Network Working Group J. Peterson
Internet-Draft Neustar
Updates: RFC8224 (if approved) October 22, 2018
Intended status: Standards Track
Expires: April 25, 2019
PASSporT Extension for Diverted Calls
draft-ietf-stir-passport-divert-04.txt
Abstract
This document extends PASSporT, which conveys cryptographically-
signed information about the people involved in personal
communications, to include an indication that a call has been
diverted from its original destination to a new one. This
information can greatly improve the decisions made by verification
services in call forwarding scenarios. Also specified here is an
encapsulation mechanism for nesting a PASSporT within another
PASSporT that assists relying parties in some diversion scenarios.
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
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This Internet-Draft will expire on April 25, 2019.
Copyright Notice
Copyright (c) 2018 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
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. PASSporT 'div' Claim . . . . . . . . . . . . . . . . . . . . 3
4. Using 'div' in SIP . . . . . . . . . . . . . . . . . . . . . 5
4.1. Authentication Service Behavior . . . . . . . . . . . . . 6
4.2. Verification Service Behavior . . . . . . . . . . . . . . 6
5. Definition of 'opt' . . . . . . . . . . . . . . . . . . . . . 7
6. 'div' and Redirection . . . . . . . . . . . . . . . . . . . . 7
7. Extending 'div' to work with Service Logic Tracking . . . . . 9
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
9.1. 'div' registration . . . . . . . . . . . . . . . . . . . 9
9.2. 'opt' registration . . . . . . . . . . . . . . . . . . . 10
10. Security Considerations . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
PASSporT [RFC8225] is a token format based on JWT [RFC7519] for
conveying cryptographically-signed information about the people
involved in personal communications; it is used with STIR [RFC8224]
to convey a signed assertion of the identity of the participants in
real-time communications established via a protocol like SIP. This
specification extends PASSporT to include an indication that a call
has been diverted from its originally destination to a new one.
Although the STIR problem statement [RFC7340] is focused on
preventing the impersonation of the caller's identity, which is a
common enabler for threats such as robocalling and voicemail hacking
on the telephone network today, it also provides a signature over the
called number as the authentication service sees it. As [RFC8224]
Section 12.1 describes, this protection over the contents of the To
header field is intended to prevent a class of cut-and-paste attacks.
If Alice calls Bob, for example, Bob might attempt to cut-and-paste
the Identity header field in Alice's INVITE into a new INVITE that
Bob sends to Carol, and thus be able to fool Carol into thinking the
call came from Alice and not Bob. With the signature over the To
header field value, the INVITE Carol sees will clearly have been
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destined originally for Bob, and thus Carol can view the INVITE as
suspect.
However, as [RFC8224] Section 12.1.1 points out, it is difficult for
Carol to confirm or reject these suspicions based on the information
she receives from the baseline PASSporT object. The common "call
forwarding" service serves as a good example of the fact that the
original called party number is not always the number to which a call
is delivered. The address in the To header field value of SIP
requests is not supposed to change, accordingly to baseline
[RFC3261], as it is the Request-URI that is supposed to be updated
when a call is retargeted, but practically speaking some operational
environments do alter the To header field. There are a number of
potential ways for intermediaries to indicate that such a forwarding
operating has taken place. The History-Info header field [RFC7044]
was created to store the Request-URIs that are discarded by a call in
transit. The SIP Diversion header field [RFC5806], though historic,
is still used for this purpose by some operators today. Neither of
these header fields provide any cryptographic assurance of secure
redirection, and they can both capture minor syntactical changes in
URIs that do not reflect a change to the actual target of a call.
This specification therefore extends PASSporT with an explicit
indication that the original called number in PASSporT no longer
reflects the destination to which a call is likely to be delivered.
Verification services and the relying parties who make authorization
decisions about communications may use this indication to confirm
that a legitimate retargeting of the call has taken place, rather
than a cut-and-paste attack. In support of this goal, this
specification also defines a nesting mechanism for PASSporTs that
allows the original unmodified PASSporT to be conveyed to relying
parties.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in [RFC2119].
