Network Working Group P. Saint-Andre
Internet-Draft Mozilla
Obsoletes: 6125 (if approved) J. Hodges
Intended status: Standards Track Google
Expires: 12 March 2022 R. Salz
Akamai Technologies
8 September 2021
Representation and Verification of Domain-Based Application Service
Identity within Internet Public Key Infrastructure Using X.509 (PKIX)
Certificates in the Context of Transport Layer Security (TLS)
draft-ietf-uta-rfc6125bis-02
Abstract
Many application technologies enable secure communication between two
entities by means of Transport Layer Security (TLS) with Internet
Public Key Infrastructure Using X.509 (PKIX) certificates. This
document specifies procedures for representing and verifying the
identity of application services in such interactions.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the Using TLS in
Applications Working Group mailing list (uta@ietf.org), which is
archived at https://mailarchive.ietf.org/arch/browse/uta/.
Source for this draft and an issue tracker can be found at
https://github.com/richsalz/draft-ietf-uta-rfc6125bis.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 12 March 2022.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Audience . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Changes since RFC 6125 . . . . . . . . . . . . . . . . . 4
1.4. How to Read This Document . . . . . . . . . . . . . . . . 5
1.5. Applicability . . . . . . . . . . . . . . . . . . . . . . 5
1.6. Overview of Recommendations . . . . . . . . . . . . . . . 6
1.7. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.7.1. In Scope . . . . . . . . . . . . . . . . . . . . . . 7
1.7.2. Out of Scope . . . . . . . . . . . . . . . . . . . . 7
1.8. Terminology . . . . . . . . . . . . . . . . . . . . . . . 9
2. Naming of Application Services . . . . . . . . . . . . . . . 12
2.1. Naming Application Services . . . . . . . . . . . . . . . 12
2.2. DNS Domain Names . . . . . . . . . . . . . . . . . . . . 13
2.3. Subject Naming in PKIX Certificates . . . . . . . . . . . 14
3. Designing Application Protocols . . . . . . . . . . . . . . . 14
4. Representing Server Identity . . . . . . . . . . . . . . . . 15
4.1. Rules . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . 16
5. Requesting Server Certificates . . . . . . . . . . . . . . . 16
6. Verifying Service Identity . . . . . . . . . . . . . . . . . 17
6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 17
6.2. Constructing a List of Reference Identifiers . . . . . . 18
6.2.1. Rules . . . . . . . . . . . . . . . . . . . . . . . . 18
6.2.2. Examples . . . . . . . . . . . . . . . . . . . . . . 20
6.3. Preparing to Seek a Match . . . . . . . . . . . . . . . . 21
6.4. Matching the DNS Domain Name Portion . . . . . . . . . . 22
6.4.1. Checking of Traditional Domain Names . . . . . . . . 22
6.4.2. Checking of Internationalized Domain Names . . . . . 22
6.4.3. Checking of Wildcard Certificates . . . . . . . . . . 23
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6.5. Matching the Application Service Type Portion . . . . . . 23
6.5.1. SRV-ID . . . . . . . . . . . . . . . . . . . . . . . 24
6.5.2. URI-ID . . . . . . . . . . . . . . . . . . . . . . . 24
6.6. Outcome . . . . . . . . . . . . . . . . . . . . . . . . . 24
6.6.1. Case #1: Match Found . . . . . . . . . . . . . . . . 24
6.6.2. Case #2: No Match Found, Pinned Certificate . . . . . 24
6.6.3. Case #3: No Match Found, No Pinned Certificate . . . 24
6.6.4. Fallback . . . . . . . . . . . . . . . . . . . . . . 25
7. Security Considerations . . . . . . . . . . . . . . . . . . . 25
7.1. Pinned Certificates . . . . . . . . . . . . . . . . . . . 25
7.2. Wildcard Certificates . . . . . . . . . . . . . . . . . . 26
7.3. Internationalized Domain Names . . . . . . . . . . . . . 26
7.4. Multiple Identifiers . . . . . . . . . . . . . . . . . . 26
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
8.1. Normative References . . . . . . . . . . . . . . . . . . 26
8.2. Informative References . . . . . . . . . . . . . . . . . 27
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30
1. Introduction
1.1. Motivation
The visible face of the Internet largely consists of services that
employ a client-server architecture in which an interactive or
automated client communicates with an application service in order to
retrieve or upload information, communicate with other entities, or
access a broader network of services. When a client communicates
with an application service using Transport Layer Security [TLS] or
Datagram Transport Layer Security [DTLS], it references some notion
of the server's identity (e.g., "the website at example.com") while
attempting to establish secure communication. Likewise, during TLS
negotiation, the server presents its notion of the service's identity
in the form of a public-key certificate that was issued by a
certification authority (CA) in the context of the Internet Public
Key Infrastructure using X.509 [PKIX]. Informally, we can think of
these identities as the client's "reference identity" and the
server's "presented identity" (these rough ideas are defined more
precisely later in this document through the concept of particular
identifiers). In general, a client needs to verify that the server's
presented identity matches its reference identity so it can
authenticate the communication.
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Many application technologies adhere to the pattern just outlined.
Such protocols have traditionally specified their own rules for
representing and verifying application service identity.
Unfortunately, this divergence of approaches has caused some
confusion among certification authorities, application developers,
and protocol designers.
Therefore, to codify secure procedures for the implementation and
deployment of PKIX-based authentication, this document specifies
recommended procedures for representing and verifying application
service identity in certificates intended for use in application
protocols employing TLS.
1.2. Audience
The primary audience for this document consists of application
protocol designers, who can reference this document instead of
defining their own rules for the representation and verification of
application service identity. Secondarily, the audience consists of
certification authorities, service providers, and client developers
from technology communities that might reuse the recommendations in
this document when defining certificate issuance policies, generating
certificate signing requests, or writing software algorithms for
identity matching.
1.3. Changes since RFC 6125
This document revises and obsoletes [VERIFY] based on the decade of
experience and changes since it was first published. The major
changes, in no particular order, include:
* All references have been updated to the current latest version.
