Internet DRAFT - draft-peterson-passport-divert
draft-peterson-passport-divert
Network Working Group J. Peterson
Internet-Draft Neustar
Intended status: Informational June 12, 2017
Expires: December 14, 2017
PASSporT Extension for Diverted Calls
draft-peterson-passport-divert-01.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 originally destination to a new one. This
information can greatly improve the decisions made by verification
services in call forwarding scenarios.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 14, 2017.
Copyright Notice
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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. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. PASSporT 'div' Claim . . . . . . . . . . . . . . . . . . . . 3
4. Using 'div' in SIP . . . . . . . . . . . . . . . . . . . . . 5
4.1. Authentication Service Behavior . . . . . . . . . . . . . 5
4.2. Verification Service Behavior . . . . . . . . . . . . . . 5
5. Extending 'div' . . . . . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. Informative References . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
PASSporT [I-D.ietf-stir-passport] 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
[I-D.ietf-stir-rfc4474bis] 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
[I-D.ietf-stir-rfc4474bis] 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 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, the INVITE Carol sees will clearly have
been destined originally for Bob, and thus Carol can view the INVITE
as suspect.
However, as [I-D.ietf-stir-rfc4474bis] 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
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that the original called party number is not always the numebr 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 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.
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 (without
removing the original PASSporT). Note that a new PASSporT is only
necessary when the cannical form of the "dest" identifier (per the
canonicalization procedures in [I-D.ietf-stir-rfc4474bis] Section 8)
changes due to this retargeting. "div" is typically populated with a
destination address found in the "dest" field of PASSporT received by
the retargeting entity. These new PASSporT 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 will look as follows:
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{ "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 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 regargeting 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 reflect that. 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:
{ "orig":{"tn":"12155551212"},
"dest":{"tn":"12155551214"},
"iat":1443208345,
"div":{"tn":"121555551213"} }
After the PASSporT header and claims have been constructed, their
signature is generated per the guidance in [I-D.ietf-stir-passport] -
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
[I-D.ietf-stir-certificates]), for all PASSporTs using the "div" type
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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.
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 behvaior
specific to their use of the "div" claim.
4.1. Authentication Service Behavior
An authentication service only adds an Identity header field
containing the "div" PASSporT type to an SIP request that already
contains at least one Identity header field; it MUST NOT add a "div"
request to an INVITE that contains no other Identity headers fields.
When adding an Identity header 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 compact form Identity header might look as follows:
Identity: ..sv5CTo05KqpSmtHt3dcEiO/1CWTSZtnG3iV+1nmurLXV/HmtyNS7Ltrg9dlxkWzo
eU7d7OV8HweTTDobV3itTmgPwCFjaEmMyEI3d7SyN21yNDo2ER/Ovgtw0Lu5csIp
pPqOg1uXndzHbG7mR6Rl9BnUhHufVRbp51Mn3w0gfUs=; \
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
headers like Diversion or History-Info; this document specifies no
specific interaction between the "div" mechanism and those SIP
headers. 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 (like History-Info) that record regargeting will likely
not require a "div" PASSporT.
4.2. Verification Service Behavior
[I-D.ietf-stir-rfc4474bis] 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 verificiation service needs to
inspect all of the valid Identity headers associated with a request,
as an Identity header containing "div" necessary refers to an earlier
PASSporT associated with the request. In particular, the
verification service must find an earlier PASSporT associated with
the call that contains a "dest" claim with a value equivalent to the
"div" claim in the current PASSporT. It is possible that this
earlier PASSporT will also contain a "div", and that it will in turn
chain to a still earlier PASSporT stored in a different Identity
header. 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 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 those policies are
outside the scope of this document.
5. Extending 'div'
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".
6. Acknowledgments
We would like to thank YOU for contributions to this problem
statement and framework.
7. IANA Considerations
This specification requests that the IANA add a new claim to the JSON
Web Token Claims registry as defined in [RFC7519].
Claim Name: "div"
Claim Description: New Target of a Call
Change Controller: IESG
Specification Document(s): [RFCThis]
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8. 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 reveal to downstream entities particulars of the
policies used to route calls through the network. But 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 have less confidence that the PASSporT was meant to be
associated with this call.
9. Informative References
[I-D.ietf-stir-certificates]
Peterson, J. and S. Turner, "Secure Telephone Identity
Credentials: Certificates", draft-ietf-stir-
certificates-14 (work in progress), May 2017.
[I-D.ietf-stir-passport]
Wendt, C. and J. Peterson, "Personal Assertion Token
(PASSporT)", draft-ietf-stir-passport-11 (work in
progress), February 2017.
[I-D.ietf-stir-rfc4474bis]
Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
"Authenticated Identity Management in the Session
Initiation Protocol (SIP)", draft-ietf-stir-rfc4474bis-16
(work in progress), February 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://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,
<http://www.rfc-editor.org/info/rfc3261>.
[RFC5806] Levy, S. and M. Mohali, Ed., "Diversion Indication in
SIP", RFC 5806, DOI 10.17487/RFC5806, March 2010,
<http://www.rfc-editor.org/info/rfc5806>.
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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,
<http://www.rfc-editor.org/info/rfc7044>.
[RFC7340] Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
Telephone Identity Problem Statement and Requirements",
RFC 7340, DOI 10.17487/RFC7340, September 2014,
<http://www.rfc-editor.org/info/rfc7340>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<http://www.rfc-editor.org/info/rfc7519>.
Author's Address
Jon Peterson
Neustar, Inc.
1800 Sutter St Suite 570
Concord, CA 94520
US
Email: jon.peterson@neustar.biz
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