draft-josefsson-rfc2538bis-01.txt   draft-ietf-dnsext-rfc2538bis.txt 
Network Working Group S. Josefsson Network Working Group S. Josefsson
Expires: July 4, 2005 Expires: December 12, 2005
Storing Certificates in the Domain Name System (DNS) Storing Certificates in the Domain Name System (DNS)
draft-josefsson-rfc2538bis-01 draft-ietf-dnsext-rfc2538bis-03
Status of this Memo Status of this Memo
This document is an Internet-Draft and is subject to all provisions By submitting this Internet-Draft, each author represents that any
of section 3 of RFC 3667. By submitting this Internet-Draft, each applicable patent or other IPR claims of which he or she is aware
author represents that any applicable patent or other IPR claims of have been or will be disclosed, and any of which he or she becomes
which he or she is aware have been or will be disclosed, and any of aware will be disclosed, in accordance with Section 6 of BCP 79.
which he or she become aware will be disclosed, in accordance with
RFC 3668.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as other groups may also distribute working documents as Internet-
Internet-Drafts. Drafts.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
This Internet-Draft will expire on July 4, 2005. This Internet-Draft will expire on December 12, 2005.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2005). Copyright (C) The Internet Society (2005).
Abstract Abstract
Cryptographic public key are frequently published and their Cryptographic public key are frequently published and their
authenticity demonstrated by certificates. A CERT resource record authenticity demonstrated by certificates. A CERT resource record
(RR) is defined so that such certificates and related certificate (RR) is defined so that such certificates and related certificate
revocation lists can be stored in the Domain Name System (DNS). revocation lists can be stored in the Domain Name System (DNS).
More information on this document, including rfcdiff output, may be This document obsolete RFC 2538.
found at <http://josefsson.org/rfc2538bis/>.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The CERT Resource Record . . . . . . . . . . . . . . . . . . . 3 2. The CERT Resource Record . . . . . . . . . . . . . . . . . . 3
2.1 Certificate Type Values . . . . . . . . . . . . . . . . . 4 2.1 Certificate Type Values . . . . . . . . . . . . . . . . . 4
2.2 Text Representation of CERT RRs . . . . . . . . . . . . . 5 2.2 Text Representation of CERT RRs . . . . . . . . . . . . . 5
2.3 X.509 OIDs . . . . . . . . . . . . . . . . . . . . . . . . 5 2.3 X.509 OIDs . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Appropriate Owner Names for CERT RRs . . . . . . . . . . . . . 6 3. Appropriate Owner Names for CERT RRs . . . . . . . . . . . . 6
3.1 Content-based X.509 CERT RR Names . . . . . . . . . . . . 7 3.1 Content-based X.509 CERT RR Names . . . . . . . . . . . . 7
3.2 Purpose-based X.509 CERT RR Names . . . . . . . . . . . . 8 3.2 Purpose-based X.509 CERT RR Names . . . . . . . . . . . . 8
3.3 Content-based PGP CERT RR Names . . . . . . . . . . . . . 8 3.3 Content-based OpenPGP CERT RR Names . . . . . . . . . . . 9
3.4 Purpose-based PGP CERT RR Names . . . . . . . . . . . . . 9 3.4 Purpose-based OpenPGP CERT RR Names . . . . . . . . . . . 9
4. Performance Considerations . . . . . . . . . . . . . . . . . . 9 3.5 Owner names for IPKIX, ISPKI, and IPGP . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 4. Performance Considerations . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10 5. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 10
7. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 10 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
8. Changes since RFC 2538 . . . . . . . . . . . . . . . . . . . . 10 7. Security Considerations . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 12 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . 11
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12 9. Changes since RFC 2538 . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Author's Address . . . . . . . . . . . . . . . . . . . . . . 13
9.1 Normative References . . . . . . . . . . . . . . . . . . . . 11 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.2 Informative References . . . . . . . . . . . . . . . . . . . 12 10.1 Normative References . . . . . . . . . . . . . . . . . . 12
B. Copying conditions . . . . . . . . . . . . . . . . . . . . . . 12 10.2 Informative References . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . 13 A. Copying conditions . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . 14
1. Introduction 1. Introduction
Public keys are frequently published in the form of a certificate and Public keys are frequently published in the form of a certificate and
their authenticity is commonly demonstrated by certificates and their authenticity is commonly demonstrated by certificates and
related certificate revocation lists (CRLs). A certificate is a related certificate revocation lists (CRLs). A certificate is a
binding, through a cryptographic digital signature, of a public key, binding, through a cryptographic digital signature, of a public key,
a validity interval and/or conditions, and identity, authorization, a validity interval and/or conditions, and identity, authorization,
or other information. A certificate revocation list is a list of or other information. A certificate revocation list is a list of
certificates that are revoked, and incidental information, all signed certificates that are revoked, and incidental information, all signed
skipping to change at page 3, line 28 skipping to change at page 3, line 28
Section 2 below specifies a CERT resource record (RR) for the storage Section 2 below specifies a CERT resource record (RR) for the storage
of certificates in the Domain Name System. of certificates in the Domain Name System.
