Web Services Security
Kerberos Token Profile 1.1
OASIS Standard Specification, 251
February August 2006
OASIS identifier:
wss-v1.1-spec-errataos-KerberosTokenProfile
Location:
http://docs.oasis-open.org/wss/v1.1
Technical Committee:
Web Service Security (WSS)
Chairs:
Kelvin Lawrence, IBM
Chris Kaler, Microsoft
Editors:
Anthony Nadalin, IBM
Chris Kaler, Microsoft
Ronald Monzillo, Sun
Phillip Hallam-Baker, Verisign
Abstract:
This document describes how to use Kerberos [Kerb] tickets (specifically the AP-REQ packet) with the WSS: SOAP Message Security [WSS] specification.
Status:
This is an OASIS Standard document produced by
the Web Services Security Technical Committee. It was approved by the
OASIS membership on 1 February 2006. Check the
current location noted above for possible errata to this document.
Technical Committee members should send comments on this specification to the technical Committee’s email list. Others should send comments to the Technical Committee by using the “Send A Comment” button on the Technical Committee’s web page at www.oasisopen.org/committees/wss.
For information on whether any patents have been disclosed that may be essential to implementing this specification, and any offers of patent licensing terms, please refer to the Intellectual Property Rights section of the Security Services TC web page (http://www.oasis-pen.org/who/intellectualproperty.shtml).
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This section is non-normative.
Table of Contents
3.3 Identifying and
Referencing Kerberos Tokens
4 Threat Model and
Countermeasures
This specification describes the use of Kerberos [Kerb] tokens with respect to the WSS: SOAP Message Security specification [WSS].
Specifically, this document defines how to encode Kerberos tickets and attach them to SOAP messages. As well, it specifies how to add signatures and encryption to the SOAP message, in accordance with WSS: SOAP Message Security, which uses and references the Kerberos tokens.
For interoperability concerns, and for some security concerns, the specification is limited to using the AP-REQ packet (service ticket and authenticator) defined by Kerberos as the Kerberos token. This allows a service to authenticate the ticket and interoperate with existing Kerberos implementations.
It should be noted that how the AP-REQ is obtained is out of scope of this specification as are scenarios involving other ticket types and user-to-user interactions.
Note that Sections 2.1, 2.2, all of 3, and indicated parts of 6 are normative. All other sections are non-normative.
This section specifies the notations, namespaces, and terminology used in this specification.
2.1 Notational Conventions
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC2119 [2119].
Namespace URIs (of the general form "some-URI") represent some application-dependent or context-dependent URI as defined in RFC2396 [URI].
This specification is designed to work with the general SOAP [S11, S12] message structure and message processing model, and should be applicable to any version of SOAP. The current SOAP 1.2 namespace URI is used herein to provide detailed examples, but there is no intention to limit the applicability of this specification to a single version of SOAP.
2.2 Namespaces
The XML namespace [XML-ns] URIs that MUST be used by implementations of this specification are as follows (note that different elements in this specification are from different namespaces):
http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd
http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd
http://docs.oasis-open.org/wss/oasis-wss-wssecurity-secext-1.1.xsd
Note that this specification does not introduce new schema elements.
The following namespaces are used in this document:
|
Prefix |
Namespace |
|
S11 |
|
|
S12 |
|
|
wsse |
http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd |
|
wsse11 |
http://docs.oasis-open.org/wss/oasis-wss-wssecurity-secext-1.1.xsd |
|
wsu |
http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd
|
|
ds |
|
|
xenc |
The URLs provided for the wsse and wsu namespaces can be used to obtain the schema files. URI fragments defined in this specification are relative to the following base URI unless otherwise specified:
http://docs.oasis-open.org/wss/oasis-wss-kerberos-token-profile-1.1
2.3 Terminology
Readers are presumed to be familiar with the terms in the Internet Security Glossary [ISG].
This specification employs the terminology defined in the WSS: SOAP Message Security Core Specification [WSS].
The following (non-normative) table defines additional acronyms and abbreviations for this document.
|
Term |
Definition |
|
SHA |
Secure Hash Algorithm |
|
SOAP |
Simple Object Access Protocol |
|
URI |
Uniform Resource Identifier |
|
XML |
Extensible Markup Language |
This section describes the profile (specific mechanisms
and procedures) for the Kerberos binding of WSS: SOAP Message Security.
