Public Key Cryptography Based User-to-User Authentication - (PKU2U)
draft-zhu-pku2u-07

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Abstract

This document defines a Generic Security Services Application Program Interface (GSS-API) mechanism based on Public Key Infrastructure (PKI) - PKU2U. This mechanism is based on Kerberos V messages and the Kerberos V GSS-API mechanism, but without requiring a Kerberos Key Distribution Center (KDC).

Table of Contents

1.  Introduction
2.  Conventions Used in This Document
3.  The PKU2U Realm Name
4.  PKU2U Principal Naming
    4.1.  GSS_C_NT_DN
    4.2.  GSS_C_NT_HOSTNAME
    4.3.  GSS_C_NT_IP_ADDR
    4.4.  GSS_C_NT_EMAIL_ADDR
    4.5.  GSS_C_NT_USER_NAME
    4.6.  GSS_KRB5_NT_PRINCIPAL_NAME
    4.7.  GSS_C_NT_ANONYMOUS
    4.8.  Matching host-based service principal names to acceptor certificates
5.  The Protocol Description and the Context Establishment Tokens
    5.1.  Context token derived from KRB_AS_REQ
    5.2.  Context token derived from KRB_AS_REP
    5.3.  Context tokens imported from RFC4121
6.  Guidelines for Credentials Selection
7.  Security Considerations
8.  Acknowledgements
9.  IANA Considerations
10.  Normative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

The Generic Security Services Application Programming Interface (GSS-API) is a generic protocol and API for providing authentication and session protection to applications. It is generic in that it supports multiple authentication mechanisms. Today there exists only one workable, widely deployed, standards-track GSS-API mechanism: the Kerberos V GSS-API mechanism [RFC1964] (Linn, J., “The Kerberos Version 5 GSS-API Mechanism,” June 1996.) [RFC4121] (Zhu, L., Jaganathan, K., and S. Hartman, “The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2,” July 2005.), which is based on Kerberos V [RFC4120] (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.). There is a need to provide a GSS-API mechanism which does not require Kerberos V Key Distribution Center (KDC) infrastructure, and which supports the use of public key cryptography, particularly Public Key Infrastructure (PKI) [RFC3280] (Housley, R., Polk, W., Ford, W., and D. Solo, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” April 2002.), including the use of public key certificates without a PKI.

This document specifies such a mechanism: the Public Key User to User mechanism (PKU2U).

PKU2U is based on building blocks taken from Kerberos V [RFC4120] (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.), PKINIT, [RFC4556] (Zhu, L. and B. Tung, “Public Key Cryptography for Initial Authentication in Kerberos (PKINIT),” June 2006.) (which in turn uses PKI [RFC3280] (Housley, R., Polk, W., Ford, W., and D. Solo, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” April 2002.)) building blocks), and the Kerberos V GSS-API mechanism [RFC1964] (Linn, J., “The Kerberos Version 5 GSS-API Mechanism,” June 1996.) [RFC4121] (Zhu, L., Jaganathan, K., and S. Hartman, “The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2,” July 2005.). In spite of using Kerberos V building blocks, PKU2U does not require any Kerberos V KDC infrastructure. And though PKU2U also uses PKI building blocks, PKU2U can be used without a PKI by pre-sharing certificates and/or pre-associating name/certificate bindings.

Therefore PKU2U can be used for true peer-to-peer authentication, as well as for PKI-based authentication.

2.  Conventions Used in This Document

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 [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

In this document, the GSS-API initiator or acceptor is referred to as the peer when the description is applicable to both the initiator and the acceptor.

3.  The PKU2U Realm Name

The PKU2U realm name is defined as a reserved Kerberos realm name per [KRB‑NAMING] (Zhu, L., “Additional Kerberos Naming Constraints,” 2007.), and it has the value of "WELLKNOWN:PKU2U".

Unless otherwise specified, the realm name in any Kerberos message used by PKU2U is the PKU2U realm name.