Section 3 discusses appropriate owner names for CERT RRs. Section 3 discusses appropriate owner names for CERT RRs.
Sections 4, 5, and 6 below cover performance, IANA, and security Sections 4, 5, and 6 below cover performance, IANA, and security
considerations, respectively. considerations, respectively.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [11]. document are to be interpreted as described in [3].
2. The CERT Resource Record 2. The CERT Resource Record
The CERT resource record (RR) has the structure given below. Its RR The CERT resource record (RR) has the structure given below. Its RR
type code is 37. type code is 37.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| type | key tag | | type | key tag |
skipping to change at page 4, line 6 skipping to change at page 4, line 6
The type field is the certificate type as define in section 2.1 The type field is the certificate type as define in section 2.1
below. below.
The algorithm field has the same meaning as the algorithm field in The algorithm field has the same meaning as the algorithm field in
DNSKEY and RRSIG RRs [10] except that a zero algorithm field DNSKEY and RRSIG RRs [10] except that a zero algorithm field
indicates the algorithm is unknown to a secure DNS, which may simply indicates the algorithm is unknown to a secure DNS, which may simply
be the result of the algorithm not having been standardized for be the result of the algorithm not having been standardized for
DNSSEC. DNSSEC.
The key tag field is the 16 bit value computed for the key embedded The key tag field is the 16 bit value computed for the key embedded
in the certificate, using the RRSIG Key Tag Algorithm described in in the certificate, using the RRSIG Key Tag algorithm described in
Appendix B of [10]. This field is used as an efficiency measure to Appendix B of [10]. This field is used as an efficiency measure to
pick which CERT RRs may be applicable to a particular key. The key pick which CERT RRs may be applicable to a particular key. The key
tag can be calculated for the key in question and then only CERT RRs tag can be calculated for the key in question and then only CERT RRs
with the same key tag need be examined. However, the key must always with the same key tag need be examined. However, the key must always
be transformed to the format it would have as the public key portion be transformed to the format it would have as the public key portion
of a DNSKEY RR before the key tag is computed. This is only possible of a DNSKEY RR before the key tag is computed. This is only possible
if the key is applicable to an algorithm (and limits such as key size if the key is applicable to an algorithm (and limits such as key size
limits) defined for DNS security. If it is not, the algorithm field limits) defined for DNS security. If it is not, the algorithm field
MUST BE zero and the tag field is meaningless and SHOULD BE zero. MUST BE zero and the tag field is meaningless and SHOULD BE zero.
