Realm Crossover for SASL and GSS-API via Diameter
draft-vanrein-diameter-sasl-04
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Rick van Rein
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Henri Manson
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Network Working Group R. Van Rein
Internet-Draft OpenFortress BV
Intended status: Informational H. Manson
Expires: July 30, 2021 Mansoft
January 26, 2021
Realm Crossover for SASL and GSS-API via Diameter
draft-vanrein-diameter-sasl-04
Abstract
SASL and GSS-API are used for authentication in many application
protocols. This specification extends them to allow credentials of a
home realm 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 https://datatracker.ietf.org/drafts/current/.
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 July 30, 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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 . . . . . . . . . . . . 5
2.4. Continued Client-to-Server Messages . . . . . . . . . . . 6
2.5. Continued Server-to-Client Messages . . . . . . . . . . . 7
2.6. Using SXOVER-PLUS with GSS-API . . . . . . . . . . . . . 7
2.7. Application Key Derivation . . . . . . . . . . . . . . . 8
3. AVP Definitions for SASL in Diameter . . . . . . . . . . . . 9
3.1. SASL-Mechanism . . . . . . . . . . . . . . . . . . . . . 9
3.2. SASL-Token . . . . . . . . . . . . . . . . . . . . . . . 10
3.3. SASL-Channel-Binding . . . . . . . . . . . . . . . . . . 10
3.4. NAS-Port-Type for SASL Services . . . . . . . . . . . . . 10
4. Diameter Session Requirements for SASL . . . . . . . . . . . 11
5. Diameter Message Requirements for SXOVER-PLUS . . . . . . . . 11
5.1. C2S-Init Requests over Diameter . . . . . . . . . . . . . 11
5.2. S2C-Init Responses over Diameter . . . . . . . . . . . . 12
5.3. C2S-Cont Requests over Diameter . . . . . . . . . . . . . 12
5.4. S2C-Cont Responses over Diameter . . . . . . . . . . . . 13
6. Running Diameter as a SASL Backend . . . . . . . . . . . . . 14
6.1. Diameter is an SCTP service . . . . . . . . . . . . . . . 14
6.2. Reliance on DANE and DNSSEC . . . . . . . . . . . . . . . 14
7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
9. Normative References . . . . . . . . . . . . . . . . . . . . 16
Appendix A. Centralised handing of SASL over Diameter . . . . . 18
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
It is common for Internet users to combine 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] is instrumental in authentication across a wide range
of application protocols; it allows those protocols to abstract from
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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 with a
backcall to the client's own domain:
+--------+ SASL +--------+ SASL +---------+
| Client |-----------> | Server | ---------> | Realm |
+--------+ AppProto +--------+ Diameter +---------+
|| || ||
john@example.com find SRV, TLSA example.com
& credential relay SASL authentication
Realm Crossover authentication:
Client John authenticates to his own Realm
while using a foreign Server.
The Diameter server in the Realm 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 combined User-Name and
validated domain form the client identity as further used in the
foreign server. The client realm 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. No server accepts externally dictated channel binding
information; the reason why it is safe to make an exception for
Diameter is that it provides no resources, which makes it an
unattractive attack target.
SASL mechanisms are not generally safe to pass over plaintext
channels. This is usually addressed by wrapping the application
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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 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 develop 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 submits a multi-session key to
his realm and receives back a keyno and encalg in the style of
Kerberos [RFC4120] along with a "keymap" blob that contain the
originally submitted multi-session key. This process may be run at
any time desired by the client; for instance, when a program first
uses the SXOVER-PLUS mechanism; it 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; at any time, they can drop the key.
By offering the SXOVER-PLUS mechanism for SASL, a foreign server
announces its willingness to validate the client's Realm as a domain,
relay SASL messages to it, trust its authentication conclusion and
User-Name and place it under the client's domain name.
Offering SXOVER-PLUS does not preclude the offering of other SASL
mechanisms; for instance, ANONYMOUS may be useful to allow clients to
choose guest access.
