Symmetric SET Transfer ProtocolOracle Corporationphil.hunt@yahoo.comMicrosofttonynad@microsoft.comInternet-Draft
This specification defines how security event tokens
(SETs) may be exchanged between a client and service provider
using HTTP POST over TLS using a symmetric format. The
specification supports three modes
of operation: "push", "pull", and "push-pull" bi-directional SET
exchange. The specification also defines a simple acknowledge
mechanism allowing parties to confirm delivery.
[EDITORS NOTE: This specification is based upon
draft-ietf-secevent-delivery and attempts to provide a
unified single MTI protocol solution satisfying all use
cases for SECEVENTS.]
This specification defines how SETs (see )
can be exchanged using HTTP over
TLS using a symmetric request/response format.
The specification supports three modes of operation: "push",
"pull", and "push-pull" bi-directional SET exchange. The
specification also defines a simple acknowledge mechanism
allowing parties to confirm delivery or attempt re-delivery.
This specification makes several simplifying assumptions:
SSTP is a symmetric protocol meaning it uses the same
HTTP content-type and JSON structure to send requests and
process responsesRecovery is provided for unacknowledged SETs for
short term or missed deliveries. Issuers are able to
re-transmit SETs in situations where a SET has been
unacknowledged (e.g. over more than one request/response
cycles).The protocol can be used for unidirectional, or bi-directional
communication avoiding the need to implement multiple
delivery methods.The use of HTTP Long Polling MAY be used in cases where
pull or bi-directional communication is needed in real time.Only one party in an exchange needs to have an addressable
fixed URI endpoint and can act as an SSTP protocol service
provider to a mobile or otherwise unrechable client.SET message exchanges are secured through the use of TLS
and some form of HTTP authorization (e.g. RFC6750, RFC7519),
and MAY in turn be signed and encrypted.This specification supports the following use-cases:Where a large entity (e.g. an Identity Provider)
needs to issue SETs to a large number of relying parties.Where a client party is behind a
firewall or otherwise network restricted location and cannot
act as a SSTP service provider. Examples include Enterprise
on-premise security systems, IoT devices which may be shielded
in restricted network environments.Where a client is mobile and thus would be
unable to maintain a permanent HTTP endpoint.This specification does not define how endpoints are
configured, nor does it define the specifics of which SET
event types are exchanged over any particular delivery
relationship.The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
"MAY", and "OPTIONAL" in this document are to be interpreted as
described in BCP 14
when, and only when, they appear in all capitals, as shown here.
For purposes of readability examples are not URL encoded.
Implementers MUST percent encode URLs as described in
Section 2.1 of
.
Throughout this documents all figures MAY contain
spaces and extra line-wrapping for readability and space
limitations. Similarly, some URI's contained within
examples, have been shortened for space and readability
reasons. All examples are non-normative.
This specification assumes terminology defined in the Security
Event Token specification
.
This specification defines the following terms:
An entity which acts as an HTTP
client as defined by which
is communicating with an SSTP server.An entity which has a permanent
endpoint reachable by a client which acts as an HTTP
server as defined by and
supports this specification.An Event is defined to be an
event as represented by a security event token (SET).
See .
A JSON numeric value representing the number of seconds from
1970-01-01T00:00:00Z UTC until the specified UTC date/time,
ignoring leap seconds. This is equivalent to the
IEEE Std 1003.1, 2013 Edition
definition "Seconds Since the Epoch", in which each day is
accounted for by exactly 86400 seconds, other than that
non-integer values can be represented. See
for details regarding date/times
in general and UTC in particular.SSTP is a symmetric protocol. As such an SSTP client
uses HTTP POST (Section 4.3.3)
with a body with Content-Type of
application/sstp+json to deliver
0 or more SETs and/or to acknowledge previously
received SETs from an SSTP server. In response, the SSTP
server returns an HTTP body with the same document-type which
may also return 0 or more SETs, acknowledgments, and errors.Requests MAY be spontaneous (in the case of push mode),
scheduled over a a periodic interval (in the case of pull),
or requests to pull MAY await new SETS using HTTP long polling
(see Section 2). An SSTP
server choosing NOT to support HTTP long polling MAY do so
by returning HTTP Status of 403 "Forbidden" (see
Section 6.5.3) if a
particular client is not authorized, or HTTP status 501
"Not implemented"
(see Section 6.6.2) if the server
does not support long polling.SSTP provides an acknowledgement capability for the
purpose of informing communications partners about which SETs
have been successfully delivered. Upon receipt of a SET and
before acknowledgement, receivers SHOULD ensure received SETs
are valid and have been retained in a manner appropriate to
the receiver's retention needs. The level and method of
retention of SETs by receivers is out-of-scope of this
specification.If after a period of time, negotiated between the client
and server, unacknowledged SETs MAY be re-transmitted.
The receiver SHOULD accept repeat SETs and acknowledge the
SETs regardless of whether the receiver believes it has
already acknowledged the SETs previously. A SET issuer
MAY limit the number of attempts to deliver a SET.
