wdiff draft-saintandre-tls-server-id-check-00.txt draft-saintandre-tls-server-id-check-01.txt

None P. Saint-Andre
Internet-Draft Cisco
Intended status: Standards Track K. Zeilenga
Expires: December 5, 2009 March 4, 2010 Isode Limited
J. Hodges
NeuStar
PayPal
R. Morgan
Internet2
June 3,
August 31, 2009

Best Practices for Checking of

Server Identities Identity Verification in the Context of
Transport Layer Security (TLS)
draft-saintandre-tls-server-id-check-00 Application Protocols
draft-saintandre-tls-server-id-check-01

Status of this Memo

This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.

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Abstract

Technologies such as Transport Layer Security (TLS) and IPsec enable
a secure connection between two entities (a "client" and a "server")
using X.509 certificates. This document specifies recommended
procedures for checking the how an entity establishing a TLS
connection, or other PKI-based interaction, with a server should
verify identity of the server identity. in such an
interaction.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3 4
3. Server Identity Check Verification Process . . . . . . . . . . . . . . . . . . . . . 3 5
3.1. Comparison of DNS Names Overview . . . . . . . . . . . . . . . . . . 4 . . . . . . . 5
3.2. Comparison of Rules . . . . . . . . . . . . . . . . . . . . . 6
3.2.1. Domain Names . . . . . . . . . . . . . . . . . . . . . 6
3.2.2. IP Addresses . . . . . . . . . . . . . . . . 5 . . . . . 7
3.2.3. Email Addresses . . . . . . . . . . . . . . . . . . . 7
3.2.4. SIP Addresses . . . . . . . . . . . . . . . . . . . . 8
3.2.5. JabberIDs . . . . . . . . . . . . . . . . . . . . . . 8
3.3. Comparison of Other subjectName Types Outcome . . . . . . . . . . . 5 . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. 9
6.1. Normative References . . . . . . . . . . . . . . . . . . . 5
5.2. 9
6.2. Informative References . . . . . . . . . . . . . . . . . . 6 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6 12

1. Introduction

Establishing

Technologies such as Transport Layer Security [TLS] and [IPSEC]
enable a TLS-based secure connection [TLS] with between two entities using the Internet
X.509 Public Key Infrastructure (PKI) as described in [X509]. In
such interactions, the entity that initiates the connection is called
a server, "client" and the entity that receives the connection is called a
"server".

Note: The terms "client" and "server" as used here refer to
security roles, not application roles; a server in the context of
TLS or any other
sort IPSec might be a "client" (i.e., a user agent) in the
context of client-server PKI-based interaction, entails, an application protocol as deployed on the part Internet.

If a client wishes to connect to a server securely, it needs to check
the identity of the client, verifying server so that it can determine if the "server's identity" based upon information
presented server is
what it claims to be, verify that there is no attacker in the middle,
etc. Typically this is done by correlating the server information presented
in its the server's certificate correlated with the information available about the
server ensconced contained in the Domain Name System (DNS).

Presently, various Internet-Drafts utilizing TLS or prescribing PKI-
based interactions, either prescribe

Different application protocols that make use of the client-server
pattern for security purposes have traditionally specified their own
procedures for checking server identity check,
or reference identities. Examples include but are
not limited to:

o The Hypertext Transfer Protocol [HTTP], for which see also
[HTTP-TLS]
o The Internet Message Access Protocol [IMAP] and the Post Office
Protocol [POP3], for which see also [USINGTLS]
o The Lightweight Directory Access Protocol [LDAP], for which see
also [LDAP-AUTH] or and its predecesor [LDAP-TLS]. [there may be
other I-Ds referencing other specs that describe the equivalent of
server identity checks]

Converging our present understanding [LDAP-TLS]
o The NETCONF Configuration Protocol [NETCONF], for which see also
[NETCONF-SSH] and [NETCONF-TLS]
o The Network News Transfer Protocol [NNTP], for which see also
[NNTP-TLS]
o The Session Initiation Protocol [SIP], for which see also
[SIP-CERTS]
o The Simple Mail Transfer Protocol [SMTP], for which see also
[SMTP-AUTH] and [SMTP-TLS]
o The Syslog Protocol [SYSLOG], for which see also [SYSLOG-TLS]
o The Extensible Messaging and Presence Protocol [XMPP], for which
see also [XMPPBIS]

Unfortunately, this divergence of approaches has caused some
confusion among developers and protocol designers. Therefore this critical aspect
document specifies recommended identity checking procedures for
application protocols produced within the Internet Standards Process,
for the purpose of PKI-
based interactions codifying secure authentication practices.

