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Realm Crossover for SASL and GSS-API via Diameter

Document Type Active Internet-Draft (individual)
Author Rick van Rein
Last updated 2022-10-14
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Network Working Group                                        R. Van Rein
Internet-Draft                                           OpenFortress BV
Intended status: Informational                           14 October 2022
Expires: 17 April 2023

           Realm Crossover for SASL and GSS-API via Diameter


   SASL and GSS-API are used for authentication in many application
   protocols.  This specification extends them to allow credentials of
   an identity domain to be used against external services.  To this
   end, it introduces end-to-end encryption for SASL that is safe to
   relay through a foreign server.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 17 April 2023.

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   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Provisions Relating to IETF Documents (
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Messages of SXOVER-PLUS . . . . . . . . . . . . . . . . . . .   4
     2.1.  Preparation for Messaging . . . . . . . . . . . . . . . .   4
     2.2.  Initial Client-to-Server Message  . . . . . . . . . . . .   5
     2.3.  Initial Server-to-Client Message  . . . . . . . . . . . .   6
     2.4.  Continued Client-to-Server Messages . . . . . . . . . . .   6
     2.5.  Continued Server-to-Client Messages . . . . . . . . . . .   7
     2.6.  Using SXOVER-PLUS with GSS-API  . . . . . . . . . . . . .   8
     2.7.  Application Key Derivation  . . . . . . . . . . . . . . .   8
   3.  AVP Definitions for SASL in Diameter  . . . . . . . . . . . .   9
     3.1.  SASL-Mechanism  . . . . . . . . . . . . . . . . . . . . .  10
     3.2.  SASL-Token  . . . . . . . . . . . . . . . . . . . . . . .  10
     3.3.  SASL-Channel-Binding  . . . . . . . . . . . . . . . . . .  11
     3.4.  Key Groups for Derived Application Keys . . . . . . . . .  11
   4.  Diameter Session Requirements for SASL  . . . . . . . . . . .  12
   5.  Diameter Message Requirements for SXOVER-PLUS . . . . . . . .  13
     5.1.  C2S-Init Requests over Diameter . . . . . . . . . . . . .  13
     5.2.  S2C-Init Responses over Diameter  . . . . . . . . . . . .  14
     5.3.  C2S-Cont Requests over Diameter . . . . . . . . . . . . .  14
     5.4.  S2C-Cont Responses over Diameter  . . . . . . . . . . . .  15
   6.  Running Diameter as a SASL Backend  . . . . . . . . . . . . .  15
     6.1.  Diameter is an SCTP service . . . . . . . . . . . . . . .  16
     6.2.  Reliance on DANE and DNSSEC . . . . . . . . . . . . . . .  16
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .  18
   Appendix A.  Centralised handing of SASL over Diameter  . . . . .  20
   Appendix B.  Support Levels for Realm Crossover . . . . . . . . .  24
   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . .  24
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  24

1.  Introduction

   It is common for Internet users to work with services from a
   varierity of providers.  An ad hoc practice has arisen of using local
   identity schemes for each of these providers.  There is no
   integration of identity systems, and the practice reduces the control
   of users over their online identity.  A solution to this is support
   for Realm Crossover
   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] and GSS-API [RFC2743] together is instrumental in
   authentication across a wide range of application protocols; it
   allows these protocols to abstract from the actual authentication

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   mechanisms, and at the same time it allows authentication mechanisms
   to not be concerned with the application protocol.  SASL can easily
   be funneled from one protocol into another, modulo a number of
   security concerns.

   Diameter and its Network Access Server application are instrumental
   in authenticating a user under a realm, while not handing over any
   resources like an application protocol would.  Furthermore, Diameter
   integrates with realm-crossing security; service can be declared
   under a domain name in a manner that is standardised, scalable and

   This can be used by a foreign server to authenticate a client by call
   the client's own domain as an authentication backend:

      +--------+    SASL     +--------+    SASL    +---------+
      | Proto  |-----------> | Foreign| ---------> | Identity|
      | Client |-----------> | Server | ---------> |  Domain |
      +--------+  AppProto   +--------+  Diameter  +---------+
          ||                     ||                    ||        find SRV, TLSA
     & credential           & relay SASL          authentication

                  Realm Crossover authentication:

            Client John authenticates to his own Domain
                   while using a foreign Server.

   The Diameter server in the domain needs to respond success or failure
   on the SASL exchange forwarded to it.  It delivers a User-Name on
   success, but not its domain.  The client domain is validated by the
   foreign server, using DANE [RFC6698].  The User-Name combines with
   the validated domain to form the client identity for use in the
   foreign server.  The domain server also validates the foreign server,
   and MAY use this for access control, and perhaps to decide on the
   release of additional AVPs.

   The client needs to assure that the authentication exchange cannot be
   relayed anywhere but to the Diameter service in his realm.  This can
   be assured with channel binding [RFC5056] [RFC5801]; the foreign
   server detects this information and relays it to the Diameter
   service.  Normally, protocol servers should not accept externally
   dictated channel binding information; the reason why it is safe to
   make an exception for Diameter is that it provides no resources,
   making it an unattractive attack target.

