Network Working Group J. Peterson
Internet-Draft NeuStar
Intended status: Standards Track C. Jennings
Expires: March 17, 2016 Cisco
E. Rescorla
RTFM, Inc.
September 14, 2015
Authenticated Identity Management in the Session Initiation Protocol
(SIP)
draft-ietf-stir-rfc4474bis-05.txt
Abstract
The baseline security mechanisms in the Session Initiation Protocol
(SIP) are inadequate for cryptographically assuring the identity of
the end users that originate SIP requests, especially in an
interdomain context. This document defines a mechanism for securely
identifying originators of SIP requests. It does so by defining new
SIP header fields for conveying a signature used for validating the
identity, and for conveying a reference to the credentials of the
signer.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 17, 2016.
Copyright Notice
Copyright (c) 2015 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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(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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 . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview of Operations . . . . . . . . . . . . . . . . . . . 6
4. Signature Generation and Validation . . . . . . . . . . . . . 7
4.1. Authentication Service Behavior . . . . . . . . . . . . . 7
4.2. Verifier Behavior . . . . . . . . . . . . . . . . . . . . 9
5. Credentials . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1. Credential Use by the Authentication Service . . . . . . 11
5.2. Credential Use by the Verification Service . . . . . . . 12
5.3. Handling Identity-Info URIs . . . . . . . . . . . . . . . 13
5.4. Credential Systems . . . . . . . . . . . . . . . . . . . 13
6. Identity Types . . . . . . . . . . . . . . . . . . . . . . . 14
6.1. Telephone Numbers . . . . . . . . . . . . . . . . . . . . 14
6.1.1. Canonicalization Procedures . . . . . . . . . . . . . 15
6.2. Domain Names . . . . . . . . . . . . . . . . . . . . . . 17
7. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . . 18
8. Extensibility . . . . . . . . . . . . . . . . . . . . . . . . 20
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 21
10. Security Considerations . . . . . . . . . . . . . . . . . . . 23
10.1. Protected Request Fields . . . . . . . . . . . . . . . . 23
10.1.1. Protection of the To Header and Retargeting . . . . 24
10.2. Unprotected Request Fields . . . . . . . . . . . . . . . 25
10.3. Malicious Removal of Identity Headers . . . . . . . . . 26
10.4. Securing the Connection to the Authentication Service . 26
10.5. Authorization and Transitional Strategies . . . . . . . 27
10.6. Display-Names and Identity . . . . . . . . . . . . . . . 29
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
11.1. Header Field Names . . . . . . . . . . . . . . . . . . . 29
11.2. Identity-Info Parameters . . . . . . . . . . . . . . . . 29
11.3. Identity-Info Algorithm Parameter Values . . . . . . . . 29
11.4. Identity-Extension Names . . . . . . . . . . . . . . . . 30
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 30
13. Changes from RFC4474 . . . . . . . . . . . . . . . . . . . . 30
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
14.1. Normative References . . . . . . . . . . . . . . . . . . 30
14.2. Informative References . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33
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1. Introduction
This document provides enhancements to the existing mechanisms for
authenticated identity management in the Session Initiation Protocol
(SIP, [RFC3261]). An identity, for the purposes of this document, is
defined as either a SIP URI, commonly a canonical address-of-record
(AoR) employed to reach a user (such as
'sip:alice@atlanta.example.com'), or a telephone number, which can be
represented as either a TEL URI [RFC3966] or as the user portion of a
SIP URI.
[RFC3261] stipulates several places within a SIP request where users
can express an identity for themselves, primarily the user-populated
From header field. However, the recipient of a SIP request has no
way to verify that the From header field has been populated
appropriately, in the absence of some sort of cryptographic
authentication mechanism. This leaves SIP vulnerable to a category
of abuses, including impersonation attacks that enable robocalling
and related problems as described in [RFC7340].
[RFC3261] specifies a number of security mechanisms that can be
employed by SIP user agents (UAs), including Digest, Transport Layer
Security (TLS), and S/MIME (implementations may support other
security schemes as well). However, few SIP user agents today
support the end-user certificates necessary to authenticate
themselves (via S/MIME, for example), and furthermore Digest
authentication is limited by the fact that the originator and
destination must share a prearranged secret. It is desirable for SIP
user agents to be able to send requests to destinations with which
they have no previous association. A cryptographic approach, like
the one described in this document, can provide a much stronger and
less spoofable assurance of identity than the Caller ID services that
the telephone network provides today.
[RFC4474] previously specified a means of signing portions of SIP
requests in order to provide that identity assurance. However, RFC
4474 was in several ways misaligned with deployment realities (see
[I-D.rosenberg-sip-rfc4474-concerns]). Most significantly, RFC 4474
did not deal well with telephone numbers as identifiers, despite
their enduring use in SIP deployments. RFC 4474 also provided a
signature over material that intermediaries in the field commonly
altered. This specification therefore revises RFC 4474 in light of
recent reconsideration of the problem space to align with the threat
model in [RFC7375].
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2. Background
Per [RFC7340], a key enabler of problems such as robocalling,
voicemail hacking, and swatting lies in an attacker's ability to
impersonate someone else. This secure operation of most SIP
applications and services depends on authorizing the source of
communications as it is represented in a SIP request. Such
authorization policies can be automated or be a part of human
operation of SIP devices. An example of the latter would be an
Internet telephone application that displays the calling party number
(and/or Caller-ID) of a caller, which a human may review to make a
policy decision before answering a call. An example of the former
would be a voicemail service that compares the identity of the caller
to a whitelist before determining whether it should allow the caller
access to recorded messages. In both of these cases, attackers might
attempt to circumvent these authorization policies through
impersonation. Since the primary identifier of the sender of a SIP
request, the From header field, can be populated arbitrarily by the
controller of a user agent, impersonation is very simple today. The
mechanism described in this document provides a strong identity
system for detecting attempted impersonation in SIP requests.
This identity architecture for SIP depends on a logical
"authentication service" which processes and signs requests; it may
be implemented either as part of a user agent or as a proxy server.
Once the sender of the message has been authenticated, the service
then adds new cryptographic information to requests to communicate to
other SIP entities that the sending user has been authenticated and
its claim of a particular identity has been authorized. A
"verification service" on the receiving end then validates this
signature and enables policy decisions to be made based on the
results of the verification.
Identities are issued to users by authorities. When a new user
becomes associated with example.com, the administrator of the SIP
service for that domain can issue them an identity in that namespace,
such as alice@example.com. Alice may then send REGISTER requests to
example.com that make her user agents eligible to receive requests
for sip:alice@example.com. In some cases, Alice may be the owner of
the domain herself, and may issue herself identities as she chooses.
But ultimately, it is the controller of the SIP service at
example.com that must be responsible for authorizing the use of names
in the example.com domain. Therefore, for the purposes of baseline
SIP, the credentials needed to prove a user is authorized to use a
particular From header field must ultimately derive from the domain
owner: either a user agent gives requests to the domain name owner in
order for them to be signed by the domain owner's credentials, or the
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user agent must possess credentials that prove in some fashion that
the domain owner has given the user agent the right to a name.
The situation is however more complicated for telephone numbers,
however. Authority over telephone numbers does not correspond
directly to Internet domains. While a user could register at a SIP
domain with a username that corresponds to a telephone number, any
connection between the administrator of that domain and the
assignment of telephone numbers is not currently reflected on the
Internet. Telephone numbers do not share the domain-scope property
described above, as they are dialed without any domain component.
