Network Working Group S. Josefsson Internet-Draft January 2005 Expires: July 2, 2005 Storing Certificates in the Domain Name System (DNS) draft-ietf-dnsext-rfc2538bis-01 Status of this Memo This document is an Internet-Draft and is subject to all provisions of section 3 of RFC 3667. By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she become aware will be disclosed, in accordance with RFC 3668. 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 working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on July 2, 2005. Copyright Notice Copyright (C) The Internet Society (2005). Abstract Cryptographic public key are frequently published and their authenticity demonstrated by certificates. A CERT resource record (RR) is defined so that such certificates and related certificate revocation lists can be stored in the Domain Name System (DNS). Josefsson Expires July 2, 2005 [Page 1] Internet-Draft Storing Certificates in the DNS January 2005 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. The CERT Resource Record . . . . . . . . . . . . . . . . . . . 3 2.1 Certificate Type Values . . . . . . . . . . . . . . . . . 4 2.2 Text Representation of CERT RRs . . . . . . . . . . . . . 5 2.3 X.509 OIDs . . . . . . . . . . . . . . . . . . . . . . . . 6 3. Appropriate Owner Names for CERT RRs . . . . . . . . . . . . . 6 3.1 Content-based X.509 CERT RR Names . . . . . . . . . . . . 7 3.2 Purpose-based X.509 CERT RR Names . . . . . . . . . . . . 8 3.3 Content-based OpenPGP CERT RR Names . . . . . . . . . . . 8 3.4 Purpose-based OpenPGP CERT RR Names . . . . . . . . . . . 9 3.5 Owner names for IPKIX, ISPKI, and IPGP . . . . . . . . . . 9 4. Performance Considerations . . . . . . . . . . . . . . . . . . 9 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10 6. Security Considerations . . . . . . . . . . . . . . . . . . . 10 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 8. Changes since RFC 2538 . . . . . . . . . . . . . . . . . . . . 11 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 12 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 9.1 Normative References . . . . . . . . . . . . . . . . . . . . 11 9.2 Informative References . . . . . . . . . . . . . . . . . . . 12 A. Copying conditions . . . . . . . . . . . . . . . . . . . . . . 12 Intellectual Property and Copyright Statements . . . . . . . . 14 Josefsson Expires July 2, 2005 [Page 2] Internet-Draft Storing Certificates in the DNS January 2005 1. Introduction Public keys are frequently published in the form of a certificate and their authenticity is commonly demonstrated by certificates and related certificate revocation lists (CRLs). A certificate is a binding, through a cryptographic digital signature, of a public key, a validity 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 OpenPGP certificates/revocations used by OpenPGP software. Section 2 below specifies a CERT resource record (RR) for the storage of certificates in the Domain Name System. Section 3 discusses appropriate owner names for CERT RRs. Sections 4, 5, and 6 below cover performance, IANA, and security considerations, respectively. 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 [10]. 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 DNSKEY and RRSIG RRs [9] except that a zero algorithm field indicates the algorithm is unknown to a secure DNS, which may simply be the result of the algorithm not having been standardized for DNSSEC. Josefsson Expires July 2, 2005 [Page 3] Internet-Draft Storing Certificates in the DNS January 2005 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 Appendix B of [9]. This field is used as an efficiency measure to 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 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 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 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. 2.1 Certificate Type Values The following values are defined or reserved: Value Mnemonic Certificate Type ----- -------- ----------- ---- 0 reserved 1 PKIX X.509 as per PKIX 2 SPKI SPKI certificate 3 PGP OpenPGP packet 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 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). (NOTE: X.509 certificates do 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 specified by the IETF SPKI working group. The PGP type indicates an OpenPGP packet as described in [5] and its extensions and successors. Two uses are to transfer public key material and revocation signatures. The data is binary, and MUST NOT be encoded into an ASCII armor. An implementation SHOULD process transferable public keys as described in section 10.1 of [5], but it MAY handle additional OpenPGP packets. Josefsson Expires July 2, 2005 [Page 4] Internet-Draft Storing Certificates in the DNS January 2005 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 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 format certificate itself. The URI SHOULD be such that a retrieval from it will lead to documentation on the format of the certificate. Recognition of private certificate types need not be based on URI equality but can use various forms of pattern matching so that, for example, subtype or version information can also be encoded into the URI. The OID private type indicates a private format certificate specified 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 the nature of the remainder of the certificate section. This can be an X.509 certificate format 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. Recognition of private certificate types need not be based on OID equality but can use various forms of pattern matching such as OID prefix. 