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   SASL tokens are not generally safe to pass over plaintext channels.
   This is usually addressed by wrapping the application protocol in
   TLS, but since that would only protect one leg of the intended realm-
   crossing authentication exchange, there is a need for end-to-end

   This specification describes a SASL mechanism named SXOVER-PLUS as an
   end-to-end encrypted tunnel around another SASL exchange.  It also
   defines how SASL can be embedded in a Diameter authentication
   exchange, which may be useful with SXOVER-PLUS or with any other SASL

   Realm Crossover for SASL is part of a series of protocol
   enhancements, as overviewed in TODO:xref target="draft-vanrein-
   internetwide-realm-crossover".  Among the potential use cases are a
   global identity scheme for general communication and group
   participation, establishment of encryption keys, all with identity
   control under individually owned domains.

2.  Messages of SXOVER-PLUS

   SXOVER-PLUS consists of a few messages that derive an encryption
   secret and then continue using it as an end-to-end encrypted tunnel
   around a standard SASL authentication exchange.  SXOVER continues to
   be active as long as the tunneled exchange does.

2.1.  Preparation for Messaging

   Before SXOVER-PLUS starts, the user uses an out-of-band protocol to
   submit a long-term key to his domain and receives back a suitable
   keyno and encalg in the style of Kerberos [RFC4120] along with a
   "keymap" blob that contains the originally submitted long-term key in
   encrypted form.  This process may be run at any time desired by the
   client, like when a program first uses the SXOVER-PLUS mechanism;
   keys may be kept for the remainder of the program run, even if this
   lasts for weeks and crosses between security realms, as a pre-
   validated key for protected contact with their realm; but at any time
   desired, the client may drop the long-term key, for example when a
   user desktop session is suspended or terminated.

   By offering the SXOVER-PLUS mechanism for SASL, a foreign server
   announces its willingness to validate the client's domain. relay SASL
   messages to it, trust its authentication conclusion and User-Name and
   treat that as a user identity of the client's domain.

   Offering SXOVER-PLUS does not preclude the offering of other SASL
   mechanisms; for instance, ANONYMOUS may be a useful option to also
   offer guest access to clients.

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2.2.  Initial Client-to-Server Message

   SXOVER-PLUS is a client-first mechanism.  The first SASL Token starts
   with "p=CHANBIND,,DOMAIN," where CHANBIND is the channel binding name
   and DOMAIN is either the fully qualified domain name of the client,
   or an e.164 Application Unique String [Section 3.1 of [RFC6116]].
   This notation is compatible with the GS2 bridge [RFC5801].

   When the client connects to the foreign service over TLS, the tls-
   exporter form [RFC9266] of channel binding is RECOMMENDED for
   protocols or their implementation that encapsulate an entire SASL
   exchange in one TLS connection.  For protocols that spread the SASL
   exchange over multiple connections it is RECOMMENDED to support both
   tls-exporter and tls-server-end-point [RFC5929].  Special
   considerations may apply as a result of software configuration per
   home realm.

   Following this is DER-encoded information for the following ASN.1

      clirnd   OCTET STRING,   -- Entropy to allow client variety
      keyno    KeyNumber,      -- Given realm and encalg, identify...
      encalg   EncryptAlg,     -- ...the key for keymap decryption...
      keymap   OCTET STRING    -- ...yielding server-acceptable data

   EncryptAlg ::= Int32
   KeyNumber  ::= UInt32

   The clirnd is a salt that should hold enough entropy to satisfy the
   client's cryptographic requirements.  The other fields result from
   the setup of the long-term key preceding SXOVER-PLUS.

   Upon reception, the server locates a key for the keyno and encalg in
   the key store for DOMAIN and uses it to decrypt keymap into entropy
   that serves as input to the random-to-key function [RFC3961], where
   the length of the decrypted keymap must match the key-generation

   The same key is constructed with random-to-key on both ends; the
   client uses the key that it originally submitted to the server.  The
   result is now on both ends, and known as key K0.

   Both ends pass K0 into the PRF+() function [Section 5.1 of [RFC6113]]
   with the entire message (the GS2 header followed by C2S-Init, which
   includes the clirnd entropy field) to produce properly sized input to
   the random-to-key function.  The result is known as key K1.  Note how

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   this is similar to the KRB-FX-C2 procedure [Section 5.1 of [RFC6113]]
   except that it is applied to a single key.  (Considering slight
   generalisation of the procedure to a list of key/pepper pairs that
   are composed with associative/commutative XOR operators.)

2.3.  Initial Server-to-Client Message

   After the client-first SASL Token, the server sends its first
   challenge.  It is encoded with DER and encrypted by K1, and contains
   the following ASN.1 structure:

      srvrnd     OCTET STRING,   -- Entropy to allow server variety
      mechlist   IA5String       -- Available SASL mechanisms

   The srvrnd is a salt that should hold enough entropy to satisfy the
   server's cryptographic requirements.  Note that the mechlist and DER
   tagging add no entropy.

   The mechlist starts the SASL exchange inside the end-to-end encrypted
   tunnel.  If this inner list uses channel binding at all, it should
   replicate the channel binding choices from the outer layer.

   The key K1 is passed into the PRF+() function [Section 5.1 of
   [RFC6113]] with the pepper set to the concatenation of the entire
   S2C-Init message and the channel binding value.  This is used to
   produce a last input to the random-to-key function.  The result is
   known as key K2.

