physical buffer zone from which
      unauthorized entities are excluded. By making the zone large
      enough, it is possible to limit the signal strength available to
      entities outside the zone to a level lower than can be received
      and read with known, state-of-the-art methods. Typically, the need
      for and size of a TEMPEST zone established by a security policy
      depends not only on the measured level of signal emitted by
      equipment, but also on the perceived threat level in the
      equipment's environment.

   $ Terminal Access Controller (TAC) Access Control System (TACACS)
      (I) A UDP-based authentication and access control protocol [R1492]
      in which a network access server receives an identifier and
      password from a remote terminal and passes them to a separate
      authentication server for verification. (See: TACACS+.)

      Tutorial: TACACS can provide service not only for network access
      servers but also routers and other networked computing devices via
      one or more centralized authentication servers. TACACS was
      originally developed for ARPANET and has evolved for use in
      commercial equipment.

   $ TESS
      (I) See: The Exponential Encryption System.

   $ The Exponential Encryption System (TESS)
      (I) A system of separate but cooperating cryptographic mechanisms
      and functions for the secure authenticated exchange of
      cryptographic keys, the generation of digital signatures, and the
      distribution of public keys. TESS uses asymmetric cryptography,
      based on discrete exponentiation, and a structure of self-
      certified public keys. [R1824]

   $ theft
      (I) /threat action/ See: secondary definitions under
      "interception" and "misappropriation".

   $ threat
      1a. (I) A potential for violation of security, which exists when
      there is an entity, circumstance, capability, action, or event

      that could cause harm. (See: dangling threat, INFOCON level,
      threat action, threat agent, threat consequence. Compare: attack,
      vulnerability.)

      1b. (N) Any circumstance or event with the potential to adversely
      affect a system through unauthorized access, destruction,
      disclosure, or modification of data, or denial of service. [C4009]
      (See: sensitive information.)

      Usage: (a) Frequently misused with the meaning of either "threat
      action" or "vulnerability". (b) In some contexts, "threat" is used
      more narrowly to refer only to intelligent threats; for example,
      see definition 2 below. (c) In some contexts, "threat" is used
      more broadly to cover both definition 1 and other concepts, such
      as in definition 3 below.

      Tutorial: A threat is a possible danger that might exploit a
      vulnerability. Thus, a threat may be intentional or not:
      -  "Intentional threat": A possibility of an attack by an
         intelligent entity (e.g., an individual cracker or a criminal
         organization).
      -  "Accidental threat": A possibility of human error or omission,
         unintended equipment malfunction, or natural disaster (e.g.,
         fire, flood, earthquake, windstorm, and other causes listed in
         [FP031]).

      The Common Criteria characterizes a threat in terms of (a) a
      threat agent, (b) a presumed method of attack, (c) any
      vulnerabilities that are the foundation for the attack, and (d)
      the system resource that is attacked. That characterization agrees
      with the definitions in this Glossary (see: diagram under
      "attack").

      2. (O) The technical and operational ability of a hostile entity
      to detect, exploit, or subvert a friendly system and the
      demonstrated, presumed, or inferred intent of that entity to
      conduct such activity.

      Tutorial: To be likely to launch an attack, an adversary must have
      (a) a motive to attack, (b) a method or technical ability to make
      the attack, and (c) an opportunity to appropriately access the
      targeted system.

      3. (D) "An indication of an impending undesirable event." [Park]

      Deprecated Definition: IDOCs SHOULD NOT use this term with
      definition 3 because the definition is ambiguous; the definition
      was intended to include the following three meanings:

      -  "Potential threat": A possible security violation; i.e., the
         same as definition 1.
      -  "Active threat": An expression of intent to violate security.
         (Context usually distinguishes this meaning from the previous
         one.)
      -  "Accomplished threat" or "actualized threat": That is, a threat
         action. Deprecated Usage: IDOCs SHOULD NOT use the term
         "threat" with this meaning; instead, use "threat action".

   $ threat action
      (I) A realization of a threat, i.e., an occurrence in which system
      security is assaulted as the result of either an accidental event
      or an intentional act. (See: attack, threat, threat consequence.)

      Tutorial: A complete security architecture deals with both
      intentional acts (i.e., attacks) and accidental events [FP031].
      (See: various kinds of threat actions defined under the four kinds
      of "threat consequence".)

   $ threat agent
      (I) A system entity that performs a threat action, or an event
      that results in a threat action.

   $ threat analysis
      (I) An analysis of the threat actions that might affect a system,
      primarily emphasizing their probability of occurrence but also
      considering their resulting threat consequences. Example: RFC
      3833. (Compare: risk analysis.)

   $ threat consequence
      (I) A security violation that results from a threat action.

      Tutorial: The four basic types of threat consequence are
      "unauthorized disclosure", "deception", "disruption", and
      "usurpation". (See main Glossary entries of each of these four
      terms for lists of the types of threat actions that can result in
      these consequences.)

   $ thumbprint
      1. (I) A pattern of curves formed by the ridges on the tip of a
      thumb. (See: biometric authentication, fingerprint.)

      2. (D) Synonym for some type of "hash result". (See: biometric
      authentication. Compare: fingerprint.)

      Deprecated Usage: IDOCs SHOULD NOT use this term with definition 2
      because that meaning mixes concepts in a potentially misleading
      way.