3. PASSporT 'div' Claim
This specification defines a new JSON Web Token claim for "div" which
indicates a previous destination for a call during its routing
process. When a retargeting entity receives a call signed with a
PASSporT, it may act as an authentication service and create a new
PASSporT containing the "div" claim to attach to the call. Note that
a new PASSporT is only necessary when the canonical form of the
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"dest" identifier (per the canonicalization procedures in [RFC8224]
Section 8) changes due to this retargeting. "div" is populated with a
destination address found in the "dest" field of PASSporT received by
the retargeting entity as well as a copy of the original PASSporT.
These new PASSporTs generated by retargeting entities MUST include
the "div" PASSporT type, and an "x5u" field pointing to a credential
that the retargeting entity controls. The new PASSporT header will
look as follows:
{ "typ":"passport",
"ppt":"div",
"alg":"ES256",
"x5u":"https://www.example.com/cert.pkx" }
A PASSporT claims object containing "div" is populated with a
modification of the original token before the call was retargeted: at
a high level, the original identifier for the called party in the
"dest" array will become the "div" claim in the new PASSporT. If the
"dest" array of the original PASSporT contains multiple identifiers,
the retargeting entity MUST select only one them to occupy the "div"
field in the new PASSporT, and in particular, it MUST select an
identifier that is within the scope of the credential that the
retargeting entity will specify in the "x5u" of the PASSporT header
(as described below).
The new target for the call selected by the retargeting entity
becomes the value of the "dest" array of the new PASSporT. The
"orig" value MUST be copied into the new PASSporT from the original
PASSporT received by the retargeting entity. The retargeting entity
SHOULD retain the "iat" value from the original PASSporT, though if
in the underlying signaling protocol (e.g. SIP) the retargeting
entity changes the date and time information in the retargeted
request, the new PASSporT should instead reflect that date and time.
"opt" (see Section 5) contains the full form of the original PASSporT
from which the "div" was generated. No other extension claims should
be copied from the original PASSporT to the "div" PASSporT.
So, for an original PASSporT of the form:
{ "orig":{"tn":"12155551212"},
"dest":{"tn":"12155551213"},
"iat":1443208345 }
If the retargeting entity is changing the target from 12155551213 to
12155551214, the new PASSporT with "div" would look as follows:
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{ "orig":{"tn":"12155551212"},
"dest":{"tn":"12155551214"},
"iat":1443208345,
"div":{"tn":"121555551213"},
"opt":"eyJ0eXAiOiJwYXNzcG9ydCIsInBwdCI6ImRpdiIsImFsZyI6IkVT \
MjU2IiwieDV1IjoiaHR0cHM6Ly93d3cuZXhhbXBsZS5jb20vY2VydC5wa3g \
ifQ==.eyJvcmlnIjp7InRuIjoiMTIxNTU1NTEyMTIifSwiZGVzdCI6eyJ0b \
iI6IjEyMTU1NTUxMjEzIn0sImlhdCI6MTQ0MzIwODM0NX0=.rq3pjT1hoRw \
akEGjHCnWSwUnshd0-zJ6F1VOgFWSjHBr8Qjpjlk-cpFYpFYsojNCpTzO3Q \
fPOlckGaS6hEck7w"} }
Note that the "div" claim may contain other elements than just a
destination, including a History-Info indicator (see Section 7).
After the PASSporT header and claims have been constructed, their
signature is generated per the guidance in [RFC8225] - except for the
credential required to sign it. While in the ordinary construction
of a PASSporT, the credential used to sign will have authority over
the identity in the "orig" claim (for example, a certificate with
authority over the telephone number in "orig" per [RFC8226]), for all
PASSporTs using the "div" type the signature MUST be created with a
credential with authority over the identity present in the "div"
claim. So for the example above, where the original "dest" is
"12155551213", the signer of the new PASSporT object MUST have
authority over that telephone number, and need not have any authority
over the telephone number present in the "orig" claim.
Instead of having multiple unlinked PASSporTs associated with a
single call, it is helpful to relying parties to nest diversion
PASSporTs, explicitly relating the original PASSporT to the diverted
one. Note that the approach of having multiple Identity headers in a
SIP request was considered in prior versions of this specification,
but it could be confusing for some verification services. The "opt"
extension is REQUIRED for use within in-band SIP use cases as well as
out-of-band [I-D.ietf-stir-oob] scenarios. Nested PASSporTs could
result in lengthy Identity headers, and some operational experience
is needed to ascertain how resilient legacy implementations will be
to large headers.