   The direct concatenation of S2C-Init with channel binding information
   is secure because of the self-descriptive size of the DER encoding of
   the former.  Also note that there is no risk of cross-polination
   between types of channel binding because the name for the type has
   been hashed into key K1 and is therefore already securely encompassed
   in the key derivation.

2.4.  Continued Client-to-Server Messages

   Further messages from the client to the server hold DER content
   encrypted with key K2, following this ASN.1 format:

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      mechsel   IA5String OPTIONAL,   -- SASL mechanism name selection
      c2s       SaslToken             -- NULL or SASL token passed
                                      -- from client to server

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

   The mechsel indicates the client's choice of a SASL mechanism, and
   MUST be in the first inner SASL message.  It initiates a new
   authentication exchange.  The c2s holds the SASL Token and is sent as
   NULL whenever the mechanism yields no token, which is distinct from
   yielding an empty token.

   The inner SASL exchange may be used to select an authorisation name
   that differs from the authentication name.  This would be subject to
   normal approval by the SASL server, but upon success the
   authorisation name would be revealed in the User-Name over Diameter,
   and the foreign server would not be told about the authentication
   name.  This can facilitate pseudonymity.

2.5.  Continued Server-to-Client Messages

   Further messages from the server to the client hold DER content
   encrypted with key K2, following this ASN.1 format:

      success  BOOLEAN DEFAULT FALSE,  -- When TRUE, s2c is an
                                       -- additional token
      s2c      SaslToken               -- NULL or SASL token from
                                       -- server to client

   The s2c field carries the SASL token if it is provided, even when it
   is empty.  If the token is absent, it carries NULL.

   General reporting of success or failure is done for SXOVER-PLUS.  But
   not all encapsulating protocols support additional data, but the
   success field makes this possible in any case.  Note that this is
   trivially supported in Diameter, by sending a SASL token as part of a
   success message.  Inside the SXOVER-PLUS tunnel it is also possible
   by setting the success flag and supplying the additional data in s2c.

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2.6.  Using SXOVER-PLUS with GSS-API

   SXOVER-PLUS can be used with GSS-API [RFC2743] instead of SASL with
   minor changes, because it is mostly similar to GS2.  This results in
   a GSS-API tunnel wrapped around SASL authentication.

   GSS-API calls [RFC2744] to gss_init_sec_context() and
   gss_accept_sec_context() MUST follow the GS2 data structure for
   channel binding information [Section 5.1 of [RFC5801]].  This means
   that only the application_data field is filled, namely with the
   "p=CHANBIND,," part of the first SASL token from client to server,
   concatenated with the application's channel binding data.  Since such
   data starts with "CHANBIND:" [RFC5056] there is some duplication of
   data, which should be validated.  This outer layer of SXOVER-PLUS
   does not support an authorization identifier; any desire to select an
   identity is to be encapsulated inside the end-to-end encrypted tunnel

   The first message transmitted as GSS-API token does not repeat the
   "p=CHANBIND,," prefix, but the "DOMAIN," and subsequent DER-encoded
   C2S-Init data is retained.  Instead, the standard GSS-API header is
   inserted, adhering to the Mechanism-Independent Token Format
   [Section 3.1 of [RFC2743]] with object identifier to identify SXOVER-PLUS.  When this object
   identifier is supplied to the call GSS_Inquire_SASLname_for_mech
   [Section 10 of [RFC5801]], the output reads "SXOVER-PLUS" (without
   the quotes).

   When the GSS-API data must be relayed to a SASL backend, then the
   "p=CHANBIND,," prefix must be reinserted after removal of the GSS-API
   header.  Realm Crossover for GSS-API works like this; it is rewritten
   to SASL and passed over Diameter in that form.

2.7.  Application Key Derivation

   SXOVER-PLUS adheres to most of the GS2 bridge, but deviates in two
   points.  First, security layers are not considered useful in GS2
   [Section 12 of [RFC5801]] because it assumes a pre-existing secure
   layer to provide this benefit.  With SXOVER-PLUS however, the end-to-
   end connection between a client and their authentication server
   differs from the single-hop connection to the foreign service, and it
   can be beneficial to extract secret key information between the
   client and foreign server.  The second deviation from GS2 is that
   SXOVER-PLUS is defined but SXOVER is not.  For these reasons, GS2-
   was not prefixed to the mechanism name.

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   In general, security layers may be derived from the key K2 by yet
   another pass through the PRF+() function [Section 5.1 of [RFC6113]].
   The pepper for this is application-specific, and the requested length
   of octet-string can also be requested by the application.  Multiple
   keys can be defined, each constructed from K2 and pepper.

   Specifically, when SXOVER-PLUS is used under GSS-API, the following
   32-byte ASCII strings may be used as pepper to derive keys for each
   of the four secure streams supported by GSS-API:

   Pepper as 32 ASCII bytes         | Purpose  | Direction
   SXOVER-PLUS/GSS-API/SIGN-C2S-KEY | signing  | client --> server
   SXOVER-PLUS/GSS-API/SIGN-S2C-KEY | signing  | client <-- server
   SXOVER-PLUS/GSS-API/WRAP-C2S-KEY | wrapping | client --> server
   SXOVER-PLUS/GSS-API/WRAP-S2C-KEY | wrapping | client <-- server

   Definitions for one application do not preclude the generation of
   keys for other applications.  It is however vital to security that
   they all use different pepper that share a SASL-authenticated session
   but distribute keys to different trusted regions within an endpoint.