Note: This document is desirable currently limited in that it will hopefully encourage
more coherence scope to the presentation
of identities in specifications and actual implementations thereof,
as well X.509 certificates as ease issued in the burden context of crafting new specifications because
this aspect has been factored out the
Public Key Infrastructure (PKI) and separately standardized.

This as applied to Transport Layer
Security [TLS]; a future version of this document extracts might address X.509
certificates as issued outside the "server identity check" section from
[LDAP-AUTH], with context of the goal PKI, non-X.509
public keys such as OpenPGP keys, presentation of becoming a stand-alone BCP document
appropriately referenceable by I-Ds identities in ways
other than in the certificate itself (e.g., certificate fingerprints
for Secure Shell as described in [SSH] or for Datagram Transport
Layer Security DTLS and Secure Real-time Transport Protocol as
described in [DTLS-SRTP]), and thus RFCs. applications other than TLS.

2. Terminology Conventions

The key words following capitalized keywords are to be interpreted as described
in [TERMS]: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", NOT";
"SHOULD", "RECOMMENDED"; "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED"; "MAY", and "OPTIONAL" in this
document
"OPTIONAL".

Most security-related terms are to be interpreted as described understood in [TERMS].

3. Server Identity Check the sense defined
in [SECTERMS]; such terms include, but are not limited to,
"assurance", "attack", "authentication", "authorization",
"certificate", "certification authority", "confidentiality",
"credential", "downgrade", "encryption", "fingerprint", "hash value",
"identity", "integrity", "signature", "security perimeter", "self-
signed certificate", "sign", "spoof", "tamper", "trust", "trust
anchor", "trust chain", "validate", "verify".

In order addition, we define the following terms to assist in understanding
the process of verifying identity:

identity set: The set of identities that are presented by the server
to prevent man-in-the-middle attacks, the client MUST verify (in the form of the server's X.509 certificate) when
the client is attempts to establish a secure connection to the
server.
identity (as type: The "natural kind" of identity to which a presented in
identity or reference identity belongs. For example, the server's Certificate
message).
reference identity might be a domain name, an IPv4 or IPv6
address, an email address, a SIP address, a JabberID, or some
other type (this specification does not yet provide a complete
taxonomy of identity types). In this section, the case of domain names, the
reference identity MUST NOT contain the wildcard character '*'
(ASCII 42) in the left-most (least significant) domain name
component or component fragment.

presented identity: A single member of the identity set.
reference identity: The client's understanding conception of the server's identity (typically
before it attempts to establish a secure connection to the server;
this is the identity used that the client expects the server to present
and to which the client makes reference when attempting to verify
the server's identity.

3. Verification Process

When a client connects to a server, it MUST verify the server's
identity (in order to prevent passive and active attacks against the
connection). By "verify identity" we mean that the client needs to
establish that at least one of the
transport connection) is called identities in the identity set
matches the reference identity.

3.1. Overview

At a high level, the client verifies the server identity in
accordance with the following rules:

1. Before connecting to the server, the "reference identity".

The client determines the
identity type (e.g., DNS name or IP address) of the reference identity and performs identity.
2. During the process of attempting to establish a comparison between secure
connection, the server MUST present its identity set to the
client in the form of an X.509 certificate [X509].
3. Upon being presented with the server's identity set, the client
MUST check the reference identity and each against the presented
identities for the purpose of finding a match. To do so, the
client iterates through all of the subjectAltName extensions it
recognizes in the server's certificate (potentially in an
application-specific preference order) and compares the value of
each extension against the corresponding type reference identity until it has either
produced a match is produced. Once a match is produced, or exhausted the server's
identity has been verified, and identities in the server identity check is
complete. Different subjectAltName set
(comparison rules for matching particular identity types are matched
provided under Section 3.2, including fallbacks to several
subjectName fields).
4. Before attempting to find a match in different
ways. Sections 3.1.3.1 - 3.1.3.3 explain how relation to compare values of
various subjectAltName types.