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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 the domain name of the client. This notation is
compatible with the GS2 bridge [RFC5056].
Following this is DER-encoded information for the following ASN.1
structure:
C2S-Init ::= [APPLICATION 1] IMPLICIT SEQUENCE {
clirnd OCTET STRING, -- Entropy to allow client variety
keyno KeyNumber, -- With realm and encalg, identifies...
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 multi-session 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 defined in RFC
3961, 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 from RFC 6113 with the
entire C2S-Init message (featuring the GS2 header and the entropy in
the clirnd 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 from RFC 6113, 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:
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S2C-Init ::= [APPLICATION 2] IMPLICIT SEQUENCE {
srvrnd OCTET STRING, -- Entropy to allow server variety
mechlist IA5String -- Available SASL mechanisms
}
The clirnd is a salt that should hold enough entropy to satisfy the
client'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. Note
that weaker channel binding choices such as tls-server-end-point may
be met with a replay-protective mechlist.
The key K1 is passed into the PRF+() function from RFC 6113 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 and will
be used to encrypt further messages, to be described as C2S-Cont and
S2C-Cont.
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:
C2S-Cont ::= [APPLICATION 3] IMPLICIT SEQUENCE {
mechsel IA5String OPTIONAL, -- SASL mechanism name selection
c2s SaslToken -- NULL or SASL token 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
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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:
S2C-Cont ::= [APPLICATION 4] IMPLICIT SEQUENCE {
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, or it explicitly carries NULL to indicate an absent token.
The success field may be set to TRUE to mark the provision of
additional data upon success, and should be taken as a hint that no
further SASL exchange is needed.
Note how this always facilitates last-sends by the SASL server. This
is trivially done in Diameter, by just adding a SASL-Token AVP to the
final success message; it is not always possible in the protocol
between the client machine and foreign server, but that may be
remedied by sending success in S2C-Cont and going through another
looping to finish.
2.6. Using SXOVER-PLUS with GSS-API
When SXOVER-PLUS is used with GSS-API instead of SASL there are only
a few changes to observe.
GSS-API Calls [RFC2744] to gss_init_sec_context() and
gss_accept_sec_context() MUST adhere to [Section 5.1 of [RFC5801]]
concerning channel binding informtion. Providing the GS2 header and
channel binding data in the application-data field involves the
"p=CHANBIND,," but not the "DOMAIN," part of the SASL header.
When transmitted as GSSAPI, only the first message changes. The
client is now referred to as initiator and the server as acceptor.
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In the first message, the initial part "p=CHANBIND,," is removed, but
the "DOMAIN," and subsequent DER-encoded C2S-Init structure are kept.
The standard GSSAPI header inserted in its place, adhering to the
Mechanism-Independent Token Format [Section 3.1 of [RFC2743]] with
object identifier 1.3.6.1.4.1.44469.666.5081.1 (TBD:GSSOID) 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).
TODO: Reconstruct SASL header or skip the "p=CHANBIND,," part in both
SASL and GSS-API? Can we tell the channel binding type?
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 secure layer that
provides 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 former and
latter. 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 from RFC 6113. 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, especially among different security
contexts.
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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 an authentication
realm.
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 1.
Normally, a successful AA-Answer would provide a User-Name AVP to
inform the server about a username NAI without a realm [Section 2.1
of [RFC4282]] 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 storage options. Sending a
pseudonym in the User-Name may be an intermediate option. In all
cases, the realm under which a successful AA-Answer is considered to
fall can be taken from the Destination-Realm handling the Network
Access Server session.
3.1. SASL-Mechanism
The SASL-Mechanism AVP has AVP Code TBD0 and is of type UTF8String,
further restricted to the following ASCII coding. The AVP can be
used to request a server's list of supported SASL mechanism names or
to relay a client-chosen SASL mechanism.
To relay a client's choice of SASL mechanism, this AVP is included in
an AA-Request message, containing the standardised string for a SASL
mechanism [Section 3.1 of [RFC4422]].
