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Versions: 00 01 02 03

Network Working Group                                        R. Van Rein
Internet-Draft                                                 ARPA2.net
Intended status: Standards Track                            May 25, 2019
Expires: November 26, 2019

                 Realm Crossover for SASL via Diameter


   SASL is used for authentication in many application protocols.  This
   specification extends it to allow credentials from a home realm to be
   used against external services.  To this end, it introduces a secure
   end-to-end wrapper for SASL traffic and a link back from to the home
   realm based on Diameter.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on November 26, 2019.

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   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  The SASL crossover mechanism SXOVER . . . . . . . . . . . . .   3
     2.1.  SXOVER initial response . . . . . . . . . . . . . . . . .   4
     2.2.  SXOVER initial challenge  . . . . . . . . . . . . . . . .   5
     2.3.  SXOVER responses  . . . . . . . . . . . . . . . . . . . .   6
     2.4.  SXOVER challenges . . . . . . . . . . . . . . . . . . . .   6
   3.  Embedding SASL in Diameter  . . . . . . . . . . . . . . . . .   7
     3.1.  AVP Definitions for SASL  . . . . . . . . . . . . . . . .   7
       3.1.1.  SASL-Mechanism  . . . . . . . . . . . . . . . . . . .   7
       3.1.2.  SASL-Token  . . . . . . . . . . . . . . . . . . . . .   8
       3.1.3.  SASL-Channel-Binding  . . . . . . . . . . . . . . . .   8
   4.  Running Diameter as a SASL Backend  . . . . . . . . . . . . .   8
     4.1.  Diameter is an SCTP service . . . . . . . . . . . . . . .   9
     4.2.  Reliance on DANE and DNSSEC . . . . . . . . . . . . . . .   9
     4.3.  Foreign Service SASL Mechanisms . . . . . . . . . . . . .  10
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   It is common for Internet users to combine services from a varierity
   of providers.  Along with this, an ad hoc practice has arisen of
   using the local identity schemes of these providers.  These are not
   integrated, and the practice tends to reduce the control of users
   over their online identity.  A solution to this is generic support
   for realm crossover, where an externally acquired service can make a
   callback to a home realm to authenticate a user's identity and use
   that for service-specific authorisation.

   SASL [RFC4422] is instrumental in authentication across a wide range
   of application protocols; it allows those protocols to abstract from
   the actual authentication mechanisms, and at the same time it allows
   authentication mechanisms to not be concerned about the application
   protocol.  SASL can easily be funneled from one protocol into
   another, modulo a number of security concerns.

   Diameter and its Network Access application are instrumental in
   authenticating a user under a realm, while not providing the
   resources that an application protocol would imply.  Moreover,
   Diameter service can be declared under a domain name in a manner that
   is standardised, scalable and secure.

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   This allows a foreign server to authenticate a client to authenticate
   with its home realm:

      +--------+    SASL     +--------+    SASL    +---------+
      | Client |-----------> | Server | ---------> |  Realm  |
      +--------+  AppProto   +--------+  Diameter  +---------+
          ||                     ||                    ||
   john@example.com        find SRV, TLSA          example.com
     & credential            relay SASL           authentication

   Diameter can send a mere notification of authentication, and the
   foreign server can use DANE [RFC6698] to validate the origin of these
   notification.  Diameter in the foreign server will authenticate to
   the home realm, which may then decide to add resources beyond the
   basic notification of authentication.

   SASL mechanisms are not generally protected against attacks by men in
   the middle named Eve.  This is usually compensated for by wrapping
   the application protocol in TLS, but since that would only protect
   one leg of the intended realm crossover, this raises a need for end-
   to-end encryption.  This can be established along with other
   credentials for the home realm, but an end-to-end mechanism needs to
   be defined.  This specification introduces a wrapper for that pupose,
   and nests a SASL exchange with the home realm under its cloak.

   Finally, to avoid the use of one authentication exchange to validate
   another, it is advisable to incorporate channel binding [RFC5056]
   [RFC5801] when making use of backends.  When passing SASL tokens
   between application protocol and Diameter backend, the channel
   binding information from the application protocol would be supplied
   as a side-note to the Diameter backcall.

2.  The SASL crossover mechanism SXOVER

   SXOVER is a SASL authentication mechanism that encrypts all
   information between a SASL client and SASL server, except for the
   realm name to which they direct the authentication.  The realm can be
   used by an foreign server to redirect SXOVER to a home realm, for
   instance using Diameter.  SXOVER does not reveal success or failure
   to this foreign server, but Diameter would release this information
   in a manner that requires no knowledge of the SASL exchange.