Identification: http://docs.oasis-open.org/wss/oasis-wss-kerberos-token-profile-1.1
3.1 Processing Model
The processing model for WSS: SOAP Message Security
with Kerberos tokens is no different from that of WSS: SOAP Message Security
with other token formats as described in WSS: SOAP Message Security.
3.2 Attaching Security Tokens
Kerberos tokens are attached to SOAP messages using WSS: SOAP Message Security by using the <wsse:BinarySecurityToken> described in WSS: SOAP Message Security. When using this element, the @ValueType attribute MUST be specified. This specification defines six values for this attribute as defined in the table below:
|
URI |
Description |
|
http://docs.oasis-open.org/wss/oasis-wss-kerberos-token-profile-1.1#Kerb erosv5_AP_REQ |
Kerberos v5 AP-REQ as defined in the Kerberos specification. This ValueType
is used when the ticket is an AP Request. |
|
http://docs.oasis-open.org/wss/oasis-wss-kerberos-token-profile-1.1#GSS_ Kerberosv5_AP_REQ |
A GSS-API Kerberos V5 mechanism token containing an KRB_AP_REQ message as defined in RFC-1964 [1964], Sec. 1.1 and its successor RFC-4121 [4121], Sec. 4.1. This ValueType is used when the ticket is an AP Request (ST + Authenticator). |
|
http://docs.oasis-open.org/wss/oasis-wss-kerberos-token-profile-1.1#Kerb erosv5_AP_REQ1510 |
Kerberos v5 AP-REQ
as defined in RFC1510. This ValueType is |
|
http://docs.oasis-open.org/wss/oasis-wss-kerberos-token-profile-1.1#GSS_ Kerberosv5_AP_REQ1510 |
A GSS-API Kerberos V5 mechanism token
containing an KRB_AP_REQ message as defined in RFC-1964, Sec. 1.1 and its
successor RFC-4121, Sec. 4.1. This ValueType is used
when the ticket is an AP Request |
|
http://docs.oasis-open.org/wss/oasis-wss-kerberos-token-profile-1.1#Kerb erosv5_AP_REQ4120 |
Kerberos v5 AP-REQ
as defined in RFC4120. This ValueType is |
|
http://docs.oasis-open.org/wss/oasis-wss-kerberos-token-profile-1.1#GSS_ Kerberosv5_AP_REQ4120 |
A GSS-API Kerberos V5 mechanism token
containing an KRB_AP_REQ message as defined in RFC-1964, Sec. 1.1 and its
successor RFC-4121, Sec. 4.1. This ValueType is used
when the ticket is an AP Request |
It should be noted that the URIs in the table above
also serve as the official URIs identifying the Kerberos tokens defined in this
specification.
All token types defined in this section use the type 0x8003 defined in RFC1964 for the checksum field of the authenticator inside the AP_REQ.
The octet sequence of either the GSS-API framed KRB_AP_REQ token or an unwrapped AP_REQ is encoded using the indicated encoding (e.g. base 64) and the result is placed inside of the <wsse:BinarySecurityToken> element.
The following example illustrates a SOAP message with a
Kerberos token.
<S11:Envelope xmlns:S11="..." xmlns:wsu="...">
<S11:Header>
<wsse:Security xmlns:wsse="...">
<wsse:BinarySecurityToken EncodingType="http://docs.
oasis-open.org/wss/2004/01/oasis-200401-wss-soap-message-security-1.0#Base64Binary"
ValueType="
http://docs.oasis-open.org/wss/oasis-wss-kerberos-token-profile-1.1#Kerb
erosv5_AP_REQ" wsu:Id="MyToken">boIBxDCCAcCgAwIBBaEDAgEOogcD...
</wsse:BinarySecurityToken>
...
</wsse:Security>
</S11:Header>
<S11:Body>
...
</S11:Body>
</S11:Envelope>
3.3 Identifying and Referencing Kerberos Tokens
A Kerberos Token is
referenced by means of the <wsse:SecurityTokenReference>
element. This mechanism, defined in WSS:
SOAP Message Security, provides different referencing mechanisms. The following list identifies the supported
and unsupported mechanisms:
The wsu:Id MAY be
specified on the <wsse:BinarySecurityToken>
element allowing the token to be directly referenced.