4.  PKU2U Principal Naming

PKU2U principal names are certificate names and subject alternative names [RFC3280] (Housley, R., Polk, W., Ford, W., and D. Solo, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” April 2002.) as they appear in the certificate of any PKU2U peer, as well as any names agreed to out of band where a certificate authenticates a name that does not appear in the certificate.

Thus certificates may be associated with multiple principal names. This presents problems for the GSS-API bindings of a PKI-based mechanism. We resolve these problems as follows:

• We define multiple GSS-API name types corresponding to several GeneralName choices [RFC3280] (Housley, R., Polk, W., Ford, W., and D. Solo, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” April 2002.), along with syntaxes, display forms, and exported name token formats for each. For most of the name-types listed below the exported name token formats consists of a BER-encoded GeneralName with the usual exported name token header as per-RFC2743. Two name-types are shared with the Kerberos V mechanism and use the Kerberos V mechanism's exported name token format.
• The cred_name of credential object acquired with a desired_name other than GSS_C_NT_NO_NAME MUST be equal to the name used as the desired_name.
• We provide that the cred_name of a default credential, or of a credential acquired with the GSS_C_NT_NO_NAME desired_name MUST be the first id-pkinit-san subject alternative name of the certificate, if there is one, or else it MUST be the DN of its certificate.
• We provide a method by which initiators can assert one of these names to the other, and we provide a default of asserting the certificate DN. We also provide a method of asserting names that do not appear in the caller's certificate. This assertion consists of a traditional Kerberos V principal name appearing in the expected 'cname' fields of various Kerberos V PDUs used to construct PKU2U security context tokens, and an optional authorization-data type consisting of an integer which identifies one of the several names of the initiator's certificate.
• We provide a method of matching host-based service principal names to acceptor certificates, so that: a) initiators need not know the particulars of an acceptor's certificates' names a priori, b) acceptors can select a credential to accept a security context with that the initiator will accept.

Thus GSS-API initiators that use the GSS_C_NO_NAME as the desired_name arguments of GSS_Acquire_cred() and GSS_Add_cred(), or GSS_C_NO_CREDENTIAL as the cred argument of GSS_Init_sec_context() will assert the selected certificate's DN, and that certificate's DN will be the name returned by GSS_Inquire_cred() and GSS_Inquire_cred_by_mech().

And portable GSS-API initiator applications using GSS_C_NT_HOSTBASED_SERVICE for naming acceptors (i.e., for importing a name to use as the targ_name input argument of GSS_Init_sec_context()) will have a reasonable chance of success in authenticating peers with certificates predating this specification.

4.1.  GSS_C_NT_DN

We introduce a new name type, GSS_C_NT_DN, with OID <TBD>, corresponding to the 'Name' ASN.1 type defined in [RFC3280] (Housley, R., Polk, W., Ford, W., and D. Solo, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” April 2002.).

The query syntax and display form for names of this type SHALL be as described in [RFC4514] (Zeilenga, K., “Lightweight Directory Access Protocol (LDAP): String Representation of Distinguished Names,” June 2006.).

There is no reasonable way to canonicalize names of this type without providing a directory against which to lookup the name. The canonical form of this name is the same as that provided to GSS_Import_name() with GSS_C_NT_DN as the input_name_type.

The exported name token format for names of this type SHALL be the DER encoding of a GeneralName with directoryName as the choice.

Implementation support for this name type is REQUIRED.

4.2.  GSS_C_NT_HOSTNAME

We introduce a new name type, GSS_C_NT_HOSTNAME, with OID <TBD>, corresponding to the 'dNSName' choice of the 'GeneralName' ASN.1 type defined in [RFC3280] (Housley, R., Polk, W., Ford, W., and D. Solo, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” April 2002.).

The query syntax for names of this type SHALL be a DNS name [RFC1034] (Mockapetris, P., “Domain names - concepts and facilities,” November 1987.) in either ACE or Unicode form [RFC3490] (Faltstrom, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA),” March 2003.).

The display and canonical form of names of this type SHALL be a DNS domainname in ACE form, with character case folded down.

The exported name token format for names of this type SHALL be the DER encoding of a GeneralName with dNSName as the choice and the DNS domainname in ACE form and case folded down.