   The first SXOVER message supplies a session key to the SASL server.
   The server responds with a list of SASL mechanisms to be used under
   the cloak of the session key.  Then, the client selects a mechanism
   and the customary exchange follows, but under protection of the
   session key.

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   Certain information in SXOVER is encrypted with the encrypt operation
   [Section 3 of [RFC3961]] with default initial state; this always
   includes integrity protection.  The value for key usage is
   KIP_KEYUSAGE_MAPPING or 1864 for the long-term key and
   KIP_KEYUSAGE_USERDATA of 1863 for the session key; these values and
   the formats exchanged are compatible with the Keyful Identity
   Protocol, which may or may not develop independently of SXOVER.

2.1.  SXOVER initial response

   The SXOVER exchange starts with an initial response message,
   traveling along the selection of SXOVER as the SASL mechanism.  This
   initial response contains the following:

   SXOVER-Initial-Response ::= SEQUENCE {
      realm    IA5String,     -- Lowercase domain name, no trailing dot
      inictr   Counter,       -- Initial counter value
      keyno    KeyNumber,     -- With realm and encalg, identifies...
      encalg   EncryptAlg,    -- ...the key for svckeymud decryption
      seskey   OCTET STRING   -- RFC 3961 encrypted, key usage 1864

   Counter ::= INTEGER (0..4294967295)             -- Unsigned 32-bit
   KeyNumber ::= INTEGER (0..4294967295)           -- Unsigned 32-bit
   EncryptAlg ::= INTEGER (-2147483648..2147483647)  -- Signed 32-bit

   The one value of interest to the foreign server is the realm, which
   it needs to determine the home realm of the client.  It finds the
   Diameter service underneath, and starts passing this SASL message and
   any that follow between the end points.  Note that this behaviour is
   specific to the SXOVER mechanism; it is just as well possible for a
   foreign server to welcome ANONYMOUS clients, which it can handle

   The inictr value is used as a bit of entropy, thus making it more
   difficult to mount replay attacks, though not at the level of
   security that a proper SASL mechanism can achieve through dynamic
   challenges and/or channel binding.

   The keyno and encalg values present identification information for a
   key at the Diameter/SASL server, and the seskey is a representation
   of a session key suitable for decryption with that identified key.

   The value of the seskey MAY be locally determined, but by default it
   SHOULD be a random seed that can serve as input to the random-to-key
   function with the required key-generation seed-length [Section 3 of
   [RFC3961]] for a session key with the same encryption algorithm
   enctype as the identified key.  The random seed is protected by

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   encryption to the identified key using the Encrypt function
   [Section 3 of [RFC3961]], which always involves authenticity.  The
   key usage number is shared from the independent KIP protocol, and is
   set to KIP_KEYUSAGE_MAPPING or 1864.

2.2.  SXOVER initial challenge

   The initial SXOVER challenge is a server's response in which it
   presents the choice of mechanism names to use under the cloak of
   SXOVER.  It does not present any other information.  The following
   information is sent as one block under protection of the seskey:

SXOVER-Initial-Challenge ::= SEQUENCE {
   ctr           Counter,                 -- Counter value is inictr+1
   realm         IA5String,               -- Confirms the realm securely
   mechlist      SEQUENCE OF IA5String,   -- Available SASL mechanisms
   chanbindmth   IA5String,               -- TODO:REALLY?
                                          -- Method of channel binding
   chanbindval   OCTET STRING             -- TODO:REALLY?
                                          -- Value for channel binding

   This message precedes the SXOVER wrapping of a SASL exchange by first
   passing the inner SASL mechanisms, which cannot be taken from the
   list provided by the foreign server.  The mechanisms listed here are
   specific for the home realm of the client, information that could not
   be known to the foreign server before learning about the targeted

   The ctr value is simply inictr incremented by 1, with a wrap-around
   to stay within an unsigned 32-bit range.  It MUST be validated by the
   SASL client.

   The realm is repeated from the SXOVER request, but this time it is
   protected by the session key.  Therefore, the SASL client MUST
   validate it before starting the wrapped SASL exchange.

