DNSIND Working Group D. Eastlake
INTERNET-DRAFT IBM
Expires October 1999
April 1999
Indirect KEY RRs in the Domain Name System (DNS)
-------- --- --- -- --- ------ ---- ------ -----
Donald E. Eastlake 3rd
Status of This Document
This draft, file name draft-ietf-dnsind-indirect-key-00.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 author.
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. 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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Abstract
[RFC 2535] defines a means for storing cryptographic public keys in
the Domain Name System. An additional code point is defined for the
algorithm field of the KEY resource record (RR) to indicate that the
key is not stored in the KEY RR but is pointed to by the KEY RR.
Encodings to indicate different types of key and pointer formats are
specified.
[This draft is moved from the DNSSEC WG as part of that WG's merger
into me DNSIND WG. It would have been draft-ietf-dnssec-indirect-
key-02.txt in the DNSSEC WG.]
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Table of Contents
Status of This Document....................................1
Abstract...................................................1
Table of Contents..........................................2
1. Introduction............................................3
2. The Indirect KEY RR Algorithm...........................3
2.1 The Target Type Field..................................4
2.2 The Target Algorithm Field.............................5
2.3 The Hash Fields........................................5
3. Performance Considerations..............................6
4. IANA Considerations.....................................6
5. Security Considerations.................................6
References.................................................7
Author's Address...........................................7
Expiration and File Name...................................8
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1. Introduction
The Domain Name System (DNS) security extensions [RFC 2535] provide
for the general storage of public keys in the domain name system via
the KEY resource record (RR). These KEY RRs are used in support of
DNS security and may be used to support other security protocols.
KEY RRs can be associated with users, zones, and hosts or other end
entities named in the DNS.
For reasons given below, it will sometimes be desireable to store a
key or keys elsewhere and merely point to it from the KEY RR.
Indirect key storage makes it possible to point to a key service via
a URL, to have a compact pointer to a larger key or set of keys, to
point to a certificate either inside DNS [RFC 2538] or outside the
DNS, and where appropriate, to store a key or key set applicable to
many DNS entries in some place and point to it from those entries.
However, to simplify DNSSEC implementation, this technique MUST NOT
be used for KEY RRs used in for verification in DNSSEC, i.e., the
value of the "protocol" field of an indirect KEY RR MUST NOT be 3.
The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD",
"RECOMMENDED", and "MAY" in this document are to be interpreted as
described in [RFC 2119].
2. The Indirect KEY RR Algorithm
Domain Name System (DNS) KEY Resource Record (RR) [RFC 2535]
algorithm number 252 is defined as the indirect key algorithm. This
algorithm MAY NOT be used for zone keys in support of DNS security.
All KEYs used in DNSSEC validation MUST be stored directly in the
DNS.
When the algorithm byte of a KEY RR has the value 252, the "public
key" portion of the RR is formated as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| target type | target alg. | hash type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| hash size | hash (variable size) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-/
| /
/ pointer (variable size) /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
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2.1 The Target Type Field
Target type specifies the type of the key containing data being
pointed at.
Target type
-----------
0 - reserved, see section 4
1 - indicates that the pointer is a domain name from which KEY RRs
[RFC 2535] should be retrieved. Name compression in the pointer
field is prohibited.
2 - indicates that the pointer is a null terminated character string
which is a URL [RFC 1738]. For exisiting data transfer URL
schemes, such as ftp, http, shttp, etc., the data is the same as
the public key portion of a KEY RR. (New URL schemes may be
defined which return multiple keys.)
3 - indicates that the pointer is a domain name from which CERT RRs
[RFC 2538] should be retrieved. Name compression in the pointer
field is prohibited.
4 - indicates that the pointer is a null terminated character string
which is a URL [RFC 1738]. For exisiting data transfer URL
schemes, such as ftp, http, shttp, etc., the data is the same as
the entire RDATA portion of a CERT RR [RFC 2538]. (New URL
schemes may be defined which return multiple such data blocks.)
5 - indicates that the pointer is a null terminated character string
which is a URL [RFC 1738]. For exisiting data transfer URL
schemes, such as ftp, http, shttp, etc., the data is a PKCS#1 [RFC
2437] format key. (New URL schemes may be defined which return
multiple keys.)
6 through 255 - available for assignment, see section 4.
256 through 511 (i.e., 256 + n) - indicate that the pointer is a null
terminated character string which is a URL [RFC 1738]. For
exisiting data transfer URL schemes, such as ftp, http, shttp,
etc., the data is a certificate of the type indicated by a CERT RR
[RFC 2538] certificate type of n. That is, target types 257, 258,
and 259 are PKIX, SPKI, and PGP certificates and target types 509
and 510 are URL and OID private certificate types. (New URL
schemes may be defined which return multiple such certificates.)
512 through 65534 - available for assignment, see section 4.
65535 reserved, see section 4.