* The TLS SNI extension is no longer new, it is commonplace.
* The only legal place for a certificate wildcard name is as the
left-most component in a domain name.
* It is no longer allowed to use the commonName RDN, known as "CN-
ID", to represent the server identity; only the subjectAltNames
extension is used.
* References to the X.500 directory, the survey of prior art, and
the sample text in Appendix A have been removed.
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1.4. How to Read This Document
This document is longer than the authors would have liked because it
was necessary to carefully define terminology, explain the underlying
concepts, define the scope, and specify recommended behavior for both
certification authorities and application software implementations.
The following sections are of special interest to various audiences:
* Protocol designers might want to first read the checklist in
Section 3.
* Certification authorities might want to first read the
recommendations for representation of server identity in
Section 4.
* Service providers might want to first read the recommendations for
requesting of server certificates in Section 5.
* Software implementers might want to first read the recommendations
for verification of server identity in Section 6.
The sections on terminology (Section 1.8), naming of application
services (Section 2), document scope (Section 1.7), and the like
provide useful background information regarding the recommendations
and guidelines that are contained in the above-referenced sections,
but are not absolutely necessary for a first reading of this
document.
1.5. Applicability
This document does not supersede the rules for certificate issuance
or validation provided in [PKIX]. Therefore, [PKIX] is authoritative
on any point that might also be discussed in this document.
Furthermore, [PKIX] also governs any certificate-related topic on
which this document is silent, including but not limited to
certificate syntax, certificate extensions such as name constraints
and extended key usage, and handling of certification paths.
This document addresses only name forms in the leaf "end entity"
server certificate, not any name forms in the chain of certificates
used to validate the server certificate. Therefore, in order to
ensure proper authentication, application clients need to verify the
entire certification path per [PKIX].
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This document also does not supersede the rules for verifying service
identity provided in specifications for existing application
protocols published prior to this document. However, the procedures
described here can be referenced by future specifications, including
updates to specifications for existing application protocols if the
relevant technology communities agree to do so.
1.6. Overview of Recommendations
To orient the reader, this section provides an informational overview
of the recommendations contained in this document.
The previous version of this specification, [VERIFY], surveyed the
current practice from many IETF standards and tried to generalize
best practices. This document takes the lessons learned in the past
decade and codifies them as best practices.
For the primary audience of application protocol designers, this
document provides recommended procedures for the representation and
verification of application service identity within PKIX certificates
used in the context of TLS.
For the secondary audiences, in essence this document encourages
certification authorities, application service providers, and
application client developers to coalesce on the following practices:
* Stop including and checking strings that look like domain names in
the subject's Common Name.
* Check DNS domain names via the subjectAlternativeName extension
designed for that purpose: dNSName.
* Move toward including and checking even more specific
subjectAlternativeName extensions where appropriate for using the
protocol (e.g., uniformResourceIdentifier and the otherName form
SRVName).
* Constrain and simplify the validation of wildcard certificates
(e.g., a certificate containing an identifier for
"*.example.com").
1.7. Scope
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1.7.1. In Scope
This document applies only to service identities associated with
fully qualified DNS domain names, only to TLS and DTLS, and only to
PKIX-based systems. As a result, the scenarios described in the
following section are out of scope for this specification (although
they might be addressed by future specifications).
1.7.2. Out of Scope
The following topics are out of scope for this specification:
* Client or end-user identities.
Certificates representing client or end-user identities (e.g., the
rfc822Name identifier) can be used for mutual authentication
between a client and server or between two clients, thus enabling
stronger client-server security or end-to-end security. However,
certification authorities, application developers, and service
operators have less experience with client certificates than with
server certificates, thus giving us fewer models from which to
generalize and a less solid basis for defining best practices.
* Identifiers other than fully qualified DNS domain names.
For example, this specification does not discuss IP addresses or
other attributes within a certificate beyond the subjectAltName
extension. The focus of this document is on application service
identities, not specific resources located at such services.
Therefore this document discusses Uniform Resource Identifiers
[URI] only as a way to communicate a DNS domain name (via the URI
"host" component or its equivalent), not as a way to communicate
other aspects of a service such as a specific resource (via the
URI "path" component) or parameters (via the URI "query"
component).
* Security protocols other than [TLS] or [DTLS].
Although other secure, lower-layer protocols exist and even employ
PKIX certificates at times (e.g., IPsec [IPSEC]), their use cases
can differ from those of TLS-based and DTLS-based application
technologies. Furthermore, application technologies have less
experience with IPsec than with TLS, thus making it more difficult
to gather feedback on proposed best practices.
* Keys or certificates employed outside the context of PKIX-based
systems.
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Some deployed application technologies use a web of trust model
based on or similar to OpenPGP [OPENPGP], or use self-signed
certificates, or are deployed on networks that are not directly
connected to the public Internet and therefore cannot depend on
Certificate Revocation Lists (CRLs) or the Online Certificate
Status Protocol [OCSP] to check CA-issued certificates. However,
the method for binding a public key to an identifier in OpenPGP
differs essentially from the method in X.509, the data in self-
signed certificates has not been certified by a third party in any
way, and checking of CA-issued certificates via CRLs or OCSP is
critically important to maintaining the security of PKIX-based
systems. Attempting to define best practices for such
technologies would unduly complicate the rules defined in this
specification.
* Certification authority policies, such as:
- What types or "classes" of certificates to issue and whether to
apply different policies for them.
- Whether to issue certificates based on IP addresses (or some
other form, such as relative domain names) in addition to fully
qualified DNS domain names.
- Which identifiers to include (e.g., whether to include SRV-IDs
or URI-IDs as defined in the body of this specification).
- How to certify or validate fully qualified DNS domain names and
application service types.
- How to certify or validate other kinds of information that
might be included in a certificate (e.g., organization name).
* Resolution of DNS domain names.
Although the process whereby a client resolves the DNS domain name
of an application service can involve several steps (e.g., this is
true of resolutions that depend on DNS SRV resource records,
Naming Authority Pointer (NAPTR) DNS resource records [NAPTR], and
related technologies such as [S-NAPTR]), for our purposes we care
only about the fact that the client needs to verify the identity
of the entity with which it communicates as a result of the
resolution process. Thus the resolution process itself is out of
scope for this specification.