Implementation support for this name type is OPTIONAL.

4.3.  GSS_C_NT_IP_ADDR

We introduce a new name type, GSS_C_NT_IP_ADDR, with OID <TBD>, corresponding to the 'iPAddress' choice of the 'GeneralName' ASN.1 type defined in [RFC3280] (Housley, R., Polk, W., Ford, W., and D. Solo, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” April 2002.).

The query syntax, and display and canonical forms for names of this type SHALL be the text representation of an IPv4 or IPv6 address (XXX add references).

The exported name token form for this name type SHALL be a DER-encoded GeneralName with the iPAddress choice.

Implementation support for this name type is OPTIONAL.

4.4.  GSS_C_NT_EMAIL_ADDR

We introduce a new name type, GSS_C_NT_EMAIL_ADDR, with OID <TBD>, corresponding to the 'rfc822Name' choice of the 'GeneralName' ASN.1 type defined in [RFC3280] (Housley, R., Polk, W., Ford, W., and D. Solo, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” April 2002.).

The query syntax and display form for names of this type SHALL be the text representation of an 'addr-spec' as defined in [RFC0822] (Crocker, D., “Standard for the format of ARPA Internet text messages,” August 1982.).

The canonical form of names of this type SHALL be the query form with case folded down.

The exported name token form for this name type SHALL be a DER-encoded GeneralName with the rfc822Name choice.

Implementation support for this name type is OPTIONAL.

4.5.  GSS_C_NT_USER_NAME

For PKU2U the generic name type GSS_C_NT_USER_NAME SHALL be an alias of GSS_C_NT_EMAIL_ADDR.

Implementation support for this name type is OPTIONAL.

4.6.  GSS_KRB5_NT_PRINCIPAL_NAME

PKU2U supports the use of GSS_KRB5_NT_PRINCIPAL_NAME names [RFC1964] (Linn, J., “The Kerberos Version 5 GSS-API Mechanism,” June 1996.). These appear as expected in various 'cname', 'crealm', 'sname' and 'srealm' fields of the Kerberos V PDUs used to construct PKU2U security context tokens. For PKU2U names of this type correspond to subject alternative names of type 'id-pkinit-san' [RFC4556] (Zhu, L. and B. Tung, “Public Key Cryptography for Initial Authentication in Kerberos (PKINIT),” June 2006.). When no such SAN appears in a certificate, then names of this type correspond to certificates as agreed out of band (e.g., there may be a table mapping certificates to Kerberos V principal names).

The canonical form of names of this type SHALL be as specified in RFC4121. (XXX Here we have two mechanisms sharing a single name type that originally was mechanism-specific. This may cause problems with some mechglue implementations; we should describe this in detail.)

Implementation support for this name type is REQUIRED.

4.7.  GSS_C_NT_ANONYMOUS

This is a generic GSS-API name-type. Implementation support for this name type is OPTIONAL. See Section 5.1 (Context token derived from KRB_AS_REQ) for more information.

4.8.  Matching host-based service principal names to acceptor certificates

Support for GSS_C_NT_HOSTBASED_SERVICE names is REQUIRED as described herein.

The query and display forms of this name type are as per-RFC2743. The canonical and exported name token forms are as per-RFC1964.

Initiators using names of type GSS_C_NT_HOSTBASED_SERVICE to identify target acceptors represent these names as Kerberos V principal names as per [RFC4121] (Zhu, L., Jaganathan, K., and S. Hartman, “The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2,” July 2005.) but with a well-known realm name of "WELLKNOWN:PKU2U" (see Section 5.1 (Context token derived from KRB_AS_REQ)).

Acceptors match such names to acceptor certificates as follows. Initiators then match the certificate chosen by the acceptor in the same manner.

Initiators can also assert host-based service names as the initiator name. In this case acceptors MUST also apply the matching rules below to validate the initiator's assertion.

