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Security Internet Engineering Task Force This document proposes an extension in the Kerberos protocol. It defines a new Authorization Data Type AD-AUTHENTICATION-INDICATOR. The purpose of introducing this data type is to include an indicator of the strength of a client's authentication in the service tickets so that the application services can use it as an input into policy decisions.

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 or Public Key Cryptography. Implementations that have Pre-Authentication mechanisms offering significantly different strengths of client authentication may choose to keep track of the strength of the authentication used as an input into policy decisions. This document proposes a new Authorization Data Type to be used to convey the authentication strength to the application services. The KDC can provide the DER encoding of the ASN.1 type defined in this document into the ad-data field of the AD-CAMMAC container. This ASN.1 type contains information about the type of authentication mechanism used by the Kerberos client to authenticate itself to the KDC.

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 the following Authorization Data element, wrapped in AD-CAMMAC, in the initial credentials and copy it from a ticket-granting ticket into service tickets: AD-AUTHENTICATION-INDICATOR 97 The corresponding ad-data field contains the DER encoding of the ASN.1 type which is defined as 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. These strings MAY be site-defined strings that do not contain a colon such as the name of the Pre-Authentication mechanism used, or alternatively URIs that reference a Level of Assurance Profile. The AD-AUTHENTICATION-INDICATOR MUST be included in the AD-CAMMAC container so that its contents can be protected. The AD-AUTHENTICATION-INDICATOR MAY safely be ignored by the applications and KDCs that do not implement this element.

AD-AUTHENTICATION-INDICATOR is wrapped in AD-CAMMAC which supersedes AD-KDC-ISSUED container. AD-CAMMAC allows both the application service and the KDC to verify the authenticity of the contained Authorization Data. A malicious service can replace AD-CAMMAC in a service ticket with a legitimate AD-CAMMAC present in some other ticket that the service has received. The KDC MUST ensure that the service does not tamper with the contents of AD-CAMMAC or the ticket by including a kdc-verifier in the containing CAMMAC. This binding protects AD-AUTHENTICATION-INDICATOR in case of constrained delegation such as S4U2Proxy extension. 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.

&rfc6113; &rfc4120; &rfc7751; &rfc2119; Microsoft &rfc6711;