* User interface issues.
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In general, such issues are properly the responsibility of client
software developers and standards development organizations
dedicated to particular application technologies (see, for
example, [WSC-UI]).
1.8. Terminology
Because many concepts related to "identity" are often too vague to be
actionable in application protocols, we define a set of more concrete
terms for use in this specification.
application service: A service on the Internet that enables
interactive and automated clients to connect for the purpose of
retrieving or uploading information, communicating with other
entities, or connecting to a broader network of services.
application service provider: An organization or individual that
hosts or deploys an application service.
application service type: A formal identifier for the application
protocol used to provide a particular kind of application service
at a domain; the application service type typically takes the form
of a Uniform Resource Identifier scheme [URI] or a DNS SRV Service
[DNS-SRV].
automated client: A software agent or device that is not directly
controlled by a human user.
delegated domain: A domain name or host name that is explicitly
configured for communicating with the source domain, by either (a)
the human user controlling an interactive client or (b) a trusted
administrator. In case (a), one example of delegation is an
account setup that specifies the domain name of a particular host
to be used for retrieving information or connecting to a network,
which might be different from the server portion of the user's
account name (e.g., a server at mailhost.example.com for
connecting to an IMAP server hosting an email address of
juliet@example.com). In case (b), one example of delegation is an
admin-configured host-to-address/address-to-host lookup table.
derived domain: A domain name or host name that a client has derived
from the source domain in an automated fashion (e.g., by means of
a [DNS-SRV] lookup).
identifier: A particular instance of an identifier type that is
either presented by a server in a certificate or referenced by a
client for matching purposes.
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identifier type: A formally defined category of identifier that can
be included in a certificate and therefore that can also be used
for matching purposes. For conciseness and convenience, we define
the following identifier types of interest, which are based on
those found in the PKIX specification [PKIX] and various PKIX
extensions.
* DNS-ID = a subjectAltName entry of type dNSName; see [PKIX]
* SRV-ID = a subjectAltName entry of type otherName whose name
form is SRVName; see [SRVNAME]
* URI-ID = a subjectAltName entry of type
uniformResourceIdentifier whose value includes both (i) a
"scheme" and (ii) a "host" component (or its equivalent) that
matches the "reg-name" rule (where the quoted terms represent
the associated [ABNF] productions from [URI]); see [PKIX] and
[URI]
interactive client: A software agent or device that is directly
controlled by a human user. (Other specifications related to
security and application protocols, such as [WSC-UI], often refer
to this entity as a "user agent".)
pinning: The act of establishing a cached name association between
the application service's certificate and one of the client's
reference identifiers, despite the fact that none of the presented
identifiers matches the given reference identifier. Pinning is
accomplished by allowing a human user to positively accept the
mismatch during an attempt to communicate with the application
service. Once a cached name association is established, the
certificate is said to be pinned to the reference identifier and
in future communication attempts the client simply verifies that
the service's presented certificate matches the pinned
certificate, as described under Section 6.6.2. (A similar
definition of "pinning" is provided in [WSC-UI].)
PKIX: PKIX is a short name for the Internet Public Key
Infrastructure using X.509 defined in RFC 5280 [PKIX], which
comprises a profile of the X.509v3 certificate specifications and
X.509v2 certificate revocation list (CRL) specifications for use
in the Internet.
PKIX-based system: A software implementation or deployed service
that makes use of X.509v3 certificates and X.509v2 certificate
revocation lists (CRLs).
PKIX certificate: An X.509v3 certificate generated and employed in
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the context of PKIX.
presented identifier: An identifier that is presented by a server to
a client within a PKIX certificate when the client attempts to
establish secure communication with the server; the certificate
can include one or more presented identifiers of different types,
and if the server hosts more than one domain then the certificate
might present distinct identifiers for each domain.
reference identifier: An identifier, constructed from a source
domain and optionally an application service type, used by the
client for matching purposes when examining presented identifiers.
Relative Distinguished Name (RDN): The ASN.1-based construction
comprising a Relative Distinguished Name (RDN), which itself is a
building-block component of Distinguished Names. See [LDAP-DN],
Section 2.
source domain: The fully qualified DNS domain name that a client
expects an application service to present in the certificate
(e.g., "www.example.com"), typically input by a human user,
configured into a client, or provided by reference such as in a
hyperlink. The combination of a source domain and, optionally, an
application service type enables a client to construct one or more
reference identifiers.
subjectAltName entry: An identifier placed in a subjectAltName
extension.
subjectAltName extension: A standard PKIX certificate extension
[PKIX] enabling identifiers of various types to be bound to the
certificate subject.
subject name: In this specification, the term refers to the name of
a PKIX certificate's subject, encoded in a certificate's subject
field (see [PKIX], Section 4.1.2.6).
TLS client: An entity that assumes the role of a client in a
Transport Layer Security [TLS] negotiation. In this specification
we generally assume that the TLS client is an (interactive or
automated) application client; however, in application protocols
that enable server-to-server communication, the TLS client could
be a peer application service.
TLS server: An entity that assumes the role of a server in a
Transport Layer Security [TLS] negotiation; in this specification
we assume that the TLS server is an application service.
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Most security-related terms in this document are to be understood in
the sense defined in [SECTERMS]; such terms include, but are not
limited to, "attack", "authentication", "authorization",
"certification authority", "certification path", "certificate",
"credential", "identity", "self-signed certificate", "trust", "trust
anchor", "trust chain", "validate", and "verify".
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Naming of Application Services
This section discusses naming of application services on the
Internet, followed by a brief tutorial about subject naming in PKIX.
2.1. Naming Application Services
This specification assumes that the name of an application service is
based on a DNS domain name (e.g., "example.com") -- supplemented in
some circumstances by an application service type (e.g., "the IMAP
server at example.com").