   The mechlist informs the SASL client of the mechanisms available for
   authentication against the SASL server.  These can be used for the
   wrapped SASL exchange.  The list is not related to any mechanism list
   that the foreign server will have sent before.  Specifically, SXOVER
   and ANONYMOUS mechanisms should not occur in the wrapped mechlist.

   TODO: The chanbindmth and chanbindval relay what channel binding
   information the SASL server obtained, and which can be used by the
   SASL client if it wants to continue.  These values MUST be verified
   against the actual context first, to ensure that they do represent
   security requirements.  If a mechanism is used that does not include

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   channel binding, then these fields can add some channel binding, but
   it is invariably better to employ channel binding that is
   cryptographically bound to the authentication operation.

2.3.  SXOVER responses

   Further SXOVER responses are essentially SASL responses and initial
   responses, encrypted under the seskey, but there is one exception;
   the first response must select a SASL mechanism.  There is a separate
   provision for sending no data, distinguishable from empty data, if
   this is desired by the SASL mechanism:

   SXOVER-Response ::= SEQUENCE {
      ctr       Counter,             -- 1 + Previous ctr value
      c2s       SaslToken,           -- NULL or token, client to server
      mechsel   IA5String OPTIONAL   -- SASL mechanism name selection

   SaslToken ::= CHOICE {
      token     OCTET STRING,
      no-token  NULL

   This is the first and later of the wrapped SASL messages sent from
   the client to the server.  When it is the first, the mechsel field
   MUST specify the SASL mechanism that the client selected from the
   mechsel issued before.  In any message, the ctr value is one more
   than the value in the previous SASL message, reduced to an unsigned
   32-bit range.

   The SaslToken in c2s is either a literal OCTET STRING with the SASL
   token to pass, or it is NULL if no token is passed.  This implements
   a distinction between an empty token and no token, as required for

2.4.  SXOVER challenges

   Further SXOVER challenges are essentially SASL challenges and initial
   challenges, encrypted under the seskey.  There is a separate
   provision for sending no data, distinguishable from empty data, if
   this is desired by the SASL mechanism:

   SXOVER-Challenge ::= SEQUENCE {
      ctr     Counter,               -- 1 + Previous ctr value
      s2c     SaslToken,             -- NULL or token, server to client
      extra   OCTET STRING OPTIONAL  -- On success, optional extra token

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   This is the first and later of the wrapped SASL messages sent from
   the server to the client.  In any message, the ctr value is one more
   than the value in the previous SASL message, reduced to an unsigned
   32-bit range.

   The SaslToken in s2c is either a literal OCTET STRING with the SASL
   token to pass, or it is NULL if no token is passed.  This implements
   a distinction between an empty token and no token, as required for

   The extra value can be passed along as a hint to the user for a
   successful authentication.  Mechanisms do not commonly use the field,
   but SASL offers it.  The distinction between an empty OCTET STRING
   and an absent value is assured through the OPTIONAL modifier.  Note
   that this value should not be passed as part of the SXOVER exchange,
   as it is part of the SASL mechanism that was selected with mechsel in
   the wrapped exchange.  SXOVER does not specify an extra value, so the
   field in the outer SASL exchange that runs SXOVER will not be used.

3.  Embedding SASL in Diameter

   SASL messages in Diameter use a number of AVPs [RFC6733] that are
   defined for this purposes.  They occur in those combinations that are
   defined for SASL.

   SASL over Diameter can only be used to relay the SXOVER mechanism to
   a home realm.  This means that no negotiation of mechanisms is needed
   at the Diameter level; this is handled under the SXOVER cloak.  The
   same holds for any negotation of channel binding; it is part of the
   cloacked SASL exchange.

3.1.  AVP Definitions for SASL

   These AVPs are added to the set that is used with the Network Access
   application, and can therefore be used in AA-Request and AA-Answer
   messages.  On top of that, the SASL-Mechanisms AVP may also occur in
   a Capabilities Exchange Answer.  The User-Name AVP MUST be supplied
   in the AA-Answer to inform the server about the user name that the
   backend decided on; the server MAY send a hint requesting a value in
   the User-Name AVP in the AA-Request.

3.1.1.  SASL-Mechanism

   The SASL-Mechanism AVP has AVP Code TBD0.  This specification only
   uses the mechanism name SXOVER as a value for this AVP.  It MUST be
   included in the first message of an SXOVER exchange sent to the home
   realm, and it SHOULD be verified by the home realm upon reception.