   $ ticket
      (I) Synonym for "capability token".

      Tutorial: A ticket is usually granted by a centralized access
      control server (ticket-granting agent) to authorize access to a
      system resource for a limited time. Tickets can be implemented
      with either symmetric cryptography (see: Kerberos) or asymmetric
      cryptography (see: attribute certificate).

   $ tiger team
      (O) A group of evaluators employed by a system's managers to
      perform penetration tests on the system.

      Deprecated Usage: It is likely that other cultures use different
      metaphors for this concept. Therefore, to avoid international
      misunderstanding, IDOCs SHOULD NOT use this term. (See: Deprecated
      Usage under "Green Book".)

   $ time stamp
      1. (I) /noun/ With respect to a data object, a label or marking in
      which is recorded the time (time of day or other instant of
      elapsed time) at which the label or marking was affixed to the
      data object. (See: Time-Stamp Protocol.)

      2. (O) /noun/ "With respect to a recorded network event, a data
      field in which is recorded the time (time of day or other instant
      of elapsed time) at which the event took place." [A1523]

      Tutorial: A time stamp can be used as evidence to prove that a
      data object existed (or that an event occurred) at or before a
      particular time. For example, a time stamp might be used to prove
      that a digital signature based on a private key was created while
      the corresponding public-key certificate was valid, i.e., before
      the certificate either expired or was revoked. Establishing this
      proof would enable the certificate to be used after its expiration
      or revocation, to verify a signature that was created earlier.
      This kind of proof is required as part of implementing PKI
      services, such as non-repudiation service, and long-term security
      services, such as audit.

   $ Time-Stamp Protocol
      (I) An Internet protocol (RFC 3161) that specifies how a client
      requests and receives a time stamp from a server for a data object
      held by the client.

      Tutorial: The protocol describes the format of (a) a request sent
      to a time-stamp authority and (b) the response that is returned
      containing a time stamp. The authority creates the stamp by

      concatenating (a) a hash value of the input data object with (b) a
      UTC time value and other parameters (policy OID, serial number,
      indication of time accuracy, nonce, DN of the authority, and
      various extensions), and then signing that dataset with the
      authority's private key as specified in CMS. Such an authority
      typically would operate as a trusted third-party service, but
      other operational models might be used.

   $ timing channel
      (I) See: covert timing channel.

   $ TKEY
      (I) A mnemonic referring to an Internet protocol (RFC 2930) for
      establishing a shared secret key between a DNS resolver and a DNS
      name server. (See: TSIG.)

   $ TLS
      (I) See: Transport Layer Security.

   $ TLSP
      (N) See: Transport Layer Security Protocol.

   $ TOE
      (N) See: target of evaluation.

   $ token
      1. (I) /cryptography/ See: cryptographic token. (Compare: dongle.)

      2. (I) /access control/ An object that is used to control access
      and is passed between cooperating entities in a protocol that
      synchronizes use of a shared resource. Usually, the entity that
      currently holds the token has exclusive access to the resource.
      (See: capability token.)

      Usage: This term is heavily overloaded in the computing
      literature; therefore, IDOCs SHOULD NOT use this term with any
      definition other than 1 or 2.

      3a. (D) /authentication/ A data object or a physical device used
      to verify an identity in an authentication process.

      3b. (D) /U.S. Government/ Something that the claimant in an
      authentication process (i.e., the entity that claims an identity)
      possesses and controls, and uses to prove the claim during the
      verification step of the process. [SP63]

      Deprecated usage: IDOCs SHOULD NOT use this term with definitions
      3a and 3b; instead, use more specifically descriptive and

      informative terms such as "authentication information" or
      "cryptographic token", depending on what is meant.

      NIST defines four types of claimant tokens for electronic
      authentication in an information system [SP63]. IDOCs SHOULD NOT
      use these four NIST terms; they mix concepts in potentially
      confusing ways and duplicate the meaning of better-established
      terms. These four terms can be avoided by using more specifically
      descriptive terms as follows:
      -  NIST "hard token": A hardware device that contains a protected
         cryptographic key. (This is a type of "cryptographic token",
         and the key is a type of "authentication information".)
      -  NIST "one-time password device token": A personal hardware
         device that generates one-time passwords. (One-time passwords
         are typically generated cryptographically. Therefore, this is a
         type of "cryptographic token", and the key is a type of
         "authentication information".)
      -  NIST "soft token": A cryptographic key that typically is stored
         on disk or some other magnetic media. (The key is a type of
         "authentication information"; "authentication key" would be a
         better description.)
      -  NIST "password token": A secret data value that the claimant
         memorizes. (This is a "password" that is being used as
         "authentication information".)

   $ token backup
      (I) A token management operation that stores sufficient
      information in a database (e.g., in a CAW) to recreate or restore
      a security token (e.g., a smart card) if it is lost or damaged.

   $ token copy
      (I) A token management operation that copies all the personality
      information from one security token to another. However, unlike in
      a token restore operation, the second token is initialized with
      its own, different local security values such as PINs and storage
      keys.

   $ token management
      (I) The process that includes initializing security tokens (e.g.,
      "smart card"), loading data into the tokens, and controlling the
      tokens during their lifecycle. May include performing key
      management and certificate management functions; generating and
      installing PINs; loading user personality data; performing card
      backup, card copy, and card restore operations; and updating
      firmware.