A security label, sometimes referred to as a confidentiality label, is a structured representation of the sensitivity of a piece of information. They are used in conjunction with a clearance, a structured representation of what information sensitivities a person (or other entity) is authorized to access, and a security policy to control access to each piece of information. For instance, message could be labeled as "SECRET", and hence requiring the sender and the receiver to have a clearance granting access to "SECRET" information. &X.841; provides a discussion of security labels, clearances, and security policy.

Sensitivity-based authorization is used in networks which operate under a set of information classification rules, such as in government defense agency networks. The standardized formats for security labels, clearances, and security policy are generalized and do have application in non-government networks.

This document describes the use of security labels in &xmpp;. The document specifies how security label metadata is carried in XMPP. It standardizes a mechanism for carrying ESS Security Labels in XMPP, as well as provides for use of other label formats. ESS Security Labels are specified in &rfc2634;. ESS Security Labels are commonly used in conjunction with &X.500; clearances and either X.841 or &SDN.801c; security policies.

Note: The &IC-ISM; label example is for illustrative purposes only.

The document details when security label metadata should or should not be provided, and how this metadata is to be processed.

This document does not provide:

• any mechanism for a client might discover the security policy enforce at its home server, or any other server;
• any mechanism for a client to discover the user's clearance, or the clearance of associated with any resource; nor
• any administrative mechanism for a client to configure configure policy, clearance, and labels of any resource.

If an entity supports the XMPP Security Label protocol, it MUST report that fact by including a service discovery feature of "urn:xmpp:sec-label:0" in response to a &xep0030; information request. Clients wishing to include a XMPP Security Label element in any stanza they generate SHOULD determine if their server supports the XMPP Security Label protocol. If their server does not support XMPP Security Label, the client SHOULD NOT generate XMPP Security Labels as the server not supporting this protocol will generally ignore XMPP Security Labels as they would any other unrecognized element.

As each service domain may have different support for security labels, servers should advertise and clients should perform appropriate discovery lookups on a per service basis.

An element, &SECURITYLABEL;, is defined to carry security label metadata. This metadata includes a security label, zero or more equivalent security labels, and optionally display marking data.

The security label metadata is carried in an &SECURITYLABEL; element. The &SECURITYLABEL; element which contains one and only one &LABEL; element, zero or more &EQUIVALENTLABEL; elements, and an optional &DISPLAYMARKING; element.

The &LABEL; contains the primary security label. It is commonly issued by the sender under the security policy of that they and their home server operating under. Each &EQUIVALENTLABEL; holds equivalent security labels under other policies. This element might be used when a recepient is known to hold a clearance under a different policy than the sender.

The &LABEL; and &EQUIVALENTLABEL; elements each require a type= attribute. The type= attribute indicates the type and encoding of the element's value. The attribute type= value 'ESS' indicates the label is the base64, as specified in &rfc4648;, encoding of the &BER; encoding of the &ASN.1; eSSSecurityLabel type as specified in &rfc2634;. Additional types may be registered (see 'XMPP Registrar Considerations').

The &DISPLAYMARKING; element contains a display string for use by implementations which are unable to utilize the applicable security policy to generate display markings. The element may optionally contain two attributes, fgcolor= and bgcolor=, whose values are HTML color strings (e.g., 'red' or '#ff0000'), for use in colorizing the display marking.

It is RECOMMENDED the server publish a catalogs of security label for use by clients.

Each catalog provided should only contain labels for which the client is allowed to use (based upon the user's authorization) in a particular context (such as in chatroom). A catalog may not be include the complete set of labels available for the use by the client in the context.

Note: the single catalog per context approach used here is likely inadequate in enviroments where there are a large number of labels in use. It is expected that a more sophisticated approach will be introduced in a subsequent revision of this specification.

As each service domain may have different support for security labels, servers should advertise and clients should perform appropriate discovery lookups on a per service basis.

To indicate the support for label catalog discovery, a server advertises the urn:xmpp:sec-label:catalog:0 feature. The following pair of examples illustrates this feature discovery.

The following example pair illustrates catalog discovery.