From the perspective of the application client or user, some names
are direct because they are provided directly by a human user (e.g.,
via runtime input, prior configuration, or explicit acceptance of a
client communication attempt), whereas other names are indirect
because they are automatically resolved by the client based on user
input (e.g., a target name resolved from a source name using DNS SRV
or NAPTR records). This dimension matters most for certificate
consumption, specifically verification as discussed in this document.
From the perspective of the application service, some names are
unrestricted because they can be used in any type of service (e.g., a
certificate might be reused for both the HTTP service and the IMAP
service at example.com), whereas other names are restricted because
they can be used in only one type of service (e.g., a special-purpose
certificate that can be used only for an IMAP service). This
dimension matters most for certificate issuance.
Therefore, we can categorize the identifier types of interest as
follows:
* A DNS-ID is direct and unrestricted.
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* An SRV-ID is typically indirect but can be direct, and is
restricted.
* A URI-ID is direct and restricted.
When implementing software, deploying services, and issuing
certificates for secure PKIX-based authentication, it is important to
keep these distinctions in mind. In particular, best practices
differ somewhat for application server implementations, application
client implementations, application service providers, and
certification authorities. Ideally, protocol specifications that
reference this document will specify which identifiers are mandatory-
to-implement by servers and clients, which identifiers ought to be
supported by certificate issuers, and which identifiers ought to be
requested by application service providers. Because these
requirements differ across applications, it is impossible to
categorically stipulate universal rules (e.g., that all software
implementations, service providers, and certification authorities for
all application protocols need to use or support DNS-IDs as a
baseline for the purpose of interoperability).
However, it is preferable that each application protocol will at
least define a baseline that applies to the community of software
developers, application service providers, and CAs actively using or
supporting that technology (one such community, the CA/Browser Forum,
has codified such a baseline for "Extended Validation Certificates"
in [EV-CERTS]).
2.2. DNS Domain Names
For the purposes of this specification, the name of an application
service is (or is based on) a DNS domain name that conforms to one of
the following forms:
1. A "traditional domain name", i.e., a fully qualified DNS domain
name or "FQDN" (see [DNS-CONCEPTS]) all of whose labels are "LDH
labels" as described in [IDNA-DEFS]. Informally, such labels are
constrained to [US-ASCII] letters, digits, and the hyphen, with
the hyphen prohibited in the first character position.
Additional qualifications apply (please refer to the above-
referenced specifications for details), but they are not relevant
to this specification.
2. An "internationalized domain name", i.e., a DNS domain name that
conforms to the overall form of a domain name (informally, dot-
separated letter-digit-hyphen labels) but includes at least one
label containing appropriately encoded Unicode code points
outside the traditional US-ASCII range. That is, it contains at
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least one U-label or A-label, but otherwise may contain any
mixture of NR-LDH labels, A-labels, or U-labels, as described in
[IDNA-DEFS] and the associated documents.
2.3. Subject Naming in PKIX Certificates
For our purposes, an application service can be identified by a name
or names carried in one or more of the following identifier types
within subjectAltName entries:
* DNS-ID
* SRV-ID
* URI-ID
The Common Name RDN MUST NOT be used to identify a service. Reasons
for this include:
* It is not strongly typed and therefore suffers from ambiguities in
interpretation.
* It can appear more than once in the Subject Name.
For similar reasons, other RDN's within the Subject Name MUST NOT be
used to identify a service.
3. Designing Application Protocols
This section provides guidelines for designers of application
protocols, in the form of a checklist to follow when reusing the
recommendations provided in this document.
* If your technology does not use DNS SRV records to resolve the DNS
domain names of application services then consider stating that
SRV-ID as defined in this document is not supported. Note that
many existing application technologies use DNS SRV records to
resolve the DNS domain names of application services, but do not
rely on representations of those records in PKIX certificates by
means of SRV-IDs as defined in [SRVNAME].
* If your technology does not use use URIs to identify application
services, then consider stating that URI-ID as defined in this
document is not supported. Note that many existing application
technologies use URIs to identify application services, but do not
rely on representation of those URIs in PKIX certificates by means
of URI-IDs.
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* If your technology disallows the wildcard character in DNS domain
names, then state the wildcard certificates as defined in this
document are not supported.
4. Representing Server Identity
This section provides rules and guidelines for issuers of
certificates.
4.1. Rules
When a certification authority issues a certificate based on the
fully qualified DNS domain name at which the application service
provider will provide the relevant application, the following rules
apply to the representation of application service identities. The
reader needs to be aware that some of these rules are cumulative and
can interact in important ways that are illustrated later in this
document.
1. The certificate SHOULD include a "DNS-ID" if possible as a
baseline for interoperability.
2. If the service using the certificate deploys a technology for
which the relevant specification stipulates that certificates
ought to include identifiers of type SRV-ID (e.g., this is true
of [XMPP]), then the certificate SHOULD include an SRV-ID.
3. If the service using the certificate deploys a technology for
which the relevant specification stipulates that certificates
ought to include identifiers of type URI-ID (e.g., this is true
of [SIP] as specified by [SIP-CERTS], but not true of [HTTP]
since [HTTP-TLS] does not describe usage of a URI-ID for HTTP
services), then the certificate SHOULD include a URI-ID. The
scheme SHALL be that of the protocol associated with the
application service type and the "host" component (or its
equivalent) SHALL be the fully qualified DNS domain name of the
service. A specification that reuses this one MUST specify which
URI schemes are to be considered acceptable in URI-IDs contained
in PKIX certificates used for the application protocol (e.g.,
"sip" but not "sips" or "tel" for SIP as described in [SIP-SIPS],
or perhaps http and https for HTTP as might be described in a
future specification).
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4. The certificate MAY include other application-specific
identifiers for types that were defined before publication of
[SRVNAME] (e.g., XmppAddr for [XMPP]) or for which service names
or URI schemes do not exist; however, such application-specific
identifiers are not applicable to all application technologies
and therefore are out of scope for this specification.
5. The certificate MAY contain more than one DNS-ID, SRV-ID, or URI-
ID as further explained under Section 7.4.
