None P. Saint-Andre
Internet-Draft Cisco
Intended status: Standards Track K. Zeilenga
Expires: December 5, 2009 Isode Limited
J. Hodges
NeuStar
R. Morgan
Internet2
June 3, 2009
Best Practices for Checking of Server Identities in the Context of
Transport Layer Security (TLS)
draft-saintandre-tls-server-id-check-00
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Abstract
This document specifies the how an entity establishing a TLS
connection, or other PKI-based interaction, with a server should
verify the server identity.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Server Identity Check . . . . . . . . . . . . . . . . . . . . . 3
3.1. Comparison of DNS Names . . . . . . . . . . . . . . . . . . 4
3.2. Comparison of IP Addresses . . . . . . . . . . . . . . . . 5
3.3. Comparison of Other subjectName Types . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Normative References . . . . . . . . . . . . . . . . . . . 5
5.2. Informative References . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6
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1. Introduction
Establishing a TLS-based connection [TLS] with a server, or any other
sort of client-server PKI-based interaction, entails, on the part of
the client, verifying the "server's identity" based upon information
presented by the server in its certificate correlated with the
information about the server ensconced in the Domain Name System
(DNS).
Presently, various Internet-Drafts utilizing TLS or prescribing PKI-
based interactions, either prescribe their own server identity check,
or reference [LDAP-AUTH] or 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 of this critical aspect of PKI-
based interactions is desirable in that it will hopefully encourage
more coherence in specifications and actual implementations thereof,
as well as ease the burden of crafting new specifications because
this aspect has been factored out and separately standardized.
This document extracts the "server identity check" section from
[LDAP-AUTH], with the goal of becoming a stand-alone BCP document
appropriately referenceable by I-Ds and thus RFCs.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [TERMS].
3. Server Identity Check
In order to prevent man-in-the-middle attacks, the client MUST verify
the server's identity (as presented in the server's Certificate
message). In this section, the client's understanding of the
server's identity (typically the identity used to establish the
transport connection) is called the "reference identity".
The client determines the type (e.g., DNS name or IP address) of the
reference identity and performs a comparison between the reference
identity and each subjectAltName value of the corresponding type
until a match is produced. Once a match is produced, the server's
identity has been verified, and the server identity check is
complete. Different subjectAltName types are matched in different
ways. Sections 3.1.3.1 - 3.1.3.3 explain how to compare values of
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various subjectAltName types.
The client may map the reference identity to a different type prior
to performing a comparison. Mappings may be performed for all
available subjectAltName types to which the reference identity can be
mapped; however, the reference identity should only be mapped 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- or admin-configured 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.
If the server identity check fails, user-oriented clients SHOULD
either notify the user (clients may give the user the opportunity to
continue with 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 is
suspect or both.
Beyond the server identity check described in this section, clients
should be prepared to do further checking to ensure that the server
is authorized to provide the service it is requested to provide. The
client may need to make use of local policy information in making
this determination.
3.1. Comparison of DNS Names
If the reference identity is an internationalized domain name,
conforming implementations MUST convert it 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 the conversion operation
specified in Section 4 of RFC 3490 as follows:
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o in step 1, the domain name SHALL be considered a "stored string";
o in step 3, set the flag called "UseSTD3ASCIIRules";
o in step 4, process each label with the "ToASCII" operation; and
o in step 5, change all label separators to U+002E (full 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 '*' (ASCII 42) wildcard character is allowed in subjectAltName
values of type dNSName, and then only as the left-most (least
significant) DNS label in that value. This wildcard matches any
left-most DNS label in the server name. That is, the subject
*.example.com matches the server names a.example.com and
b.example.com, but does not match example.com or a.b.example.com.
3.2. Comparison of IP Addresses
When the reference identity is an IP address, the identity MUST be
converted to the "network byte order" octet string representation
[IP] [IPv6]. For IP Version 4, as specified in RFC 791, the octet
string will contain exactly four octets. For IP Version 6, as
specified in RFC 2460, the octet string will contain exactly sixteen
octets. This octet string is then compared against subjectAltName
values of type iPAddress. A match occurs if the reference identity
octet string and value octet strings are identical.
3.3. Comparison of Other subjectName Types
Client implementations MAY support matching against subjectAltName
values of other types as described in other documents.
4. Security Considerations
To follow.
5. References
5.1. 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.
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[IPv6] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[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 for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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., and M. Wahl, "Lightweight
Directory Access Protocol (v3): Extension for Transport
Layer Security", RFC 2830, May 2000.
Authors' Addresses
Peter Saint-Andre
Cisco
Email: psaintan@cisco.com
Kurt D. Zeilenga
Isode Limited
Email: Kurt.Zeilenga@Isode.COM
Jeff Hodges
NeuStar
Email: Jeff.Hodges@KingsMountain.com
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RL 'Bob' Morgan
UWashington/Internet2
Email: rlmorgan@washington.edu
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