The sensitivity-based access control decisions discussed herein are to be made independently of other access control decisions or other facilities. That is, the sensitivity-based access control decisions are not conditional on other factors.

It is intended that &SECURITYLABEL; elements are only used as prescribed by this document, or other formal specifications. Any other use of &SECURITYLABEL; SHOULD be viewed as a protocol violation. The stanza SHOULD be discarded with, if approrpriate, an error response. Such error responses SHOULD NOT include content from the violating stanza, excepting that necessary to well-formed error responses.

When use of a &SECURITYLABEL; element is prescribed, that use is RECOMMENDED. Absence of a &SECURITYLABEL; element implies the stanza has the default label as specified in the governing security policy. Given that the governing policy may not specify a default label, hence denying access to the stanza, supporting clients SHOULD provide a &SECURITYLABEL; element where prescribed.

Typically, a client would allow the user to choose populate the &SECURITYLABEL; from one of from a small set of security labels selections known to it (through configuration and/or discovery and/or other means), such as from a pull-down menu. That selection would include appropriate values for the &LABEL;, &DISPLAYMARKING;, and &EQUIVALENTLABEL; elements.

A policy-aware client may provide the user with an interface allowing the user to produce custom labeling data for inclusion in this set. A policy-aware client SHOULD preclude the user from producing &LABEL; values which the user's own clearance does not grant access to, and SHOULD preclude sending any label which the user's own clearance does not grant access to. Each &EQUIVALENTLABEL; value, if any, MUST be equivalent under an equivalent policy to the &LABEL;. The &DISPLAYMARKING; element SHOULD be set the display marking prescribed for the &LABEL; under the governing policy, or, if the governing policy prescribes no display marking for the &LABEL;, absent.

A client which receives a stanza with &SECURITYLABEL; element is to promiently display the &DISPLAYMARKING; value. A policy-aware may alternatively promiently display the marking for the &LABEL; prescribed by the governing policy.

Each server is expected to make a number of sensitivity-based authorization decisions. Each decision is made by evaluating an Access Control Decision Function (ACDF) with a governing policy, a clearance, and a security label. The ACDF yields either Grant or Deny.

If the user holds a valid clearance (known to the server) under the governing policy, the clearance input is the user's clearance. Otherwise, if the governing policy provides a default clearance, the clearance input is the default clearance. Otherwise, the clearance input is the nil clearance. The nil clearance is a clearance for which the ACDF always returns Deny when given as the clearance input.

If the stanza contains a &SECURITYLABEL; element and the either the &LABEL; element or one of the &EQUIVALENTLABEL; elements contain an appropriate label, that label input is that label. Otherwise, the label input is the default label provided the governing policy or, if no default label is provided, the nil label. The nil label is a label for which the ACDF always returns Deny when given as the label input.

The term "effective clearance" and "effective label" refer, respectively, to the clearance and label provided as input to the ACDF.

Not all sensitivity-based authorization decisions an XMPP server might make involve a user clearance and/or stanza label. A server may only provide service to users which hold an appropriate clearance as determined by calling the ACDF with the user's clearance and a label associated with the service. A clearance might also be associated with the service to restrict the set of labels may be used in labeling stanzas. Labels and clearances can also be associated with network interfaces, remote servers, chatrooms, pubsub notes.

This extension is itself is extensible. In particular, the &LABEL; and &EQUIVALENTLABEL; elements are designed to hold a range of security labels formats. XML namespaces SHOULD be used to avoid name clashes.

This document is all about authorization, a key aspect of security. Hence, security considerations are discussed through this document.

Security labels generally should be securely bound to the object. This may be accomplished through use of &xmppe2e; signing, or possibly other signing mechanisms.

Certain XMPP stanzas, such as &PRESENCE; stanzas, are not themselves subject to any sensitity-based authorization decisions, and may be forwarded throughout the XMPP network. The content of these stanzas should not contain information requiring sensitivity-based dissemination controls.

This document requires no interaction with &IANA;.

It is requested the ®ISTRAR; add the extension's namespaces and schemas to appropriate XMPP registries.