Network Working Group J. Peterson
Internet-Draft Neustar
Intended status: Informational March 5, 2018
Expires: September 6, 2018
PASSporT Extension for Diverted Calls
draft-ietf-stir-passport-divert-02.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.
Status of This Memo
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This Internet-Draft will expire on September 6, 2018.
Copyright Notice
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. PASSporT 'div' Claim . . . . . . . . . . . . . . . . . . . . 3
3.1. Nesting the original PASSporT in 'div' . . . . . . . . . 5
4. Using 'div' in SIP . . . . . . . . . . . . . . . . . . . . . 6
4.1. Authentication Service Behavior . . . . . . . . . . . . . 6
4.2. Verification Service Behavior . . . . . . . . . . . . . . 6
5. 'div' and Redirection . . . . . . . . . . . . . . . . . . . . 7
6. Extending 'div' to work with Service Logic Tracking . . . . . 8
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
9. Security Considerations . . . . . . . . . . . . . . . . . . . 9
10. Informative References . . . . . . . . . . . . . . . . . . . 9
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
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
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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 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 canonical form of the "dest" identifier (per the
canonicalization procedures in [RFC8224] 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, though it may include other elements as well,
including a copy of the original PASSporT. These new PASSporT
generated by retargeting entities MUST include the "div" PASSporT
type, and an "x5u" field pointing to a credential that the
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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.
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"} }
Note that the "div" claim may contain other elements than just a
destination, including a copy of the original PASSporT (see
Section 3.1). After the PASSporT header and claims have been
constructed, their signature is generated per the guidance in
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[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.
3.1. Nesting the original PASSporT in 'div'
For some use cases, rather than having multiple unconnected PASSporTS
associated with a single call, it makes more sense to nest the
PASSporTs, explicitly relating two PASSporTs to one another. For
example, when storing a PASSporT with "div" at a Call Placement
Service (CPS) for STIR out-of-band [I-D.ietf-stir-oob] scenarios,
clients MUST include an "opt" element within "div". "opt" contains
the full form of the original PASSporT from which the "div" was
generated. If the diverting entity originally received that PASSporT
encrypted, it MUST decrypt it before storing it in "opt." The entire
"div" PASSporT would than be signed and re-encrypted normally for
storage at an out-of-band Call Placement Service (CPS).
A "div" PASSporT containing the "opt" would look as follows:
{ "orig":{"tn":"12155551212"},
"dest":{"tn":"12155551214"},
"iat":1443208345,
"div":{"tn":"121555551213",
"opt":"eyJhbGciOiJFUzI1NiIsInR5cCI6InBhc3Nwb3J0IiwieDV1I \
joiaHR0cHM6Ly9jZXJ0LmV4YW1wbGUub3JnL3Bhc3Nwb3J0LmNlciJ9.eyJ
kZXN0Ijp7InVyaSI6WyJzaXA6YWxpY2VAZXhhbXBsZS5jb20iXX0sImlhdC \
I6IjE0NDMyMDgzNDUiLCJvcmlnIjp7InRuIjoiMTIxNTU1NTEyMTIifX0.r \
q3pjT1hoRwakEGjHCnWSwUnshd0-zJ6F1VOgFWSjHBr8Qjpjlk-cpFYpFYs \
ojNCpTzO3QfPOlckGaS6hEck7w"} }
The "opt" extension is RECOMMENDED for use within in-band SIP use
cases as well. The alternative, having multiple Identity headers in
a SIP request, could be confusing for some verification services.
However, nested PASSporTs could result in lengthy Identity headers,
and some operational experience is needed to ascertain how viable
multiple layers of nesting will be.
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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.
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.
Note that the authentication service doing so does not remove or
replace any existing Identity header fields, it simply adds a new
one. 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 compact form Identity header field to add to the message
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
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 (like History-Info) that record retargeting 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.
In order to use the "div" extension, a verification service needs to
inspect all of the valid Identity header field values associated with
a request, as an Identity header field value containing "div"
necessary refers to an earlier PASSporT already in the message. In
particular, the verification service must find a PASSporT associated
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with the call, one created earlier, 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 field value. 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, per [RFC8224] Section 6.2.1,
that decision is a matter of local policy.
Note that Identity header fields are not ordered in a SIP request,
and in a case where there is a multiplicity of Identity header fields
in a request, some sorting may be required to match divert PASSporTs
to their originals.
5. '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 could choose to redirect with a 3xx response code instead
of retargeting. In ordinary operations, a redirection poses no
difficult for the operations of baseline STIR: when the UAC receives
the 3xx response, it will initiate a new request to the new target
(typically carried in the Contact header field 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 requirement 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 to attest that 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.
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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; and thus the redirecting UAS
needs to include one nested PASSporT per new target. 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 per each Contact URI in the 3xx. Often that may
mean just one "div" PASSporT, but for some deployment scenarios, it
could require an impractical number of combinations.
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.
6. 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"} }
Past experience has shown that there may be additional information
about the motivation for retargeting that relying parties might
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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".
7. Acknowledgments
We would like to thank Robert Sparks for contributions to this
document.
8. 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]
9. 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.
10. Informative References
[I-D.ietf-stir-oob]
Rescorla, E. and J. Peterson, "STIR Out-of-Band
Architecture and Use Cases", draft-ietf-stir-oob-01 (work
in progress), October 2017.
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[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>.
[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>.
[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>.
[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>.
[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>.
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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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