   The key sharing mechanism from [RFC6734] may be used to distribute
   key material from the Identity Domain to the Foreign Server, based on
   a Key-Type for SXOVER-PLUS and using the pepper as the Key-Name.
   Each of the peppers for the GSS-API use case is packaged in its own
   Key group.

3.  AVP Definitions for SASL in Diameter

   SASL messages in Diameter use a number of AVPs [Section 4 of
   [RFC6733]] that are combined to relay SASL to a Destination-Realm
   that is set to the client's domain name.  The domain name may be
   derived from the client's phone number with the ENUM procedure.

   These AVPs are added to the set that is used with the Network Access
   Server application [RFC7155], and can therefore be used in AA-Request
   and AA-Answer messages.  They are always sent with the Mandatory Flag
   set to 0.  When they are not recognised upon reception, they will be
   silently igored.

   Normally, a successful AA-Answer would provide a User-Name AVP to
   inform the server about a utf8-username NAI without a utf8-realm
   [Section 2.2 of [RFC7542]] under which the client is identified;
   without the User-Name an anonymous session is the only available
   option, possibly leading to reduced service and/or limited data
   retention.  Sending a pseudonym in the User-Name may be an
   intermediate option.  In all cases, the domain under which an

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   authenticated user name is defined can be taken from the Destination-
   Realm handling the Network Access Server session; with the domain
   also written in UTF-8, the parts may be combined in the nai grammar
   [Section 2.2 of [RFC7542].

   The Identity Domain may choose to send back key material as part of a
   successful AA-Answer, using [RFC6113].  Triggers to do this are not
   specified hierein, but possible reasons could be founded on user
   identity or Foreign Server identity.

3.1.  SASL-Mechanism

   The SASL-Mechanism AVP has AVP Code TBD0 and is of type UTF8String,
   further restricted to a list of zero or more SASL mechanism names in
   their standardised notation [Section 3.1 of [RFC4422]] separated by a
   space character U+0020.

   To retrieve a server's list of supported SASL mechanisms, this AVP is
   included in an AA-Request message, containing an empty list of SASL
   mechanism names, so an empty string.  When SASL is supported by the
   server, it responds with the list of currently available SASL

   Clients MAY retrieve the server's supported mechanism list without
   actually attempting authentication in the same session; this can be a
   caching mechanism for a given combination of Destination-Realm,
   Origin-Realm and Origin-Host.  An abort of such a session by the
   server indicates that the cache entry has expired, and should be
   retrieved anew for a following attempt.

   To relay a client's choice of SASL mechanism, this AVP is included in
   an AA-Request message, containing a single SASL mechanism name.  This
   MAY be done in another session than the one that retrieved the
   supported SASL mechanisms from the server, as long as origin and use
   have a matching Destination-Realm, Origin-Realm and Origin-Host.

   When the supported SASL mechanism list on a server is changed, any
   open sessions that depend on one or more of the modified mechanisms
   SHOULD be aborted by the server.

   Diameter peers MUST NOT send the SASL-Mechanism AVP unless they also
   process SASL-Token and SASL-Channel-Binding AVPs for any sessions
   with the same Destination-Realm.

3.2.  SASL-Token

   The SASL-Token AVP has AVP Code TBD1 and is of type OctetString.  It
   may be passed in AA-Request and AA-Answer messages.

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   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.

   The interpretation of a SASL-Token is subject to the SASL mechanism
   selection by the client.  This is relayed with a SASL-Mechanism AVP,
   which MUST be part of each Network Access Server session, no later
   than the first SASL-Token exchange in that session.

3.3.  SASL-Channel-Binding

   The SASL-Channel-Binding AVP has AVP Code TBD2 and is of type
   OctetString.  The AVP contains the literal channel binding
   information for a SASL mechanism, and may be sent in an AA-Request
   that also holds a SASL-Mechanism AVP that lists a single SASL

   Without Realm Crossover, a SASL identity provider can source channel
   binding information from the underlying communications channel.  This
   information is not available to a SASL backend running Diameter.  To
   enable channel binding between the end points, and thereby
   authentication between the SASL 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.

   Note that SASL requires channel binding information when the client-
   selected SASL-Mechanism AVP ends in -PLUS.  Different kinds of
   channel binding exist, and their representations are distinguished
   with an IANA-registered prefix.  As a result, more than one SASL-
   Channel-Binding AVP can be safely included in one AA-Request.
   Servers MAY refrain from learning the client-chosen kind of channel
   binding from the SASL exchange, but SHOULD then transmit all the
   kinds that they support to avoid authentication failure.

3.4.  Key Groups for Derived Application Keys

   The key derivation mechanism in Section 2.7 can be used to find keys
   that the Proto Client and Foreign Server can share.  Such keys are
   derived from key material that is not visible to the Foreign Server,
   so it can only be passed back to the Foreign Server as part of an AA-

   Keys MAY be passed in a successful AA-Answer using the general
   framework for sharing key material [RFC6734].  This groups key
   information under a Key AVP.  Keys derived with the procedure in
   Section 2.7 use a Key group containing these AVPs:

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   Key-Type  is set to the value TBD3 for SXOVER-PLUS, as registered by
         IANA.  This drives the interpretation of the following AVPs in
         the same Key group.