A <wsse:KeyIdentifier>
element MAY be used which specifies the identifier for the Kerberos
ticket. This value is computed as the SHA1
of the pre-encoded octets that were used to form the contents of the <wsse:BinarySecurityToken>
element. The <wsse:KeyIdentifier>
element contains the encoded form the of the KeyIdentifier which
is defined as the base64 encoding of the
SHA1 result.
Key Name references MUST
NOT be used.
When a Kerberos Token is referenced using <wsse:SecurityTokenReference> the @wsse11:TokenType attribute SHOULD be specified. If the @wsse11:TokenType is specified its value MUST be the URI that identifies the Kerberos token type as defined for a corresponding BinarySecurityToken/@ValueType attribute. The Reference/@ValueType attribute is not required. If specified, its value MUST be equivalent to that of the @wsse11:TokenType attribute..
The <wsse:SecurityTokenReference> element from which the reference is made contains the <wsse:KeyIdentifier> element. The <wsse:KeyIdentifier> element MUST have a ValueType attribute on the <wsse:KeyIdentifier> element with the value #Kerberosv5APREQSHA1 and its contents MUST be the SHA1 of GSS-API framed KRB_AP_REQ token or unwrapped AP-REQ, as appropriate, encoded as per the <wsse:KeyIdentifier> element’s EncodingType attribute.
|
Reference Identifier |
ValueType |
Description |
|
Kerberos v5 AP-REQ |
http://docs.oasis-open.org/wss/oasis-wss-kerberos-token-profile-1.1#Kerb erosv5APREQSHA1 |
SHA1 of the v5
AP-REQ octets, either GSS-API framed KRB_AP_REQ token or just the Kerberos
AP-REQ. |
The following example illustrates using ID
references to a Kerberos token:
<S11:Envelope xmlns:S11="..." xmlns:wsse="..." xmlns:wsu="...">
<S11:Header>
<wsse:Security>
<wsse:BinarySecurityToken EncodingType="http://docs.
oasis-open.org/wss/2004/01/oasis-200401-wss-soap-message-security-1.0#Base64Binary" ValueType="http://docs.oasis-open.org/wss/oasis-wss-kerberos-token-profile-1.1#Kerberosv5_AP_REQ" wsu:Id="MyToken">
boIBxDCCAcCgAwIBBaEDAgEOogcD...
</wsse:BinarySecurityToken>
...
<wsse:SecurityTokenReference TokenType="http://docs.oasis-open.org/wss/oasis-wss-kerberos-toke
n-profile-1.1#Kerberosv5_AP_REQ">>
<wsse:Reference URI="#MyToken" ValueType="http://docs.oasis-open.org/wss/oasis-wss-kerberos-token-profile-1.1#Kerberosv5_AP_REQ">
</wsse:Reference>
</wsse:SecurityTokenReference>
...
</wsse:Security>
</S11:Header>
<S11:Body>
...
</S11:Body>
</S11:Envelope>
The AP-REQ packet is included in the initial
message to the service, but need not be attached to subsequent messages
exchanged between the involved parties.
Consequently, the KeyIdentifier
reference mechanism SHOULD be used on subsequent exchanges as illustrated in
the example below:
<S11:Envelope xmlns:S11="..." xmlns:wsse="..." xmlns:wsu="...">
<S11:Header>
<wsse:Security>
...
<wsse:SecurityTokenReference> wsse11:TokenType=http://docs.oasis-open.org/wss/oasis-wss-kerberos-token-profile-1.1#Kerberosv5_AP_REQ>
<wsse:KeyIdentifier
ValueType="http://docs.oasis-open.org/wss/oasis-wss-kerberos-token-profile-1.1#Kerb
erosv5APREQSHA1">GbsDt+WmD9XlnUUWbY/nhBveW8I=
</wsse:KeyIdentifier>
</wsse:SecurityTokenReference>
...
</wsse:Security>
</S11:Header>
<S11:Body>
...
</S11:Body>
</S11:Envelope>
3.4 Authentication
When a Kerberos ticket is referenced as a signature key, the signature algorithm [DSIG] MUST be a hashed message authentication code.