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   Its purpose is mostly to distinguish this specification from
   potential future specifications to encapsulate SASL in Diameter.

   Though not used in this specification, this AVP may also be supplied
   from the home realm to the Diameter client to hold a space-separated
   list of SASL mechanisms.

3.1.2.  SASL-Token

   The SASL-Token AVP has AVP Code TBD1.  Note that SASL requires
   distinction between empty and absent tokens; absent SASL tokens are
   represented by absence of the SASL-Token AVP and empty SASL tokens
   are represented as a present SASL-Token AVP with zero content bytes.

3.1.3.  SASL-Channel-Binding

   The SASL-Channel-Binding AVP has AVP Code TBD2.  It SHOULD appear
   along the first SASL-Token AVP for a Network Access session.  The AVP
   may occur more than once, to indicate support of multiple forms of
   channel binding.

   When the client connects to the foreign service over TLS, the tls-
   unique form [RFC5929] of channel binding is RECOMMENDED.  Specific
   foreign servers may however be exempted by the home realm.

   The contents of this AVP are:

   name  is the standard name of the channel binding information,
         followed by a zero-valued byte.

   value contains the bytes of the channel binding information.

   Normally, channel binding information should be sourced from the
   underlying communications channel, but this information is not
   available to backend running Diameter.  To enable channel binding
   between the end points, the foreign server incorporates the channel
   binding information that the client can use in its connection to the
   foreign server.  This is useful to mitigate replay attacks, which is
   why its use is RECOMMENDED.  Channel binding provides better
   guarantees than the simple inictr/ctr mechanism used in SXOVER.

4.  Running Diameter as a SASL Backend

   Following are a few practical considerations in relation to the
   Diameter connectivity for SASL.

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4.1.  Diameter is an SCTP service

   Diameter is primarily an SCTP-based protocol [RFC6733], for reasons
   of scalabaility and efficiency.  SASL Diameter benefits from these
   properties and embraces the SCTP carrier.  Operating system support
   for SCTP is wide-spread, but parts of network infrastructure may not
   support it, and that may cause implementations to add a fallback to
   more traditional protocols.  Standards offer two options for doing

   Diameter can fallback to run over TCP, which is mostly of use to
   client-only machines, but it sacrifices several benefits of the SCTP
   carrier.  Since the SASL Diameter embedding typically involves no
   client systems, this option is NOT RECOMMENDED.

   SCTP may be run over a UDP transport using port 9899 [RFC6951], which
   does not sacrifice much; it only inserts a UDP header before each
   message.  This is a reasonable expectation of foreign servers as well
   as home realms, so this additional option is RECOMMENDED for
   situations where a fallback for plain SCTP is desired.  It is
   standardised as a socket option SCTP_REMOTE_UDP_ENCAPS_PORT, and only
   involves a small repetition in code, with a minor change between the

4.2.  Reliance on DANE and DNSSEC

   Diameter always involves the use of TLS, but there is a number of
   choices concerning the validation of connections through DNSSEC and
   DANE.  It is the home realm's prerogative what level of protection it
   upholds for its client identities, but any foreign server can choose
   to raise the bar by setting a minimum standard.

   DNSSEC is a useful protection mechanism for the _diameter._sctp SRV
   records that lead to the Diameter host and its port for the home
   realm.  This does not protect against forged IP addresses, port
   mappings or routing.  To protect against this as well, a TLSA record
   for the service host and port, along with the _sctp protocol label,
   should be used as specified for DANE [RFC6698].

   Home realms that choose to be light on such measures risk that
   identities are forged, in spite of their use of TLS.  Foreign servers
   MAY choose to reject such home realms, or alternatively be more
   inquisitive about the certificates used.

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4.3.  Foreign Service SASL Mechanisms

   A foreign server MUST offer SXOVER if it wants to support realm
   crossover via Diameter as specified herein.  In addition, it MAY
   offer SASL mechanisms that it resolves locally.

   The ANONYMOUS method for SASL [RFC4505] may be offered for guest
   access.  The PLAIN method [RFC4616] continues to be ill-advised,
   especially with modern methods such as SCRAM [RFC5802] to address the
   needs of local accounts with password validation.

