Network Working Group P. Hoffman Internet-Draft VPN Consortium Intended status: Standards Track J. Schlyter Expires: February 25, 2011 Kirei AB W. Kumari A. Langley Google August 24, 2010 Using Secure DNS to Associate Certificates with Domain Names For TLS draft-hoffman-keys-linkage-from-dns-01 Abstract TLS and DTLS uses certificates for authenticating the server. Users want their applications to verify that the certificate provided by the TLS server is in fact associated with the domain name they expect. Instead of trusting a certificate authority to have made this association correctly, the user might instead trust the authoritative DNS server for the domain name to make that association. This document describes how to use secure DNS to associate the TLS server's certificate with the the intended domain name. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on February 25, 2011. Copyright Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Hoffman, et al. Expires February 25, 2011 [Page 1] Internet-Draft DNS Cert Linkage for TLS August 2010 Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. 1. Introduction The first response from the server in TLS may contain a certificate. In order for the TLS client to authenticate that it is talking to the expected TLS server, the client must validate that this certificate is associated with the domain name used by the client to get to the server. Currently, the client must extract the domain name from one of many places in the certificate, must trust the trust anchor upon which the server's certificate is rooted, and must perform correct validation on the certificate. This document applies to both TLS [RFC5246] and DTLS [4347bis]. In order to make the document more readable, it mostly only talks about "TLS", but in all cases, it means "TLS or DTLS". Some people want a different way to authenticate the association of the server's certificate with the intended domain name without trusting the CA. Given that the DNS administrator for a domain name is authorized to give identifying information about the zone, it makes sense to allow that administrator to also make an authoritative binding between the domain name and a certificate that might be used by a host at that domain name. The easiest way to do this is to use the DNS. A certificate association is a cryptographic hash of a certificate (sometimes called a "fingerprint"). That is, a hash is taken of the certificate, and that hash is the certificate association. The type of hash function used can be chosen by the DNS administrator. DNSSEC, which is defined in RFCs 4033, 4034, and 4035 ([RFC4033], [RFC4034], and [RFC4035]), uses cryptographic keys and digital signatures to provide authentication of DNS data. Information retrieved from the DNS and that is validated using DNSSEC is thereby proved to be the authoritative data. This document defines a secure method to associate the certificate that is obtained from the TLS server with a domain name using DNS protected by DNSSEC. Because the certificate association was Hoffman, et al. Expires February 25, 2011 [Page 2] Internet-Draft DNS Cert Linkage for TLS August 2010 retrieved based on a DNS query, the domain name in the query is by definition associated with the certificate. This document only relates to securely getting the DNS information for the certificate association using DNSSEC; other secure DNS mechanisms are out of scope. The DNSSEC signature MUST be validated on all responses in order to assure the proof of origin of the data. This document only relates to securely associating certificates for TLS and DTLS; other security protocols are handled in other documents. For example, keys for IPsec are covered in [RFC4025] and keys for SSH are covered in [RFC4255]. 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 RFC 2119 [RFC2119]. This document is being discussed on the "keyassure" mailing list; see . 2. Getting TLS Certificate Associations from the DNS with the CERT RR This section describes the TLSFP Certificate Type of the CERT RR. The CERT RR [RFC4398] allows expansion by defining new certificate types. The new TLSFP certificate type is defined here. A query on a domain name for the CERT RR can return one or more records of the type CERT, and zero or more of those CERT responses can be of type TLSFP. o The TLSFP certificate type is TBD. o The key tag and algorithm fields are both set to zero. The format of the TLSFP certificate is a binary record, which MUST be in the order defined here, is: o A one-octet value, called "hash type", specifying the type of hash algorithm used for the certificate association. This value has the same values as those of the DS RR, as defined in [RFC4034] and its successors. o A one-octet value, called "validation preference", specifying the preferences for further validation of the certificate in TLS. A certificate association that contains a validation preference whose value is 1 indicates that the TLS administrator believes that it is sufficient to match the certificate association with the hash of certificate received in the TLS negotiation, and that Hoffman, et al. Expires February 25, 2011 [Page 3] Internet-Draft DNS Cert Linkage for TLS August 2010 no further certificate validation is necessary. A certificate association that contains a validation preference whose value is 0 indicates that the TLS administrator believes that it is not sufficient to match the certificate association with the hash of certificate received in the TLS negotiation, and that normal certificate validation is necessary. Note that the validation preference is an advisory, and it is not mandatory for a TLS client to follow the advice. o A variable-length set of bytes containing the hash of the associated certificate. For example: www.example.com. IN CERT TLSFP 0 0 ( AQGa+FZd8sAeS03ca8xDigDQ ePgJnQvgMe/kKyf8rzluiQ== ) 3. Use of TLS Certificate Associations from the DNS in TLS In order to use one or more TLS certificate associations obtained from the DNS, an application MUST assure that the certificates were obtained using DNS protected by DNSSEC. There may be other methods to securely obtain certificates in DNS, but those methods are not covered by this document. An application that requests TLS certificate associations using the method described in the previous section obtains zero or more certificate associations. If the application receives zero certificate associations, it process TLS in the normal fashion. If a certificate association contains a hash type that is not understood by the TLS client, that certificate association MUST be completely ignored. If a match between the certificate association(s) and the server's end entity certificate in TLS is not found, the TLS client MUST abort the