This document thus assumes the existence of a separate means of
establishing authority over telephone numbers, for cases where the
telephone number is the identity of the user. As with SIP URIs, the
necessary credentials to prove authority for a name might reside
either in the endpoint or at some intermediary.
This document specifies a means of sharing a cryptographic assurance
of end-user SIP identity in an interdomain or intradomain context.
It relies on the authentication service adding to requests a SIP
header, the Identity header, which contains that cryptographic
assurance. In order to assist in the validation of the Identity
header, this specification also describes an Identity-Info header
that can be used by the recipient of a request to recover the
credentials of the signer. Note that the scope of this document is
limited to providing this identity assurance for SIP requests;
solving this problem for SIP responses is outside the scope of this
work (see [RFC4916]).
This specification allows either a user agent or a proxy server to
provide the authentication service function and/or the verification
service function. To maximize end-to-end security, it is obviously
preferable for end-users to acquire their own credentials; if they
do, their user agents can act as authentication services. However,
for some deployments end-user credentials may be neither practical
nor affordable, given the potentially large number of SIP user agents
(phones, PCs, laptops, PDAs, gaming devices) that may be employed by
a single user. In such environments, synchronizing keying material
across multiple devices may be prohibitively complex and require
quite a good deal of additional endpoint behavior. Managing several
credentials for the various devices could also be burdensome. In
these cases, implementation the authentication service at an
intermediary may be more practical. This trade-off needs to be
understood by implementers of this specification.
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3. Overview of Operations
This section provides an informative (non-normative) high-level
overview of the mechanisms described in this document.
Imagine a case where Alice, who has the home proxy of example.com and
the address-of-record sip:alice@example.com, wants to communicate
with Bob at sip:bob@example.org. They have no prior relationship,
and Bob implements best practices to prevent impersonation attacks.
Alice generates an INVITE and places her identity, in this case her
address-of-record, in the From header field of the request. She then
sends an INVITE over TLS to an authentication service proxy for the
example.com domain.
The authentication service authenticates Alice (possibly by sending a
Digest authentication challenge) and validates that she is authorized
to assert the identity that she populated in the From header field.
This value is Alice's AoR, but in other cases it could be some
different value that the proxy server has authority over, such as a
telephone number. The proxy then computes a hash over some
particular headers and fields, including part of the From header
field of the message. This hash is signed with the appropriate
credential for the identity (example.com, in the
sip:alice@example.com case) and inserted in a new header field in the
SIP message, the 'Identity' header.
The proxy, as the holder of the private key for the example.com
domain, is asserting that the originator of this request has been
authenticated and that she is authorized to claim the identity that
appears in the From header field. The proxy also inserts a companion
header field, Identity-Info, that tells Bob how to acquire keying
material necessary to validate its credentials (a public key), in
case he doesn't already have it.
When Bob's domain receives the request, it verifies the signature
provided in the Identity header, and thus can validate that the
authority over the identity in the From header field authenticated
the user, and permitted the user to assert that From header field
value. This same validation operation may be performed by Bob's user
agent server (UAS). As the request has been validated, it is
rendered to Bob. If the validation was unsuccessful, some other
treatment would be applied by the receiving domain.
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4. Signature Generation and Validation
4.1. Authentication Service Behavior
This document specifies a role for SIP entities called an
authentication service. The authentication service role can be
instantiated by an intermediary such as a proxy server or by a user
agent. Any entity that instantiates the authentication service role
MUST possess the private key of one or more credentials that can be
used to sign for a domain or a telephone number (see Section 5.1).
Intermediaries that instantiate this role MUST be capable of
authenticating one or more SIP users who can register for that
identity. Commonly, this role will be instantiated by a proxy
server, since these entities are more likely to have a static
hostname, hold corresponding credentials, and have access to SIP
registrar capabilities that allow them to authenticate users. It is
also possible that the authentication service role might be
instantiated by an entity that acts as a redirect server, but that is
left as a topic for future work.
Entities instantiating the authentication service role perform the
following steps, in order, to generate an Identity header for a SIP
request:
Step 1:
First, the authentication service must determine whether it is
authoritative for the identity of the sender of the request. In
ordinary operations, the authentication service decides this by
inspecting the URI value from the addr-spec component of From header
field; this URI will be referred to here as the 'identity field'. If
the identity field contains a SIP or SIP Secure (SIPS) URI, and the
user portion is not a telephone number, the authentication service
MUST extract the hostname portion of the identity field and compare
it to the domain(s) for which it is responsible (following the
procedures in RFC 3261 [RFC3261], Section 16.4). If the identity
field uses the TEL URI scheme [RFC3966], or the identity field is a
SIP or SIPS URI with a telephone number in the user portion, the
authentication service determines whether or not it is responsible
for this telephone number; see Section 6.1 for more information. An
authentication service proceeding with a signature over a telephone
number MUST then follow the canonicaliaation procedures described in
Section 6.1.1. If the authentication service is not authoritative
for the identity in question, it SHOULD process and forward the
request normally, but it MUST NOT follow the steps below to add an
Identity header. An authentication service MUST NOT add an Identity
header to a request that already has one.
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Step 2:
The authentication service MUST then determine whether or not the
sender of the request is authorized to claim the identity given in
the identity field. In order to do so, the authentication service
MUST authenticate the sender of the message. Some possible ways in
which this authentication might be performed include:
If the authentication service is instantiated by a SIP
intermediary (proxy server), it may authenticate the request with
the authentication scheme used for registration in its domain
(e.g., Digest authentication).
If the authentication service is instantiated by a SIP user agent,
a user agent may authenticate its own user through any system-
specific means, perhaps simply by virtue of having physical access
to the user agent.
Authorization of the use of a particular username or telephone number
in the user part of the From header field is a matter of local policy
for the authentication service, see Section 5.1 for more information.
Note that this check is performed only on the addr-spec in the
identity field (e.g., the URI of the sender, like
'sip:alice@atlanta.example.com'); it does not convert the display-
name portion of the From header field (e.g., 'Alice Atlanta').
Authentication services MAY check and validate the display-name as
well, and compare it to a list of acceptable display-names that may
be used by the sender; if the display-name does not meet policy
constraints, the authentication service could return a 403 response
code. In this case, the reason phrase should indicate the nature of
the problem; for example, "Inappropriate Display Name". However, the
display-name is not always present, and in many environments the
requisite operational procedures for display-name validation may not
exist, so no normative guidance is given here. For more information,
see Section 10.6.
Step 3:
An authentication service MUST add a Date header field to SIP
requests if one is not already present. The authentication service
MUST ensure that any preexisting Date header in the request is
accurate. Local policy can dictate precisely how accurate the Date
must be; a RECOMMENDED maximum discrepancy of sixty seconds will
ensure that the request is unlikely to upset any verifiers. If the
Date header contains a time different by more than one minute from
the current time noted by the authentication service, the
authentication service SHOULD reject the request. This behavior is
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not mandatory because a user agent client (UAC) could only exploit
the Date header in order to cause a request to fail verification; the
Identity header is not intended to provide a source of non-
repudiation or a perfect record of when messages are processed.
Finally, the authentication service MUST verify that the Date header
falls within the validity period of its credential.
See Section 7 for information on how the Date header field assists
verifiers.
Step 4:
The authentication service MAY add extend the identity mechanism by
adding one or more Identity-Extension headers to the request. Only
implementations that extend this base mechanism MAY populate this
header field and add this signature. See Section 8.
Step 5:
Subsequently, the authentication service MUST form the identity
signature and add an Identity header to the request containing this
signature. After the Identity header has been added to the request,
the authentication service MUST also add an Identity-Info header.