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2.2 The Target Algorithm Field
The algorithm field is as defined in [RFC 2535]. If non-zero, it
specifies the algorithm type of the target key or keys pointed. If
zero, it does not specify what algorithm the target key or keys apply
to.
2.3 The Hash Fields
If the indirecting KEY RRset [RFC 2181, 2535] is retrieved from an
appropriately secure DNS zone with a resolver implementing DNS
security, then there would be a high level of confidence in the
entire value of the KEY RRset including any direct keys. This may or
may not be true of any indirect key pointed to. If an indirect key
is embodied in a certificate or retrieved via a secure protocol such
as SHTTP, it may also be secure. But an indirecting KEY RR could,
for example, simply have an FTP URL pointing to a binary key stored
elsewhere, the retrieval of which would not be secure.
The hash option in algorithm 252 KEY RRs provides a means of
extending the security of the indirecting KEY RR to the actual key
material pointed at. By including a hash in a secure indirecting RR,
this secure hash can be checked against the hash of the actual keying
material
Type Hash Algorithm
---- --------------
0 indicates no hash present
1 MD5 [RFC 1321]
2 SHA-1
3 RIPEMD
4-252 available, see section 4
253 private, domain name (see below)
254 private, OID (see below)
255 reserved
Codes 253 and 254 indicate that a private, proprietary, local, or
experimental hash algorithm is used. For code 253, the hash field
begins with a wire encoded domain name (with compression prohibited)
that indicates the algorithm to use. For code 254, the hash field
begins with a one byte unsigned OID length followed by a BER encoded
OID which indicates the algorithm to use.
The hash size field is an unsigned octet count of the hash field size
less the length of any code 253 or 254 prefix. For some hash
algorithms it may be fixed by the algorithm choice but this will not
always be the case. For example, hash size is used to distinguish
between RIPEMD-128 (16 octets) and RIPEMD-160 (20 octets). If the
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hash algorithm field is 0, the hash size MUST be zero and no hash
octets are present.
The hash field itself is variable size with its length specified by
the hash size field and any code 253 or 254 prefix.
3. Performance Considerations
With current public key technology, an indirect key will sometimes be
shorter than the keying material it points at. In addition, there
can be cases where a single indirect KEY RR points to multiple keys
elsewhere. This may improve DNS performance in the retrieval of the
initial KEY RR. However, an additional retrieval step then needs to
be done to get the actually keying material which must be added to
the overall time to get the public key.
4. IANA Considerations
IETF consensus, standards action, and similar terms in this section
are as define in [RFC 2434].
KEY RR algorithm number 252 was already reserved for indirect keys in
RFC 2535.
An IETF standards action is required to allocate target type codes
hex x0000, x0006 through x00FF, x0200 through x0FFF, and xFFFF.
Codes in the range x1000 through x7FFF can be allocated by an IETF
consensus. Codes x8000 through xFEFF are available on a first come
first serve basis. Codes xFF00 through xFFFE are available for
experimentation or private local use without allocation. Use of
codes in this block may result in conflicts outside such experiment
or locality.
An IETF consensus is required to allocate an indirect KEY RR hash
algorithm code in the range 4-252 and a standards action is required
to allocate hash algorithm code 255. Codes 253 and 254 should cover
requirements for local, private, or proprietary algorithms.
5. Security Considerations
The indirecting step of using an indirect KEY RR adds complexity and
additional steps where security could go wrong. If the indirect key
RR was retrieved from a zone that was insecure for the resolver, you
have no security. If the indirect key RR, although secure itself,
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point to a key which can not be securely retrieved and is not
validateted by a secure hash in the indirect key RR, you have no
security.
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 1321 - R. Rivest, "The MD5 Message-Digest Algorithm", April 1992.
RFC 1738 - T. Berners-Lee, L. Masinter & M. McCahill, "Uniform
Resource Locators (URL)", December 1994.
RFC 2119 - "Key words for use in RFCs to Indicate Requirement
Levels", S. Bradner. March 1997.
RFC 2181 - R. Elz, R. Bush, "Clarifications to the DNS
Specification", July 1997.
RFC 2434 - T. Narten, H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", October 1998.
RFC 2437 - B. Kaliski, J. Staddon, "PKCS #1: RSA Cryptography
Specifications Version 2.0", October 1998.
RFC 2535 - D. Eastlake, "Domain Name System Security Extensions",
March 1999.
RFC 2538 - D. Eastlake, O. Gudmundsson, "Storing Certificates in the
Domain Name System (DNS)", March 1999.
Author's Address
Donald E. Eastlake 3rd
IBM
65 shindegan Hill Road, RR #1
Carmel, NY 10512 USA
Telephone: +1-914-784-7913 (w)
+1-914-276-2668 (h)
FAX: +1-914-784-3833 (w)
EMail: dee3@us.ibm.com
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Expiration and File Name
This draft expires October 1999.
Its file name is draft-ietf-dnsind-indirect-key-00.txt.
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