1. If the acceptor has a certificate with an id-pkinit-san subject alternative name matching the initiator-provided acceptor name, then the certificate matches.
2. If the acceptor has a certificate with a dNSName SAN that matches the hostname part of the host-based service principal name, and either the anyExtendedKeyUsage extended key usage (EKU), or no EKU is present, or an EKU is present which corresponds to the service part of the host-based service principal name, then the certificate matches. The id-kp-serverAuth EKU SHALL be considered to match the 'HTTP' service name. (See Section 9 (IANA Considerations), IANA considerations, where the GSS-API service name registry is extended to include an EKU for each service name.)
3. Implementations SHOULD, subject to local configuration, allowing matches where the CN of the DN of a cert matches the hostname part of the host-based service name, for some or all service names. This feature is needed to allow the use of existing web certificates.
4. If there is an out-of-band binding of the peer's host-based service name to its certificate, then the certificate matches.

5.  The Protocol Description and the Context Establishment Tokens

The PKU2U mechanism is a GSS-API mechanism based on [RFC4120] (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.), [RFC4556] (Zhu, L. and B. Tung, “Public Key Cryptography for Initial Authentication in Kerberos (PKINIT),” June 2006.) and [RFC4121] (Zhu, L., Jaganathan, K., and S. Hartman, “The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2,” July 2005.).

The per-message tokens of the PKU2U mechanism are the same as those of the Kerberos V GSS-API mechanism [RFC4121] (Zhu, L., Jaganathan, K., and S. Hartman, “The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2,” July 2005.).

The PKU2U security context token exchange consists of KRB-AS-REQ and KRB-AS-REP (and KRB-ERROR) Kerberos KDC PDUs (with minor changes/requirements described below) as context tokens, with the acceptor as the KDC, followed by context tokens from [RFC4121] (Zhu, L., Jaganathan, K., and S. Hartman, “The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2,” July 2005.) using the Kerberos V Ticket PDU issued by the acceptor-as-KDC. PKINIT [RFC4556] (Zhu, L. and B. Tung, “Public Key Cryptography for Initial Authentication in Kerberos (PKINIT),” June 2006.) is the only acceptable pre-authentication method. Caching that ticket issued by the acceptor allows subsequent security context exchanges between the same to peers to use a single context token round-trip -- a "fast restart" feature.

PKU2U differs from Kerberos V with PKINIT in several minor ways:

• KDC PDUs are not exchanged as usual in Kerberos, but wrapped as GSS-API context tokens
• PKU2U allows the use of out-of-band binding of certificates to principal names
• PKU2U does not require the use of KDC certificates
• PKU2U adds pa-data types for carrying the initiator's assertion of its name and the targ_name passed to GSS_Init_sec_context()

PKU2U differs from the Kerberos V GSS-API mechanism in several ways:

• KDC PDUs are not exchanged as usual in Kerberos, but wrapped as GSS-API context tokens
• PKU2U allows the use of principal names matching PKI naming
• PKU2U adds an extension to the RFC4121 initial context token for binding the AP-REQ to the AS exchange that precedes is (that is, when the initiator has to request a ticket from the acceptor)
• The number of round-trips can vary. If the initiator already has a ticket for the acceptor then the context token exchange will be half a round-trip or one round-trip, as per RFC4121. Otherwise one or two round-trips are added for the AS exchanges needed to acquire a ticket. Note that two AS exchanges may be required when the initiator's initial choice of certificate does not match the acceptor's trust anchors, in which case the acceptor will reply with a KRB-ERROR with TD-TRUSTED-CERTIFIERS indicating what the acceptor's trust anchors are, and then the initiator can engage in a second AS exchange within the same context token exchange.

To recapitulate, the acceptor and the initiator communicate by tunneling the authentication service exchange messages through the use of the GSS-API tokens and application traffic. The reliable delivery of the authentication service exchange messages at the GSS-API token level is mandatory. In the event of message loss, message duplication, or out of order message delivery, the security context MUST fail to establish.

All context establishment tokens MUST follow the InitialContextToken syntax defined in Section 3.1 of [RFC2743] (Linn, J., “Generic Security Service Application Program Interface Version 2, Update 1,” January 2000.). PKU2U is identified by the Objection Identifier (OID) id-kerberos-pku2u.