   Key-Name  is the pepper for the derived key.  Every Key group MUST
         contain precisely one Key-Name AVP.  To distrubute multiple
         keys, separate Key groups MAY be used.

   Keying-Material  is a sufficiently long prefix of the PRF+() output
         to accomplish a desired task.  There is no risk in sending a
         bit more than required, so administrators can set a value that
         is simply high enough in practice at a minor computational
         penalty.  Applications SHOULD extract a prefix that suits their
         cryptographic mode.  Applications SHOULD NOT split the Keying-
         Material into multiple keys, because that reduces
         administrative control over cryptographic facilitation and it
         may hamper the ability to update cryptographic modes.

   Key-Lifetime  MAY be added if and when appropriate.  Care should be
         taken that this may cut short an application session, and that
         this may be construed as a form of instability.

   Key-SPI  MAY be added if and when appropriate.

   This specification does not detail administrative procedures for when
   to pass keys, or which peppers should be applied.  Configuration
   settings may be locally defined, and they may incorporate the Client
   Identity and/or the Foreign Server identity.

4.  Diameter Session Requirements for SASL

   Any exchange under the Network Access Server application must include
   a Session-ID.  There MAY be two kinds of session, and whether they
   are combined is an implementation requirement.

   A session can probe a SASL mechanism list as supported by a
   Destination-Realm for a given Origin-Realm and Origin-Host.  This
   mechanism MAY be assumed valid for any other sessions with these same
   three AVPs, for as long as this session is open.

   A session can make at most one SASL authentication attempt.  This is
   initiated with a SASL-Mechanism AVP that conveys precisely one SASL
   mechanism name in the first token.  The same Diameter message MAY
   convey a SASL-Token AVP in support of client-first mechanisms.  The
   same Diameter message MUST convey one or more SASL-Channel-Binding
   AVPs if the SASL-Mechanism ends in -PLUS.  Further messages in the
   session MUST NOT have the SASL-Mechanism AVP, MUST NOT have the SASL-
   Channel-Binding AVP and MAY have zero or one SASL-Token AVP.

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   It is possible for a session that probed a SASL mechanism list to
   continue as an authentication attempt.  In this case, the SASL
   mechanism list collapses to the one choice made by the client, and
   other sessions cannot rely on the entire mechanism list.  The server
   MAY close the session if it drops support for the client-selected
   SASL mechanism.

   Alternatively, a session that probed a SASL mechanism list can be
   kept open, and the obtained SASL mechanism list is then considered
   stable for sessions that use the same combination of Destination-
   Realm, Origin-Realm and Origin-Host.  This may be used to cache
   mechanism lists.  The server SHOULD close this session if it changes
   the mechanism list, thus invalidating the previously submitted
   mechanism list.  As long as the client has the mechanism list open,
   it MAY use that list for sessions that directly enter into an
   authentication attempt.

5.  Diameter Message Requirements for SXOVER-PLUS

   This section explains how the various SXOVER-PLUS messages are
   forwarded over Diameter by the foreign server.  The foreign server is
   connected to the SASL client, possibly over a TLS connection or a
   protocol under GSS-API protection, and relays requests over Diameter,
   usually over SCTP with DTLS.

   Diameter servers eventually provide success and failure responses,
   based on the corresponding final results from a SASL implementation
   that they in turn use.  Before the final result is reached, the SASL
   implementation may impose a challenge that will be reproduced over
   Diameter, passing challenge and response tokens over Diameter on
   behalf of SASL.

5.1.  C2S-Init Requests over Diameter

   To send C2S-Init the Diameter client MUST include at least the
   following AVPs in an AA-Request [Section 3.1 of [RFC7155]]:

   Destination-Realm  is the client's identity domain, replicated here
         for Diameter routing purposes; SXOVER-PLUS conveys this value
         in plaintext, and it is normally copied literally;

         when the client's identity domain consists of only digits, it
         MUST considered an international phone number; to transform it
         into a domain name, it is prefixed with a plus sign "+" to form
         an e.164 address, and then transformed as under ENUM
         [Section 3.2 of [RFC6116]] to derive the value for the
         Destination-Realm AVP, based on which Diameter's customary
         routing rules apply.  It is RECOMMENDED to continue to use the

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         international number as a domain in feedback to users, and only
         use the ENUM-mapped domain in backends, where they serve domain
         lookup and Diameter routing purposes.

   SASL-Mechanism  MUST be set to the fixed string SXOVER-PLUS for this
         SASL mechanism's name;

   SASL-Token  MUST be set to the GS2 header and C2S-Init;

   SASL-Channel-Binding  MUST be set to the channel binding bytes for
         the connection in which the SASL client attempts
         authentication, adhering to the channel binding mechanism named
         in the gs2-header in the SASL-Token.

   It is possible to extend the message with more AVPs.  Unless
   described herein, the SASL implementation ignores them.

