Internet-Draft Diameter SASL October 2022
Van Rein Expires 17 April 2023 [Page]
Network Working Group
Intended Status:
R. Van Rein
OpenFortress BV

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 working documents as Internet-Drafts. The list of current Internet-Drafts is at

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any 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.

Table of Contents

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, 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 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 secure.

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.

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 encryption.

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 mechanism.

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.

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 structure:

   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 seed-length.

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 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:

   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.

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 of SXOVER-PLUS.

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.

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 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 mechanisms.

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.

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 mechanism.

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-Answer.

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:

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.
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.
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.
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.
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.

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]]:

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 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.
MUST be set to the fixed string SXOVER-PLUS for this SASL mechanism's name;
MUST be set to the GS2 header and C2S-Init;
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:

MUST be set to an error or failure code [Section 7.1 of [RFC6733]].

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

MUST be set to the value DIAMETER_MULTI_ROUND_AUTH [Section 7.1.1 of [RFC6733]];
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:

MUST be set to the C2S-Cont value from the SASL client;
MUST NOT be sent;
MUST NOT be sent;
MAY be sent but MUST NOT be processed when received by implementations of this specification;
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:

is set to the value DIAMETER_MULTI_ROUND_AUTH [Section 7.1.1 of [RFC6733]];
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:

is set to an error or failure code [Section 7.1 of [RFC6733]].

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

is set to a success code [Section 7.1.2 of [RFC6733]];
is included when the SASL exchange produced an additional token upon success [Section 4 of [RFC4422]];
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.

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 this.

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 attempts.

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 level.

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 DANE is RECOMMENDED.

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 accepted.

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 server.

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 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" registry:

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

van Rein, R., "InternetWide Identities with Realm Crossover", Work in Progress, Internet-Draft, draft-vanrein-internetwide-realm-crossover-01, , <>.
Linn, J., "Generic Security Service Application Program Interface Version 2, Update 1", RFC 2743, DOI 10.17487/RFC2743, , <>.
Wray, J., "Generic Security Service API Version 2 : C-bindings", RFC 2744, DOI 10.17487/RFC2744, , <>.
Raeburn, K., "Encryption and Checksum Specifications for Kerberos 5", RFC 3961, DOI 10.17487/RFC3961, , <>.
Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos Network Authentication Service (V5)", RFC 4120, DOI 10.17487/RFC4120, , <>.
Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple Authentication and Security Layer (SASL)", RFC 4422, DOI 10.17487/RFC4422, , <>.
Williams, N., "On the Use of Channel Bindings to Secure Channels", RFC 5056, DOI 10.17487/RFC5056, , <>.
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, , <>.
Altman, J., Williams, N., and L. Zhu, "Channel Bindings for TLS", RFC 5929, DOI 10.17487/RFC5929, , <>.
Hartman, S. and L. Zhu, "A Generalized Framework for Kerberos Pre-Authentication", RFC 6113, DOI 10.17487/RFC6113, , <>.
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, , <>.
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, , <>.
Fajardo, V., Ed., Arkko, J., Loughney, J., and G. Zorn, Ed., "Diameter Base Protocol", RFC 6733, DOI 10.17487/RFC6733, , <>.
Zorn, G., Wu, Q., and V. Cakulev, "Diameter Attribute-Value Pairs for Cryptographic Key Transport", RFC 6734, DOI 10.17487/RFC6734, , <>.
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, , <>.
Zorn, G., Ed., "Diameter Network Access Server Application", RFC 7155, DOI 10.17487/RFC7155, , <>.
DeKok, A., "The Network Access Identifier", RFC 7542, DOI 10.17487/RFC7542, , <>.
Whited, S., "Channel Bindings for TLS 1.3", RFC 9266, DOI 10.17487/RFC9266, , <>.

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 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.
   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.
-- 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.


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 Binding.

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

Author's Address

Rick van Rein
OpenFortress BV
Haarlebrink 5