All context establishment tokens consist of some Kerberos V PDU or another, prefixed with a two-octet token type ID, and the InitialContextToken header (see above).

The PKU2U OID is:

            id-kerberos-pku2u ::=
            { iso(1) org(3) dod(6) internet(1) security(5) kerberosV5(2)
            pku2u(7) }

The innerToken described in section 3.1 of [RFC2743] (Linn, J., “Generic Security Service Application Program Interface Version 2, Update 1,” January 2000.) and subsequent GSS-API mechanism tokens have the following formats: it starts with a two-octet token-identifier (TOK_ID), followed by a Kerberos message. The TOK_ID values for the KRB_AS_REQ message and the KRB_AS_REP message are defined in the table blow:

		    Token         TOK_ID Value in Hex
		    -----------------------------------------------
		    KRB_AS_REQ          05 00
		    KRB_AS_REP          06 00

The TOK_ID values for all other Kerberos messages are the same as defined in [RFC4121] (Zhu, L., Jaganathan, K., and S. Hartman, “The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2,” July 2005.).

By using anonymous PKINIT [KRB‑ANON] (Zhu, L. and P. Leach, “Kerberos Anonymity Support,” 2007.), PKU2U can provide server-authentication without revealing the client's identity.

5.1.  Context token derived from KRB_AS_REQ

When the initiator does not have a service ticket to the acceptor, it requests a ticket from the acceptor instead of the KDC by constructing a KRB_AS_REQ PDU [RFC4120] (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.) and using it as the context token, with a token type ID prefixed. This will be the initiator's initial context token, therefore it MUST also have the standard header bearing the OID of the mechanism being used (in this case, PKU2U's OID).

If the initiator wishes to assert a name of type GSS_C_NT_ANONYMOUS then it MUST set the cname field to WELLKNOWN/WELLKNOWN with name type KRB_NT_WELLKNOWN [KRB‑ANON] (Zhu, L. and P. Leach, “Kerberos Anonymity Support,” 2007.), and it MUST NOT use a certificate [KRB‑ANON] (Zhu, L. and P. Leach, “Kerberos Anonymity Support,” 2007.). If the initiator wishes to assert a name of type GSS_KRB5_NT_PRINCIPAL_NAME, then it MUST set the cname and realm fields of the KRB_AS_REQ to match. Otherwise the initiator MUST add a pa-data element (see below) stating the name that the initiator wishes to assert, it MUST set the cname field to the anonymous principal name, and it MUST set the realm field to "WELLKNOWN:PKU2U" with type "other" [KRB‑NAMING] (Zhu, L., “Additional Kerberos Naming Constraints,” 2007.).

If the targ_name passed to GSS_Init_sec_context() is of GSS_C_NT_ANONYMOUS type, then the sname field of the AS-REQ SHALL be set to WELLKNOWN/WELLKNOWN with name type KRB_NT_WELLKNOWN [KRB‑ANON] (Zhu, L. and P. Leach, “Kerberos Anonymity Support,” 2007.). If the targ_name passed to GSS_Init_sec_context() is of type GSS_C_NT_HOSTBASED_NAME then the initiator sets the sname field to match the parsed name as per [RFC4121] (Zhu, L., Jaganathan, K., and S. Hartman, “The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2,” July 2005.), and there is no notion of server realm name in this case. In all other cases the initiator MUST produce the exported name token for the given targ_name and MUST include it as the pa-value of an pa-data element whose pa-type SHALL be <TBD>, and the initiator MUST set the sname field of the AS-REQ to WELLKNOWN/WELLKNOWN with name type KRB_NT_WELLKNOWN [KRB-ANON] -- the presence of the targ_name pa-data element means the targ_name is not of type GSS_C_NT_ANONYMOUS.

The token type ID for this token SHALL be 05 00.