The Identity-Info header contains a URI from which the authentication
service's credential can be acquired; see Section 5.3 for more on
credential acquisition. Details on the syntax of both of these
headers are provided in Section 7.
Finally, the authentication service MUST forward the message
normally.
4.2. Verifier Behavior
This document specifies a logical role for SIP entities called a
verification service, or verifier. When a verifier receives a SIP
message containing an Identity header, it inspects the signature to
verify the identity of the sender of the message. Typically, the
results of a verification are provided as input to an authorization
process that is outside the scope of this document. If an Identity
header is not present in a request, and one is required by local
policy (for example, based on a per-sending-domain policy, or a per-
sending-user policy), then a 428 'Use Identity Header' response MUST
be sent.
In order to verify the identity of the sender of a message, an entity
acting as a verifier MUST perform the following steps, in the order
here specified.
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Step 1:
In order to determine whether the signature for the identity field
should be over the entire URI or just a canonicalized telephone
number, the verification service MUST follow the canonicalization
process described in Section 6.1.1. That section also describes the
procedures the verification service MUST follow to determine if the
signer is authoritative for a telephone number. For domains, the
verifier MUST follow the process described in Section 6.2 to
determine if the signer is authoritative for the identity field.
Step 2:
The verifier must first ensure that it possesses the proper keying
material to validate the signature in the Identity header field,
which usually involves dereferencing the Identity-Info header. See
Section 5.2 for more information on these procedures.
Step 3:
The verifier MUST must furthermore ensure that the value of the Date
header meets local policy for freshness (usually, within sixty
seconds) and that it falls within the validity period of the
credential used to sign the Identity header. For more on the attacks
this prevents, see Section 10.1.
Step 4:
The verifier MUST validate the signature in the Identity header
field, following the procedures for generating the hashed digest-
string described in Section 7. If a verifier determines that the
signature on the message does not correspond to the reconstructed
digest-string, then a 438 'Invalid Identity Header' response MUST be
returned.
Step 5:
If the request contains one or more Identity-Extension headers, then
if the verifier supports the included extension(s), it SHOULD verify
any associated fields following the procedures specified in that
extension (see Section 8). If a verifier determines that such a
signature in the message does not correspond to the reconstructed
digest-string, then a 438 'Invalid Identity Header' response SHOULD
be returned. If the verifier does not support the extension(s), then
the verifier takes no further action.
The handling of the message after the verification process depends on
how the implementation service is implemented, and on local policy.
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This specification does not propose any authorization policy for user
agents or proxy servers to follow based on the presence of a valid
Identity header, the presence of an invalid Identity header, or the
absence of an Identity header, but it is anticipated that local
policies could involve making different forwarding decisions in
intermediary implementations, or changing how the user is alerted, or
how identity is rendered, in user agent implementations.
5. Credentials
5.1. Credential Use by the Authentication Service
In order to act as an authentication service, a SIP entity must have
access to the private keying material of one or more credentials that
cover domain names or telephone numbers. These credentials may
represent authority over an entire domain (such as example.com) or
potentially a set of domains enumerated by the credential.
Similarly, a credential may represent authority over a single
telephone number or a range of telephone numbers. The way that the
scope of a credential is expressed is specific to the credential
mechanism.
Authorization of the use of a particular username or telephone number
in the identity field is a matter of local policy for the
authentication service, one that depends greatly on the manner in
which authentication is performed. For non-telephone number user
parts, one policy might be as follows: the username given in the
'username' parameter of the Proxy-Authorization header MUST
correspond exactly to the username in the From header field of the
SIP message. However, there are many cases in which this is too
limiting or inappropriate; a realm might use 'username' parameters in
Proxy-Authorization that do not correspond to the user-portion of SIP
From headers, or a user might manage multiple accounts in the same
administrative domain. In this latter case, a domain might maintain
a mapping between the values in the 'username' parameter of Proxy-
Authorization and a set of one or more SIP URIs that might
legitimately be asserted for that 'username'. For example, the
username can correspond to the 'private identity' as defined in Third
Generation Partnership Project (3GPP), in which case the From header
field can contain any one of the public identities associated with
this private identity. In this instance, another policy might be as
follows: the URI in the From header field MUST correspond exactly to
one of the mapped URIs associated with the 'username' given in the
Proxy-Authorization header. This is a suitable approach for
telephone numbers in particular.
This specification could also be used with credentials that cover a
single name or URI, such as alice@example.com or
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sip:alice@example.com. This would require a modification to
authentication service behavior to operate on a whole URI rather than
a domain name. Because this is not believed to be a pressing use
case, this is deferred to future work, but implementors should note
this as a possible future direction.
Exceptions to such authentication service policies arise for cases
like anonymity; if the AoR asserted in the From header field uses a
form like 'sip:anonymous@example.com' (see [RFC3323]), then the
'example.com' proxy might authenticate only that the user is a valid
user in the domain and insert the signature over the From header
field as usual.
5.2. Credential Use by the Verification Service
In order to act as a verification service, a SIP entity must have a
way to acquire and retain credentials for authorities over particular
domain names and/or telephone numbers or number ranges.
Dereferencing the Identity-Info header (as described in the next
section) MUST be supported by all verification service
implementations to create a baseline means of credential acquisition.
Provided that the credential used to sign a message is not previously
known to the verifier, SIP entities SHOULD discover this credential
by dereferencing the Identity-Info header, unless they have some more
other implementation-specific way of acquiring the needed keying
material, such as an offline store of periodically-updated
credentials. If the URI in the Identity-Info header cannot be
dereferenced, then a 436 'Bad Identity-Info' response MUST be
returned.
This specification does not propose any particular policy for a
verification service to determine whether or not the holder of a
credential is the appropriate party to sign for a given SIP identity.
Guidance on this is deferred to the credential mechanism
specifications, which must meet the requirements in Section 5.4.
Verification service implementations supporting this specification
SHOULD have some means of retaining credentials (in accordance with
normal practices for credential lifetimes and revocation) in order to
prevent themselves from needlessly downloading the same credential
every time a request from the same identity is received. Credentials
cached in this manner may be indexed in accordance with local policy:
for example, by their scope, or the URI given in the Identity-Info
header field value. Further consideration of how to cache
credentials is deferred to the credential mechanism specifications.
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5.3. Handling Identity-Info URIs
An Identity-Info header MUST contain a URI which dereferences to a
resource that contains the public key components of the credential
used by the authentication service to sign a request. It is
essential that a URI in the Identity-Info header be dereferencable by
any entity that could plausibly receive the request. For common
cases, this means that the URI must be dereferencable by any entity
on the public Internet. In constrained deployment environments, a
service private to the environment might be used instead.
Beyond providing a means of accessing credentials for an identity,
the Identity-Info header further serves as a means of differentiating
which particular credential was used to sign a request, when there
are potentially multiple authorities eligible to sign. For example,
imagine a case where a domain implements the authentication service
role for a range of telephone and a user agent belonging to Alice has
acquired a credential for a single telephone number within that
range. Either would be eligible to sign a SIP request for the number
in question. Verification services however need a means to
differentiate which one performed the signature. The Identity-Info
header performs that function.
If the optional "canon" parameter is present, it contains the result
of the number canonicalization process performed by the
authentication service (see Section 6.1.1) on the identity in the
From. This value is provided purely informationally as an
optimization for the verification service. The verification service
MAY compute its own canonicalization of the number and compare this
to the value in the "canon" parameter before performing any
cryptographic functions in order to ascertain whether or not the two
ends agree on the canonical number form.