4. Using 'div' in SIP
This section specifies SIP-specific usage for the "div" PASSporT type
and its handling in the SIP Identity header field "ppt" parameter
value. Other using protocols of PASSporT may define behavior
specific to their use of the "div" claim.
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4.1. Authentication Service Behavior
An authentication service only adds an Identity header field value
containing the "div" PASSporT type to a SIP request that already
contains at least one Identity header field value; it MUST NOT add a
"div" PASSporT to an INVITE that contains no Identity headers field.
As the authentication service will be adding a new PASSporT that
contains an encapsulation of the original, it SHOULD remove the
original request's Identity header field value before forwarding.
Note that a request may contain multiple Identity header field values
generated by different authorities; as a consequence, the retargeting
authentication service may need to perform this operation on multiple
existing PASSporTs, adding a "div" PASSporT per PASSporT in the
original. When adding an Identity header field with a PASSporT
object containing a "div" claim, SIP authentication services MUST
also add a "ppt" parameter to that Identity header with a value of
"div". The resulting full form Identity header field to add to the
message might look as follows:
Identity: eyJhbGciOiJFUzI1NiIsInR5cCI6InBhc3Nwb3J0IiwieDV1IjoiaHR0cHM6Ly9 \
jZXJ0LmV4YW1wbGUub3JnL3Bhc3Nwb3J0LmNlciJ9.eyJvcmlnIjp7InRuIjoiMTIxNTU1NTE \
yMTIifSwiZGVzdCI6eyJ0biI6IjEyMTU1NTUxMjE0In0sImlhdCI6MTQ0MzIwODM0NSwiZGl2 \
Ijp7InRuIjoiMTIxNTU1NTUxMjEzIn0sIm9wdCI6ImV5SjBlWEFpT2lKd1lYTnpjRzl5ZENJc \
0luQndkQ0k2SW1ScGRpSXNJbUZzWnlJNklrVlRNalUySWl3aWVEVjFJam9pYUhSMGNITTZMeT \
kzZDNjdVpYaGhiWEJzWlM1amIyMHZZMlZ5ZEM1d2EzZ2lmUT09LmV5SnZjbWxuSWpwN0luUnV \
Jam9pTVRJeE5UVTFOVEV5TVRJaWZTd2laR1Z6ZENJNmV5SjBiaUk2SWpFeU1UVTFOVFV4TWpF \
ekluMHNJbWxoZENJNk1UUTBNekl3T0RNME5YMD0ucnEzcGpUMWhvUndha0VHakhDbldTd1Vuc \
2hkMC16SjZGMVZPZ0ZXU2pIQnI4UWpwamxrLWNwRllwRllzb2pOQ3BUek8zUWZQT2xja0dhUz \
ZoRWNrN3cifX0=.rq3pjT1hoRwakEGjHCnWSwUnshd0-zJ6F1VOgFWSjHBr8Qjpjlk-cpFYpF \
YsojNCpTzO3QfPOlckGaS6hEck7w;
info=<https://biloxi.example.org/biloxi.cer>;alg=ES256;ppt="div"
A SIP authentication service typically will derive the new value of
"dest" from a new Request-URI that is set for the SIP request before
it is forwarded. Older values of the Request-URI may appear in
header fields like Diversion or History-Info; this document specifies
no specific interaction between the "div" mechanism and those SIP
header fields. Note as well that because PASSporT operates on
canonicalized telephone numbers and normalized URIs, many smaller
changes to the syntax of identifiers that might be captured by other
mechanisms that record retargeting (like History-Info) will likely
not require a "div" PASSporT.
4.2. Verification Service Behavior
[RFC8224] Section 6.2 Step 5 requires that specifications defining
"ppt" values describe any additional verifier behavior. The behavior
specified for the "div" value of "ppt" is as follows.