The a particular
presented identity, the client may MAY map the reference identity to
a different type prior
to performing identity type. Such a comparison. Mappings may mapping MAY be performed for all
any available subjectAltName types type to which the reference identity
can be mapped; however, the reference identity should only SHOULD be mapped
only to types for which the mapping is either inherently secure
(e.g., extracting the DNS name from a URI to compare with a
subjectAltName of type dNSName) or for which the mapping is
performed in a secure manner (e.g., using DNSSEC, or using user-
configured or admin-configured host-
to-address/address-to-host host-to-address/address-to-host
lookup tables).

The server's identity may also be verified by comparing the reference
identity to the Common Name (CN) [LDAP-SCHEMA] value in the leaf
Relative Distinguished Name (RDN) of the subjectName field of the
server's certificate. This comparison is performed using the rules
for comparison of DNS names in Section 3.1.3.1, below, with the
exception that no wildcard matching is allowed. Although the use of
the Common Name value is existing practice, it is deprecated, and
Certification Authorities are encouraged to provide subjectAltName
values instead. Note that the TLS implementation may represent DNs
in certificates according to X.500 or other conventions. For
example, some X.500 implementations order the RDNs in a DN using a
left-to-right (most significant to least significant) convention
instead of LDAP's right-to-left convention.

5. If the server identity check fails, user-oriented clients SHOULD
either notify the user (clients may give the user the opportunity to
continue set has more than one member, a match with any of
the LDAP session in this case) or close the transport
connection and indicate that the server's identity is suspect.
Automated clients SHOULD close the transport connection and then
return or log an error indicating that the server's identity presented identities is
suspect or both. acceptable.

Note: Beyond the server identity check described in this section,
clients
should be prepared to do might complete further checking to ensure that the server
is authorized to provide the service it is requested to provide.
The client may might need to make use of local policy information in
making this determination.

3.1.

3.2. Comparison of DNS Rules

3.2.1. Domain Names

If the reference identity is a domain name as defined by [RFC1034]
and [RFC1035] for "traditional" domain names or by [IDNA] for
internationalized domain names, then the client can match the
reference identity against subjectAltName extensions of type dNSName
and SRVName [SRVNAME] according to the following rules.

If the reference identity is a "traditional" domain name, then
matching of reference identity against the presented identity is
performed by comparing the set of domain components using a case-
insensitive ASCII comparison.

If the reference identity is an internationalized domain name,
conforming implementations then
an implementation MUST convert it the reference identity to the ASCII
Compatible Encoding (ACE) format as specified in Section 4 of [IDNA]
before comparison with subjectAltName values of type dNSName. Specifically,
conforming implementations MUST perform dNSName;
specifically, the conversion operation specified in Section 4 of RFC 3490
[IDNA] MUST be performed as follows:

o in step 1, the domain name SHALL be considered a "stored string"; string"
o in step 3, set the flag called "UseSTD3ASCIIRules"; "UseSTD3ASCIIRules"
o in step 4, process each label with the "ToASCII" operation; and operation
o in step 5, change all label separators to U+002E (full stop). stop)

After performing the "to-ASCII" conversion, the DNS labels and names
MUST be compared for equality according to the rules specified in
Section 3 of RFC3490.

The [IDNA].

A dNSName MAY contain the wildcard character '*' (ASCII 42) 42). The
wildcard character is allowed in subjectAltName
values of type dNSName, and then applies only as to the left-most (least significant) DNS label in that value. This wildcard
domain name component or component fragment and matches any
left-most DNS label in the server name. That is, the subject single
component or component fragment. For instance, a dNSName of
*.example.com matches the server names a.example.com and
b.example.com, foo.example.com but does not match bar.foo.example.com or
example.com itself; similarly, a dNSName of baz*.example.net matches
baz1.example.net and baz2.example.net but not qux.example.net or a.b.example.com.