5.4. Credential Systems
This document makes no recommendation for the use of any specific
credential system. Today, there are two primary credential systems
in place for proving ownership of domain names: certificates (e.g.,
X.509 v3, see [RFC5280]) and the domain name system itself (e.g.,
DANE, see [RFC6698]). It is envisioned that either could be used in
the SIP identity context: an Identity-Info header could for example
give an HTTP URL of the form 'application/pkix-cert' pointing to a
certificate (following the conventions of [RFC2585]). The Identity-
Info headers may use the DNS URL scheme (see [RFC4501]) to designate
keys in the DNS.
While no comparable public credentials exist for telephone numbers,
either approach could be applied to telephone numbers. A credential
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system based on certificates is given in
[I-D.ietf-stir-certificates]. One based on the domain name system is
given in [I-D.kaplan-stir-cider].
In order for a credential system to work with this mechanism, its
specification must detail:
which URIs schemes the credential will use in the Identity-Info
header, and any special procedures required to dereference the
URIs
how the verifier can learn the scope of the credential
any special procedures required to extract keying material from
the resources designated by the URI
any algorithms that would appear in the Identity-Info "alg"
parameter other than 'rsa-sha256.' Note that per the IANA
Considerations of RFC 4474, new algorithms can only be specified
by Standards Action
SIP entities cannot reliably predict where SIP requests will
terminate. When choosing a credential scheme for deployments of this
specification, it is therefore essential that the trust anchor(s) for
credentials be widely trusted, or that deployments restrict the use
of this mechanism to environments where the reliance on particular
trust anchors is assured by business arrangements or similar
constraints.
Note that credential systems must address key lifecycle management
concerns: were a domain to change the credential available at the
Identity-Info URI before a verifier evaluates a request signed by an
authentication service, this would cause obvious verifier failures.
When a rollover occurs, authentication services SHOULD thus provide
new Identity-Info URIs for each new credential, and SHOULD continue
to make older key acquisition URIs available for a duration longer
than the plausible lifetime of a SIP transaction (a minute would most
likely suffice).
6. Identity Types
6.1. Telephone Numbers
Since many SIP applications provide a Voice over IP (VoIP) service,
telephone numbers are commonly used as identities in SIP deployments.
In order for telephone numbers to be used with the mechanism
described in this document, authentication services must enroll with
an authority that issues credentials for telephone numbers or
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telephone number ranges, and verification services must trust the
authority employed by the authentication service that signs a
request. Enrollment procedures and credential management are outside
the scope of this document.
In the longer term, it is possible that some directory or other
discovery mechanism may provide a way to determine which
administrative domain is responsible for a telephone number, and this
may aid in the signing and verification of SIP identities that
contain telephone numbers. This is a subject for future work.
In order to work with any such authorities, authentication and
verification services must be able to identify when a request should
be signed by an authority for a telephone number, and when it should
be signed by an authority for a domain. Telephone numbers most
commonly appear in SIP header field values in the username portion of
a SIP URI (e.g., 'sip:+17005551008@chicago.example.com;user=phone').
The user part of that URI conforms to the syntax of the TEL URI
scheme (RFC 3966 [RFC3966]). It is also possible for a TEL URI to
appear in the SIP To or From header field outside the context of a
SIP or SIPS URI (e.g., 'tel:+17005551008'). In both of these cases,
it's clear that the signer must have authority over the telephone
number, not the domain name of the SIP URI. It is also possible,
however, for requests to contain a URI like
'sip:7005551000@chicago.example.com'. It may be non-trivial for a
service to ascertain in this case whether the URI contains a
telephone number or not.
6.1.1. Canonicalization Procedures
In order to determine whether or not the user portion of a SIP URI is
a telephone number, authentication services and verification services
must perform the following canonicalization procedure on any SIP URI
they inspect which contains a wholly numeric user part. Note that
the same procedures are followed for creating the canonical form of
URIs found in both the From and To header field values.
First, implementations must assess if the user-portion of the URI
constitutes a telephone number. In some environments, numbers
will be explicitly labeled by the use of TEL URIs or the
'user=phone' parameter, or implicitly by the presence of the '+'
indicator at the start of the user-portion. Absent these
indications, if there are numbers present in the user-portion,
implementations may also detect that the user-portion of the URI
contains a telephone number by determining whether or not those
numbers would be dialable or routable in the local environment --
bearing in mind that the telephone number may be a valid E.164
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number, a nationally-specific number, or even a private branch
exchange number.
Once an implementation has identified a telephone number, it must
construct a number string. Implementations MUST drop any leading
+'s, any internal dashes, parentheses or other non-numeric
characters, excepting only the leading "#" or "*" keys used in
some special service numbers (typically, these will appear only in
the To header field value). This MUST result in an ASCII string
limited to "#", "*" and digits without whitespace or visual
separators.
Next, an implementation must assess if the number string is a
valid, globally-routable number with a leading country code. If
not, implementations SHOULD convert the number into E.164 format,
adding a country code if necessary; this may involve transforming
the number from a dial string (see [RFC3966]), removing any
national or international dialing prefixes or performing similar
procedures. It is only in the case that an implementation cannot
determine how to convert the number to a globally-routable format
that this step may be skipped.
In some cases, further transformations MAY be made in accordance
with specific policies used within the local domain. For example,
one domain may only use local number formatting and need to
convert all To/From user portions to E.164 by prepending country-
code and region code digits; another domain might prefix usernames
with trunk-routing codes and need to remove the prefix. Also, in
some networks, the P-Asserted-Identity header field value is used
in lieu of the From header field to convey the telephone number of
the sender of a request; while it is not envisioned that most of
those networks would or should make use of the Identity mechanism
described in this specification, where they do, local policy might
therefore dictate that the canonical string derive from the P-
Asserted-Identity header field rather than the From. In any case
where local policy canonicalizes the number into a form different
from how it appears in the From header field, the use of the
"canon" parameter by authentication services is RECOMMENDED, but
because "canon" itself could then divulge information about users
or networks, implementers should be mindful of the guidelines in
Section 9.
The resulting canonical number string will be used as input to the
hash calculation during signing and verifying processes.
The ABNF of this number string is:
tn-spec = [ "#" / "*" ] 1*DIGIT
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If the result of this procedure forms a complete telephone number,
that number is used for the purpose of creating and signing the
digest-string by both the authentication service and verification
service. Practically, entities that perform the authentication
service role will sometimes alter the telephone numbers that appear
in the To and From header field values, converting them to this
format (though note this is not a function that [RFC3261] permits
proxy servers to perform). The authentication service MAY also add
the result of the canonicalization process of the From header field
value to the "canon" parameter of the Identity-Info header. If the
result of the canonicalization of the From header field value does
not form a complete telephone number, the authentication service and
verification service should treat the entire URI as a SIP URI, and
apply a domain signature per the procedures in Section 6.2.
6.2. Domain Names
When a verifier processes a request containing an Identity-Info
header with a domain signature, it must compare the domain portion of
the URI in the From header field of the request with the domain name
that is the subject of the credential acquired from the Identity-Info
header. While it might seem that this should be a straightforward
process, it is complicated by two deployment realities. In the first
place, credentials have varying ways of describing their subjects,
and may indeed have multiple subjects, especially in 'virtual
hosting' cases where multiple domains are managed by a single
application. Secondly, some SIP services may delegate SIP functions
to a subordinate domain and utilize the procedures in RFC 3263
[RFC3263] that allow requests for, say, 'example.com' to be routed to
'sip.example.com'. As a result, a user with the AoR
'sip:jon@example.com' may process requests through a host like
'sip.example.com', and it may be that latter host that acts as an
authentication service.