5.2.  S2C-Init Responses over Diameter

   When SASL fails to initialise in response to the C2S-Init passed in
   an AA-Request, then the AA-Answer MUST represent that in the
   following AVP:

   Result-Code  MUST be set to an error or failure code [Section 7.1 of

   The initialisation of SASL forms a S2C-Init response, and an AA-
   Answer MUST be sent with the following AVPs:

   Result-Code  MUST be set to the value DIAMETER_MULTI_ROUND_AUTH
         [Section 7.1.1 of [RFC6733]];

   SASL-Token  MUST be set to the S2C-Init value.

5.3.  C2S-Cont Requests over Diameter

   The C2S-Cont message is any further message that the SASL client
   passes to the foreign server.  It MUST be forwarded as a Diameter AA-
   Request with the following AVPs:

   SASL-Token  MUST be set to the C2S-Cont value from the SASL client;

   SASL-Mechanism  MUST NOT be sent;

   SASL-Channel-Binding  MUST NOT be sent;

   User-Name  MAY be sent but MUST NOT be processed when received by
         implementations of this specification;

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   User-Password  MOST NOT be sent.

5.4.  S2C-Cont Responses over Diameter

   S2C-Cont tokens are produced as output from continued SASL processing
   based on C2S-Cont tokens found in AA-Request messages.

   If the SASL exchange is not final, then the AA-Answer MUST represent
   that in the following AVPs:

   Result-Code  is set to the value DIAMETER_MULTI_ROUND_AUTH
         [Section 7.1.1 of [RFC6733]];

   SASL-Token  MUST be included, and set to the S2C-Cont value.

   If the SASL exchange fails, then the AA-Answer MUST represent that in
   the following AVP:

   Result-Code  is set to an error or failure code [Section 7.1 of

   If the SASL exchange succeeds, then the AA-Answer MUST represent that
   in the following AVPs:

   Result-Code  is set to a success code [Section 7.1.2 of [RFC6733]];

   SASL-Token  is included when the SASL exchange produced an additional
         token upon success [Section 4 of [RFC4422]];

   User-Name  may be provided, and then contains the utf8-username part
         of a NAI [RFC7542], but not a utf8-realm; normally, this is the
         authentication identity for which the inner SASL mechanism
         succeeded, but if an authorization identity string was supplied
         and approved, then that is used instead; finally, there may be
         circumstances that call for sending no User-Name, such as when
         the inner SASL mechanism was ANONYMOUS (as that does not yield
         an authenticated user identity).

   Further AVPs may be included in a successful AA-Answer.  Examples are
   access control list information and backend tunnel creation.  No
   meaning is assigned herein to such additional AVPs.

6.  Running Diameter as a SASL Backend

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

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6.1.  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 transport.  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
   poorly connected client machines, but this sacrifices several
   benefits of the SCTP carrier.  SASL Diameter embeddings typically
   involve no client systems, so 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 identity domain, so this additional option is RECOMMENDED for
   situations where an alternative for native 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

6.2.  Reliance on DANE and DNSSEC

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

   DNSSEC offers a protection mechanism for the _diameters._sctp SRV
   records that lead to the Diameter host and its port for the identity
   domain.  DNSSEC can also protect any following AAAA and A records.
   DNSSEC does not protect against forged IP routes or hijacked 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,
   can be used as specified for DANE [RFC6698].  This use of DNSSEC and

   When identity domains choose to be light on these measures they risk
   that their user identities are hijacked, in spite of the use of DTLS
   or TLS.  Foreign servers MAY choose to reject such identity domains,
   or alternatively be more restrictive about the certificates that are

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7.  Security Considerations

   The SASL mechanism SXOVER-PLUS separates the authentication of a
   client identity into a domain and a user name underneath it.  The
   user name is validated by an identity server whose authority to
   identify users for the domain is authenticated by the relying foreign

   From the perspective of the foreign server, assurance of an identity
   is the vital aspect of the SXOVER-PLUS flow that it forwards over
   Diameter.  Through DTLS or TLS, with DNSSEC and DANE to validate the
   certificate it uses, the link from an identity domain to the Diameter
   connection can be verified, so the relying server can be certain
   about the domain under which its backend connection resides.  By
   receiving a response over that connection to a known-authoritative
   server for the domain, the user name can also be trusted.  The
   relying server continues to treat the user name and domain as a pair
   the for identification of the user.

   Channel binding is normally limited to two parties only, and
   forwarding such information is not a trivial idea.  The fact that the
   forwarding connection is encrypted, and known to lead to an
   authoritative server for an identity domain does help.  The foreign
   server relies on proper authentication, and has no interest in
   bypassing authentication, and it would be doing that by adopting
   channel binding information from anywhere else.

   From the perspective of the client and the identity domain, the
   safety of the SASL credentials is paramount.  When addressing a
   foreign server that is not part of the identity domain, clients
   therefore MUST NOT rely on mechanisms that might leak credentials.
   Two mechanisms that are safe to use are ANONYMOUS, which passes no
   credentials and yields no privileges, and SXOVER-PLUS, which applies
   end-to-end encryption to another SASL mechanism that may or may not
   be secure.

   The SXOVER-PLUS mechanism uses channel binding to ensure that the
   authentication is specific to a stream.  The level to which this is
   secure depends on the channel binding mechanism.  Therefore, in spite
   of end-to-end encryption, most use cases will want a secure carrier
   such as TLS between the client and foreign server.