2.1 Certificate Type Values 2.1 Certificate Type Values
The following values are defined or reserved: The following values are defined or reserved:
Value Mnemonic Certificate Type Value Mnemonic Certificate Type
----- -------- ----------- ---- ----- -------- ----------- ----
0 reserved 0 reserved
1 PKIX X.509 as per PKIX 1 PKIX X.509 as per PKIX
2 SPKI SPKI certificate 2 SPKI SPKI certificate
3 PGP OpenPGP packet 3 PGP OpenPGP packet
4-252 available for IANA assignment 4 IPKIX The URL of an X.509 data object
5 ISPKI The URL of an SPKI certificate
6 IPGP The URL of an OpenPGP packet
7-252 available for IANA assignment
253 URI URI private 253 URI URI private
254 OID OID private 254 OID OID private
255-65534 available for IANA assignment 255-65534 available for IANA assignment
65535 reserved 65535 reserved
The PKIX type is reserved to indicate an X.509 certificate conforming The PKIX type is reserved to indicate an X.509 certificate conforming
to the profile being defined by the IETF PKIX working group. The to the profile being defined by the IETF PKIX working group. The
certificate section will start with a one byte unsigned OID length certificate section will start with a one byte unsigned OID length
and then an X.500 OID indicating the nature of the remainder of the and then an X.500 OID indicating the nature of the remainder of the
certificate section (see 2.3 below). (NOTE: X.509 certificates do certificate section (see 2.3 below). (NOTE: X.509 certificates do
not include their X.500 directory type designating OID as a prefix.) not include their X.500 directory type designating OID as a prefix.)
The SPKI type is reserved to indicate a certificate formated as to be The SPKI type is reserved to indicate the SPKI certificate format
specified by the IETF SPKI working group. [13], for use when the SPKI documents are moved from experimental
status.
The PGP type indicates an OpenPGP packet as described in [5] and its The PGP type indicates an OpenPGP packet as described in [6] and its
extensions and successors. Two uses are to transfer public key extensions and successors. Two uses are to transfer public key
material and revocation signatures. The data is binary, and MUST NOT material and revocation signatures. The data is binary, and MUST NOT
be encoded into an ASCII armor. An implementation SHOULD process be encoded into an ASCII armor. An implementation SHOULD process
transferable public keys as described in section 10.1 of [5], but it transferable public keys as described in section 10.1 of [5], but it
MAY handle additional OpenPGP packets. MAY handle additional OpenPGP packets.
The IPKIX, ISPKI and IPGP types indicate a URL which will serve the
content that would have been in the "certificate, CRL or URL" field
of the corresponding (PKIX, SPKI or PGP) packet types. These types
are known as "indirect". These packet types MUST be used when the
content is too large to fit in the CERT RR, and MAY be used at the
implementations discretion. They SHOULD NOT be used where the entire
UDP packet would have fit in 512 bytes.
The URI private type indicates a certificate format defined by an The URI private type indicates a certificate format defined by an
absolute URI. The certificate portion of the CERT RR MUST begin with absolute URI. The certificate portion of the CERT RR MUST begin with
a null terminated URI [4] and the data after the null is the private a null terminated URI [5] and the data after the null is the private
format certificate itself. The URI SHOULD be such that a retrieval format certificate itself. The URI SHOULD be such that a retrieval
from it will lead to documentation on the format of the certificate. from it will lead to documentation on the format of the certificate.
Recognition of private certificate types need not be based on URI Recognition of private certificate types need not be based on URI
equality but can use various forms of pattern matching so that, for equality but can use various forms of pattern matching so that, for
example, subtype or version information can also be encoded into the example, subtype or version information can also be encoded into the
URI. URI.
The OID private type indicates a private format certificate specified The OID private type indicates a private format certificate specified
by a an ISO OID prefix. The certificate section will start with a by a an ISO OID prefix. The certificate section will start with a
one byte unsigned OID length and then a BER encoded OID indicating one byte unsigned OID length and then a BER encoded OID indicating
skipping to change at page 5, line 32 skipping to change at page 5, line 44
The RDATA portion of a CERT RR has the type field as an unsigned The RDATA portion of a CERT RR has the type field as an unsigned
decimal integer or as a mnemonic symbol as listed in section 2.1 decimal integer or as a mnemonic symbol as listed in section 2.1
above. above.
The key tag field is represented as an unsigned decimal integer. The key tag field is represented as an unsigned decimal integer.