To request a server's list of supported SASL mechanisms, this AVP
containing an empty string is included in an AA-Request message. The
AA-Answer response message may then either (0) fail with
DIAMETER_AVP_UNSUPPORTED if the SASL-Mechanism AVP was not
recognised, (1) contain a SASL-Mechanism AVP holding an empty string
to indicate that no current SASL mechanism is available, or (2)
contain a SASL-Mechanism AVP holding one or more standard SASL
mechanisms [Section 3.1 of [RFC4422]] separated by a single U+0020
space character.
It is possible that the list of supported SASL mechanisms depends on
other AVPs in an AA-Request asking for that list. When practicing
realm crossover, the Destination-Realm SHOULD be used to indicate the
destination of the request, and Origin-Realm or Origin-Host may
influece the offered SASL mechanism list.
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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.
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.
Note that SASL requires channel binding information when the SASL-
Mechanism AVP ends in -PLUS. Also note that different kinds of
channel binding exist, and that they all start with a unique prefix
registered with IANA. As a result, more than one SASL-Channel-
Binding AVP may be included in one AA-Request. Servers MAY refrain
from learning the client-chosed kind of channel binding from the SASL
exchange, but SHOULD then transmit all the kinds that they support to
avoid authentication failure.
3.4. NAS-Port-Type for SASL Services
The NAS-Port-Type AVP exists with AVP Code 61, and its values
enumerate possible interpretations for the NAS-Port and NAS-Port-Id
AVPs. The value TBD3 is used in the NAS-Port-Type AVP in AA-
Requests, with the following interpretation results.
The NAS-Port-Id carries a SASL service name, which often translates
to a standardised protocol name such as "imap". Other values MAY be
agreed on when all components agree.
The NAS-Port carries a trunk number, and may be used to reference a
previously negotiated relation between a foreign service and an
authentication server.
The form of a NAS-Port-Id assumes an implicit agreement, usually
founded in standards. This makes it into a portable option, and
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suitable for public services. The NAS-Port option may be used when
discrimination between foreign services is desired, in which case the
expectation of prior agreement also makes sense.
4. Diameter Session Requirements for SASL
Probes for SASL mechanism lists SHOULD be sent outside of a Diameter
session, and the response MAY be influenced by the Destination-Realm,
Origin-Realm and Origin-Host AVPs. It SHOULD NOT be varied for other
reasons.
Non-empty SASL-Mechanism AVPs, as well as any SASL-Token and SASL-
Channel-Binding AVPs SHOULD NOT be sent outside of a Diameter
session. The first AA-Request in this session SHOULD hold the SASL-
Mechanism and MAY hold the SASL-Channel-Binding; these two AVPs
SHOULD NOT occur in later messages in the same session. There MAY be
a SASL-Token AVP in any AA-Request or AA-Answer anywhere in the
Diameter session.
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 requested realm, replicated here
for Diameter routing purposes; SXOVER-PLUS conveys this value
in plaintext;
SASL-Mechanism MUST be set to the fixed string SXOVER-PLUS for this
SASL mechanism's name;
SASL-Token MUST be set to the C2S-Init and optional C2S-Cont as it
produced by the SASL client;
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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 that the client
and server can agree on.
The C2S-Init Request is likely to hold other Diameter AVPs for
general housekeeping of the Diameter base protocol and NAS
application, such as the Session-Id. Though User-Name and User-
Password would be sent with password-based Diameter mechanisms, they
MUST be ignored by implementations of SASL over Diameter when they
appear in C2S-Init messages.
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
[RFC6733]].
Upon initialisation of SASL, the normal response is a list of
mechanisms that the client may use. If the AA-Request sent along a
C2S-Cont that guessed an available mechanism and if that extension is
acceptable to the server, then further processing will be as defined
for S2C-Cont, below. Otherwise, the remainder of this section
applies.
