Authentication Indicator in Kerberos Tickets

Kerberos allows secure interaction among users and services over a network. It supports a variety of authentication mechanisms using its pre-authentication framework. The Kerberos authentication service has been architected to support password-based authentication as well as multi-factor authentication using one-time password devices, public-key cryptography and other pre-authentication schemes. Implementations that offer pre-authentication mechanisms supporting significantly different strengths of client authentication may choose to keep track of the strength of the authentication that was used, for use as an input into policy decisions. This document specifies a new authorization data type to convey authentication strength information to application services. Elements of this type appear within an AD-CAMMAC container.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

The KDC MAY include authorization data of ad-type 97, wrapped in AD-CAMMAC, in initial credentials. The KDC MAY copy it from a ticket-granting ticket into service tickets. The corresponding ad-data field contains the DER encoding of the following ASN.1 type: AD-AUTHENTICATION-INDICATOR ::= SEQUENCE OF UTF8String Each UTF8String value is a short string that indicates that a particular set of requirements was met during the initial authentication. These strings are intended to be compared against known values. They are not intended to store structured data. Each string MUST be either: * A URI which references a Level of Assurance Profile * A site-defined string, which MUST NOT contain a colon, whose meaning is determined by the realm administrator. Authorization data elements of type AD-AUTHENTICATION-INDICATOR MUST be included in an AD-CAMMAC container so that their contents can be verified as originating from the KDC. Elements of type AD-AUTHENTICATION-INDICATOR MAY safely be ignored by applications and KDCs that do not implement this element.

Elements of type AD-AUTHENTICATION-INDICATOR are wrapped in AD-CAMMAC containers. AD-CAMMAC supersedes AD-KDC-ISSUED, and allows both application services and the KDC to verify the authenticity of the contained authorization data. KDC implementations MUST use AD-CAMMAC verifiers as described in the the security considerations of RFC 7751 to ensure that AD-AUTHENTICATION-INDICATOR elements are not modified by an attacker. Application servers MUST validate the AD-CAMMAC container before making authorization decisions based on AD-AUTHENTICATION-INDICATOR elements. Application servers MUST NOT make authorization decisions based on AD-AUTHENTICATION-INDICATOR elements which appear outside of AD-CAMMAC containers. Using multiple strings in AD-AUTHENTICATION-INDICATOR may lead to ambiguity when a service tries to make a decision based on the AD-AUTHENTICATION-INDICATOR values. This ambiguity can be avoided if indicator values are always used as a positive indication of certain requirements being met during the initial authentication. For example, if a "without-password" indicator is inserted whenever authentication occurs without a password, a service might assume this is an indication that a higher-strength client authentication occurred. However, this indicator might also be inserted when no authentication occurred at all (such as anonymous PKINIT). Service evaluation of site-defined indicators MUST consider the realm of original authentication in order to avoid cross-realm indicator collision. Failure to enforce this property can result in invalid authorization.

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