The algorithm field is represented as an unsigned decimal integer or The algorithm field is represented as an unsigned decimal integer or
a mnemonic symbol as listed in [10]. a mnemonic symbol as listed in [10].
The certificate / CRL portion is represented in base 64 [8] and may The certificate / CRL portion is represented in base 64 [14] and may
be divided up into any number of white space separated substrings, be divided up into any number of white space separated substrings,
down to single base 64 digits, which are concatenated to obtain the down to single base 64 digits, which are concatenated to obtain the
full signature. These substrings can span lines using the standard full signature. These substrings can span lines using the standard
parenthesis. parenthesis.
Note that the certificate / CRL portion may have internal sub-fields Note that the certificate / CRL portion may have internal sub-fields
but these do not appear in the master file representation. For but these do not appear in the master file representation. For
example, with type 254, there will be an OID size, an OID, and then example, with type 254, there will be an OID size, an OID, and then
the certificate / CRL proper. But only a single logical base 64 the certificate / CRL proper. But only a single logical base 64
string will appear in the text representation. string will appear in the text representation.
skipping to change at page 6, line 36 skipping to change at page 6, line 46
Following some of the guidelines below may result in the use in DNS Following some of the guidelines below may result in the use in DNS
names of characters that require DNS quoting which is to use a names of characters that require DNS quoting which is to use a
backslash followed by the octal representation of the ASCII code for backslash followed by the octal representation of the ASCII code for
the character such as \000 for NULL. the character such as \000 for NULL.
The choice of name under which CERT RRs are stored is important to The choice of name under which CERT RRs are stored is important to
clients that perform CERT queries. In some situations, the client clients that perform CERT queries. In some situations, the client
may not know all information about the CERT RR object it wishes to may not know all information about the CERT RR object it wishes to
retrieve. For example, a client may not know the subject name of an retrieve. For example, a client may not know the subject name of an
X.509 certificate, or the e-mail address of the owner of an OpenPGP X.509 certificate, or the e-mail address of the owner of an OpenPGP
key. Further, the client may only know the hostname of a service key. Further, the client might only know the hostname of a service
that uses X.509 certificates or the OpenPGP key id of an OpenPGP key. that uses X.509 certificates or the Key ID of an OpenPGP key.
This motivate describing two different owner name guidelines. We This motivates describing two different owner name guidelines. We
call the two rules content-based owner names and purpose-based owner call the two rules content-based owner names and purpose-based owner
names. A content-based owner name is derived from the content of the names. A content-based owner name is derived from the content of the
CERT RR data; for example the Subject field in an X.509 certificate CERT RR data; for example the Subject field in an X.509 certificate
or the User ID field in OpenPGP keys. A purpose-based owner name is or the User ID field in OpenPGP keys. A purpose-based owner name is
selected to be a name that clients that wishes to retrieve CERT RRs selected to be a name that clients that wishes to retrieve CERT RRs
knows; for example the host name of a X.509 protected service or a are expected to know; for example the host name of a X.509 protected
OpenPGP key id of an OpenPGP key. Note that in some situations, the service or a Key ID of an OpenPGP key. Note that in some situations,
content-based and purpose-based owner name can be the same; for the content-based and purpose-based owner name can be the same; for
example when a client look up keys based on e-mail addresses for example when a client look up keys based on e-mail addresses for
incoming e-mail. incoming e-mail.
[Editorial note: Purpose-based owner name guidelines were introduced Implementations SHOULD use the purpose-based owner name guidelines
in RFC 2538bis. Earlier, in RFC 2538, only content-based owner name described in this document, and MAY use CNAMEs at content-based owner
guidelines were described. Implementation experience suggested that names (or other names), pointing to the purpose-based owner name.
the content-based owner name guidelines were not generally
applicable. It was realized that purpose-based owner name guidelines
were required to use CERT RRs in some ways.]