handshake with an "access_denied" error. If the TLS server authenticates itself with a self-signed certificate, it SHOULD be sure that the validation preference in the CERT RR is set to 1. If the TLS server administrator believes that there is information in its certificate that is relevant to the TLS client other than the public key (such as a extended value (EV) name), it SHOULD be sure that the validation preference in the CERT RR is set to 0. Hoffman, et al. Expires February 25, 2011 [Page 4] Internet-Draft DNS Cert Linkage for TLS August 2010 4. IANA Considerations This document requests that IANA allocates one certificate type from the CERT RR certificate type registry. The type is to be allocated from the 'IETF Consensus' range. Decimal type: TBD Type: TLSFP Meaning: TLS Certificate Associations 5. Security Considerations [[ TBD. This section will need to describe, at least, the "attack" where a DNS administrator goes rogue and changes both the A and CERT records for a domain name. Also will discuss the need for secure DNS. ]] 6. Acknowledgements Many of the ideas in this document have been discussed over many years. More recently, the ideas have been discussed by the authors and others in a more focused fashion. In particular, some of the ideas here originated with Paul Vixie, Dan Kaminsky, Jeff Hodges, Simon Josefsson, among others. 7. References 7.1. Normative References [4347bis] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security version 1.2", draft-ietf-tls-rfc4347-bis (work in progress), July 2010. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, March 2005. [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Resource Records for the DNS Security Extensions", RFC 4034, March 2005. Hoffman, et al. Expires February 25, 2011 [Page 5] Internet-Draft DNS Cert Linkage for TLS August 2010 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Protocol Modifications for the DNS Security Extensions", RFC 4035, March 2005. [RFC4398] Josefsson, S., "Storing Certificates in the Domain Name System (DNS)", RFC 4398, March 2006. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008. 7.2. Informative References [RFC4025] Richardson, M., "A Method for Storing IPsec Keying Material in DNS", RFC 4025, March 2005. [RFC4255] Schlyter, J. and W. Griffin, "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints", RFC 4255, January 2006. Appendix A. Ideas Considered But Not Necessarily Chosen This appendix will list some of the ideas that have been kicked around in this space and give a few paragraphs why they weren't chosen for the current version this proposal. The following is a placeholder for the list that will be filled out more in future versions of this document: o A flag that indicates that the certificate with the associated key must be signed by a trusted CA. Briefly: this was not added because it is up to the TLS server to decide which type of certificate it wants to serve up. Serving a self-signed certificate would effectively disable traditional PKIX validation, whereas serving a certificate signed by a trusted CA would require both validation by DNSSEC and the trusted CA. o A flag that indicates that all connections to this server need to be TLS secured. Briefly: this is a good idea but it is not related to the key of the certificate given in TLS, and thus should be indicated in a different method. o Giving keys instead of hashes of keys. Briefly: TLS requires that the server gives a certificate, and some systems use the metadata from a CA-signed certificate for display, so there is value to always looking in the certificate. o Hashes of keys vs. hashes of certificates. Briefly: we have changed our minds (at least once) on this. Our original thinking Hoffman, et al. Expires February 25, 2011 [Page 6] Internet-Draft DNS Cert Linkage for TLS August 2010 was that there are many reasons why someone might change their certificate while leaving the public key alone, and those changes should not have to force them to change the DNS record because they do not actually change what the TLS client cares about; thus, use hashes of keys. Our new thinking is that there are certificate semantics that we want to pass (namely, should the client actually do the certificate validation), and attaching those semantics to keys is confusing; thus, use hashes of certificates. o List TLS/DTLS ports or services for which the certificate is associated. Briefly: we had this in an earlier version of this document but got rid of it when it was pointed out that this is an edge case, and most servers differentiate these services by domain names such as "mail.example.com" and "www.example.com". o Different ways of encoding this information in the DNS. Briefly: we considered a new RR type and coming up with an encoding of the TXT RR type, but didn't see any significant advantage of them over using the CERT RR, and there were disadvantages. A disadvantage of a new RR type is getting DNS servers and clients to recognize it; a disadvantage of coming up with a new TXT format is that doing so prevents wildcards. There is a lot more to discuss here, but the authors are now happy with a new sub-type for the CERT RR. o Having the hash be over the TLS certificate structure instead of just the end-entity certificate. Briefly: the TLS certificate structure currently allows a chain of PKIX certificates, and the semantics of what is being associated in a chain is not clear. Further, the structure might be changed in the future (such as to allow a group of web-of-trust OpenPGP certificates), and the semantics of what is being associated would become even less clear. Appendix B. Changes between -00 and -01 Change the association from being a hash of the key of a PKIX certificate to being a hash of a certificate (PKIX or other). This, of course, makes large changes throughout the document. Expanded the document to cover DTLS as well. Added a pointer to the keyassure mailing list. Removed the proposals for two alternate formats (the TLSFP Resource Record and the TXT record encoding). Added a bit to Appendix A about this. Hoffman, et al. Expires February 25, 2011 [Page 7] Internet-Draft DNS Cert Linkage for TLS August 2010 Got rid of the specification for ports within a single domain name. Made the hash type one octet and used the DS registry instead of defining our own. Added "Necessarily" chosen in the title of Appendix A to show that we might (continue to) change our minds after discussion. Added Simon Josefsson to the acknowledgements. Authors' Addresses Paul Hoffman VPN Consortium Email: paul.hoffman@vpnc.org Jakob Schlyter Kirei AB Email: jakob@kirei.se Warren Kumari Google Email: warren@kumari.net Adam Langley Google Email: agl@google.com Hoffman, et al. Expires February 25, 2011 [Page 8]