draft-ietf-stir-certificates-00.txt   draft-ietf-stir-certificates-01.txt 
Network Working Group J. Peterson STIR Working Group J. Peterson
Internet-Draft NeuStar Internet-Draft NeuStar
Intended status: Standards Track S. Turner Intended status: Standards Track S. Turner
Expires: April 26, 2015 IECA Expires: September 25, 2015 IECA
October 23, 2014 March 24, 2015
Secure Telephone Identity Credentials: Certificates Secure Telephone Identity Credentials: Certificates
draft-ietf-stir-certificates-00.txt draft-ietf-stir-certificates-01.txt
Abstract Abstract
In order to prove ownership of telephone numbers on the Internet, In order to prove ownership of telephone numbers on the Internet,
some kind of public infrastructure needs to exist that binds some kind of public infrastructure needs to exist that binds
cryptographic keys to authority over telephone numbers. This cryptographic keys to authority over telephone numbers. This
document describes a certificate-based credential system for document describes a certificate-based credential system for
telephone numbers, which could be used as a part of a broader telephone numbers, which could be used as a part of a broader
architecture for managing telephone numbers as identities in architecture for managing telephone numbers as identities in
protocols like SIP. protocols like SIP.
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on April 26, 2015. This Internet-Draft will expire on September 22, 2015.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 Table of Contents
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Enrollment and Authorization . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
3.1. Certificate Scope and Structure . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Provisioning Private Keying Material . . . . . . . . . . 5 3. Enrollment and Authorization . . . . . . . . . . . . . . . . . 3
4. Acquiring Credentials to Verify Signatures . . . . . . . . . 5 3.1. Certificate Scope and Structure . . . . . . . . . . . . . 4
4.1. Verifying Certificate Scope . . . . . . . . . . . . . . . 6 3.2. Provisioning Private Keying Material . . . . . . . . . . . 5
4.2. Certificate Freshness and Revocation . . . . . . . . . . 8 4. Acquiring Credentials to Verify Signatures . . . . . . . . . . 5
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8 4.1. Verifying Certificate Scope . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 4.2. Certificate Freshness and Revocation . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8 5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
8. Informative References . . . . . . . . . . . . . . . . . . . 9 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. Informative References . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction 1. Introduction
As is discussed in the STIR problem statement [13], the primary As is discussed in the STIR problem statement [13], the primary
enabler of robocalling, vishing, swatting and related attacks is the enabler of robocalling, vishing, swatting and related attacks is the
capability to impersonate a calling party number. The starkest capability to impersonate a calling party number. The starkest
examples of these attacks are cases where automated callees on the examples of these attacks are cases where automated callees on the
PSTN rely on the calling number as a security measure, for example to Public Switched Telephone Network (PSTN) rely on the calling number
access a voicemail system. Robocallers use impersonation as a means as a security measure, for example to access a voicemail system.
of obscuring identity; while robocallers can, in the ordinary PSTN, Robocallers use impersonation as a means of obscuring identity; while
block (that is, withhold) their caller identity, callees are less robocallers can, in the ordinary PSTN, block (that is, withhold)
likely to pick up calls from blocked identities, and therefore their caller identity, callees are less likely to pick up calls from
appearing to calling from some number, any number, is preferable. blocked identities, and therefore appearing to calling from some
Robocallers however prefer not to call from a number that can trace number, any number, is preferable. Robocallers however prefer not to
back to the robocaller, and therefore they impersonate numbers that call from a number that can trace back to the robocaller, and
are not assigned to them. therefore they impersonate numbers that are not assigned to them.
One of the most important components of a system to prevent One of the most important components of a system to prevent
impersonation is an authority responsible for issuing credentials to impersonation is an authority responsible for issuing credentials to
parties who control telephone numbers. With these credentials, parties who control telephone numbers. With these credentials,
parties can prove that they are in fact authorized to use telephony parties can prove that they are in fact authorized to use telephony
numbers, and thus distinguish themselves from impersonators unable to numbers, and thus distinguish themselves from impersonators unable to
present credentials. This document describes a credential system for present credentials. This document describes a credential system for
telephone numbers based on X.509 version 3 certificates in accordance telephone numbers based on X.509 version 3 certificates in accordance
with [7]. While telephone numbers have long been a part of the X.509 with [7]. While telephone numbers have long been a part of the X.509
standard, the certificates described in this document may contain standard, the certificates described in this document may contain
telephone number blocks or ranges, and accordingly it uses an telephone number blocks or ranges, and accordingly it uses an
alternate syntax. alternate syntax.