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In order to use the "div" extension, a verification service needs to
inspect any nested PASSporT objects within PASSporTs it validates, as
an Identity header field value containing "div" necessarily refers to
an earlier PASSporT nested. If a nested PASSporT within a "div"
PASSporT contains a "dest" claim with a value not equivalent to the
"div" claim in the "div" PASSporT, that "div" PASSporT SHOULD NOT be
considered valid. It is possible that this nested PASSporT will also
contain a "div", and that it will in turn chain to a still earlier
PASSporT nested within it. Ultimately, by looking at this chain of
transformations and validating the associated signatures, the
verification service will be able to ascertain that the appropriate
parties were responsible for the retargeting of the call to its
ultimate destination; this can help the verification service to
determine that the original PASSporT in the call was not simply used
in a cut-and-paste attack. This will help relying parties to make
any associated authorization decisions in terms of how the call will
be treated - though, per [RFC8224] Section 6.2.1, that decision is a
matter of local policy.
5. Definition of 'opt'
The presence of an original PASSporT claims object element,
designated as "opt", signifies that a PASSporT encapsulates another
entire PASSporT within it, typically a PASSporT that was transformed
in some way to create the current PASSporT. Relying parties may need
to consult the encapsulated PASSporT in order to validate the
identity of a caller. "opt" as defined in this specification may be
used by future PASSporT extensions as well as in conjunction with
"div".
"opt" MUST contain a quoted base64 encoded full-form PASSporT; it
MUST NOT contain a compact form PASSporT. For an example of a "div"
PASSporT containing "opt," see Section 3.
6. 'div' and Redirection
The "div" mechanism exists primarily to prevent false negatives at
verification services when an arriving SIP request, due to
intermediary retargeting, does not appear to be intended for its
eventual recipient, because its "dest" value designates a different
original destination.
Any intermediary that assigns a new target to a request can, instead
of retargeting and forwarding the request, instead redirect with a
3xx response code. In ordinary operations, a redirection poses no
difficulties for the operations of baseline STIR: when the UAC
receives the 3xx response, it will initiate a new request to the new
target (typically the target carried in the Contact header field
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value of the 3xx), and the "dest" of the PASSporT created for the new
request will match that new target. As no impersonation attack can
arise from this case, it creates no new requirements for STIR.
However, some UACs record the original target of a call with
mechanisms like History-Info [RFC7044] or Diversion [RFC5806], and
may want to leverage STIR to demonstrate to the ultimate recipient
that the call has been redirected securely: that is, that the
original destination was the one that sent the redirection message
that led to the recipient receiving the request. The semantics of
the PASSporT necessary for that assertion are the same as those for
the "div" retargeting cases above. The only wrinkle is that the
PASSporT needs to be generated by the redirecting entity and sent
back to the originating user agent client within the 3xx response.
This introduces more complexity than might immediately be apparent.
In the first place, a 3xx response can convey multiple targets
through the Contact header field value; to accommodate this, the
"div" PASSporT MAY include one "dest" array value per Contact, but if
the retargeting entity wants to keep the Contact list private from
targets, it may need to generate one PASSporT per Contact. Bear in
mind as well that the original SIP request could have carried
multiple Identity header field values that had been added by
different authentication services in the request path, so a
redirecting entity might need to generate one nested "div" PASSporT
per each PASSporT in the original request. Often this will mean just
one "div" PASSporT, but for some deployment scenarios, it could
require an impractical number of combinations. But in very complex
call routing scenarios, attestation of source identity would only add
limited value anyway.
STIR-aware intermediaries that redirect requests MAY therefore convey
one or more PASSporTs in the backwards direction within Identity
headers. This document consequently updates [RFC8224] to permit
carrying Identity headers in SIP 300-class responses. It is left to
authentication services to determine which Identity headers should be
copied into any new requests resulting from the redirection, if any:
use of these Identity headers by entities receiving a 3xx response is
OPTIONAL.
Finally, note that if an intermediary in the response path consumes
the 3xx and explores new targets itself while performing sequential
forking, it will effectively retarget the call on behalf of the
redirecting server, and this will create the same need for "div"
PASSporTs as any other retargeted call.