To meet the second of these problems, a domain that deploys an
authentication service on a subordinate host MUST be willing to
supply that host with the private keying material associated with a
credential whose subject is a domain name that corresponds to the
domain portion of the AoRs that the domain distributes to users.
Note that this corresponds to the comparable case of routing inbound
SIP requests to a domain. When the NAPTR and SRV procedures of RFC
3263 are used to direct requests to a domain name other than the
domain in the original Request-URI (e.g., for 'sip:jon@example.com',
the corresponding SRV records point to the service
'sip1.example.org'), the client expects that the certificate passed
back in any TLS exchange with that host will correspond exactly with
the domain of the original Request-URI, not the domain name of the
host. Consequently, in order to make inbound routing to such SIP
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services work, a domain administrator must similarly be willing to
share the domain's private key with the service. This design
decision was made to compensate for the insecurity of the DNS, and it
makes certain potential approaches to DNS-based 'virtual hosting'
unsecurable for SIP in environments where domain administrators are
unwilling to share keys with hosting services.
A verifier MUST evaluate the correspondence between the user's
identity and the signing credential by following the procedures
defined in RFC 2818 [RFC2818], Section 3.1. While RFC 2818 [RFC2818]
deals with the use of HTTP in TLS and is specific to certificates,
the procedures described are applicable to verifying identity if one
substitutes the "hostname of the server" in HTTP for the domain
portion of the user's identity in the From header field of a SIP
request with an Identity header.
7. Header Syntax
Baseline RFC4474 defined the Identity and Identity-Info headers.
This document updates that specification and adds the optional
Identity-Extension header (the grammar for which appears in
Section 8). Identity and Identity-Info are REQUIRED for securing
requests with this specification, and may appear only once in a
request, while Identity-Extension can be present multiple times. The
grammar for the first two headers is (following the ABNF [RFC4234] in
RFC 3261 [RFC3261]):
Identity = "Identity" HCOLON signed-identity-digest
signed-identity-digest = LDQUOT *base64-char RDQUOT
Identity-Info = "Identity-Info" HCOLON ident-info
*( SEMI ident-info-params )
ident-info = LAQUOT absoluteURI RAQUOT
ident-info-params = ident-info-alg / canonical-str / ident-info-extension
ident-info-alg = "alg" EQUAL token
canonical-str = "canon" EQUAL tn-spec
ident-info-extension = generic-param
base64-char = ALPHA / DIGIT / "/" / "+"
This follows the original specification of Identity and Identity-Info
in RFC4474, except for the addition of the "canon" parameter. Note
that in RFC4474, the signed-identity-digest was given as quoted
32LHEX, whereas here it is given as a quoted sequence of base64-char.
The signed-identity-digest is a signed hash of a canonical string
generated from certain components of a SIP request. To create the
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contents of the signed-identity-digest, the following elements of a
SIP message MUST be placed in a bit-exact string in the order
specified here, separated by a vertical line, "|" or %x7C, character:
First, the identity. If the user part of the AoR in the From
header field of the request contains a telephone number, then the
canonicalization of that number goes into the first slot (see
Section 6.1.1). Otherwise, the first slot contains the AoR of the
UA sending the message as taken from addr-spec of the From header
field.
Second, the target. If the user part of the AoR in the To header
field of the request contains a telephone number, then the
canonicalization of that number goes into the second slot (again,
see Section 6.1.1). Otherwise, the second slot contains the addr-
spec component of the To header field, which is the AoR to which
the request is being sent.
Third, the request method.
Fourth, the Date header field, with exactly one space each for
each SP and the weekday and month items case set as shown in the
BNF of RFC 3261 [RFC3261]. RFC 3261 specifies that the BNF for
weekday and month is a choice amongst a set of tokens. The RFC
4234 [RFC4234] rules for the BNF specify that tokens are case
sensitive. However, when used to construct the canonical string
defined here, the first letter of each week and month MUST be
capitalized, and the remaining two letters must be lowercase.
This matches the capitalization provided in the definition of each
token. All requests that use the Identity mechanism MUST contain
a Date header.
Fifth, if the request contains an SDP message body, and if that
SDP contains one or more "a=fingerprint" attributes, the value(s)
of the attributes if they differ. Each attribute value consists
of all characters following the colon after "a=fingerprint"
including the algorithm description and hexadecimal key
representation, any whitespace, carriage returns, and "/" line
break indicators. If multiple non-identical "a=fingerprint"
attributes appear in an SDP body, then all non-identical
attributes values MUST be concatenated, with no separating
character, after sorting the values in alphanumeric order. If the
SDP body contains no "a=fingerprint" attribute, the fifth element
MUST be empty, containing no whitespace, resulting in a "||" in
the signed-identity-digest.
Sixth, the Identity-Extension header field value(s), if there is
at least one Identity-Extension header field in the request. If
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multiple Identity-Extension header fields are in the request, they
MUST be concatenated after sorting the header field values in
alphanumeric order, with each entry separated by a vertical line,
"|" or %x7C. If the message contains no Identity-Extension
header, then the sixth slot MUST be empty, containing no
whitespace, resulting in a "||" in the signed-identity-digest.
characters.
For more information on the security properties of these headers, and
why their inclusion mitigates replay attacks, see Section 10 and
[RFC3893]. The precise formulation of this digest-string is,
therefore (following the ABNF[RFC4234] in RFC 3261 [RFC3261]):
digest-string = ( addr-spec / tn-spec ) "|" ( addr-spec / tn-spec ) "|"
Method "|" SIP-date "|" [ sdp-fingerprint ] "|"
[ signed-identity-extension-digest ]
sdp-fingerprint = byte-string
For the definition of 'tn-spec' see Section 6.1.1.
After the digest-string is formed, it MUST be hashed and signed with
the certificate of authority over the identity. The hashing and
signing algorithm is specified by the 'alg' parameter of the
Identity-Info header (see below for more information on Identity-Info
header parameters). This document defines only one value for the
'alg' parameter: 'rsa-sha256'; further values MUST be defined in a
Standards Track RFC, see Section 11.3 for more information. All
implementations of this specification MUST support 'rsa-sha256'.
When the 'rsa-sha256' algorithm is specified in the 'alg' parameter
of Identity-Info, the hash and signature MUST be generated as
follows: compute the results of signing this string with
sha1WithRSAEncryption as described in RFC 3370 [RFC3370] and base64
encode the results as specified in RFC 3548 [RFC3548]. A 2048-bit or
longer RSA key MUST be used. The result of the digest-string hash is
placed in the Identity header field.
The 'absoluteURI' portion of the Identity-Info header MUST contain a
URI; see Section 5.3 for more on choosing how to advertise
credentials through Identity-Info.
8. Extensibility
As future requirements may warrant increasing the scope of the
Identity mechanism, this specification defines an optional Identity-
Extension header. Each extension header field value MUST consist of
a right hand side identifying the extension, an equals sign, and then
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a left hand side consisting of a signature over an element in a SIP
request.
Future specifications that define extensions to the Identity
mechanism must explicitly designate which elements of a SIP request
are to be signed, how a canonical string of those elements is
generated by both the authentication service and the verifier, and
the mechanism and algorithms used to generate the signature (it is
RECOMMENDED that these follow the algorithm choice of this
specification). Note that per verifier behavior in Section 4.2,
verifying an extension is always optional. An authentication service
cannot assume that verifiers will understand any given extension.
Verifiers that do support an extension may then trigger appropriate
application-level behavior in the presence of a signature over
additional part of the SIP request; authors of Identity extensions
should provide appropriate extension-specific guidance to application
developers on this point.