The pa-data element for the initiator's name assertion SHALL be of ad-type <TBD> and its ad-value SHALL consist of the BER encoding of the following:

			InitiatorNameAssertion ::= CHOICE {
			    -- -1 -> certificate DN
			    -- 0..16384 -> subjectAltName named by
			    --             this index
			    sanIndex INTEGER (-1..16384),
			    nameNotInCert GeneralName -- from RFC3280
			}

The acceptor MUST validate this token as a Kerberos V KDC would validate a PKINIT AS-REQ, and also MUST check that the initiator's asserted name is present in the initiator's certificate or otherwise bound to the initiator's certificate by out-of-band negotiation (e.g., by a table lookup). Failure to validate this token MUST cause GSS_Accept_sec_context() to return an error and, optionally, to output a KRB-ERROR context token as per-RFC4121.

5.2.  Context token derived from KRB_AS_REP

When the initiator's initial context token is a KRB_AS_REQ then the acceptor MUST reply with either a KRB-ERROR token as per [RFC4121] (Zhu, L., Jaganathan, K., and S. Hartman, “The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2,” July 2005.) or a token derived from a KRB_AS_REP PDU [RFC4120] (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.) constructed to respond to the initiator's KRB_AS_REQ.

The acceptor MUST only accept PKINIT pre-authentication. If the initiator's KRB_AS_REQ token is valid, and the initiator's asserted name is bound to the certificate (see Section 5.1 (Context token derived from KRB_AS_REQ) and Section 4 (PKU2U Principal Naming)) then the acceptor MUST select a certificate (if it has more than one) which matches the initiator's targ_name (or any certificate at all, if the initiator's targ_name is the anonymous name), and then the acceptor MUST construct a KRB_AS_REP as usual for PKINIT, except that there is no requirement to use a KDC certificate.

The initiator then validates this token according to Section 3.1.5 of [RFC4120] (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.) and Section 3.2.4 of [RFC4556] (Zhu, L. and B. Tung, “Public Key Cryptography for Initial Authentication in Kerberos (PKINIT),” June 2006.). The inclusion of the EKU KeyPurposeId [RFC3280] (Housley, R., Polk, W., Ford, W., and D. Solo, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” April 2002.) id-pkinit-KPKdc in the X.509 certificate in the response is not applicable when PKU2U is used because there is no KDC involved in this protocol. The initiator MUST verify that the acceptor's certificate matches the targ_name passed in to GSS_Init_sec_context().

5.3.  Context tokens imported from RFC4121

Once the initiator has a Kerberos V Ticket for the acceptor the security context token exchange will continue with those of the Kerberos V GSS-API mechanism [RFC4121] (Zhu, L., Jaganathan, K., and S. Hartman, “The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2,” July 2005.) with the following modifications:

• the mechanism OID of PKU2U SHALL be used instead of that of the Kerberos V GSS-API mechanism;
• a sub-session key MUST be included in the initiator's Authenticator;
• if the initiator's initial context token is a KRB_AS_REQ token (i.e., not KRB_AP_REQ token), then the Exts field in the Authenticator of the KRB_AP_REQ-derived token [GSS‑EXTS] (Emery, S., “Kerberos Version 5 GSS-API Channel Binding Hash Agility,” 2007.) MUST contain an extension of the type GSS_EXTS_FINISHED (extension type ID <TBA>) and the extension data contains a DER-encoded value of the Kerberos V 'Checksum' type, where the checksum (actually, a MAC, see [RFC3961] (Raeburn, K., “Encryption and Checksum Specifications for Kerberos 5,” February 2005.)) is taken over all the preceding context tokens in this exchange (including the InitialContextToken header), concatenated in chronological order (remember, GSS-API context token exchanges are synchronous). The key usage number for this checksum is KEY_USAGE_FINISHED (number 41).

The acceptor MUST process this token as usual for RFC4121, except that if the context token exchange included an AS eschange, then the acceptor MUST also validate the GSS_EXTS_FINISHED and return an error if it is not valid or not present. But if this is the initial context token then the acceptor MUST return an error if GSS_EXTS_FINISHED is present.