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7. Extending 'div' to work with Service Logic Tracking
It is anticipated that "div" may be used in concert with History-Info
[RFC7044] in some deployments. It may not be clear from the "orig"
and "dest" values which History-Info header a given PASSporT
correlates to, especially because some of the target changes tracked
by History-Info will not be reflected in a "div" PASSporT (see
Section 1). Therefore an "hi" element may appear in "div"
corresponding to the History-Info header field index parameter value.
So for a History-Info header with an index value of "1.2.1", the
claims object of the corresponding PASSporT with "div" might look
like:
{ "orig":{"tn":"12155551212"},
"dest":{"tn":"12155551214"},
"iat":1443208345,
"div":{"tn":"121555551213",
"hi":"1.2.1"}
"opt":[...]" }
Past experience has shown that there may be additional information
about the motivation for retargeting that relying parties might
consider when making authorization decisions about a call, see for
example the "reason" associated with the SIP Diversion header field
[RFC5806]. Future extensions to this specification might incorporate
reasons into "div".
8. Acknowledgments
We would like to thank Eric Burger, Dave Hancock, Chris Wendt and
Robert Sparks for contributions to this document.
9. IANA Considerations
This specification requests that the IANA add two new claims to the
JSON Web Token Claims registry as defined in [RFC7519].
9.1. 'div' registration
Claim Name: "div"
Claim Description: New Target of a Call
Change Controller: IESG
Specification Document(s): [RFCThis]
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9.2. 'opt' registration
Claim Name: "opt"
Claim Description: Encapsulated JSON token
Change Controller: IESG
Specification Document(s): [RFCThis]
10. Security Considerations
This specification describes a security feature, and is primarily
concerned with increasing security when calls are forwarded.
Including information about how calls were retargeted during the
routing process can allow downstream entities to infer particulars of
the policies used to route calls through the network. However,
including this information about forwarding is at the discretion of
the retargeting entity, so if there is a requirement to keep the
original called number confidential, no PASSporT should be created
for that retargeting - the only consequence will be that downstream
entities will be unable to correlate an incoming call with the
original PASSporT without access to some prior knowledge of the
policies that could have caused the retargeting.
Any extension that makes PASSporTs larger creates a potential
amplification mechanism for SIP-based DDoS attacks. Since diversion
PASSporTs are created as a part of normal forwarding activity, this
risk arises at the discretion of the retargeting domain: simply using
3xx response redirections rather than retargeting (with supply a
"div" per Section 6) mitigates the potential impact. Under unusual
traffic loads, even domains that might ordinarily retarget requests
can switch to redirection.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.
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[RFC7044] Barnes, M., Audet, F., Schubert, S., van Elburg, J., and
C. Holmberg, "An Extension to the Session Initiation
Protocol (SIP) for Request History Information", RFC 7044,
DOI 10.17487/RFC7044, February 2014,
<https://www.rfc-editor.org/info/rfc7044>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>.
[RFC8224] Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
"Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 8224,
DOI 10.17487/RFC8224, February 2018,
<https://www.rfc-editor.org/info/rfc8224>.
[RFC8225] Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,
<https://www.rfc-editor.org/info/rfc8225>.
[RFC8226] Peterson, J. and S. Turner, "Secure Telephone Identity
Credentials: Certificates", RFC 8226,
DOI 10.17487/RFC8226, February 2018,
<https://www.rfc-editor.org/info/rfc8226>.
11.2. Informative References
[I-D.ietf-stir-oob]
Rescorla, E. and J. Peterson, "STIR Out-of-Band
Architecture and Use Cases", draft-ietf-stir-oob-03 (work
in progress), July 2018.
[RFC5806] Levy, S. and M. Mohali, Ed., "Diversion Indication in
SIP", RFC 5806, DOI 10.17487/RFC5806, March 2010,
<https://www.rfc-editor.org/info/rfc5806>.
[RFC7340] Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
Telephone Identity Problem Statement and Requirements",
RFC 7340, DOI 10.17487/RFC7340, September 2014,
<https://www.rfc-editor.org/info/rfc7340>.
Author's Address
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Jon Peterson
Neustar, Inc.
1800 Sutter St Suite 570
Concord, CA 94520
US
Email: jon.peterson@team.neustar
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