Identity-Extension = "Identity-Extension" HCOLON identity-extension-string
identity-extension-string = identity-extension-name EQUAL *base64-char
identity-extension-name = token
signed-identity-extension-digest = LDQUOT *base64-char RDQUOT
Defining a new Identity-Extension requires a Standards Action; see
Section 11.4.
9. Privacy Considerations
The purpose of this mechanism is to provide a strong identification
of the originator of a SIP request, specifically a cryptographic
assurance that the URI given in the From header field value can
legitimately be claimed by the originator. This URI may contain a
variety of personally identifying information, including the name of
a human being, their place of work or service provider, and possibly
further details. The intrinsic privacy risks associated with that
URI are, however, no different from those of baseline SIP. Per the
guidance in [RFC6973], implementors should make users aware of the
privacy trade-off of providing secure identity.
The identity mechanism presented in this document is compatible with
the standard SIP practices for privacy described in [RFC3323]. A SIP
proxy server can act both as a privacy service and as an
authentication service. Since a user agent can provide any From
header field value that the authentication service is willing to
authorize, there is no reason why private SIP URIs that contain
legitimate domains (e.g., sip:anonymous@example.com) cannot be signed
by an authentication service. The construction of the Identity
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header is the same for private URIs as it is for any other sort of
URIs.
Note, however, that even when using anonymous SIP URIs, an
authentication service must possess a certificate corresponding to
the host portion of the addr-spec of the From header field of the
request; accordingly, using domains like 'anonymous.invalid' will not
be possible for privacy services that also act as authentication
services. The assurance offered by the usage of anonymous URIs with
a valid domain portion is "this is a known user in my domain that I
have authenticated, but I am keeping its identity private". The use
of the domain 'anonymous.invalid' entails that no corresponding
authority for the domain can exist, and as a consequence,
authentication service functions for that domain are meaningless.
[RFC3325] defines the "id" priv-value token, which is specific to the
P-Asserted-Identity header. The sort of assertion provided by the P-
Asserted-Identity header is very different from the Identity header
presented in this document. It contains additional information about
the sender of a message that may go beyond what appears in the From
header field; P-Asserted-Identity holds a definitive identity for the
sender that is somehow known to a closed network of intermediaries
that presumably the network will use this identity for billing or
security purposes. The danger of this network-specific information
leaking outside of the closed network motivated the "id" priv-value
token. The "id" priv-value token has no implications for the
Identity header, and privacy services MUST NOT remove the Identity
header when a priv-value of "id" appears in a Privacy header.
The optional "canon" parameter of the Identity-Info header specified
in this document provides a canonicalized form of the telephone
number of the originator of a call. In some contexts, local policy
may be used to populate a "canon" that may differ substantially from
the original From header field. Depending on those policies,
potentially the "canon" parameter might divulge information about the
originating network or user that might not appear elsewhere in the
SIP request. Were it to be used to reflect the contents of the P-
Asserted-Identity header field, for example, then "canon" would need
to be removed when the P-Asserted-Identity header is removed to avoid
any such leakage outside of a trust domain. Since, in those
contexts, the canonical form of the sender's identity could not be
reassembled by a verifier, and thus the Identity signature validation
process would fail, using P-Asserted-Identity with the Identity
"canon" parameter in this fashion is NOT RECOMMENDED outside of
environments where SIP requests will never leave the trust domain.
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Finally, note that unlike [RFC3325], the mechanism described in this
specification adds no information to SIP requests that has privacy
implications.
10. Security Considerations
This document describes a mechanism that provides a signature over
the Date header field of SIP requests, parts of the To and From
header fields, the request method, and when present any media keying
material in the message body. In general, the considerations related
to the security of these headers are the same as those given in
[RFC3261] for including headers in tunneled 'message/sip' MIME bodies
(see Section 23 in particular). The following section details the
individual security properties obtained by including each of these
header fields within the signature; collectively, this set of header
fields provides the necessary properties to prevent impersonation.
It adddresses the solution-specific attacks again in-band solutions
enumerated in [RFC7375] Section 4.1.
10.1. Protected Request Fields
The From header field value indicates the identity of the sender of
the message, and the SIP address-of-record URI, or an embedded
telephone number, in the From header field is the identity of a SIP
user, for the purposes of this document. This is the key piece of
information that this mechanism secures; the remainder of the signed
parts of a SIP request are present to provide reference integrity and
to prevent certain types of cut-and-paste attacks.
The Date header field value protects against cut-and-paste attacks,
as described in [RFC3261], Section 23.4.2. Implementations of this
specification MUST NOT deem valid a request with an outdated Date
header field (the RECOMMENDED interval is that the Date header must
indicate a time within 60 seconds of the receipt of a message). Note
that per baseline [RFC3261] behavior, servers keep state of recently
received requests, and thus if an Identity header is replayed by an
attacker within the Date interval, verifiers can detect that it is
spoofed; because a message with an identical Date from the same
source had recently been received.
The To header field value provides the identity of the SIP user that
this request originally targeted. Providing the To header field in
the Identity signature serves two purposes: first, it prevents cut-
and-paste attacks in which an Identity header from legitimate request
for one user is cut-and-pasted into a request for a different user;
second, it preserves the starting URI scheme of the request, which
helps prevent downgrade attacks against the use of SIPS. The To
offers additional protection against cut-and-paste attacks beyond the
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Date header field: for example, without a signature over the To, an
attacker who receives a call from a target could immediately forward
the INVITE to the target's voicemail service within the Date
interval, and the voicemail service would have no way knowing that
the Identity header it received had been originally signed for a call
intended for a different number. However, note the caveats below in
Section 10.1.1.
Without the request method, an INVITE request could be cut- and-
pasted by an attacker and transformed into a MESSAGE request without
changing any fields covered by the Identity header, and moreover
requests within a transaction (for example, a re-INVITE) could be
replayed in potentially confusing or malicious ways to enable
impersonation.
When signing a request that contains a fingerprint of keying material
in SDP for DTLS-SRTP [RFC5763], this mechanism always provides a
signature over that fingerprint. This signature prevents certain
classes of impersonation attacks in which an attacker forwards or
cut-and-pastes a legitimate request: although the target of the
attack may accept the request, the attacker will be unable to
exchange media with the target as they will not possess a key
corresponding to the fingerprint. For example there are some baiting
attacks, launched with the REFER method or through social
engineering, where the attacker receives a request from the target
and reoriginates it to a third party: these might not be prevented by
only a signature over the From, To and Date, but could be prevented
by securing a fingerprint for DTLS-SRTP. While this is a different
form of impersonation than is commonly used for robocalling,
ultimately there is little purpose in establishing the identity of
the user that originated a SIP request if this assurance is not
coupled with a comparable assurance over the contents of the
subsequent communication. This signature also, per [RFC7258],
reduces the potential for passive monitoring attacks against the SIP
media. In environments where DTLS-SRTP is unsupported, however, no
field is signed and no protections are provided.
10.1.1. Protection of the To Header and Retargeting
The mechanism in this document provides a signature over the identity
information in the To header field value of requests. This provides
a means for verifiers to detect replay attacks where a signed request
originally sent to one target is modified and then forwarded by an
attacker to another, unrelated target. Armed with the original value
of the To header field, the recipient of a request may compare it to
their own identity in order to determine whether or not the identity
information in this call might have been replayed. However, any
request may be legitimately retargeted as well, and as a result
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legitimate requests may reach a SIP endpoint whose user is not
identified by the URI designated in the To header field value. It is
therefore difficult for any verifier to decide whether or not some
prior retargeting was "legitimate." Retargeting can also cause
confusion when identity information is provided for requests sent in
the backwards in a dialog, as the dialog identifiers may not match
credentials held by the ultimate target of the dialog. For further
information on the problems of response identity see
[I-D.peterson-sipping-retarget].