3.2. Comparison
example.net itself.

In addition to checking the subjectAltName extensions of type dNSName
and SRVNAME, the client MAY as a fallback check the value of the
Common Name (CN) (see [LDAP-SCHEMA]) as presented in the subjectName
component of the server's X.509 certificate. In existing
certificates, the CN is often used for encapsulating a domain name;
for example, consider the following subjectName:

cn=www.example.com, ou=Web Services, c=GB

Here the Common Name is "www.example.com" and the client could choose
to compare the reference identity against that CN.

When comparing the referenced identity against the Common Name, the
client MUST follow the comparison rules described above for
subjectAltName extensions of type dNSName and SRVName, with the
exception that no wildcard matching is allowed.

In order to match domain names, a client MUST NOT check Relative
Distinguished Names (RDNs) other than the Common Name; in particular,
this means that a series of Domain Component (DC) attributes MUST NOT
be checked (because the order of Domain Components is not guaranteed,
certain attacks are possible if DC attributes are checked).

3.2.2. IP Addresses

When

If the reference identity is an IP address, address as defined by [IP] or
[IPv6], then the client can match the reference identity against
subjectAltName extensions of type iPaddress according to the
following rules.

The reference identity MUST be converted to the "network byte order"
octet string representation
[IP] [IPv6]. For representation; for IP Version 4, as specified in RFC 791, 4 the octet string will
contain exactly four octets. For octets, and for IP Version 6, as
specified in RFC 2460, 6 the octet string
will contain exactly sixteen octets. This octet string is The client then compared against subjectAltName
values of type iPAddress. A compares this
octet string, where a match occurs if the reference identity
octet string and value
presented identity octet strings are identical.

3.3. Comparison

3.2.3. Email Addresses

If the reference identity is an email address as defined by [EMAIL],
then the client SHOULD compare the reference identity against the
value of Other subjectName Types

Client implementations the "rfc822Name" subjectAltName extension described in
[X509].

The client MAY support matching also compare the reference identity against the value
of the "E" attribute of the subjectName as described in [CRMF].

3.2.4. SIP Addresses

If the reference identity is a SIP address as defined by [SIP], then
the client SHOULD compare map the reference identity to a domain name
or email address and proceed as described for those identity types,
or proceed as described in [SIP-CERTS].

3.2.5. JabberIDs

If the reference identity is a JabberID as defined by [XMPP], then
the client SHOULD compare the reference identity against the value of
the "id-on-xmppAddr" subjectAltName
values extension of other types type otherName
described in [XMPP], or proceed as described in other documents. [XMPPBIS].

3.3. Outcome

The outcome of the checking procedure is one of the following:

Case #1: The client finds at least one presented identity that
matches the reference identity; the entity MUST use this as the
validated identity of the server.
Case #2: The client finds no subjectAltName that matches the
reference identity but a human user has permanently accepted the
certificate during a previous connection attempt; the client MUST
verify that the cached certificate was presented and MUST notify
the user if the certificate has changed since the last time that a
secure connection was successfully negotiated.
Case #3: The client finds no subjectAltName that matches the
reference identity and a human user has not permanently accepted
the certificate during a previous connection attempt; the client
MUST NOT use the presented identity (if any) as the validated
identity of the server and instead MUST proceed as described in
the next section. Instead, if the client is a user-oriented
application, then it MUST either (1) automatically terminate the
connection with a bad certificate error or (2) show the
certificate (including the entire certificate chain) to the user
and give the user the choice of terminating the connecting or
accepting the certificate temporarily (i.e., for this connection
attempt only) or permanently (i.e., for all future connection
attempts) and then continuing with the connection; if a user
permanently accepts a certificate in this way, the client MUST
cache the certificate (or some non-forgeable representation such
as a hash value) and in future connection attempts behave as in
Case #2. (It is the resposibility of the human user to verify the
hash value or fingerprint of the certificate with the peer over a
trusted communication layer.) If the client is an automated
application, then it SHOULD terminate the connection with a bad
certificate error and log the error to an appropriate audit log;
an automated application MAY provide a configuration setting that
disables this check, but MUST provide a setting that enables the
check.

4. Security Considerations

To follow.

5. IANA Considerations

This document has no actions for the IANA.

6. References

5.1.

6.1. Normative References

[IDNA] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.

[IP] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.

[IPv6] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.

[TERMS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[X509] 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, May 2008.

