INTERNET-DRAFT CERTs in the DNS
November 1997
Expires May 1998
Storing Certificates in the Domain Name System
------- ------------ -- --- ------ ---- ------
Donald E. Eastlake 3rd
Olafur Gudmundsson
Status of This Document
This draft, file name draft-ietf-dnssec-certs-01.txt, is intended to
be become a Proposed Standard RFC. Distribution of this document is
unlimited. Comments should be sent to the DNSSEC mailing list <dns-
security@tis.com> or to the authors.
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
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D. Eastlake, O. Gudmundsson [Page 1]
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Abstract
Cryptographic public key are frequently published and their
authenticity demonstrated by certificate systems. A CERT resource
record (RR) is defined so that such certificates and certificate
revocation lists can be conveniently stored in the Domain Name System
(DNS).
RFC 2065 specifies a Proposed Standard for storing cryptographic
public keys in the DNS via the KEY resource record (RR). In
addition to defining the CERT RR as above, a certificate flag bit is
also allocated out of the KEY RR flag field to indicate that a key
may be authenticated by one or more CERT RRs stored under the same
owner name as the KEY RR.
A separate document, draft-ietf-dnssec-indirect-key-*.txt, provides
aaditional ways of references keys or certificates within or outside
the DNS.
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Table of Contents
Status of This Document....................................1
Abstract...................................................2
Table of Contents..........................................3
1. Introduction............................................4
2. The CERT Resource Record................................5
2.1 Certificate Type Values................................5
2.2 Text Representation of CERT RRs........................6
2.3 X.509 OIDs.............................................7
3. The KEY Resorce Record CERT Flag Bit....................8
4. Appropriate Owner Names for CERT RRs....................9
4.1 X.509 CERT RR Names....................................9
4.2 PGP CERT RR Names......................................9
5. Performance Considerations.............................10
6. Security Considerations................................10
References................................................11
Authors Addresses.........................................11
Expiration and File Name..................................11
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1. Introduction
Public keys are frequently published in the form of a certificate and
their authenticity is commonly demonstrated by certificates and
certificate revocation lists (CRLs). A certificate is a binding,
through a cryptographic digital signature, of a public key, a validty
interval and/or conditions, and identity, authorization, or other
information. A certificate revocation list is a list of certificates
that are revoked and incidental information all signed by the signer
(issuer) of the revoked certificates. Examples are X.509
certificates/CRLs in the X.500 directory system or PGP
certificates/revocations used by PGP software.
Section 2 below specifies a CERT resource record (RR) for the storage
of certificates in the Doman Name System.
Section 3 below specifies a certificate flag bit in the KEY RR [RFC
2065] to hint at the presence of a certificate authenticating the
key.
Section 4 discusses appropriate owner names for CERT RRs when their
owner name is not constrained by a KEY RR with the CERT flag bit on.
Sections 5 and 6 below cover performance and security considerations,
respectively.
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2. The CERT Resource Record
The CERT resource record (RR) has the structure given below. Its RR
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
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| algorithm | /
+---------------+ certificate or CRL /
/ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
The type field is the certificate type as define in section 2.1
below.
The algorithm field has the same meaning as the algorithm field in
KEY and SIG RRs [RFC 2065] except that a zero algorithm field
indicates that the algorithm is unkown or not an alogirthm that is
known to have been standardized for DNSSEC.
The key tag field is the 16 bit value computed for the key embedded
in the certificate as specified in the DNSSEC Standard [RFC 2065].
This field is used as an efficiency measure to pick which CERT RRs
may be applicable to a particular KEY RR. The key tag can be
calculated for the KEY RR in question and then only CERT RRs with the
same key tag need be examined. In general, the key in a certificate
must be transformed to the format it would have as the public key
portion of a KEY 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 limits) defined for DNS security. If it is not, the tag
field is meaningless and SHOULD BE zero and the algorithm field MUST
BE zero.
2.1 Certificate Type Values
The following values are initially defined or reserved:
Value Mnemonic Certificate Type
----- -------- ----------- ----
0 reserved
1 PKIX X.509 as per PKIX
2 SPKI SPKI cert
3 PGP PGP cert
4-252 available for IANA assignment
253 URL URL private
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254 OID OID private
255-65534 available for IANA assignment
65535 reserved
The PKIX type is reserved to indicate an X.509 certificate conforming
to the profile being defined by the IETF PKIX working group. The
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
certificate section (see 2.3 below).
The SPKI type is reserved to indicate a certificate formated as to be
specified by the IETF SPKI working group.
The PGP type indicates a Pretty Good Privacy certificate as described
in RFC 1991 and its extensions and successors.
The URL private type indicates a format certificate defined by a URL
prefix. The certificate portion of the CERT RR MUST begin with a
null terminated URL [RFC 1738] and the data after the null is the
private format certificate itself. The URL SHOULD be such that a
retrieval from it will lead to documentation on the format of the
certificate.
The OID private type indicates a private format certificate specified
by an ISO OID prefix. The certificate section will start with a one
byte unsigned OID length and then an OID indicating the nature of the
remainder of the certificate section. This can be an X.509
certificate or some other format. X.509 certificates that conform to
the IETF PKIX profile SHOULD be indicated by the PKIX type, not the
OID private type.
2.2 Text Representation of CERT RRs
The RDATA portion of a CERT RR has the type field as an unsigned
integer or as a mnemonic symbol as listed in section 2.1 above.
The key tag field is represented as an unsigned integer.
The algorithm field is represented as an unsigned integer or a
mnemonic symbol as listed in RFC 2065 or other RFCs suplanting or
supplemental to RFC 2065.