A receiving party (client or server) of SETs SHOULD parse
and validate each SET to meet its own requirements and SHOULD
acknowledge receipt in a timely (e.g. minutes) fashion so
that the issuer may mark the SETs as received. Receivers
SHOULD acknowledge receipt before taking any local actions
based on the SETs to avoid unnecessary delay in
acknowledgement to avoid unnecessary re-transmission.The body (or message) of an SSTP request or response is a
Content-Type of
application/sstp+json which is
a JSON document consisting of the following optional JSON
attributes:
A JSON boolean
parameter which indicates whether the receiver SHOULD
return SETs in its upcoming response. When part of an
HTTP Request, it indicates the SSTP server SHOULD
return with SETs in its response (and optionally wait).
When asserted as false by an
SSTP server in its response, it indicates that the SSTP
server is not requesting SETs in the next client request.
If omitted, this attribute SHALL have a default value of
true.
An OPTIONAL JSON
boolean value which when true
has the effect of declining HTTP long polling. A value
of false
indicates the request is to be treated as an HTTP Long Poll (see
Section 2). When asserted
in an HTTP response, the value indicates the SSTP has
more information and the client SHOULD NOT wait before
initiating its next request. When omitted, the default
value of false SHALL be
assumed.A JSON object that contains one
or more nested JSON attributes. Each nested attribute
corresponds to the jti of a SET to
be delivered and whose value is a JSON String containing the
value of the encoded corresponding SET. If there are no
SETs to be transmitted, the attribute MAY be omitted.An array of Strings that each
correspond to the jti of a
successfully received SET by the client. If there are no
outstanding SETs to acknowledge, the attribute MAY be omitted.
When acknowledging a SET, the issuer is released from
any obligation to retain the SET (e.g. for a future re-try).A JSON Object that contains
one or more nested JSON attributes that correspond to the
jti of each invalid SET received.
The value of each is a JSON object whose contents is an
err attribute and
description attribute whose value
correspond to the errors described in .
If there are no errors to acknowledge, the attribute MAY
be omitted.Three examples are provided where:A client pushes SETs to a server using the
sets JSON attribute in its
request and receives "ack" values in response from the SSTP server.A polling client that requests SETs from a a server and uses
the ack parameter in
its request to acknowledge SETs from a previous HTTP
request, and receives new SETs in the response.A client and SSTP server bi-directionally exchange
SETs using both the sets and
ack JSON attributes in both the request
and response messages.In this example, a client posts SETs to an SSTP
server which in turn acknowledges the transferred
SETs in its response.In response to the above two requests, the SSTP
server responds immediately if
respondImmediately is true or
SETs are available. If no SETs are available at the
time of the request and respondImmediately
is false, the SSTP server delays response until a SET is
available.As described in a JSON
document is returned containing the JSON attribute
sets.In the above example, two SETs whose jti
are 4d3559ec67504aaba65d40b0363faad8
and 3d0c3cf797584bd193bd0fb1bd4e7d30
are delivered.This variation is typically used when a client needs to
acknowledge received SETs on a separate thread
from one receiving SETs.An SSTP client acknowledges previously received SETs
but indicates it does not want to receive SETs in the
current request/response by setting the
returnEvents
attribute to false.This variation allows a client to simultaneously
acknowledge previously received SETs and wait for the next
group of SETs in a single HTTP request.In the case where errors are detected in previously
received SETs, the client (or server) uses the
setErrs attribute to indicate errors
in its request.
In push-pull mode, JSON attributes sets,
ack and setErrs
are used in both HTTP request and response messages between
client and SSTP server.Following the response from the SSTP server, the client
would subsequently repeats the request-response cycle by
acknowledging the SET identified by a jti
value of 6f332aefc730400a9f645d36a12ba4ab.If a SET is invalid, the following error codes are defined:Err ValueDescriptionjsonInvalid JSON object.jwtParseInvalid or unparsable JWT or JSON structure.jwtHdrIn invalid JWT header was detected.jwtCryptoUnable to parse due to unsupported algorithm.jwsSignature was not validated.jweUnable to decrypt JWE encoded data.jwtAudInvalid audience value.jwtIssIssuer not recognized.setTypeAn unexpected Event type was received.setParseInvalid structure was encountered such as an
inability to parse or an incomplete set of event claims.setDataSET event claims incomplete or invalid.directionalThe SSTP does not support transfer of
SETs in the requested direction.An error response has a Content-Type
of application/sstp+json which
is a JSON document that provides details about the error. The JSON document
includes the JSON attributes: A value which is a keyword that
describes the error (see ).A human-readable text that provides
additional diagnostic information.
When included as part of an HTTP Status 400 response, the above
JSON is the HTTP response body in the JSON attribute
setErrs (see ).
The SET delivery methods described in this specification are
based upon HTTP and depend on the use of TLS and/or standard
HTTP authentication and authorization schemes as per
. For example, the following
methodologies could be used among others: SSTP
server MAY negotiate TLS mutual client authentication.
See Section 7.3. Bearer tokens
MAY be used when combined with TLS and a token
framework such as OAuth 2.0 .
For security considerations regarding the use of bearer tokens in
SET delivery see .Usage of basic
authentication should be avoided due to its use of a single factor
that is based upon a relatively static, symmetric secret.