Any means for authentication services or verifiers to anticipate
retargeting is outside the scope of this document, and likely to have
equal applicability to response identity as it does to requests in
the backwards direction within a dialog. Consequently, no special
guidance is given for implementers here regarding the 'connected
party' problem (see [RFC4916]); authentication service behavior is
unchanged if retargeting has occurred for a dialog-forming request.
Ultimately, the authentication service provides an Identity header
for requests in the backwards dialog when the user is authorized to
assert the identity given in the From header field, and if they are
not, an Identity header is not provided. And per the threat model of
[RFC7375], resolving problems with 'connected' identity has little
bearing on detecting robocalling or related impersonation attacks.
10.2. Unprotected Request Fields
RFC4474 originally had protections for the Contact, Call-ID and CSeq.
These are removed from RFC4474bis. The absence of these header
values creates some opportunities for determined attackers to
impersonate based on cut-and-paste attacks; however, the absence of
these headers does not seem impactful to preventing the simple
unauthorized claiming of an identity for the purposes of robocalling,
voicemail hacking, or swatting, which is the primary scope of the
current document.
It might seem attractive to provide a signature over some of the
information present in the Via header field value(s). For example,
without a signature over the sent-by field of the topmost Via header,
an attacker could remove that Via header and insert its own in a cut-
and-paste attack, which would cause all responses to the request to
be routed to a host of the attacker's choosing. However, a signature
over the topmost Via header does not prevent attacks of this nature,
since the attacker could leave the topmost Via intact and merely
insert a new Via header field directly after it, which would cause
responses to be routed to the attacker's host "on their way" to the
valid host, which has exactly the same end result. Although it is
possible that an intermediary-based authentication service could
guarantee that no Via hops are inserted between the sending user
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agent and the authentication service, it could not prevent an
attacker from adding a Via hop after the authentication service, and
thereby preempting responses. It is necessary for the proper
operation of SIP for subsequent intermediaries to be capable of
inserting such Via header fields, and thus it cannot be prevented.
As such, though it is desirable, securing Via is not possible through
the sort of identity mechanism described in this document; the best
known practice for securing Via is the use of SIPS.
10.3. Malicious Removal of Identity Headers
In the end analysis, the Identity, Identity-Info and Identity-
Extension headers cannot protect themselves. Any attacker could
remove these headers from a SIP request, and modify the request
arbitrarily afterwards. However, this mechanism is not intended to
protect requests from men-in-the-middle who interfere with SIP
messages; it is intended only to provide a way that the originators
of SIP requests can prove that they are who they claim to be. At
best, by stripping identity information from a request, a man-in-the-
middle could make it impossible to distinguish any illegitimate
messages he would like to send from those messages sent by an
authorized user. However, it requires a considerably greater amount
of energy to mount such an attack than it does to mount trivial
impersonations by just copying someone else's From header field.
This mechanism provides a way that an authorized user can provide a
definitive assurance of his identity that an unauthorized user, an
impersonator, cannot.
One additional respect in which the Identity-Info header cannot
protect itself is the 'alg' parameter. The 'alg' parameter is not
included in the digest-string, and accordingly, a man-in-the-middle
might attempt to modify the 'alg' parameter. Once again, it is
important to note that preventing men-in-the-middle is not the
motivation for this mechanism. Moreover, changing the 'alg' would at
worst result in some sort of bid-down attack, and at best cause a
failure in the verifier. Note that only one valid 'alg' parameter is
defined in this document and that thus there is currently no weaker
algorithm to which the mechanism can be bid down. 'alg' has been
incorporated into this mechanism for forward-compatibility reasons in
case the current algorithm exhibits weaknesses, and requires swift
replacement, in the future.
10.4. Securing the Connection to the Authentication Service
In the absence of user agent-based authentication services, the
assurance provided by this mechanism is strongest when a user agent
forms a direct connection, preferably one secured by TLS, to an
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intermediary-based authentication service. The reasons for this are
twofold:
If a user does not receive a certificate from the authentication
service over the TLS connection that corresponds to the expected
domain (especially when the user receives a challenge via a
mechanism such as Digest), then it is possible that a rogue server
is attempting to pose as an authentication service for a domain
that it does not control, possibly in an attempt to collect shared
secrets for that domain. A similar practice could be used for
telephone numbers, though the application of certificates for
telephone numbers to TLS is left as a matter for future study.
Without TLS, the various header field values and the body of the
request will not have integrity protection when the request
arrives at an authentication service. Accordingly, a prior
legitimate or illegitimate intermediary could modify the message
arbitrarily.
Of these two concerns, the first is most material to the intended
scope of this mechanism. This mechanism is intended to prevent
impersonation attacks, not man-in-the-middle attacks; integrity over
the header and bodies is provided by this mechanism only to prevent
replay attacks. However, it is possible that applications relying on
the presence of the Identity header could leverage this integrity
protection, especially body integrity, for services other than replay
protection.
Accordingly, direct TLS connections SHOULD be used between the UAC
and the authentication service whenever possible. The opportunistic
nature of this mechanism, however, makes it very difficult to
constrain UAC behavior, and moreover there will be some deployment
architectures where a direct connection is simply infeasible and the
UAC cannot act as an authentication service itself. Accordingly,
when a direct connection and TLS are not possible, a UAC should use
the SIPS mechanism, Digest 'auth-int' for body integrity, or both
when it can. The ultimate decision to add an Identity header to a
request lies with the authentication service, of course; domain
policy must identify those cases where the UAC's security association
with the authentication service is too weak.
10.5. Authorization and Transitional Strategies
Ultimately, the worth of an assurance provided by an Identity header
is limited by the security practices of the authentication service
that issues the assurance. Relying on an Identity header generated
by a remote administrative domain assumes that the issuing domain
uses recommended administrative practices to authenticate its users.
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However, it is possible that some authentication services will
implement policies that effectively make users unaccountable (e.g.,
ones that accept unauthenticated registrations from arbitrary users).
The value of an Identity header from such authentication services is
questionable. While there is no magic way for a verifier to
distinguish "good" from "bad" signers by inspecting a SIP request, it
is expected that further work in authorization practices could be
built on top of this identity solution; without such an identity
solution, many promising approaches to authorization policy are
impossible. That much said, it is RECOMMENDED that authentication
services based on proxy servers employ strong authentication
practices.
One cannot expect the Identity and Identity-Info headers to be
supported by every SIP entity overnight. This leaves the verifier in
a compromising position; when it receives a request from a given SIP
user, how can it know whether or not the sender's domain supports
Identity? In the absence of ubiquitous support for identity, some
transitional strategies are necessary.
A verifier could remember when it receives a request from a domain
or telephone number that uses Identity, and in the future, view
messages received from that sources without Identity headers with
skepticism.
A verifier could consult some sort of directory that indications
whether a given caller should have a signed identity. There are a
number of potential ways in which this could be implemented. This
is left as a subject for future work.
In the long term, some sort of identity mechanism, either the one
documented in this specification or a successor, must become
mandatory-to-use for the SIP protocol; that is the only way to
guarantee that this protection can always be expected by verifiers.
Finally, it is worth noting that the presence or absence of the
Identity headers cannot be the sole factor in making an authorization
decision. Permissions might be granted to a message on the basis of
the specific verified Identity or really on any other aspect of a SIP
request. Authorization policies are outside the scope of this
specification, but this specification advises any future
authorization work not to assume that messages with valid Identity
headers are always good.