   Key sharing in Diameter's AA-Answer messages relays sensitive
   information, using a TLS connection for Diameter between the Foreign
   Server and the Identity Domain.  The parties already validated mutual
   identities before this is done.  Moreover, the keys are specific to
   the session, and involve entropy from each side, so that each can
   constrain the reuse across sessions on the other side.  Applications

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   that want to protect from use of the derived keys by administrators
   in the Identity Domain may choose to mix the key material with
   material exchanged outside their focus, such as an ECDH key exchange
   between the Proto Client and the Foreign Server.  Such key exchanges
   are not secure from Quantum Computing on their own, but proper mixing
   with the shared key adds such protection and the only remaining
   concern may be an Identity Domain that is run by a party that is
   sufficiently rich to own a Quantum Computer.  Given that Identity
   Domains can be run by arbitrary parties, this is a controllable risk.
   (This assumes that TLS will independently evolve to mitigate Quantum
   Computer risk.)

8.  IANA Considerations

   This specification defines three AVP Codes for use with Diameter.
   IANA is requested to register 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)

   This specification defines a Key-Type value that IANA is requested to
   register under the Key-Type AVP Values in the Authentication,
   Authorization, and Accounting (AAA) Parameters registry:

   AVP Value | Attribute Name | Reference
   TBD3      | SXOVER-PLUS    | (this spec)

   This specification defines a SASL mechanism named SXOVER-PLUS.  IANA
   is requested to register the following in the Simple Authentication
   and Security Layer (SASL) Mechanisms registry under SASL Mechanisms:

Mechanism   | Usage  | Reference  | Owner
SXOVER-PLUS | COMMON | (this doc) | Rick van Rein <>

9.  Normative References

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              van Rein, R., "InternetWide Identities with Realm
              Crossover", Work in Progress, Internet-Draft, draft-
              vanrein-internetwide-realm-crossover-01, 6 October 2022,

   [RFC2743]  Linn, J., "Generic Security Service Application Program
              Interface Version 2, Update 1", RFC 2743,
              DOI 10.17487/RFC2743, January 2000,

   [RFC2744]  Wray, J., "Generic Security Service API Version 2 :
              C-bindings", RFC 2744, DOI 10.17487/RFC2744, January 2000,

   [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for
              Kerberos 5", RFC 3961, DOI 10.17487/RFC3961, February
              2005, <>.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              DOI 10.17487/RFC4120, July 2005,

   [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, <>.

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

   [RFC6113]  Hartman, S. and L. Zhu, "A Generalized Framework for
              Kerberos Pre-Authentication", RFC 6113,
              DOI 10.17487/RFC6113, April 2011,

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   [RFC6116]  Bradner, S., Conroy, L., and K. Fujiwara, "The E.164 to
              Uniform Resource Identifiers (URI) Dynamic Delegation
              Discovery System (DDDS) Application (ENUM)", RFC 6116,
              DOI 10.17487/RFC6116, March 2011,

   [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, <>.

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

   [RFC6734]  Zorn, G., Wu, Q., and V. Cakulev, "Diameter Attribute-
              Value Pairs for Cryptographic Key Transport", RFC 6734,
              DOI 10.17487/RFC6734, 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,

   [RFC7155]  Zorn, G., Ed., "Diameter Network Access Server
              Application", RFC 7155, DOI 10.17487/RFC7155, April 2014,

   [RFC7542]  DeKok, A., "The Network Access Identifier", RFC 7542,
              DOI 10.17487/RFC7542, May 2015,

   [RFC9266]  Whited, S., "Channel Bindings for TLS 1.3", RFC 9266,
              DOI 10.17487/RFC9266, July 2022,

Appendix A.  Centralised handing of SASL over Diameter

   This section is non-normative.

   Within foreign server networks, it can be useful to centralise
   Diameter handling in one host, where service-neutral pooling of
   connections to remote peers can improve efficiency and security.
   Diameter could facilitate this directly, but that would add quite a
   bit of handling logic to various foreign servers.  The following

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   ASN.1 module was therefore designed as the simplest possible request/
   answer protocol that could run over a TCP connection between a
   foreign service host and a nearby/trusted centralised host running
   its side of Diameter.

   The protocol can also be used over SCTP.  In this case, a user
   message can be defined to contain precisely one DiaSASL-Request in
   downstream direction, or one DiaSASL-Answer in upstream direction,
   and sent with the SCTP_UNORDERED flag.

   There is no standardised support for key exchange.  This being an
   internal protocol, it is better to leave this to local practices,
   which are presumed secure under internal supervision.


   -- ## SASL ready for Diameter
   -- This is targeted at Diameter backends and avoids loading all of
   -- Diameter into applications.

   -- Open a connection; return is DiaSASL-Open-Answer.
   -- The service-realm defines the context of the
   -- identity provider; this is where Diameter requests
   -- should be send, and it helps to determine what
   -- sasl-mechanisms may be used.
   -- The front-end is identified by a service-trunk code
   -- (for the long-term relation between a front-end and
   -- back-end) and/or a service-proto protocol that can
   -- be used while driving SASL (it could be the "imap"
   -- part before the "imap/"PrincipalName
   -- for a service in a Kerberos Ticket).
      service-realm   [1] UTF8String,
      service-trunk   [8] INTEGER   OPTIONAL,
      service-proto   [9] IA5String OPTIONAL

   -- Close a connection; session-id identifies which
   -- and there is no response.  This is ignored when the
   -- session-id is unknown; the call is not required
   -- after a DiaSASL-Authn-Answer that sets a value for
   -- final-comerr, but it is harmless when sent anyway.