3.1 Content-based X.509 CERT RR Names 3.1 Content-based X.509 CERT RR Names
Some X.509 versions permit multiple names to be associated with Some X.509 versions permit multiple names to be associated with
subjects and issuers under "Subject Alternate Name" and "Issuer subjects and issuers under "Subject Alternate Name" and "Issuer
Alternate Name". For example, x.509v3 has such Alternate Names with Alternate Name". For example, x.509v3 has such Alternate Names with
an ASN.1 specification as follows: an ASN.1 specification as follows:
GeneralName ::= CHOICE { GeneralName ::= CHOICE {
otherName [0] INSTANCE OF OTHER-NAME, otherName [0] INSTANCE OF OTHER-NAME,
skipping to change at page 7, line 39 skipping to change at page 7, line 47
The recommended locations of CERT storage are as follows, in priority The recommended locations of CERT storage are as follows, in priority
order: order:
1. If a domain name is included in the identification in the 1. If a domain name is included in the identification in the
certificate or CRL, that should be used. certificate or CRL, that should be used.
2. If a domain name is not included but an IP address is included, 2. If a domain name is not included but an IP address is included,
then the translation of that IP address into the appropriate then the translation of that IP address into the appropriate
inverse domain name should be used. inverse domain name should be used.
3. If neither of the above it used but a URI containing a domain 3. If neither of the above it used but a URI containing a domain
name is present, that domain name should be used. name is present, that domain name should be used.
4. If none of the above is included but a character string name is 4. If none of the above is included but a character string name is
included, then it should be treated as described for PGP names in included, then it should be treated as described for OpenPGP
3.2 below. names below.
5. If none of the above apply, then the distinguished name (DN) 5. If none of the above apply, then the distinguished name (DN)
should be mapped into a domain name as specified in [3]. should be mapped into a domain name as specified in [4].
Example 1: Assume that an X.509v3 certificate is issued to /CN=John Example 1: Assume that an X.509v3 certificate is issued to /CN=John
Doe/DC=Doe/DC=com/DC=xy/O=Doe Inc/C=XY/ with Subject Alternative Doe/DC=Doe/DC=com/DC=xy/O=Doe Inc/C=XY/ with Subject Alternative
names of (a) string "John (the Man) Doe", (b) domain name john- names of (a) string "John (the Man) Doe", (b) domain name john-
doe.com, and (c) uri <https://www.secure.john-doe.com:8080/>. Then doe.com, and (c) uri <https://www.secure.john-doe.com:8080/>. Then
the storage locations recommended, in priority order, would be the storage locations recommended, in priority order, would be
1. john-doe.com, 1. john-doe.com,
2. www.secure.john-doe.com, and 2. www.secure.john-doe.com, and
3. Doe.com.xy. 3. Doe.com.xy.
skipping to change at page 8, line 25 skipping to change at page 8, line 33
3.2 Purpose-based X.509 CERT RR Names 3.2 Purpose-based X.509 CERT RR Names
It is difficult for clients that do not already posses a certificate It is difficult for clients that do not already posses a certificate
to reconstruct the content-based owner name that should be used to to reconstruct the content-based owner name that should be used to
retrieve the certificate. For this reason, purpose-based owner names retrieve the certificate. For this reason, purpose-based owner names
are recommended in this section. Because purpose-based owner names are recommended in this section. Because purpose-based owner names
by nature depend on the specific scenario, or purpose, for which the by nature depend on the specific scenario, or purpose, for which the
certificate will be used, there are more than one recommendation. certificate will be used, there are more than one recommendation.
The following table summarize the purpose-based X.509 CERT RR owner The following table summarize the purpose-based X.509 CERT RR owner
name guidelines. name guidelines for use with S/MIME [16], SSL/TLS [11], and IPSEC
[12].
Scenario Owner name Scenario Owner name
------------------------------------------------------------------- -------------------------------------------------------------------
S/MIME Certificate Standard translation of RFC 822 email address. S/MIME Certificate Standard translation of RFC 822 email address.
Example: A S/MIME certificate for Example: A S/MIME certificate for
"postmaster@example.org" will use a standard "postmaster@example.org" will use a standard
hostname translation of the owner name, hostname translation of the owner name,
i.e. "postmaster.example.org". i.e. "postmaster.example.org".