4.2. Examples
Consider a simple website at "www.example.com", which is not
discoverable via DNS SRV lookups. Because HTTP does not specify the
use of URIs in server certificates, a certificate for this service
might include only a DNS-ID of "www.example.com".
Consider an IMAP-accessible email server at the host
"mail.example.net" servicing email addresses of the form
"user@example.net" and discoverable via DNS SRV lookups on the
application service name of "example.net". A certificate for this
service might include SRV-IDs of "_imap.example.net" and
"_imaps.example.net" (see [EMAIL-SRV]) along with DNS-IDs of
"example.net" and "mail.example.net".
Consider a SIP-accessible voice-over-IP (VoIP) server at the host
"voice.example.edu" servicing SIP addresses of the form
"user@voice.example.edu" and identified by a URI of
<sip:voice.example.edu>. A certificate for this service would
include a URI-ID of "sip:voice.example.edu" (see [SIP-CERTS]) along
with a DNS-ID of "voice.example.edu".
Consider an XMPP-compatible instant messaging (IM) server at the host
"im.example.org" servicing IM addresses of the form
"user@im.example.org" and discoverable via DNS SRV lookups on the
"im.example.org" domain. A certificate for this service might
include SRV-IDs of "_xmpp-client.im.example.org" and "_xmpp-
server.im.example.org" (see [XMPP]), a DNS-ID of "im.example.org",
and an XMPP-specific "XmppAddr" of "im.example.org" (see [XMPP]).
5. Requesting Server Certificates
This section provides rules and guidelines for service providers
regarding the information to include in certificate signing requests
(CSRs).
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In general, service providers are encouraged to request certificates
that include all of the identifier types that are required or
recommended for the application service type that will be secured
using the certificate to be issued.
If the certificate might be used for any type of application service,
then the service provider is encouraged to request a certificate that
includes only a DNS-ID.
If the certificate will be used for only a single type of application
service, then the service provider is encouraged to request a
certificate that includes a DNS-ID and, if appropriate for the
application service type, an SRV-ID or URI-ID that limits the
deployment scope of the certificate to only the defined application
service type.
If a service provider offering multiple application service types
(e.g., a World Wide Web service, an email service, and an instant
messaging service) wishes to limit the applicability of certificates
using SRV-IDs or URI-IDs, then the service provider is encouraged to
request multiple certificates, i.e., one certificate per application
service type. Conversely, the service provider is discouraged from
requesting a single certificate containing multiple SRV-IDs or URI-
IDs identifying each different application service type. This
guideline does not apply to application service type "bundles" that
are used to identify manifold distinct access methods to the same
underlying application (e.g., an email application with access
methods denoted by the application service types of "imap", "imaps",
"pop3", "pop3s", and "submission" as described in [EMAIL-SRV]).
6. Verifying Service Identity
This section provides rules and guidelines for implementers of
application client software regarding algorithms for verification of
application service identity.
6.1. Overview
At a high level, the client verifies the application service's
identity by performing the actions listed below (which are defined in
the following subsections of this document):
1. The client constructs a list of acceptable reference identifiers
based on the source domain and, optionally, the type of service
to which the client is connecting.
2. The server provides its identifiers in the form of a PKIX
certificate.
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3. The client checks each of its reference identifiers against the
presented identifiers for the purpose of finding a match.
4. When checking a reference identifier against a presented
identifier, the client matches the source domain of the
identifiers and, optionally, their application service type.
Naturally, in addition to checking identifiers, a client might
complete further checks to ensure that the server is authorized to
provide the requested service. However, such checking is not a
matter of verifying the application service identity presented in a
certificate, and therefore methods for doing so (e.g., consulting
local policy information) are out of scope for this document.
6.2. Constructing a List of Reference Identifiers
6.2.1. Rules
The client MUST construct a list of acceptable reference identifiers,
and MUST do so independently of the identifiers presented by the
service.
The inputs used by the client to construct its list of reference
identifiers might be a URI that a user has typed into an interface
(e.g., an HTTPS URL for a website), configured account information
(e.g., the domain name of a particular host or URI used for
retrieving information or connecting to a network, which might be
different from the DNS domain name portion of a username), a
hyperlink in a web page that triggers a browser to retrieve a media
object or script, or some other combination of information that can
yield a source domain and an application service type.
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The client might need to extract the source domain and application
service type from the input(s) it has received. The extracted data
MUST include only information that can be securely parsed out of the
inputs (e.g., parsing the fully qualified DNS domain name out of the
"host" component (or its equivalent) of a URI or deriving the
application service type from the scheme of a URI) or information
that is derived in a manner not subject to subversion by network
attackers (e.g., pulling the data from a delegated domain that is
explicitly established via client or system configuration, resolving
the data via [DNSSEC], or obtaining the data from a third-party
domain mapping service in which a human user has explicitly placed
trust and with which the client communicates over a connection or
association that provides both mutual authentication and integrity
checking). These considerations apply only to extraction of the
source domain from the inputs; naturally, if the inputs themselves
are invalid or corrupt (e.g., a user has clicked a link provided by a
malicious entity in a phishing attack), then the client might end up
communicating with an unexpected application service.
For example, given an input URI of <sips:alice@example.net>, a client
would derive the application service type "sip" from the scheme and
parse the domain name "example.net" from the host component.
Each reference identifier in the list SHOULD be based on the source
domain and SHOULD NOT be based on a derived domain (e.g., a host name
or domain name discovered through DNS resolution of the source
domain). This rule is important because only a match between the
user inputs and a presented identifier enables the client to be sure
that the certificate can legitimately be used to secure the client's
communication with the server. There is only one scenario in which
it is acceptable for an interactive client to override the
recommendation in this rule and therefore communicate with a domain
name other than the source domain: because a human user has "pinned"
the application service's certificate to the alternative domain name
as further discussed under Section 6.6.4 and Section 7.1. In this
case, the inputs used by the client to construct its list of
reference identifiers might include more than one fully qualified DNS
domain name, i.e., both (a) the source domain and (b) the alternative
domain contained in the pinned certificate.