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;
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SASL-Channel-Binding MUST NOT be sent;
User-Name MAY be sent but MUST NOT be processed when received by
implementations of this specification;
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; when
responding to accepted optimisation for the initial round-trip
then the S2C-Init token MUST be prefixed 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
[RFC6733]].
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 username part of a
NAI [RFC4282], but not a realm; when an authorization identity
string was provided [Section 3.4.1 of [RFC4422]] and approved
by the SASL exchange, then this will be used instead of the
authentication idenity. This mechanism may be used to request
the use of a pseudonym as well as to signal the willingness to
return this AVP.
Further AVPs may be included in a successful AA-Answer, but their
meaning is not defined herein. Applications might range from access
control lists to backend tunnel creation.
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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
client-only machines, but then 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 home realms, so this additional option is RECOMMENDED for
situations where a fallback for plain SCTP is desired. It is
standardised as a socket option SCTP_REMOTE_UDP_ENCAPS_PORT, and only
involves a small repetition in code, with a minor change between the
attempts.
6.2. Reliance on DANE and DNSSEC
Diameter always involves the use of TLS, but there is a number of
choices concerning the validation of connections through DNSSEC and
DANE. It is the home realm's prerogative what level of protection it
upholds for its client identities, but any foreign server MAY choose
to raise the bar by setting a minimal accepable level.
DNSSEC offers a protection mechanism for the _diameter._sctp SRV
records that lead to the Diameter host and its port for the home
realm. This does not protect against forged IP addresses, port
mappings or routing. To protect against this as well, a TLSA record
for the service host and port, along with the _sctp protocol label,
can be used as specified for DANE [RFC6698]. This use of DNSSEC and
DANE is RECOMMENDED.
Home realms that choose to be light on such measures risk that
identities are forged, in spite of their use of TLS. Foreign servers
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MAY choose to reject such home realms, or alternatively be more
inquisitive about the certificates used.
7. Security Considerations
The SASL mechanism SXOVER-PLUS separates the authentication of a
foreign identity into its realm and the username underneath it. The
realm is authenticated by the relying server, such as the proposed
foreign server, whereas the username is obtained from a backend realm
server that is known to be responsible for that realm.
From the perspective of the foreign server, assurance of an identity
is the vital aspect of the SXOVER-PLUS flow that it relays over
Diameter. Through TLS or DTLS, with DNSSEC and DANE to validate the
certificate it uses, the link from a realm (which is read as a domain
name) to the Diameter connection can be verified, so the relying
server can be certain about the realm under which the backend
connection resides. By receiving a response over that connection to
a known-authoritative server for the realm, the username can also be
trusted. The relying server continues to treat the username and
realm 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 a claimed realm does help. The intermediate
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 home realm, the safety of
the SASL credentials is paramount. When addressing a foreign server,
which is not part of the home realm, 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 assigns no
rights, 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.
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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 new value for the NAS-Port-Type AVP to
indicate a new interpretation of values passed in NAS-Port and NAS-
Port-Id AVPs. IANA is requested to register the following value in
the RADIUS Types registry, under Values for RADIUS Attribute 61, NAS-
Port-Type:
Value | Description | Reference
------+----------------------------+------------
TBD3 | SASL Authenticated Service | (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 spec) | Rick van Rein <rick@openfortress.nl>
9. Normative References
[I-D.vanrein-internetwide-realm-crossover]
Rein, R., "InternetWide Identities with Realm Crossover",
draft-vanrein-internetwide-realm-crossover-00 (work in
progress), September 2020.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743,
DOI 10.17487/RFC2743, January 2000,
<https://www.rfc-editor.org/info/rfc2743>.
[RFC2744] Wray, J., "Generic Security Service API Version 2 :
C-bindings", RFC 2744, DOI 10.17487/RFC2744, January 2000,
<https://www.rfc-editor.org/info/rfc2744>.
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[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, DOI 10.17487/RFC3961, February
2005, <https://www.rfc-editor.org/info/rfc3961>.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
DOI 10.17487/RFC4120, July 2005,
<https://www.rfc-editor.org/info/rfc4120>.
[RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
Network Access Identifier", RFC 4282,
DOI 10.17487/RFC4282, December 2005,
<https://www.rfc-editor.org/info/rfc4282>.
[RFC4422] Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
Authentication and Security Layer (SASL)", RFC 4422,
DOI 10.17487/RFC4422, June 2006,
<https://www.rfc-editor.org/info/rfc4422>.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,
<https://www.rfc-editor.org/info/rfc5056>.
[RFC5554] Williams, N., "Clarifications and Extensions to the
Generic Security Service Application Program Interface
(GSS-API) for the Use of Channel Bindings", RFC 5554,
DOI 10.17487/RFC5554, May 2009,
<https://www.rfc-editor.org/info/rfc5554>.
[RFC5801] Josefsson, S. and N. Williams, "Using Generic Security
Service Application Program Interface (GSS-API) Mechanisms
in Simple Authentication and Security Layer (SASL): The
GS2 Mechanism Family", RFC 5801, DOI 10.17487/RFC5801,
July 2010, <https://www.rfc-editor.org/info/rfc5801>.
[RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,
<https://www.rfc-editor.org/info/rfc5929>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
2012, <https://www.rfc-editor.org/info/rfc6698>.
[RFC6733] Fajardo, V., Ed., Arkko, J., Loughney, J., and G. Zorn,
Ed., "Diameter Base Protocol", RFC 6733,
DOI 10.17487/RFC6733, October 2012,
<https://www.rfc-editor.org/info/rfc6733>.
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[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,
<https://www.rfc-editor.org/info/rfc6951>.
[RFC7155] Zorn, G., Ed., "Diameter Network Access Server
Application", RFC 7155, DOI 10.17487/RFC7155, April 2014,
<https://www.rfc-editor.org/info/rfc7155>.
Appendix A. Centralised handing of SASL over Diameter
This section is non-normative.
Within foreign service networks, it can make sense to centralised
Diameter handling on one host, where service-neutral pooling of
connections to client realms can improve efficiency. Though Diameter
could do this, but adds a fair bit of handling logic to a foreign
service. The following ASN.1 module was designed as the simplest
possible query/response protocol that could sit between a foreign
services and a nearby/trusted centralised host running its side of
Diameter.
Quick-DiaSASL DEFINITIONS EXPLICIT TAGS ::= BEGIN
-- ## 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/imap.example.com"PrincipalName
-- for a service in a Kerberos Ticket).
--
DiaSASL-Open-Request ::= [APPLICATION 10] IMPLICIT SEQUENCE {
service-realm [1] UTF8String,
service-trunk [8] INTEGER OPTIONAL,
service-proto [9] IA5String OPTIONAL
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}
-- 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.
--
DiaSASL-Close-Request ::= [APPLICATION 11] IMPLICIT SEQUENCE {
session-id [2] OCTET STRING
}
-- Relay an authentication request message; response is
-- DiaSASL-Authn-Answer with a copied session-id.
--
DiaSASL-Authn-Request ::= [APPLICATION 12] IMPLICIT SEQUENCE {
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.
--
DiaSASL-Open-Answer ::= [APPLICATION 13] IMPLICIT SEQUENCE {
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.
--
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-- 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.
--
DiaSASL-Authn-Answer ::= [APPLICATION 14] IMPLICIT SEQUENCE {
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
}
-- ## The API of diasasl
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-- An `diasasl` API consists of a number of easy calls:
-- http://quick-sasl.arpa2.net/group__quickdiasasl.html
-- It should be a modest extension to existing software.
END
Appendix B. Acknowledgements
Thanks to Nico Williams for input on the GS2 bridge and Channel
Binding.
Authors' Addresses
Rick van Rein
OpenFortress BV
Haarlebrink 5
Enschede, Overijssel 7544 WP
The Netherlands
Email: rick@openfortress.nl
Henri Manson
Mansoft
Castorstraat 30
Enschede, Overijssel 7521 JS
The Netherlands
Email: info@mansoft.nl
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