SSL Certificate Hostname of the SSL server. TLS Certificate Hostname of the TLS server.
IPSEC Certificate Hostname of the IPSEC machine, and/or IPSEC Certificate Hostname of the IPSEC machine, and/or for
for the in-addr.arpa reverse lookup IP address. IPv4 or IPv6 addresses the fully qualified
domain name in the appropriate reverse domain.
CRLs Hostname of the issuing CA. An alternative approach for IPSEC is to store raw public keys [15].
3.3 Content-based PGP CERT RR Names 3.3 Content-based OpenPGP CERT RR Names
OpenPGP signed keys (certificates) use a general character string OpenPGP signed keys (certificates) use a general character string
User ID [5]. However, it is recommended by PGP that such names User ID [6]. However, it is recommended by OpenPGP that such names
include the RFC 2822 [7] email address of the party, as in "Leslie include the RFC 2822 [8] email address of the party, as in "Leslie
Example <Leslie@host.example>". If such a format is used, the CERT Example <Leslie@host.example>". If such a format is used, the CERT
should be under the standard translation of the email address into a should be under the standard translation of the email address into a
domain name, which would be leslie.host.example in this case. If no domain name, which would be leslie.host.example in this case. If no
RFC 2822 name can be extracted from the string name no specific RFC 2822 name can be extracted from the string name no specific
domain name is recommended. domain name is recommended.
If a user has more than one email address, the CNAME type can be used If a user has more than one email address, the CNAME type can be used
to reduce the amount of data stored in the DNS. For example: to reduce the amount of data stored in the DNS. For example:
$ORIGIN example.org. $ORIGIN example.org.
smith IN CERT PGP 0 0 <OpenPGP binary> smith IN CERT PGP 0 0 <OpenPGP binary>
john.smith IN CNAME smith john.smith IN CNAME smith
js IN CNAME smith js IN CNAME smith
3.4 Purpose-based PGP CERT RR Names 3.4 Purpose-based OpenPGP CERT RR Names
Applications that receive an OpenPGP packet but do not know the email Applications that receive an OpenPGP packet containing encrypted or
address of the sender will have difficulties guessing the correct signed data but do not know the email address of the sender will have
owner name, and cannot use the content-based owner name guidelines. difficulties constructing the correct owner name and cannot use the
However, the OpenPGP packet typically contain the Key ID of the key. content-based owner name guidelines. However, these clients commonly
In these situations, it is recommended to use an owner name derived know the key fingerprint or the Key ID. The key ID is found in
from the Key ID. For example: OpenPGP packets, and the key fingerprint is commonly found in
auxilliary data that may be available. For these situations, it is
recommended to use an owner name identical to the key fingerprint and
key ID expressed in hexadecimal [14]. For example:
$ORIGIN example.org. $ORIGIN example.org.
0424D4EE81A0E3D119C6F835EDA21E94B565716F IN CERT PGP ...
F835EDA21E94B565716F IN CERT PGP ... F835EDA21E94B565716F IN CERT PGP ...
B565716F IN CNAME F835EDA21E94B565716F B565716F IN CERT PGP ...
As before, if the same key material is stored at several owner names, If the same key material is stored at several owner names, the use of
using CNAME can be used to avoid data duplication. CNAME may be used to avoid data duplication. Note that CNAME is not
always applicable, because it map an owner names to the other for all
purposes, and this may be sub-optimal when two keys with the same Key
ID are stored.
3.5 Owner names for IPKIX, ISPKI, and IPGP
These types are stored under the same owner names, both purpose- and
content-based, as the PKIX, SPKI and PGP types, respectively.
4. Performance Considerations 4. Performance Considerations
Current Domain Name System (DNS) implementations are optimized for Current Domain Name System (DNS) implementations are optimized for
small transfers, typically not more than 512 bytes including small transfers, typically not more than 512 bytes including
overhead. While larger transfers will perform correctly and work is overhead. While larger transfers will perform correctly and work is
underway to make larger transfers more efficient, it is still underway to make larger transfers more efficient, it is still
advisable at this time to make every reasonable effort to minimize advisable at this time to make every reasonable effort to minimize
the size of certificates stored within the DNS. Steps that can be the size of certificates stored within the DNS. Steps that can be
taken may include using the fewest possible optional or extensions taken may include using the fewest possible optional or extensions
fields and using short field values for variable length fields that fields and using short field values for variable length fields that
must be included. must be included.