Using the combination of fully qualified DNS domain name(s) and
application service type, the client constructs a list of reference
identifiers in accordance with the following rules:
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* The list SHOULD include a DNS-ID. A reference identifier of type
DNS-ID can be directly constructed from a fully qualified DNS
domain name that is (a) contained in or securely derived from the
inputs (i.e., the source domain), or (b) explicitly associated
with the source domain by means of user configuration (i.e., a
derived domain).
* If a server for the application service type is typically
discovered by means of DNS SRV records, then the list SHOULD
include an SRV-ID.
* If a server for the application service type is typically
associated with a URI for security purposes (i.e., a formal
protocol document specifies the use of URIs in server
certificates), then the list SHOULD include a URI-ID.
Which identifier types a client includes in its list of reference
identifiers is a matter of local policy. For example, in certain
deployment environments, a client that is built to connect only to a
particular kind of service (e.g., only IM services) might be
configured to accept as valid only certificates that include an SRV-
ID for that application service type; in this case, the client would
include only SRV-IDs matching the application service type in its
list of reference identifiers (not, for example, DNS-IDs). By
contrast, a more lenient client (even one built to connect only to a
particular kind of service) might include both SRV-IDs and DNS-IDs in
its list of reference identifiers.
6.2.2. Examples
A web browser that is connecting via HTTPS to the website at
"www.example.com" would have a single reference identifier: a DNS-ID
of "www.example.com".
A mail user agent that is connecting via IMAPS to the email service
at "example.net" (resolved as "mail.example.net") might have three
reference identifiers: an SRV-ID of "_imaps.example.net" (see
[EMAIL-SRV]), and DNS-IDs of "example.net" and "mail.example.net".
(A legacy email user agent would not support [EMAIL-SRV] and
therefore would probably be explicitly configured to connect to
"mail.example.net", whereas an SRV-aware user agent would derive
"example.net" from an email address of the form "user@example.net"
but might also accept "mail.example.net" as the DNS domain name
portion of reference identifiers for the service.)
A voice-over-IP (VoIP) user agent that is connecting via SIP to the
voice service at "voice.example.edu" might have only one reference
identifier: a URI-ID of "sip:voice.example.edu" (see [SIP-CERTS]).
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An instant messaging (IM) client that is connecting via XMPP to the
IM service at "im.example.org" might have three reference
identifiers: an SRV-ID of "_xmpp-client.im.example.org" (see [XMPP]),
a DNS-ID of "im.example.org", and an XMPP-specific "XmppAddr" of
"im.example.org" (see [XMPP]).
6.3. Preparing to Seek a Match
Once the client has constructed its list of reference identifiers and
has received the server's presented identifiers in the form of a PKIX
certificate, the client checks its reference identifiers against the
presented identifiers for the purpose of finding a match. The search
fails if the client exhausts its list of reference identifiers
without finding a match. The search succeeds if any presented
identifier matches one of the reference identifiers, at which point
the client SHOULD stop the search.
Before applying the comparison rules provided in the following
sections, the client might need to split the reference identifier
into its DNS domain name portion and its application service type
portion, as follows:
* A reference identifier of type DNS-ID does not include an
application service type portion and thus can be used directly as
the DNS domain name for comparison purposes. As an example, a
DNS-ID of "www.example.com" would result in a DNS domain name
portion of "www.example.com".
* For a reference identifier of type SRV-ID, the DNS domain name
portion is the Name and the application service type portion is
the Service. As an example, an SRV-ID of "_imaps.example.net"
would be split into a DNS domain name portion of "example.net" and
an application service type portion of "imaps" (mapping to an
application protocol of IMAP as explained in [EMAIL-SRV]).
* For a reference identifier of type URI-ID, the DNS domain name
portion is the "reg-name" part of the "host" component (or its
equivalent) and the application service type portion is the
application service type associated with the scheme name matching
the [ABNF] "scheme" rule from [URI] (not including the ':'
separator). As previously mentioned, this document specifies that
a URI-ID always contains a "host" component (or its equivalent)
containing a "reg-name". (Matching only the "reg-name" rule from
[URI] limits verification to DNS domain names, thereby
differentiating a URI-ID from a uniformResourceIdentifier entry
that contains an IP address or a mere host name, or that does not
contain a "host" component at all.) Furthermore, note that
extraction of the "reg-name" might necessitate normalization of
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the URI (as explained in [URI]). As an example, a URI-ID of
"sip:voice.example.edu" would be split into a DNS domain name
portion of "voice.example.edu" and an application service type of
"sip" (associated with an application protocol of SIP as explained
in [SIP-CERTS]).
Detailed comparison rules for matching the DNS domain name portion
and application service type portion of the reference identifier are
provided in the following sections.
6.4. Matching the DNS Domain Name Portion
The client MUST match the DNS domain name portion of a reference
identifier according to the following rules (and SHOULD also check
the application service type as described under Section 6.5). The
rules differ depending on whether the domain to be checked is a
"traditional domain name" or an "internationalized domain name" (as
defined under Section 2.2). Furthermore, to meet the needs of
clients that support presented identifiers containing the wildcard
character "*", we define a supplemental rule for such "wildcard
certificates".
6.4.1. Checking of Traditional Domain Names
If the DNS domain name portion of a reference identifier is a
"traditional domain name", then matching of the reference identifier
against the presented identifier is performed by comparing the set of
domain name labels using a case-insensitive ASCII comparison, as
clarified by [DNS-CASE] (e.g., "WWW.Example.Com" would be lower-cased
to "www.example.com" for comparison purposes). Each label MUST match
in order for the names to be considered to match, except as
supplemented by the rule about checking of wildcard labels
(Section 6.4.3).
6.4.2. Checking of Internationalized Domain Names
If the DNS domain name portion of a reference identifier is an
internationalized domain name, then an implementation MUST convert
any U-labels [IDNA-DEFS] in the domain name to A-labels before
checking the domain name. In accordance with [IDNA-PROTO], A-labels
MUST be compared as case-insensitive ASCII. Each label MUST match in
order for the domain names to be considered to match, except as
supplemented by the rule about checking of wildcard labels
(Section 6.4.3; but see also Section 7.2 regarding wildcards in
internationalized domain names).