When a Kerberos ticket is referenced as an encryption key, the encryption algorithm MUST be a symmetric encryption algorithm.
The value of the signature or encryption key is constructed from the value of the Kerberos sub-key when it is present in the authenticator or a session key from the ticket if the sub-key is absent, either by using the Kerberos sub-key or session key directly or using a key derived from that key using a mechanism agreed to by the communicating parties.
3.5 Encryption
When a Kerberos ticket is referenced as an encryption key, the encryption algorithm MUST be a symmetric encryption algorithm.
The value of the signature or encryption key is constructed from the value of the Kerberos sub-key when it is present in the authenticator or a session key from the ticket if the sub-key is absent, either by using the Kerberos sub-key or session key directly or using a key derived from that key using a mechanism agreed to by the communicating parties..
3.6 Principal Name
Kerberos principal name definition and mapping of non-Kerberos names to Kerberos V principal names are out of scope of this document.
3.7 Error Codes
When using Kerberos tokens, it is RECOMMENDED to use
the error codes defined in the WSS: SOAP Message Security specification. However, implementations MAY use custom
errors, defined in private namespaces if they desire. Care should be taken not to introduce
security vulnerabilities in the errors returned.
The use of Kerberos assertion tokens with WSS: SOAP
Message Security introduces no new message-level threats beyond those
identified for Kerberos itself or by WSS: SOAP Message Security with other
types of security tokens.
One potential threat is that of key re-use. The mechanisms described in WSS: SOAP Message
Security can be used to prevent replay of the message; however, it is possible
that for some service scopes, there are host security concerns of key hijacking
within a Kerberos infrastructure. The
use of the AP-REQ and its associated authenticator and sequencer mitigate this
threat.
Message alteration and eavesdropping can be addressed
by using the integrity and confidentiality mechanisms described in WSS: SOAP
Message Security. Replay attacks can be
addressed by using message timestamps and caching, as well as other
application-specific tracking mechanisms.
For Kerberos tokens ownership is verified by use of keys, so man-in-the-middle
attacks are generally mitigated.
It is strongly recommended that GSS wrapped AP-REQ be used
or that unwrapped AP-REQ be combined with timestamp be
used to prevent replay attack.
It is strongly recommended that all relevant and
immutable message data be signed to prevent replay attacks.
It should be noted that transport-level security MAY be used to protect the message and the security token in cases where neither a GSS-API framed KRB_AP_REQ token or an unwrapped AP-REQ combined with timestamp and signature are being used.
The following are normative references
[2119] S. Bradner, "Key words for use in RFCs to
Indicate Requirement Levels," RFC 2119,
[Kerb] J. Kohl and C. Neuman, "The Kerberos Network
Authentication Service (V5)," RFC 1510, September 1993, http://www.ietf.org/rfc/rfc1510.txt
.
[KEYWORDS] S.
Bradner, "Key words for use in RFCs to Indicate Requirement Levels," RFC
2119,
[S11] W3C Note, "SOAP:
Simple Object Access Protocol 1.1," 08 May 2000.
[S12] W3C Recommendation, "SOAP
Version 1.2 Part 1: Messaging Framework", 23 June 2003.
[URI] T. Berners-Lee, R. Fielding,
L. Masinter, "Uniform Resource Identifiers (URI): Generic Syntax,"
RFC 3986, MIT/LCS, Day Software, Adobe Systems, January 2005.
[WSS] A. Nadalin et al., Web Services Security: SOAP Message
Security 1.1 (WS-Security 2004), OASIS Standard, http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-soap-message-security-1.1.pdf.
[1964] J.
Linn , The Kerberos Version 5 GSS-API Mechanism, RFC 1964, June 1996.
[4121] L,
Zhu, K. Jaganathan, S. Hartman, The Kerberos Version 5 Generic Security Service
Application Program Interface (GSS-API) Mechanism: Version 2, RFC 4121, July
2005.
The following are non-normative references
[ISG] Informational RFC 2828, "Internet
Security Glossary," May 2000.
[XML-ns] W3C Recommendation, "Namespaces in XML," 14 January 1999.
[DSIG] D.
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Rev |
Date |
By Whom |
What |
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errata |
08-25-2006 |
Anthony Nadalin |
Issue 456 |