In the STIR in-band architecture, two basic types of entities need In the STIR in-band architecture, two basic types of entities need
access to these credentials: authentication services, and access to these credentials: authentication services, and
verification services (or verifiers); see [15]. An authentication verification services (or verifiers); see [15]. An authentication
service must be operated by an entity enrolled with the certificate service must be operated by an entity enrolled with the certification
authority (see Section 3), whereas a verifier need only trust the authority (see Section 3), whereas a verifier need only trust the
root certificate of the authority, and have a means to acquire and root certificate of the authority, and have a means to acquire and
validate certificates. validate certificates.
This document attempts to specify only the basic elements necessary This document attempts to specify only the basic elements necessary
for this architecture. Only through deployment experience will it be for this architecture. Only through deployment experience will it be
possible to decide directions for future work. possible to decide directions for future work.
2. Terminology 2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED", In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in RFC 2119 [1] and RFC 6919 [2]. described in RFC 2119 [1] and RFC 6919 [2].
3. Enrollment and Authorization 3. Enrollment and Authorization
This document assumes a threefold model for certificate enrollment. This document assumes a threefold model for certificate enrollment.
The first enrollment model is one where the certificate authority The first enrollment model is one where the certification authority
acts in concert with national numbering authorities to issue (CA) acts in concert with national numbering authorities to issue
credentials to those parties to whom numbers are assigned. In the credentials to those parties to whom numbers are assigned. In the
United States, for example, telephone number blocks are assigned to United States, for example, telephone number blocks are assigned to
Local Exchange Carriers (LECs) by the North American Numbering Plan Local Exchange Carriers (LECs) by the North American Numbering Plan
Administrator (NANPA), who is in turn directed by the national Administrator (NANPA), who is in turn directed by the national
regulator. LECs may also receive numbers in smaller allocations, regulator. LECs may also receive numbers in smaller allocations,
through number pooling, or via an individual assignment through through number pooling, or via an individual assignment through
number portability. LECs assign numbers to customers, who may be number portability. LECs assign numbers to customers, who may be
private individuals or organizations - and organizations take private individuals or organizations - and organizations take
responsibility for assigning numbers within their own enterprise. responsibility for assigning numbers within their own enterprise.
The second enrollment model is one where a certificate authority The second enrollment model is one where a certification authority
requires that an entity prove control by means of some sort of test. requires that an entity prove control by means of some sort of test.
For example, an authority might send a text message to a telephone For example, an authority might send a text message to a telephone
number containing a URL (which might be deferenced by the recipient) number containing a URL (which might be deferenced by the recipient)
as a means of verifying that a user has control of terminal as a means of verifying that a user has control of terminal
corresponding to that number. Checks of this form are frequently corresponding to that number. Checks of this form are frequently
used in commercial systems today to validate telephone numbers used in commercial systems today to validate telephone numbers
provided by users. This is comparable to existing enrollment systems provided by users. This is comparable to existing enrollment systems
used by some certificate authorities for issuing S/MIME credentials used by some certificate authorities for issuing S/MIME credentials
for email by verifying that the party applying for a credential for email by verifying that the party applying for a credential
receives mail at the email address in question. receives mail at the email address in question.
The third enrollment model is delegation: that is, the holder of a The third enrollment model is delegation: that is, the holder of a
certificate (assigned by either of the two methods above) may certificate (assigned by either of the two methods above) may
delegate some or all of their authority to another party. In some delegate some or all of their authority to another party. In some
cases, multiple levels of delegation could occur: a LEC, for example, cases, multiple levels of delegation could occur: a LEC, for example,
might delegate authority to customer organization for a block of 100 might delegate authority to customer organization for a block of 100
numbers, and the organization might in turn delegate authority for a numbers, and the organization might in turn delegate authority for a
particular number to an individual employee. This is analogous to particular number to an individual employee. This is analogous to
delegation of organizational identities in traditional hierarchical delegation of organizational identities in traditional hierarchical
PKIs who use the name constraints extension [3]; the root CA Public Key Infrastructures (PKIs) who use the name constraints
delegates names in sales to the sales department CA, names in extension [3]; the root CA delegates names in sales to the sales
development to the development CA, etc. As lengthy certificate department CA, names in development to the development CA, etc. As
delegation chains are brittle, however, and can cause delays in the lengthy certificate delegation chains are brittle, however, and can
verification process, this document considers optimizations to reduce cause delays in the verification process, this document considers
the complexity of verification. optimizations to reduce the complexity of verification.