The certificate portion is represented in base 64 [RFC 2065] and may
be divided up into any number of white space separated substrings,
down to single base 64 digits, which are concatenated to obtain the
full signature. These substrings can span lines using the standard
parenthesis.
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Note that the certificate (or CRL) portion may have internal sub-
fields 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 the certificate proper. But only a single logical base 64 string
will appear in the text representation.
2.3 X.509 OIDs
OIDs have been defined in connection with the X.500 directory for
user certificates, certification authority certificates, revocations
of certification authority, and revocations of user certificates.
The following table lists the OIDs and their length prefixed hex
format for use in CERT RRs:
id-at-userCertificate
= { joint-iso-ccitt(2) ds(5) at(4) 36 }
== 0x 03 55 04 24
id-at-cACertificate
= { joint-iso-ccitt(2) ds(5) at(4) 37 }
== 0x 03 55 04 25
id-at-authorityRevocationList
= { joint-iso-ccitt(2) ds(5) at(4) 38 }
== 0x 03 55 04 26
id-at-certificateRevocationList
= { joint-iso-ccitt(2) ds(5) at(4) 39 }
== 0x 03 55 04 27
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3. The KEY Resorce Record CERT Flag Bit
Bit 4 in the KEY resource record (RR) is defined as the certificate
flag bit. It indicates that any software which is sensitive to or
wishes to process certificates and/or certificate revocation lists
should do an additional retrieval from the Domain Name System for
CERT RRs with the same owner name as the KEY RR in question. The
presence of the CERT bit does not require that there is a CERT record
for that KEY stored in DNS, it is just a hint.
When KEY RRs are presented in text form, the certificate flag bit may
be symbolically presented as the mnemonic "CERT".
Note: DNS Security as used to secure the information (RRs) in the
Domain Name System does not use and never depends on CERT RRs.
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4. Appropriate Owner Names for CERT RRs
CERT RR's to be found via the KEY RR CERT flag bit described in
section 3 above must be stored under the same name as the KEY RR in
question. However, there may be CERT RRs that are not constrained by
a KEY RR with the CERT flag bit and it may be desired to name a CERT
RR so that it can be found with some convenience. It is recommended
that certificate CERT RRs be stored under a domain name related to
their subject and that certificate revocation list CERT RRs be stored
under a domain name related to their issuer.
4.1 X.509 CERT RR Names
X.509 has versions some of which permit multiple names to be
associated with subjects and issuers under "Subject Alternate Name"
and "Issuer Alternate Name".
If a domain name is included in the identification in the certificate
or CRL, that should be used. If a domain name is not included but an
IP address is included, then the translation of that IP address into
the appropriate inverse domain name should be used. If neither of
the above it used but a URI containing a domain name is present, that
domain name should be used. 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 4.2 below. If none of the above apply,
then the distinguished name (DN) that is required by X.509 should be
mapped into a domain name as specified in RFC ????.
4.2 PGP CERT RR Names
PGP signed keys (certificates) use a general character string name
[RFC 1991]. However, it is recommended by PGP that such names
include the RFC 822 email address of the party, as in "Leslie Example
<Leslie@host.example>". If such a format is used, the CERT 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 RFC 822
name can be extracted from the string name no specific domain name is
recommended.
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5. Performance Considerations
Current Domain Name System (DNS) implementations are optimized for
small transfers, typically not more than 512 bytes including
overhead. While larger transfers will perform correctly and work is
underway to make larger transfers more efficient, it is still
advisable at this time to make every reasonable effort to minimize
the size of certificates stored within the DNS. Steps that can be
taken may include using the fewest possible optional or extentions
fields and using short field values for variable length fields that
must be included.
6. Security Considerations
By definition, certificates contains their own authenticating
signature. Thus it is reasonable to store certificates in non-secure
DNS zones or to retrieve certificates from DNS with DNS security
checking not implemented or deferred for efficiency. The results can
be trusted if the certificate chain is verified back to a known
trusted key and this conforms with the user's security policy.
Alternatively, if certificates are retrieved from a secure DNS zone
with DNS security checking enabled and are verified by DNS security,
the key within the retrieved certificate MAY be trusted without
verifying the certificate chain if this conforms with the user's
security policy.
CERT RRs are not used in connection with securing the DNS security
additions so there are no security considerations related to CERT RRs
and securing the DNS itself.
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References
RFC 1034 - P. Mockapetris, "Domain Names - Concepts and Facilities",
STD 13, November 1987.
RFC 1035 - P. Mockapetris, "Domain Names - Implementation and
Specifications", STD 13, November 1987.
RFC 1738 - T. Berners-Lee, L. Masinter & M. McCahill, "Uniform
Resource Locators (URL)", December 1994.
RFC 1991 - D. Atkins, W. Stallings & P. Zimmermann, "PGP Message
Exchange Formats", August 1996.
RFC 2065 - D. Eastlake, C. Kaufman, "Domain Name System Security
Extensions", 01/03/1997.
draft-ietf-dnssec-indirect-key-*.txt
Authors Addresses
Donald E. Eastlake 3rd
CyberCash, Inc.
318 Acton Street
Carlisle, MA 01741 USA
Telephone: +1 978 287 4877
+1 703 620-4200 (main office, Reston, VA)
FAX: +1 978 371 7148
EMail: dee@cybercash.com
Olafur Gudmundsson
Trusted Information Systems
3060 Washington Road, Route 97
Glenwood, MD 21738
Telephone: +1 301 854 6889
EMail: ogud@tis.com
Expiration and File Name
This draft expires May 1998.
Its file name is draft-ietf-dnssec-certs-01.txt.
D. Eastlake, O. Gudmundsson [Page 11]