Implementers SHOULD combine the use of basic authentication with
other factors. The security considerations of HTTP BASIC, are well
documented in and SHOULD be considered
along with using signed SETs (see SET Payload Authentication below).In scenarios
where SETs are signed and
the delivery method is HTTP POST,
SSTP clients MAY elect to use Basic Authentication or not
to use HTTP or TLS based authentication at all. See
for considerations.As per Section 4.1 of, a SET
delivery endpoint SHALL indicate supported HTTP authentication
schemes via the WWW-Authenticate header.Because SET Delivery describes a simple function, authorization
for the ability to pick-up or deliver SETs can be derived by
considering the identity of the SET issuer, or via an authentication
method above. This specification considers authentication as a
feature to prevent denial-of-service attacks. Because SETs are
not commands (see ), event receivers are free to ignore SETs that
are not of interest.For illustrative purposes only, SET delivery examples show an OAuth2
bearer token value in the authorization header.
This is not intended to imply that bearer tokens are
preferred. However, the use of bearer tokens in the specification does
reflect common practice. When using bearer tokens or proof-of-possession tokens that
represent an authorization grant such as issued by OAuth (see ), implementers SHOULD consider the type of
authorization granted, any authorized scopes (see Section 3.3 of ), and the security subject(s) that SHOULD be mapped
from the authorization when considering local access control rules.
Section 6 of the OAuth Assertions draft , documents common scenarios for
authorization including:Clients using an assertion to authenticate and/or act on behalf
of itself;Clients acting on behalf of a user; and,A Client acting on behalf of an anonymous user (e.g., see next
section).When using OAuth authorization tokens, implementers MUST take
into account the threats and countermeasures documented in the
security considerations for the use of client authorizations (see
Section 8 of ). When using
other token formats or frameworks, implementers MUST take into account
similar threats and countermeasures, especially those documented by
the relevant specifications.In scenarios where HTTP authorization or TLS mutual authentication
are not used or are considered weak, JWS signed SETs SHOULD be
used (see and
Security Considerations). This enables event receivers
to validate that the SET issuer is authorized to deliver SETs.
SET delivery depends on the use of Hypertext Transfer Protocol and thus
subject to the security considerations of HTTP Section 9 and its related specifications.As stated in Section 2.7.1, an
HTTP requestor MUST NOT generate the userinfo
(i.e., username and password) component (and its "@" delimiter) when
an "http" URI reference is generated with a message as they are now
disallowed in HTTP.SETs contain sensitive information that is considered PII
(e.g. subject claims). Therefore, event issuers and
receivers MUST require the use of a transport-layer
security mechanism. Event delivery endpoints MUST support TLS
1.2 and MAY support additional
transport-layer mechanisms meeting its security requirements.
When using TLS, the client MUST perform a TLS/SSL server
certificate check, per . Implementation
security considerations for TLS can be found in "Recommendations for
Secure Use of TLS and DTLS" .When using authorization tokens such as those issued by OAuth 2.0
, implementers MUST take into account threats
and countermeasures documented in Section 8 of .Due to the possibility of interception, Bearer tokens MUST be
exchanged using TLS.Bearer tokens MUST have a limited lifetime that can be determined
directly or indirectly (e.g., by checking with a validation service)
by the service provider. By expiring tokens, clients are forced to
obtain a new token (which usually involves re-authentication) for
continued authorized access. For example, in OAuth2, a client MAY use
OAuth token refresh to obtain a new bearer token after authenticating
to an authorization server. See Section 6 of .Implementations supporting OAuth bearer tokens need to factor in
security considerations of this authorization method . Since security is only as good
as the weakest link, implementers also need to consider authentication
choices coupled with OAuth bearer tokens. The security considerations
of the default authentication method for OAuth bearer tokens, HTTP
BASIC, are well documented in , therefore implementers
are encouraged to prefer stronger authentication methods. Designating
the specific methods of authentication and authorization are
out-of-scope for the delivery of SET tokens, however this
information is provided as a resource to implementers.If a SET needs to be retained for audit purposes, JWS MAY
be used to provide verification of its authenticity.Event transmitters SHOULD attempt to filter SETs issued
so that the content is targeted to the specific business and
protocol needs of receivers.When sharing personally identifiable information or information
that is otherwise considered confidential to affected users, event
transmitters and receivers MUST have the appropriate legal agreements
and user consent or terms of service in place.The propagation of subject identifiers can be perceived as personally
identifiable information. Where possible, event transmitters and receivers
SHOULD devise approaches that prevent propagation -- for example, the
passing of a hash value that requires the subscriber to already know
the subject.There are no IANA considerations.The Open Group Base Specifications Issue 7Institute of Electrical and Electronics EngineersOpenID Connect Core 1.0NRIAssertions and Protocols for the OASIS Security Assertion Markup Language (SAML) V2.0Internet2The editor would like to thank the participants in the the SECEVENTS
working group for their support of this specification.This specification is based on and ideally replaces draft-ietf-secevent-delivery,
and we thank its contributors Annabelle Backman, Marius Scurtescu, and Morteza Ansari.Draft 00 - PH - Original