The GSS_EXTS_FINISHED (along with the ticket) binds the second part of the context token exchange to the first, and it binds the pa-data used in the request as well (this needs to be done because PKINIT does not bind pa-data other than PKINIT pa-data from the request). GSS_EXTS_FINISHED also protects all otherwise unauthenticated plaintext in Kerberos V PDUs. Note that GSS_EXTS_FINISHED also protects the mechanism OID in the InitialContextToken header.

			KEY_USAGE_FINISHED            41

6.  Guidelines for Credentials Selection

If a peer, either the initiator or the acceptor, has multiple pairs of public-key private keys, a choice is to be made in choosing the best fit. The trustedCertifiers field in the PA-PK-AS-REQ structure [RFC4556] (Zhu, L. and B. Tung, “Public Key Cryptography for Initial Authentication in Kerberos (PKINIT),” June 2006.) SHOULD be filled by the initiator, to provide hints for guiding the selection of an appropriate certificate chain by the acceptor.

If the initiator's X.509 certificate cannot be validated according to [RFC3280] (Housley, R., Polk, W., Ford, W., and D. Solo, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” April 2002.), the acceptor SHOULD send back the TD-TRUSTED-CERTIFIERS structure [RFC4556] (Zhu, L. and B. Tung, “Public Key Cryptography for Initial Authentication in Kerberos (PKINIT),” June 2006.) that provides hints for guiding the selection of an appropriate certificate by the initiator. In this case GSS_Accept_sec_context() returns GSS_S_CONTINUE_NEEDED, and the initiator gets to try again in its subsequent AS-REQ token.

The GSS-API does not provide a way to make this credential selection interactive, however, whenever the context allows for direct interaction of the mechanism with the user then it is RECOMMENDED that implementations do so.

If the certificates cannot be selected interactively, and multiple certificates can be used, it is RECOMMENDED that initiators fail the context establishment thus avoid confusions caused by an unexpected programmatic selection. Users should be able to retry using a specific credential (this requires that distinct credentials have distinct names that can be used to acquire each credential separately).

7.  Security Considerations

The security considerations in [RFC4556] (Zhu, L. and B. Tung, “Public Key Cryptography for Initial Authentication in Kerberos (PKINIT),” June 2006.) apply here. This mechanism relaxes some requirements of PKINIT and adds a device for protecting otherwise unauthenticated plaintext in the protocol -- it is crucial that this device be faithfully implemented. It is also crucial that both the initiator and the acceptor MUST be able to verify the binding between the signing key and the associated identity.

8.  Acknowledgements

The authors would like to thank Jeffrey Hutzelman for his insightful comments on the earlier revisions of this document.

In addition, the following individuals have provided review comments for this document: Nicolas Williams, Sam Hartman, Leif Johansson, Olga Kornievskaia, Martin Rex, and Sunil Gottumukkala.

Ari Medvinsky provided help in editing the initial revisions of this document.

The text for the DN mapping is compiled directly from the email discussions among the following individuals: Howard Chu, Martin Rex, Nicolas Williams, Jeffrey Hutzelman, Kevin Coffman, Henry B. Hotz, Leif Johansson, and Olga Kornievskaia. Howard and Jeffery clearly illustrated the challenges in creating a unique mapping, while Nicolas and Martin demonstrated the relevance and interactions to GSS-API and Kerberos.

9.  IANA Considerations

Section 3 (The PKU2U Realm Name) defines the PKU2U realm. The IANA registry for the reserved names should be updated to reference this document.

This document defines GSS_EXTS_FINISHED extension type. The corresponding IANA registry need to be updated to reference this document. The following single registration should be added in the registry for "Kerberos V GSS-API mechanism extension types": GSS_EXTS_FINISHED, "GSS-API token checksum", "Extension to provide a checksum for GSS-API tokens", the RFC # of this document.

This document also expands the existing GSS-API service name registry, that IANA maintains, to have a an assignment of one or more OIDs for each service name. The id-kp-serverAuth OID [RFC3280] (Housley, R., Polk, W., Ford, W., and D. Solo, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” April 2002.) is immediately registered as the OID for the "HTTP" service name.

10. Normative References

Authors' Addresses

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