6.2. Informative References

[CRMF] Schaad, J., "Internet X.509 Public Key Infrastructure
Certificate Request Message Format (CRMF)", RFC 4211,
September 2005.

[DTLS-SRTP]
McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for Secure
Real-time Transport Protocol (SRTP)",
draft-ietf-avt-dtls-srtp-07 (work in progress),
February 2009.

[EMAIL] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.

[HTTP] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

[HTTP-TLS]
Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

[IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.

[IPSEC] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.

[LDAP] Sermersheim, J., "Lightweight Directory Access Protocol
(LDAP): The Protocol", RFC 4511, June 2006.

[LDAP-AUTH]
Harrison, R., "Lightweight Directory Access Protocol
(LDAP): Authentication Methods and Security Mechanisms",
RFC 4513, June 2006.

[LDAP-SCHEMA]
Sciberras, A., "Lightweight Directory Access Protocol
(LDAP): Schema for User Applications", RFC 4519,
June 2006.

[TERMS] Bradner, S., "Key words

[LDAP-TLS]
Hodges, J., Morgan, R., and M. Wahl, "Lightweight
Directory Access Protocol (v3): Extension for use Transport
Layer Security", RFC 2830, May 2000.

[NETCONF] Enns, R., "NETCONF Configuration Protocol", RFC 4741,
December 2006.

[NETCONF-SSH]
Wasserman, M. and T. Goddard, "Using the NETCONF
Configuration Protocol over Secure SHell (SSH)", RFC 4742,
December 2006.

[NETCONF-TLS]
Badra, M., "NETCONF over Transport Layer Security (TLS)",
RFC 5539, May 2009.

[NNTP] Feather, C., "Network News Transfer Protocol (NNTP)",
RFC 3977, October 2006.

[NNTP-TLS]
Murchison, K., Vinocur, J., and C. Newman, "Using
Transport Layer Security (TLS) with Network News Transfer
Protocol (NNTP)", RFC 4642, October 2006.

[POP3] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, May 1996.

[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.

[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.

[SECTERMS]
Shirey, R., "Internet Security Glossary, Version 2",
RFC 4949, August 2007.

[SIP] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.

[SIP-CERTS]
Gurbani, V., Lawrence, S., and B. Laboratories, "Domain
Certificates in RFCs to Indicate
Requirement Levels", BCP 14, the Session Initiation Protocol (SIP)",
draft-ietf-sip-domain-certs-04 (work in progress),
May 2009.

[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 2119, 5321,
October 2008.

[SMTP-AUTH]
Siemborski, R. and A. Melnikov, "SMTP Service Extension
for Authentication", RFC 4954, July 2007.

[SMTP-TLS]
Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.

[SRVNAME] Santesson, S., "Internet X.509 Public Key Infrastructure
Subject Alternative Name for Expression of Service Name",
RFC 4985, August 2007.

[SSH] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.

[SYSLOG] Gerhards, R., "The Syslog Protocol", RFC 5424, March 1997. 2009.

[SYSLOG-TLS]
Miao, F., Ma, Y., and J. Salowey, "Transport Layer
Security (TLS) Transport Mapping for Syslog", RFC 5425,
March 2009.

[TLS] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.

5.2. Informative References

[LDAP-TLS]
Hodges, J., Morgan, R.,

[USINGTLS]
Newman, C., "Using TLS with IMAP, POP3 and M. Wahl, "Lightweight
Directory Access ACAP",
RFC 2595, June 1999.

[XMPP] Saint-Andre, P., Ed., "Extensible Messaging and Presence
Protocol (v3): Extension for Transport
Layer Security", (XMPP): Core", RFC 2830, May 2000. 3920, October 2004.

[XMPPBIS] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", draft-ietf-xmpp-3920bis-01 (work
in progress), August 2009.

Authors' Addresses

Peter Saint-Andre
Cisco

Email: psaintan@cisco.com

Kurt D. Zeilenga
Isode Limited

Email: Kurt.Zeilenga@Isode.COM

Jeff Hodges
NeuStar
PayPal

Email: Jeff.Hodges@KingsMountain.com
RL 'Bob' Morgan
UWashington/Internet2

Email: rlmorgan@washington.edu