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6.4.3. Checking of Wildcard Certificates
A client MAY match the reference identifier against a presented
identifier whose DNS domain name portion contains the wildcard
character "*" in a label (following the description of labels and
domain names in [DNS-CONCEPTS]), provided these requirements are met:
1. There is only one wildcard character.
2. The wildcard character appears only as the content of the left-
most label.
3. The wildcard character is not embedded in an A-label or U-label
[IDNA-DEFS] of an internationalized domain name [IDNA-PROTO].
A wildcard in a presented identifier can only match exactly one label
in a reference identifier. Note that this is not the same as DNS
wildcard matching, where the "*" label always matches at least one
whole label and sometimes more. See [DNS-CONCEPTS], Section 4.3.3
and [DNS-WILDCARDS].
For information regarding the security characteristics of wildcard
certificates, see Section 7.2.
6.5. Matching the Application Service Type Portion
When a client checks identifiers of type SRV-ID and URI-ID, it MUST
check not only the DNS domain name portion of the identifier but also
the application service type portion. The client does this by
splitting the identifier into the DNS domain name portion and the
application service type portion (as described under Section 6.3),
then checking both the DNS domain name portion (as described under
Section 6.4) and the application service type portion as described in
the following subsections.
Implementation Note: An identifier of type SRV-ID or URI-ID provides
an application service type portion to be checked, but that portion
is combined only with the DNS domain name portion of the SRV-ID or
URI-ID itself. For example, if a client's list of reference
identifiers includes an SRV-ID of "_xmpp-client.im.example.org" and a
DNS-ID of "apps.example.net", the client would check (a) the
combination of an application service type of "xmpp-client" and a DNS
domain name of "im.example.org" and (b) a DNS domain name of
"apps.example.net". However, the client would not check (c) the
combination of an application service type of "xmpp-client" and a DNS
domain name of "apps.example.net" because it does not have an SRV-ID
of "_xmpp-client.apps.example.net" in its list of reference
identifiers.
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6.5.1. SRV-ID
The application service name portion of an SRV-ID (e.g., "imaps")
MUST be matched in a case-insensitive manner, in accordance with
[DNS-SRV]. Note that the "_" character is prepended to the service
identifier in DNS SRV records and in SRV-IDs (per [SRVNAME]), and
thus does not need to be included in any comparison.
6.5.2. URI-ID
The scheme name portion of a URI-ID (e.g., "sip") MUST be matched in
a case-insensitive manner, in accordance with [URI]. Note that the
":" character is a separator between the scheme name and the rest of
the URI, and thus does not need to be included in any comparison.
6.6. Outcome
The outcome of the matching procedure is one of the following cases.
6.6.1. Case #1: Match Found
If the client has found a presented identifier that matches a
reference identifier, then the service identity check has succeeded.
In this case, the client MUST use the matched reference identifier as
the validated identity of the application service.
6.6.2. Case #2: No Match Found, Pinned Certificate
If the client does not find a presented identifier matching any of
the reference identifiers but the client has previously pinned the
application service's certificate to one of the reference identifiers
in the list it constructed for this communication attempt (as
"pinning" is explained under Section 1.8), and the presented
certificate matches the pinned certificate (including the context as
described under Section 7.1), then the service identity check has
succeeded.
6.6.3. Case #3: No Match Found, No Pinned Certificate
If the client does not find a presented identifier matching any of
the reference identifiers and the client has not previously pinned
the certificate to one of the reference identifiers in the list it
constructed for this communication attempt, then the client MUST
proceed as described under Section 6.6.4.
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6.6.4. Fallback
If the client is an interactive client that is directly controlled by
a human user, then it SHOULD inform the user of the identity mismatch
and automatically terminate the communication attempt with a bad
certificate error; this behavior is preferable because it prevents
users from inadvertently bypassing security protections in hostile
situations.
Some interactive clients give advanced users the option of proceeding
with acceptance despite the identity mismatch. Although this
behavior can be appropriate in certain specialized circumstances, it
needs to be handled with extreme caution, for example by first
encouraging even an advanced user to terminate the communication
attempt and, if the advanced user chooses to proceed anyway, by
forcing the user to view the entire certification path before
proceeding.
Otherwise, if the client is an automated application not directly
controlled by a human user, then it SHOULD terminate the
communication attempt with a bad certificate error and log the error
appropriately. An automated application MAY provide a configuration
setting that disables this behavior, but MUST enable the behavior by
default.
7. Security Considerations
7.1. Pinned Certificates
As defined under Section 1.8, a certificate is said to be "pinned" to
a DNS domain name when a user has explicitly chosen to associate a
service's certificate with that DNS domain name despite the fact that
the certificate contains some other DNS domain name (e.g., the user
has explicitly approved "apps.example.net" as a domain associated
with a source domain of "example.com"). The cached name association
MUST take account of both the certificate presented and the context
in which it was accepted or configured (where the "context" includes
the chain of certificates from the presented certificate to the trust
anchor, the source domain, the application service type, the
service's derived domain and port number, and any other relevant
information provided by the user or associated by the client).
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7.2. Wildcard Certificates
Wildcard certificates, those that have an identifier with "*" as the
left-most DNS label, automatically vouch for any single-label host
names within their domain, but not multiple levels of domains. This
can be convenient for administrators but also poses the risk of
vouching for rogue or buggy hosts. See for example [Defeating-SSL]
(beginning at slide 91) and [HTTPSbytes] (slides 38-40).
Protection against a wildcard that identifies a so-called "public
suffix" (e.g., "*.co.uk" or "*.com") is beyond the scope of this
document.
7.3. Internationalized Domain Names
Allowing internationalized domain names can lead to the inclusion of
visually similar (so-called "confusable") characters in certificates;
for discussion, see for example [IDNA-DEFS].
7.4. Multiple Identifiers
A given application service might be addressed by multiple DNS domain
names for a variety of reasons, and a given deployment might service
multiple domains or protocols. The client SHOULD use the TLS Server
Name Identification (SNI) extension as discussed in [TLS],
Section 4.4.2.2.