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      session-id   [2] OCTET STRING

   -- Relay an authentication request message; response is
   -- DiaSASL-Authn-Answer with a copied session-id.
      session-id             [2] OCTET STRING,
      sasl-mechanism         [3] IA5String OPTIONAL,
      sasl-channel-binding   [4] OCTET STRING OPTIONAL,
      sasl-token             [5] OCTET STRING OPTIONAL

   -- This is the response to a DiaSASL-Open-Request.
   -- The final-comerr is set when Diameter was conclusive.
   -- It is an error code from com_err to allow for errors,
   -- but it may be sufficient to know that 0 indicates success
   -- and everything else is a failure.
   -- The service-realm is copied from the Diasasl-Open-Request
   -- so it can be used to match; the session-id will continue
   -- to identify this session in requests and responses.
   -- The sasl-mechanisms holds a space-separated string of
   -- SASL mechanism names.
      final-comerr      [0] INTEGER (-2147483648..2147483647) OPTIONAL,
                            -- Only set when Diameter was conclusive.
                            -- 0 for success, different for failure.
                            -- The code is a com_err code, so int32_t.
      service-realm     [1] UTF8String,
      session-id        [2] OCTET STRING,
      sasl-mechanisms   [3] IA5String

   -- This is the response to a DiaSASL-Authn-Request.
   -- The final-result is only set if Diameter was conclusive.
   -- It is an error code from com_err to allow for errors,
   -- but it may be sufficient to know that 0 indicates success
   -- and everything else is a failure.
   -- Only a successful authentication response can hold values
   -- for client-userid and client-domain.  The latter overrides
   -- the initial realm, which was provided in the open call,
   -- but may be substituted as a result of Realm Crossover.

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   -- The client-userid is the local part and may be absent on
   -- anonymous sessions; the client-userid value is approved
   -- by the local Diameter peer as having come from a Diameter
   -- Diameter peer that tends to client-domain.
      final-comerr   [0] INTEGER (-2147483648..2147483647) OPTIONAL,
                         -- Only set when Diameter was conclusive.
                         -- 0 for success, different for failure.
                         -- The code is a com_err code, so int32_t.
      session-id     [2] OCTET STRING,
      sasl-token     [5] OCTET STRING OPTIONAL,
      client-userid  [6] UTF8String OPTIONAL,
      client-domain  [7] UTF8String OPTIONAL

   -- Requests are Open, Close and Authn requests.  This simple
   -- CHOICE differentiates between the variants.
   -- Note that no extra tags are needed; the [APPLICATION n]
   -- tag can be used, or the presence of fields in variants.
   DiaSASL-Request ::= CHOICE {
       open-request   DiaSASL-Open-Request,
       close-request  DiaSASL-Close-Request,
       authn-request  DiaSASL-Authn-Request

   -- Answers are sent in response to Open and Authn requests.
   -- This simple CHOICE differentiates between the variants.
   -- Note that no extra tags are needed; the [APPLICATION n]
   -- tag can be used, or the presence of fields in variants.
   DiaSASL-Answer ::= CHOICE {
       open-answer    DiaSASL-Open-Answer,
       authn-answer   DiaSASL-Authn-Answer

   -- ## A simple API for DiaSASL

   -- A `diasasl` API only needs a small number of calls:
   -- This presents only a modest extension to existing software,
   -- and easily merges into a variety of concurrency models.


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Appendix B.  Support Levels for Realm Crossover

   This section is non-normative.

   There are a few levels of support at which Realm Crossover for SASL
   can be used.  An informal description of these levels can be useful
   for communication purposes.

   Level 0 constitutes the normal mode with local SASL authentication.
   This works well when clients are treated as local users of the
   foreign server.  Authentication is therefore carried out on the
   foreign server.

   Level 1/2 relays SASL authentication tokens to a statically
   configured backend, perhaps specific for a host name or resource
   path.  The client is treated as a user of the statically configured
   backend, which may serve their own domain.  This setup can be used
   for virtual hosting of a service without the need to hold
   authentication data.

   Level 1 supports SASL mechanisms for Realm Crossover like SXOVER-PLUS
   and relays the SASL information to the DOMAIN embedded in the first
   SASL token.  In this case, clients can present their own identity
   regardless of configuration on the foreign server; the foreign server
   welcomes a user to Bring Your Own IDentity.

   The Diameter formalisms are required for level 1/2 and level 1, but
   are an internal choice at level 0.  In all cases, the Quick-DiaSASL
   definition in Appendix A may be used to locally concentrate SASL
   authentication; the receiving end may be a local SASL identity
   provider for level 0 and would be a local Diameter node in level 1/2
   and level 1.

Appendix C.  Acknowledgements

   Thanks to Henri Manson for believing in this work, and making its
   first implementation, while interrogating the protocol and helping to
   improve it.

   Thanks to Nico Williams for input on the GS2 bridge and Channel

   Thanks to NLNet and the NGI Pointer project of the European Union for
   each funding parts of this work.

Author's Address

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   Rick van Rein
   OpenFortress BV
   Haarlebrink 5

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