2.2 Text Representation of CERT RRs 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 above. The key tag field is represented as an unsigned decimal integer. The algorithm field is represented as an unsigned decimal integer or a mnemonic symbol as listed in [9]. The certificate / CRL portion is represented in base 64 [11] 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. Note that the certificate / 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 Josefsson Expires July 2, 2005 [Page 5] Internet-Draft Storing Certificates in the DNS January 2005 the certificate / CRL 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, their BER encoding, 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 3. Appropriate Owner Names for CERT RRs It is recommended that certificate CERT RRs be stored under a domain name related to their subject, i.e., the name of the entity intended to control the private key corresponding to the public key being certified. It is recommended that certificate revocation list CERT RRs be stored under a domain name related to their issuer. 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 backslash followed by the octal representation of the ASCII code for the character such as \000 for NULL. The choice of name under which CERT RRs are stored is important to clients that perform CERT queries. In some situations, the client 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 X.509 certificate, or the e-mail address of the owner of an OpenPGP key. Further, the client might only know the hostname of a service that uses X.509 certificates or the Key ID of an OpenPGP key. This motivates describing two different owner name guidelines. We call the two rules content-based owner names and purpose-based owner Josefsson Expires July 2, 2005 [Page 6] Internet-Draft Storing Certificates in the DNS January 2005 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 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 are expected to know; for example the host name of a X.509 protected service or a Key ID of an OpenPGP key. Note that in some situations, 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 incoming e-mail. Implementations SHOULD use the purpose-based owner name guidelines described in this document, and MAY use CNAMEs at content-based owner names (or other names), pointing to the purpose-based owner name. 3.1 Content-based X.509 CERT RR Names Some X.509 versions permit multiple names to be associated with subjects and issuers under "Subject Alternate Name" and "Issuer Alternate Name". For example, x.509v3 has such Alternate Names with an ASN.1 specification as follows: GeneralName ::= CHOICE { otherName [0] INSTANCE OF OTHER-NAME, rfc822Name [1] IA5String, dNSName [2] IA5String, x400Address [3] EXPLICIT OR-ADDRESS.&Type, directoryName [4] EXPLICIT Name, ediPartyName [5] EDIPartyName, uniformResourceIdentifier [6] IA5String, iPAddress [7] OCTET STRING, registeredID [8] OBJECT IDENTIFIER } The recommended locations of CERT storage are as follows, in priority order: 1. If a domain name is included in the identification in the certificate or CRL, that should be used. 2. 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. 3. If neither of the above it used but a URI containing a domain name is present, that domain name should be used. 4. If none of the above is included but a character string name is included, then it should be treated as described for OpenPGP names below. 5. If none of the above apply, then the distinguished name (DN) should be mapped into a domain name as specified in [3]. Josefsson Expires July 2, 2005 [Page 7] Internet-Draft Storing Certificates in the DNS January 2005 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 names of (a) string "John (the Man) Doe", (b) domain name john- doe.com, and (c) uri . Then the storage locations recommended, in priority order, would be 1. john-doe.com, 2. www.secure.john-doe.com, and 3. Doe.com.xy. Example 2: Assume that an X.509v3 certificate is issued to /CN=James Hacker/L=Basingstoke/O=Widget Inc/C=GB/ with Subject Alternate names of (a) domain name widget.foo.example, (b) IPv4 address 10.251.13.201, and (c) string "James Hacker ". Then the storage locations recommended, in priority order, would be 1. widget.foo.example, 2. 201.13.251.10.in-addr.arpa, and 3. hacker.mail.widget.foo.example. 3.2 Purpose-based X.509 CERT RR Names It is difficult for clients that do not already posses a certificate to reconstruct the content-based owner name that should be used to retrieve the certificate. For this reason, 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 certificate will be used, there are more than one recommendation. The following table summarize the purpose-based X.509 CERT RR owner name guidelines. Scenario Owner name ------------------------------------------------------------------- S/MIME Certificate Standard translation of RFC 822 email address. Example: A S/MIME certificate for "postmaster@example.org" will use a standard hostname translation of the owner name, i.e. "postmaster.example.org". SSL Certificate Hostname of the SSL server. IPSEC Certificate Hostname of the IPSEC machine, and/or for the in-addr.arpa reverse lookup IP address. An alternative approach for IPSEC is to store raw public keys [12]. 3.3 Content-based OpenPGP CERT RR Names OpenPGP signed keys (certificates) use a general character string Josefsson Expires July 2, 2005 [Page 8] Internet-Draft Storing Certificates in the DNS January 2005 User ID [5]. However, it is recommended by OpenPGP that such names include the RFC 2822 [7] email address of the party, as in "Leslie 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 2822 name can be extracted from the string name no specific domain name is recommended. 