5. IANA Considerations The RDATA field in the DNS protocol may only hold data of size 65535
octets (64kb) or less. This means that each CERT RR cannot contain
more than 64kb worth of payload, even if the corresponding
certificate or certificate revocation list is larger. This document
address this by defining "indirect" data types for each normal type.
Certificate types 0x0000 through 0x00FF and 0xFF00 through 0xFFFF can 5. Contributors
only be assigned by an IETF standards action [6]. This document
assigns 0x0001 through 0x0003 and 0x00FD and 0x00FE. Certificate
types 0x0100 through 0xFEFF are assigned through IETF Consensus [6]
based on RFC documentation of the certificate type. The availability
of private types under 0x00FD and 0x00FE should satisfy most
requirements for proprietary or private types.
6. Security Considerations The majority of this document is copied verbatim from RFC 2538, by
Donald Eastlake 3rd and Olafur Gudmundsson.
6. Acknowledgements
Thanks to David Shaw and Michael Graff for their contributions to
earlier works that motivated, and served as inspiration for, this
document.
This document was improved by suggestions and comments from Olivier
Dubuisson, Olaf M. Kolkman, Ben Laurie, Samuel Weiler, and Florian
Weimer. No doubt the list is incomplete. We apologize to anyone we
left out.
7. Security Considerations
By definition, certificates contain their own authenticating By definition, certificates contain their own authenticating
signature. Thus it is reasonable to store certificates in non-secure signature. Thus it is reasonable to store certificates in non-secure
DNS zones or to retrieve certificates from DNS with DNS security DNS zones or to retrieve certificates from DNS with DNS security
checking not implemented or deferred for efficiency. The results MAY checking not implemented or deferred for efficiency. The results MAY
be trusted if the certificate chain is verified back to a known be trusted if the certificate chain is verified back to a known
trusted key and this conforms with the user's security policy. trusted key and this conforms with the user's security policy.
Alternatively, if certificates are retrieved from a secure DNS zone Alternatively, if certificates are retrieved from a secure DNS zone
with DNS security checking enabled and are verified by DNS security, with DNS security checking enabled and are verified by DNS security,
the key within the retrieved certificate MAY be trusted without the key within the retrieved certificate MAY be trusted without
verifying the certificate chain if this conforms with the user's verifying the certificate chain if this conforms with the user's
security policy. security policy.
CERT RRs are not used in connection with securing the DNS security When the URI type is used, it should be understood that it introduces
additions so there are no security considerations related to CERT RRs an additional indirection that may allow for a new attack vector.
and securing the DNS itself. One method to secure that indirection is to include a hash of the
certificate in the URI itself.
7. Open Issues CERT RRs are not used by DNSSEC [9] so there are no security
considerations related to CERT RRs and securing the DNS itself.
1. How to handle PGP certificates larger than 64kb? In If DNSSEC is used then the non-existence of a CERT RR, and
draft-josefsson-cert-openpgp I outline one approach, but it may consequently certificates or revocation lists, can be securely
not be the best one. asserted. Without DNSSEC, this is not possible.
2. Whether to enforce owner name guidelines with SHOULD/MUST. From
David Shaw (on OpenPGP): "One of the things that struck me when
reading this draft is that while there are several suggested ways
to name keys in DNS, there is no one canonical name as a SHOULD
or MUST. I suggest that the key fingerprint be the canonical
name, and all others be CNAMEs pointing to the fingerprint
name.". From Sean P. Turner (on PKIX): "Should "recommended" be
"RECOMMENDED" in the 1st and 2nd sentences?" referring to the
text in section 3 that recommend appropriate owner names.
3. Should the document suggest use of both full fingerprints, 4/8
byte OpenPGP key id owner names? Perhaps only fingerprint
version.