To accommodate the workaround that was needed before the development
of the SNI extension, this specification allows multiple DNS-IDs,
SRV-IDs, or URI-IDs in a certificate.
8. References
8.1. Normative References
[DNS-CONCEPTS]
Mockapetris, P.V., "Domain names - concepts and
facilities", STD 13, RFC 1034, DOI 10.17487/RFC1034,
November 1987, <https://doi.org/10.17487/RFC1034>.
[DNS-SRV] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<https://doi.org/10.17487/RFC2782>.
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[DNS-WILDCARDS]
Lewis, E., "The Role of Wildcards in the Domain Name
System", RFC 4592, DOI 10.17487/RFC4592, July 2006,
<https://doi.org/10.17487/RFC4592>.
[IDNA-DEFS]
Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<https://doi.org/10.17487/RFC5890>.
[IDNA-PROTO]
Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891,
DOI 10.17487/RFC5891, August 2010,
<https://doi.org/10.17487/RFC5891>.
[LDAP-DN] Zeilenga, K., Ed., "Lightweight Directory Access Protocol
(LDAP): String Representation of Distinguished Names",
RFC 4514, DOI 10.17487/RFC4514, June 2006,
<https://doi.org/10.17487/RFC4514>.
[PKIX] 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,
<https://doi.org/10.17487/RFC5280>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://doi.org/10.17487/RFC2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://doi.org/10.17487/RFC8174>.
[SRVNAME] Santesson, S., "Internet X.509 Public Key Infrastructure
Subject Alternative Name for Expression of Service Name",
RFC 4985, DOI 10.17487/RFC4985, August 2007,
<https://doi.org/10.17487/RFC4985>.
[URI] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://doi.org/10.17487/RFC3986>.
8.2. Informative References
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[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://doi.org/10.17487/RFC5234>.
[Defeating-SSL]
Marlinspike, M., "New Tricks for Defeating SSL in
Practice", BlackHat DC, February 2009,
<http://www.blackhat.com/presentations/bh-dc-
09/Marlinspike/BlackHat-DC-09-Marlinspike-Defeating-
SSL.pdf>.
[DNS-CASE] Eastlake 3rd, D., "Domain Name System (DNS) Case
Insensitivity Clarification", RFC 4343,
DOI 10.17487/RFC4343, January 2006,
<https://doi.org/10.17487/RFC4343>.
[DNSSEC] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<https://doi.org/10.17487/RFC4033>.
[DTLS] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://doi.org/10.17487/RFC6347>.
[EMAIL-SRV]
Daboo, C., "Use of SRV Records for Locating Email
Submission/Access Services", RFC 6186,
DOI 10.17487/RFC6186, March 2011,
<https://doi.org/10.17487/RFC6186>.
[EV-CERTS] CA/Browser Forum, "Guidelines For The Issuance And
Management Of Extended Validation Certificates", October
2009, <http://www.cabforum.org/Guidelines_v1_2.pdf>.
[HTTP] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://doi.org/10.17487/RFC7230>.
[HTTP-TLS] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<https://doi.org/10.17487/RFC2818>.
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[HTTPSbytes]
Sokol, J. and R. Hansen, "HTTPS Can Byte Me", BlackHat Abu
Dhabi, November 2010, <https://media.blackhat.com/bh-ad-
10/Hansen/Blackhat-AD-2010-Hansen-Sokol-HTTPS-Can-Byte-Me-
slides.pdf>.
[IPSEC] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <https://doi.org/10.17487/RFC4301>.
[NAPTR] Mealling, M., "Dynamic Delegation Discovery System (DDDS)
Part Three: The Domain Name System (DNS) Database",
RFC 3403, DOI 10.17487/RFC3403, October 2002,
<https://doi.org/10.17487/RFC3403>.
[OCSP] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP",
RFC 6960, DOI 10.17487/RFC6960, June 2013,
<https://doi.org/10.17487/RFC6960>.
[OPENPGP] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880,
DOI 10.17487/RFC4880, November 2007,
<https://doi.org/10.17487/RFC4880>.
[S-NAPTR] Daigle, L. and A. Newton, "Domain-Based Application
Service Location Using SRV RRs and the Dynamic Delegation
Discovery Service (DDDS)", RFC 3958, DOI 10.17487/RFC3958,
January 2005, <https://doi.org/10.17487/RFC3958>.
[SECTERMS] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://doi.org/10.17487/RFC4949>.
[SIP] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://doi.org/10.17487/RFC3261>.
[SIP-CERTS]
Gurbani, V., Lawrence, S., and A. Jeffrey, "Domain
Certificates in the Session Initiation Protocol (SIP)",
RFC 5922, DOI 10.17487/RFC5922, June 2010,
<https://doi.org/10.17487/RFC5922>.
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[SIP-SIPS] Audet, F., "The Use of the SIPS URI Scheme in the Session
Initiation Protocol (SIP)", RFC 5630,
DOI 10.17487/RFC5630, October 2009,
<https://doi.org/10.17487/RFC5630>.
[TLS] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://doi.org/10.17487/RFC8446>.
[US-ASCII] American National Standards Institute, "Coded Character
Set - 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
[VERIFY] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://doi.org/10.17487/RFC6125>.
[WSC-UI] Saldhana, A. and T. Roessler, "Web Security Context: User
Interface Guidelines", World Wide Web Consortium LastCall
WD-wsc-ui-20100309, March 2010,
<http://www.w3.org/TR/2010/WD-wsc-ui-20100309>.
[XMPP] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
March 2011, <https://doi.org/10.17487/RFC6120>.
Acknowledgements
We gratefully acknowledge everyone who contributed to the previous
version of this document, [VERIFY].
Authors' Addresses
Peter Saint-Andre
Mozilla
United States of America
Email: stpeter@mozilla.com
Jeff Hodges
Google
United States of America
Email: jdhodges@google.com
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Rich Salz
Akamai Technologies
United States of America
Email: rsalz@akamai.com
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