   The HTTP protocol does not yet support SASL, and it is not optimal
   from a security viewpoint to integrate credentials in the dynamic
   environment of HTML, where dynamic content from potentially
   undesirable origins come together in a manner not controllable to the
   end user.  One remedy is to use HTTP and its authentication methods
   that match with SASL, such as SCRAM for HTTP [RFC7804].  Another
   remedy is to switch to generic SASL embedding in HTTP
   [TODO:REF:draft-vanrein-httpauth-sasl] and gain replay protection
   through channel binding.

   Many application protocols offer richer semantics than HTTP, making
   them better targets for automation.  Their reliance on SASL has made
   them less tractable as a service to third parties.  One reason for
   introducing SXOVER is in the hope to make it possible to have those
   semantically rich applications as a third-party offering.

5.  Security Considerations

   From the perspective of the client and the home realm, the safety of
   the SASL credentials is paramount.  Since not all SASL mechanisms are
   safe from inspection by the foreign server, and since TLS cannot help
   there either, there is a need for some caution.

   The limitation of the Diameter carrier for SASL to SXOVER reduces
   this risk, by only authenticating SASL mechanisms under end-to-end
   encryption between the client and home realm.  It is generally
   understood that clients must not send unprotected SASL authentication
   attempts to arbitrary parties, but SXOVER adds a facility that is
   safe for clients to use in this manner.  The SXOVER mechanism could
   even be used without TLS protection.

   From the perspective of the foreign server, the security concern is
   to be certain of an identity.  The home realm sends this information
   back when SXOVER authentication succeeds, and the communication doing
   so is protected with TLS.  The certificate of the Diameter server can
   be validated, and for cautious home realms there could be an
   additional check based on DANE.

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6.  IANA Considerations

   This specification defines three AVP Codes for use with Diameter.
   IANA registers the following AVP Codes for them in the
   "Authentication, Authorization, and Accounting (AAA) Parameters"

   AVP Code | Attribute Name       | Reference
   TBD0     | SASL-Mechanism       | (this spec)
   TBD1     | SASL-Token           | (this spec)
   TBD2     | SASL-Channel-Binding | (this spec)

7.  References

7.1.  Normative References

   [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for
              Kerberos 5", RFC 3961, DOI 10.17487/RFC3961, February
              2005, <https://www.rfc-editor.org/info/rfc3961>.

   [RFC4422]  Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
              Authentication and Security Layer (SASL)", RFC 4422,
              DOI 10.17487/RFC4422, June 2006,

   [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
              Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,

   [RFC5801]  Josefsson, S. and N. Williams, "Using Generic Security
              Service Application Program Interface (GSS-API) Mechanisms
              in Simple Authentication and Security Layer (SASL): The
              GS2 Mechanism Family", RFC 5801, DOI 10.17487/RFC5801,
              July 2010, <https://www.rfc-editor.org/info/rfc5801>.

   [RFC5929]  Altman, J., Williams, N., and L. Zhu, "Channel Bindings
              for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <https://www.rfc-editor.org/info/rfc6698>.

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   [RFC6733]  Fajardo, V., Ed., Arkko, J., Loughney, J., and G. Zorn,
              Ed., "Diameter Base Protocol", RFC 6733,
              DOI 10.17487/RFC6733, October 2012,

   [RFC6951]  Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
              Control Transmission Protocol (SCTP) Packets for End-Host
              to End-Host Communication", RFC 6951,
              DOI 10.17487/RFC6951, May 2013,

7.2.  Informative References

   [RFC4505]  Zeilenga, K., "Anonymous Simple Authentication and
              Security Layer (SASL) Mechanism", RFC 4505,
              DOI 10.17487/RFC4505, June 2006,

   [RFC4616]  Zeilenga, K., Ed., "The PLAIN Simple Authentication and
              Security Layer (SASL) Mechanism", RFC 4616,
              DOI 10.17487/RFC4616, August 2006,

   [RFC5802]  Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
              "Salted Challenge Response Authentication Mechanism
              (SCRAM) SASL and GSS-API Mechanisms", RFC 5802,
              DOI 10.17487/RFC5802, July 2010,

   [RFC7804]  Melnikov, A., "Salted Challenge Response HTTP
              Authentication Mechanism", RFC 7804, DOI 10.17487/RFC7804,
              March 2016, <https://www.rfc-editor.org/info/rfc7804>.

Appendix A.  Acknowledgements

   Thanks go to TODO for useful discussions during the creation of this

Author's Address

   Rick van Rein
   Haarlebrink 5
   Enschede, Overijssel  7544 WP
   The Netherlands

   Email: rick@openfortress.nl

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