8. Changes since RFC 2538 8. IANA Considerations
Certificate types 0x0000 through 0x00FF and 0xFF00 through 0xFFFF can
only be assigned by an IETF standards action [7]. This document
assigns 0x0001 through 0x0006 and 0x00FD and 0x00FE. Certificate
types 0x0100 through 0xFEFF are assigned through IETF Consensus [7]
based on RFC documentation of the certificate type. The availability
of private types under 0x00FD and 0x00FE should satisfy most
requirements for proprietary or private types.
The CERT RR reuses the DNS Security Algorithm Numbers registry. In
particular, the CERT RR requires that algorithm number 0 remain
reserved, as described in Section 2. The IANA is directed to
reference the CERT RR as a user of this registry and value 0, in
particular.
9. Changes since RFC 2538
1. Editorial changes to conform with new document requirements, 1. Editorial changes to conform with new document requirements,
including splitting reference section into two parts and updating including splitting reference section into two parts and
references to point at latest versions. updating the references to point at latest versions, and to add
some additional references.
2. Improve terminology. For example replace "PGP" with "OpenPGP", 2. Improve terminology. For example replace "PGP" with "OpenPGP",
to align with RFC 2440. to align with RFC 2440.
3. In section 2.1, clarify that OpenPGP public key data are binary, 3. In section 2.1, clarify that OpenPGP public key data are binary,
not the ASCII armored format, and reference 10.1 in RFC 2440 on not the ASCII armored format, and reference 10.1 in RFC 2440 on
how to deal with OpenPGP keys, and acknowledge that how to deal with OpenPGP keys, and acknowledge that
implementations may handle additional packet types. implementations may handle additional packet types.
4. Clarify that integers in the representation format are decimal. 4. Clarify that integers in the representation format are decimal.
5. Replace KEY/SIG with DNSKEY/RRSIG etc, to align with DNSSECbis 5. Replace KEY/SIG with DNSKEY/RRSIG etc, to align with DNSSECbis
terminology. terminology. Improve reference for Key Tag Algorithm
calculations.
6. Add examples that suggest use of CNAME to reduce bandwidth. 6. Add examples that suggest use of CNAME to reduce bandwidth.
7. In section 3, add three paragraphs that discuss "content-based" 7. In section 3, appended the last paragraphs that discuss
vs "purpose-based" owner names. Add section 3.2 for "content-based" vs "purpose-based" owner names. Add section 3.2
purpose-based X.509 CERT owner names, and section 3.4 for for purpose-based X.509 CERT owner names, and section 3.4 for
purpose-based OpenPGP CERT owner names. purpose-based OpenPGP CERT owner names.
8. Added size considerations.
9. The SPKI types has been reserved, until RFC 2692/2693 is moved
from the experimental status.
10. Added indirect types IPKIX, ISPKI, and IPGP.
9. References 10. References
9.1 Normative References 10.1 Normative References
[1] Mockapetris, P., "Domain names - concepts and facilities", STD [1] Mockapetris, P., "Domain names - concepts and facilities",
13, RFC 1034, November 1987. STD 13, RFC 1034, November 1987.
[2] Mockapetris, P., "Domain names - implementation and [2] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987. specification", STD 13, RFC 1035, November 1987.
[3] Kille, S., Wahl, M., Grimstad, A., Huber, R. and S. Sataluri, [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Kille, S., Wahl, M., Grimstad, A., Huber, R., and S. Sataluri,
"Using Domains in LDAP/X.500 Distinguished Names", RFC 2247, "Using Domains in LDAP/X.500 Distinguished Names", RFC 2247,
January 1998. January 1998.
[4] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform [5] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396, August Resource Identifiers (URI): Generic Syntax", RFC 2396,
1998. August 1998.
[5] Callas, J., Donnerhacke, L., Finney, H. and R. Thayer, "OpenPGP
Message Format", RFC 2440, November 1998.
[6] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
[7] Resnick, P., "Internet Message Format", RFC 2822, April 2001. [6] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,