[TBD] Future versions of this specification may address adding a [TBD] Future versions of this specification may address adding a
level of assurance indication to certificates to differentiate those level of assurance indication to certificates to differentiate those
enrolled from proof-of-possession versus delegation. enrolled from proof-of-possession versus delegation.
[TBD] Future versions of this specification may also discuss methods [TBD] Future versions of this specification may also discuss methods
of partial delegation, where certificate holders delegate only part of partial delegation, where certificate holders delegate only part
of their authority. For example, individual assignees may want to of their authority. For example, individual assignees may want to
delegate to a service authority for text messages associated with delegate to a service authority for text messages associated with
their telephone number, but not for other functions. their telephone number, but not for other functions.
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Moreover, service providers may wish to have multiple certificates Moreover, service providers may wish to have multiple certificates
with the same scope of authority. For example, a service provider with the same scope of authority. For example, a service provider
with several regional gateway systems may want each system to be with several regional gateway systems may want each system to be
capable of signing for each of their numbers, but not want to have capable of signing for each of their numbers, but not want to have
each system share the same private key. each system share the same private key.
The set of telephone numbers for which a particular certificate is The set of telephone numbers for which a particular certificate is
valid is expressed in the certificate through a certificate valid is expressed in the certificate through a certificate
extension; the certificate's extensibility mechanism is defined in extension; the certificate's extensibility mechanism is defined in
RFC 5280 but the telephone number authorization extension is defined [7] but the telephone number authorization extension is defined in
in this document. this document.
3.2. Provisioning Private Keying Material 3.2. Provisioning Private Keying Material
In order for authentication services to sign calls via the procedures In order for authentication services to sign calls via the procedures
described in [15], they must possess a private key corresponding to a described in [15], they must possess a private key corresponding to a
certificate with authority over the calling number. This certificate with authority over the calling number. This
specification does not require that any particular entity sign specification does not require that any particular entity sign
requests, only that it be an entity with an appropriate private key; requests, only that it be an entity with an appropriate private key;
the authentication service role may be instantiated by any entity in the authentication service role may be instantiated by any entity in
a SIP network. For a certificate granting authority only over a a SIP network. For a certificate granting authority only over a
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4.1. Verifying Certificate Scope 4.1. Verifying Certificate Scope
The subjects of these certificates are the administrative entities to The subjects of these certificates are the administrative entities to
whom numbers are assigned or delegated. When a verifier is whom numbers are assigned or delegated. When a verifier is
validating a caller's identity, local policy always determines the validating a caller's identity, local policy always determines the
circumstances under which any particular subject may be trusted, but circumstances under which any particular subject may be trusted, but
for the purpose of validating a caller's identity, this certificate for the purpose of validating a caller's identity, this certificate
extension establishes whether or not a signer is authorized to sign extension establishes whether or not a signer is authorized to sign
for a particular number. for a particular number.
The TN Authorization List certificate extension is identified by the The telephone number (TN) Authorization List certificate extension is
following object identifier: identified by the following object identifier:
id-ce-TNAuthList OBJECT IDENTIFIER ::= { TBD } id-ce-TNAuthList OBJECT IDENTIFIER ::= { TBD }
The TN Authorization List certificate extension has the following The TN Authorization List certificate extension has the following
syntax: syntax:
TNAuthorizationList ::= SEQUENCE SIZE (1..MAX) OF TNAuthorization TNAuthorizationList ::= SEQUENCE SIZE (1..MAX) OF TNAuthorization
TNAuthorization ::= SEQUENCE SIZE (1..MAX) OF TNEntry TNAuthorization ::= SEQUENCE SIZE (1..MAX) OF TNEntry
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id-ce-TNAuthList OBJECT IDENTIFIER ::= { TBD } id-ce-TNAuthList OBJECT IDENTIFIER ::= { TBD }
The TN Authorization List certificate extension has the following The TN Authorization List certificate extension has the following
syntax: syntax:
TNAuthorizationList ::= SEQUENCE SIZE (1..MAX) OF TNAuthorization TNAuthorizationList ::= SEQUENCE SIZE (1..MAX) OF TNAuthorization
TNAuthorization ::= SEQUENCE SIZE (1..MAX) OF TNEntry TNAuthorization ::= SEQUENCE SIZE (1..MAX) OF TNEntry
TNEntry ::= CHOICE { TNEntry ::= CHOICE {
spid ServiceProviderIdentifierList, spid ServiceProviderIdentifierList,
range TelephoneNumberRange, range TelephoneNumberRange,
one E164Number } one E164Number }
ServiceProviderIdentifierList ::= SEQUENCE SIZE (1..3) OF ServiceProviderIdentifierList ::= SEQUENCE SIZE (1..3) OF
OCTET STRING OCTET STRING
-- When all three are present: SPID, Alt SPID, and Last Alt SPID -- When all three are present: SPID, Alt SPID, and Last Alt SPID
TelephoneNumberRange ::= SEQUENCE { TelephoneNumberRange ::= SEQUENCE {
start E164Number, start E164Number,
count INTEGER } count INTEGER }
E164Number ::= IA5String (SIZE (1..15)) (FROM ("0123456789")) E164Number ::= IA5String (SIZE (1..15)) (FROM ("0123456789"))
[TBD- do we really need to do IA5String? The alternative would be [TBD] Do we really need to do IA5String? The alternative would be
UTF8String, e.g.: UTF8String (SIZE (1..15)) (FROM ("0123456789")) ] UTF8String, e.g.: UTF8String (SIZE (1..15)) (FROM ("0123456789"))
The TN Authorization List certificate extension indicates the The TN Authorization List certificate extension indicates the
authorized phone numbers for the call setup signer. It indicates one authorized phone numbers for the call setup signer. It indicates one
or more blocks of telephone number entries that have been authorized or more blocks of telephone number entries that have been authorized
for use by the call setup signer. There are three ways to identify for use by the call setup signer. There are three ways to identify
the block: 1) a Service Provider Identifier (SPID) can be used to the block: 1) a Service Provider Identifier (SPID) can be used to
indirectly name all of the telephone numbers associated with that indirectly name all of the telephone numbers associated with that
service provider, 2) telephone numbers can be listed in a range, and service provider, 2) telephone numbers can be listed in a range, and
3) a single telephone number can be listed. 3) a single telephone number can be listed.
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4.2. Certificate Freshness and Revocation 4.2. Certificate Freshness and Revocation
The problem of certificate freshness gains a new wrinkle in the The problem of certificate freshness gains a new wrinkle in the
telephone number context, because verifiers must establish not only telephone number context, because verifiers must establish not only
that a certificate remains valid, but also that the certificate's that a certificate remains valid, but also that the certificate's
scope contains the telephone number that the verifier is validating. scope contains the telephone number that the verifier is validating.
Dynamic changes to number assignments can occur due to number Dynamic changes to number assignments can occur due to number
portability, for example. So even if a verifier has a valid cached portability, for example. So even if a verifier has a valid cached
certificate for a telephone number (or a range containing the certificate for a telephone number (or a range containing the
number), the verifier must determine that the entity that the signer number), the verifier must determine that the entity that signed is
is still a proper authority for that number. still a proper authority for that number.
This document therefore recommends the use of OCSP in high-volume To verify the status of the certificate, the verifier needs the
environments for validating the freshness of certificates, per [12]. certificate, which is included with the call, and they need to:
[TBD - depending on our algorithm choices this profile may need to be
further profiled.] o Rely on short-lived certificates and not check the certificate's
status, or
o Rely on status information from the authority; there are three
common mechanisms employed by CAs:
* Certificate Revocation Lists (CRLs) [7],
* Online Certificate Status Protocol (OCSP) [RFC6560], and
* Server-based Certificate Validation Protocol (SCVP) [RFC5055].
The tradeoff between short lived certificates and using status
information is the former's burden is on the front end (i.e.,
enrollment) and the latter's burden is on the back end (i.e.,
verification). Both impact call setup time, but it is assumed that
performing enrollment for each call is more of an impact that using
status information. This document therefore recommends relying on
status information.
When relying on status information, the verifier needs to obtain the
status information but before that can happen the verifier needs to
know where to locate it. Placing the location of the status
information in the certificate makes the certificate larger but it
eases the client workload. The CRL Distribution Point certificate
extension includes the location of the CRL and the Authority
Information Access certificate extension includes the location of
OCSP and/or SCVP servers; both of these extensions are defined in
[7]. In all cases, the status information location is provided in
the form of an URI.