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: $ORIGIN example.org. smith IN CERT PGP 0 0 john.smith IN CNAME smith js IN CNAME smith 3.4 Purpose-based OpenPGP CERT RR Names Applications that receive an OpenPGP packet containing encrypted or signed data but do not know the email address of the sender will have difficulties constructing the correct owner name and cannot use the content-based owner name guidelines. However, these clients commonly know the key fingerprint or the Key ID. The key ID is found in 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 [11]. For example: $ORIGIN example.org. 0424D4EE81A0E3D119C6F835EDA21E94B565716F IN CERT PGP ... F835EDA21E94B565716F IN CERT PGP ... B565716F IN CERT PGP ... If the same key material is stored at several owner names, the use of 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 Current Domain Name System (DNS) implementations are optimized for Josefsson Expires July 2, 2005 [Page 9] Internet-Draft Storing Certificates in the DNS January 2005 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 extensions fields and using short field values for variable length fields that must be included. 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. 5. Acknowledgements The majority of this document is copied verbatim from RFC 2538, by Donald Eastlake 3rd and Olafur Gudmundsson. The author wishes to thank David Shaw and Michael Graff for their contributions to the earlier work that motivated this revised document. This document was improved by suggestions and comments from Olivier Dubuisson, Ben Laurie, Samuel Weiler, and Florian Weimer. No doubt the list is incomplete. We apologize to anyone we left out. 6. Security Considerations By definition, certificates contain 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 MAY 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. When the URI type is used, it should be understood that it introduces an additional indirection that may allow for a new attack vector. One method to secure that indirection is to include a hash of the certificate in the URI itself. Josefsson Expires July 2, 2005 [Page 10] Internet-Draft Storing Certificates in the DNS January 2005 CERT RRs are not used by DNSSEC [8] so there are no security considerations related to CERT RRs and securing the DNS itself. If DNSSEC [8] is used then the non-existence of a CERT RR, and consequently certificates or revocation lists, can be securely asserted. Without DNSSEC, this is not possible. 7. IANA Considerations Certificate types 0x0000 through 0x00FF and 0xFF00 through 0xFFFF can only be assigned by an IETF standards action [6]. This document assigns 0x0001 through 0x0006 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. 8. Changes since RFC 2538 1. Editorial changes to conform with new document requirements, including splitting reference section into two parts and updating the references to point at latest versions, and to add some additional references. 2. Improve terminology. For example replace "PGP" with "OpenPGP", to align with RFC 2440. 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 how to deal with OpenPGP keys, and acknowledge that implementations may handle additional packet types. 4. Clarify that integers in the representation format are decimal. 5. Replace KEY/SIG with DNSKEY/RRSIG etc, to align with DNSSECbis terminology. Improve reference for Key Tag Algorithm calculations. 6. Add examples that suggest use of CNAME to reduce bandwidth. 7. In section 3, appended the last paragraphs that discuss "content-based" vs "purpose-based" owner names. Add section 3.2 for purpose-based X.509 CERT owner names, and section 3.4 for purpose-based OpenPGP CERT owner names. 8. Added size considerations. 9. Added indirect types IPKIX, ISPKI, and IPGP. 9. References 9.1 Normative References [1] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987. Josefsson Expires July 2, 2005 [Page 11] Internet-Draft Storing Certificates in the DNS January 2005 [2] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [3] Kille, S., Wahl, M., Grimstad, A., Huber, R. and S. Sataluri, "Using Domains in LDAP/X.500 Distinguished Names", RFC 2247, January 1998. [4] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifiers (URI): Generic Syntax", RFC 2396, 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. [8] Arends, R., Austein, R., Massey, D., Larson, M. and S. Rose, "DNS Security Introduction and Requirements", draft-ietf-dnsext-dnssec-intro-13 (work in progress), October 2004. [9] Arends, R., "Resource Records for the DNS Security Extensions", draft-ietf-dnsext-dnssec-records-11 (work in progress), October 2004. 9.2 Informative References [10] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [11] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 3548, July 2003. [12] Richardson, M., "A method for storing IPsec keying material in DNS", draft-ietf-ipseckey-rr-11 (work in progress), July 2004. Author's Address Simon Josefsson EMail: simon@josefsson.org Appendix A. Copying conditions Regarding the portion of this document that was written by Simon Josefsson Expires July 2, 2005 [Page 12] Internet-Draft Storing Certificates in the DNS January 2005 Josefsson ("the author", for the remainder of this section), the author makes no guarantees and is not responsible for any damage resulting from its use. The author grants irrevocable permission to anyone to use, modify, and distribute it in any way that does not diminish the rights of anyone else to use, modify, and distribute it, provided that redistributed derivative works do not contain misleading author or version information. Derivative works need not be licensed under similar terms. Josefsson Expires July 2, 2005 [Page 13] Internet-Draft Storing Certificates in the DNS January 2005 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. 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Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Josefsson Expires July 2, 2005 [Page 14]