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10.6. Display-Names and Identity
As a matter of interface design, SIP user agents might render the
display-name portion of the From header field of a caller as the
identity of the caller; there is a significant precedent in email
user interfaces for this practice. Securing the display-name
component of the From header field value is outside the scope of this
document, but may be the subject of future work, such as an extension.
11. IANA Considerations
This document relies on the headers and response codes defined in RFC
4474. It also retains the requirements for the specification of new
algorithms or headers related to the mechanisms described in that
document.
11.1. Header Field Names
This document also specifies a new SIP header called Identity-
Extension. Its syntax is given in Section 8. A registry for
Identity-Extension names is defined in Section 11.4.
Header Name: Identity-Extension
Compact Form: N/A
11.2. Identity-Info Parameters
The IANA has already created a registry for Identity-Info header
parameters. This specification defines a new value called "canon" as
defined in Section 5.3.
11.3. Identity-Info Algorithm Parameter Values
The IANA has already created a registry for Identity-Info 'alg'
parameter values. This registry is to be prepopulated with a single
entry for a value called 'rsa-sha256', which describes the algorithm
used to create the signature that appears in the Identity header.
Registry entries must contain the name of the 'alg' parameter value
and the specification in which the value is described. New values
for the 'alg' parameter may be defined only in Standards Track RFCs.
RFC4474 defined the 'rsa-sha1' value for this registry. That value
is hereby deprecated, and should be treated as such. It is not
believed that any implementations are making use of this value.
Future specifications may consider elliptical curves for smaller key
sizes.
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11.4. Identity-Extension Names
This specification requests that the IANA create a new registry for
Identity-Extension names. The registry will consist solely of a list
of names mapped to the Standards Track RFCs in which those extensions
are defined.
The syntax of Identity-Extension names is given in Section 8.
Registering a new Identity-Extension name requires a Standards
Action.
This specification does not provide any initial values for Identity-
Extension names.
12. Acknowledgments
The authors would like to thank Stephen Kent, Brian Rosen, Alex
Bobotek, Paul Kyzviat, Jonathan Lennox, Richard Shockey, Martin
Dolly, Andrew Allen, Hadriel Kaplan, Sanjay Mishra, Anton Baskov,
Pierce Gorman, David Schwartz, Philippe Fouquart, Michael Hamer,
Henning Schulzrinne, and Richard Barnes for their comments.
13. Changes from RFC4474
The following are salient changes from the original RFC 4474:
Generalized the credential mechanism; credential enrollment and
acquisition is now outside the scope of this document
Reduced the scope of the Identity signature to remove CSeq, Call-
ID, Contact, and the message body
Added any DTLS-SRTP fingerprint in SDP as a mandatory element of
the digest-string
Added the Identity-Extension header and extensibility mechanism
Deprecated 'rsa-sha1' in favor of new baseline signing algorithm
14. References
14.1. Normative References
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<http://www.rfc-editor.org/info/rfc2818>.
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[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>.
[RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation
Protocol (SIP): Locating SIP Servers", RFC 3263,
DOI 10.17487/RFC3263, June 2002,
<http://www.rfc-editor.org/info/rfc3263>.
[RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
DOI 10.17487/RFC3280, April 2002,
<http://www.rfc-editor.org/info/rfc3280>.
[RFC3370] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, DOI 10.17487/RFC3370, August 2002,
<http://www.rfc-editor.org/info/rfc3370>.
[RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers",
RFC 3966, DOI 10.17487/RFC3966, December 2004,
<http://www.rfc-editor.org/info/rfc3966>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>.
14.2. Informative References
[I-D.ietf-stir-certificates]
Peterson, J., "Secure Telephone Identity Credentials:
Certificates", draft-ietf-stir-certificates-02 (work in
progress), July 2015.
[I-D.kaplan-stir-cider]
Kaplan, H., "A proposal for Caller Identity in a DNS-based
Entrusted Registry (CIDER)", draft-kaplan-stir-cider-00
(work in progress), July 2013.
[I-D.peterson-sipping-retarget]
Peterson, J., "Retargeting and Security in SIP: A
Framework and Requirements", draft-peterson-sipping-
retarget-00 (work in progress), February 2005.
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[I-D.rosenberg-sip-rfc4474-concerns]
Rosenberg, J., "Concerns around the Applicability of RFC
4474", draft-rosenberg-sip-rfc4474-concerns-00 (work in
progress), February 2008.
[RFC2585] Housley, R. and P. Hoffman, "Internet X.509 Public Key
Infrastructure Operational Protocols: FTP and HTTP",
RFC 2585, DOI 10.17487/RFC2585, May 1999,
<http://www.rfc-editor.org/info/rfc2585>.
[RFC3323] Peterson, J., "A Privacy Mechanism for the Session
Initiation Protocol (SIP)", RFC 3323,
DOI 10.17487/RFC3323, November 2002,
<http://www.rfc-editor.org/info/rfc3323>.
[RFC3325] Jennings, C., Peterson, J., and M. Watson, "Private
Extensions to the Session Initiation Protocol (SIP) for
Asserted Identity within Trusted Networks", RFC 3325,
DOI 10.17487/RFC3325, November 2002,
<http://www.rfc-editor.org/info/rfc3325>.
[RFC3548] Josefsson, S., Ed., "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, DOI 10.17487/RFC3548, July 2003,
<http://www.rfc-editor.org/info/rfc3548>.
[RFC3893] Peterson, J., "Session Initiation Protocol (SIP)
Authenticated Identity Body (AIB) Format", RFC 3893,
DOI 10.17487/RFC3893, September 2004,
<http://www.rfc-editor.org/info/rfc3893>.
[RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, DOI 10.17487/RFC4234,
October 2005, <http://www.rfc-editor.org/info/rfc4234>.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474,
DOI 10.17487/RFC4474, August 2006,
<http://www.rfc-editor.org/info/rfc4474>.
[RFC4501] Josefsson, S., "Domain Name System Uniform Resource
Identifiers", RFC 4501, DOI 10.17487/RFC4501, May 2006,
<http://www.rfc-editor.org/info/rfc4501>.
[RFC4916] Elwell, J., "Connected Identity in the Session Initiation
Protocol (SIP)", RFC 4916, DOI 10.17487/RFC4916, June
2007, <http://www.rfc-editor.org/info/rfc4916>.
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[RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
for Establishing a Secure Real-time Transport Protocol
(SRTP) Security Context Using Datagram Transport Layer
Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
2010, <http://www.rfc-editor.org/info/rfc5763>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
2012, <http://www.rfc-editor.org/info/rfc6698>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013,
<http://www.rfc-editor.org/info/rfc6973>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <http://www.rfc-editor.org/info/rfc7258>.
[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>.
[RFC7375] Peterson, J., "Secure Telephone Identity Threat Model",
RFC 7375, DOI 10.17487/RFC7375, October 2014,
<http://www.rfc-editor.org/info/rfc7375>.
Authors' Addresses
Jon Peterson
Neustar, Inc.
1800 Sutter St Suite 570
Concord, CA 94520
US
Email: jon.peterson@neustar.biz
Cullen Jennings
Cisco
400 3rd Avenue SW, Suite 350
Calgary, AB T2P 4H2
Canada
Email: fluffy@iii.ca
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Eric Rescorla
RTFM, Inc.
2064 Edgewood Drive
Palo Alto, CA 94303
USA
Phone: +1 650 678 2350
Email: ekr@rtfm.com
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