CRLs are an obviously attractive solution because they are supported
by every CA. CRLs have a reputation of being quite large (10s of
MBytes) because CAs issue one with all of their revoked certificates
but CRLs do support a variety of mechanisms to scope the size of the
CRLs based on revocation reasons (e.g., key compromise vs CA
compromise), user certificates only, and CA certificates only as well
as just operationally deciding to keep the CRLs small. Scoping the
CRL though introduces other issues (i.e., does the RP have all of the
CRL partitions). CAs in this system will likely all create CRLs for
audit purposes but it not recommended that they be relying upon for
status information. Instead, one of the two "online" options is
recommended. Between the two, OCSP is much more widely deployed and
this document therefore recommends the use of OCSP in high-volume
environments for validating the freshness of certificates, based on
[12]. Note that OCSP responses have three possible values: good,
revoked, or unknown.
[TBD] HVE OCSP requires SHA-1 be used as the hash algorithm, we're
obviously going to change this to be SHA-256.
[TBD] What would happen in the unknown case?
The wrinkle here is that OCSP only provides status information it
does not indicate whether the certificate's is authorized for the
telephone number that the verifier is validating. There's two ways
to ask the authorization question:
o For this certificate, is the following number currently in its
scope of validity?
o What are the numbers associated with this certificate?
The former seems to lend itself to piggybacking on the status
mechanism; since the verifier is already asking an authority about
the certificate's status why not use that mechanism instead of
creating a new service that requires additional round trips. Like
most PKIX-developed protocols, OCSP is extensible; OCSP supports
request extensions (OCSP supports sending multiple requests at once)
and per-request extensions. It seems unlikely that the verifier will
be requesting authorization checks on multiple callers in one request
so a per-request extension is what is needed. But, support for any
particular extension is optional and the HVE OCSP profile [12]
prohibits the use of per-request extensions so there is some
additional work required to modify existing OCSP responders.
The extension mechanism itself is fairly straightforward and it's
based on the X.509 v3 certificate extensions: an OID, a criticality
flag, and ASN.1 syntax as defined by the OID. The OID would be
registered in the IANA PKIX arc, the criticality would likely be set
to critical (i.e., if the OCSP responder doesn't understand the
extension stop processing), and the syntax can be anything we desire.
Applying the KISS principle, the syntax could simply be the TN being
asserted by caller. The responder could then determine whether the
TN asserted in the OCSP per-request extension is still authorized for
the certificate referred to in the certificate request field; the
reference is a tuple of hash algorithm, issuer name hash, issuer key
hash, and serial number.
The second option seems more like a query response type of
interaction and could be initiated through a URI included in the
certificate. Luckily, the AIA extension supports such a mechanism;
it's an OID to identify the "access method" and an "access location",
which would most most likely be a URI. The verifier would then
follow the URI to ascertain whether the list of TNs authorized for
use by the caller. There are obviously some privacy considerations
with this approach.
The need to check the authorizations in another round-trip is also
something to consider because it will add to the call setup time.
OCSP implementations commonly pre-generate responses and to speed up
HTTPS connections the server provides OCSP responses for each
certificate in their hierarchy. If possible, both of these OCSP
concepts should be adopted.
Ideally, once a certificate has been acquired by a verifier, some Ideally, once a certificate has been acquired by a verifier, some
sort of asynchronous mechanism could notify and update the verifier sort of asynchronous mechanism could notify and update the verifier
if the scope of the certificate changes. While not all possible if the scope of the certificate changes. While not all possible
categories of verifiers could implement such behavior, some sort of categories of verifiers could implement such behavior, some sort of
event-driven notification of certificate status is another potential event-driven notification of certificate status is another potential
subject of future work. subject of future work.
5. Acknowledgments 5. Acknowledgments
Russ Housley, Brian Rosen, Cullen Jennings and Eric Rescorla provided Russ Housley, Brian Rosen, Cullen Jennings and Eric Rescorla provided
key input to the discussions leading to this document. key input to the discussions leading to this document.
6. IANA Considerations 6. IANA Considerations
This memo includes no request to IANA. This memo includes no request to IANA at this time. If we define an
OCSP extension or AIA access method then we'll need an OID from the
PKIX.
7. Security Considerations 7. Security Considerations
This document is entirely about security. For further information on This document is entirely about security. For further information on
certificate security and practices, see RFC 3280 [5], in particular certificate security and practices, see RFC 3280 [5], in particular
its Security Considerations. its Security Considerations.
8. Informative References 8. Informative References
[1] Bradner, S., "Key words for use in RFCs to Indicate [1] Bradner, S., "Key words for use in RFCs to Indicate
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