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Title:draft-delany-domainkeys-base-04.txtdraft-delany-domainkeys-base-05.txt Yahoo! Inc Expires: 26SeptemberDecember 200627 March25 June 2006 Domain-based Email Authentication UsingPublic-KeysPublic Keys Advertised in the DNS (DomainKeys) Status of this Memo 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 becomes aware will be disclosed, in accordance with Section 6 of BCP 79. This document may not be modified, and derivative works of it may not be created, except to publish it as an RFC and to translate it into languages other than English. 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/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Abstract "DomainKeys" creates a domain-level authentication framework for email by usingpublic-keypublic key technology and the DNS to prove the provenance and contents of an email. This document defines a framework for digitally signing email on a per-domain basis. The ultimate goal of this framework is to unequivocally prove and protect identity while retaining the semantics of Internet email as it is known today. Proof and protection of email identity may assist in the global control of "spam" and "phishing". Delany ExpiresSeptember,December, 2006 [Page 1] Internet-Draft DomainKeysMarchJune 2006 Purpose of Submission The DomainKeys specification was a primary source from which the DomainKeys Identified Mail [DKIM] specification has been derived. The purpose in submitting this draft is as an historical reference for deployed implementations while the DKIM draft evolves. When DKIM has stabilized, this draft should be submitted as an historical or informational RFC. Delany ExpiresSeptember,December, 2006 [Page 2] Internet-Draft DomainKeysMarchJune 2006 Table of Contents 1. Introduction 1.1 Lack of authentication is damaging Internet email 1.2 Digitally signed email creates credible domain authentication 1.3Public-keysPublic keys in the DNS 1.4 Initial deployment is likely at the border MTA 1.5 Conveying verification results toUAsMUAs 1.6 Technical minutiae are not completely covered 1.7 Motivation 1.8 Benefits of DomainKeys 1.9 Definitions 1.10 Requirements Notation 2. DomainKeys overview 3. DomainKeys detailed view 3.1 Determining the sending address of an email 3.2 Retrieving thepublic-keypublic key given the sending domain 3.2.1 Introducing "selectors" 3.2.2Public-keyPublic key signing and verification algorithm 3.2.3Public-keyPublic key representation in the DNS 3.2.4 Key sizes 3.3 Storing the signature in the email header 3.4 Preparation of email for transit and signing 3.4.1 Preparation for transit 3.4.2 Canonicalization for signing 3.4.2.1 The "simple" canonicalization algorithm 3.4.2.2 The "nofws" canonicalization algorithm 3.5 The signing process 3.5.1 Identifying the sending domain 3.5.2 Determining if an email should be signed 3.5.3 Selecting aprivate-keyprivate key and corresponding selector information 3.5.4 Calculating the signature value 3.5.5 Prepending the "DomainKey-Signature:" header 3.6 Policy statement of sending domain3.6.1 Domain policy is nascent 3.6.2 Interim sending domain policy3.7 The verification process 3.7.1 Presumption that headers are notre-orderedreordered 3.7.2 Verification should render a binary result 3.7.3 Selecting the most appropriate "DomainKey-Signature:" header 3.7.4 Retrieve thepublic-keypublic key based on the signature information 3.7.5 Verify the signature 3.7.6 Retrieving sending domain policy 3.7.7 Applying local policy 3.8 Conveying verification results toUAsMUAs 4. Example of use 4.1 The user composes an emailDelany Expires September, 2006 [Page 3] Internet-Draft DomainKeys March 20064.2 The email is signed 4.3 The email signature is verified Delany Expires December, 2006 [Page 3] Internet-Draft DomainKeys June 2006 5. Association with a Certificate Authority 5.1 The "DomainKey-X509:" header 6. Topics for discussion 6.1 The benefits of selectors 6.2 Canonicalization of email 6.3 Mailing lists 6.4 Roving users 7. Security Considerations 7.1 DNS 7.1.1 The DNS is not currently secure 7.1.2 DomainKeys creates additional DNS load 7.2 Key Management 7.3 Implementation risks 7.4 Privacy assumptions with forwarding addresses 7.5 Cryptographic processing is computationally intensive 8. The trial 8.1 Goals 8.2 Constraints on participation 8.3 Results of trial thus far 9.NotesNote toImplementors 9.1implementors regarding TXT records 10. References 10.1 Normative References 10.2 Informative References Appendix A - Syntax rules for the tag=value format AcknowledgmentsChange HistoryEditor's Address Intellectual Property and CopyrightNoticeStatements Delany ExpiresSeptember,December, 2006 [Page 4] Internet-Draft DomainKeysMarchJune 2006 1. Introduction This document proposes an authentication framework for email that storespublic-keyspublic keys in the DNS and digitally signs email on a domain basis. Separate documents discuss how this framework can be extended to validate the delivery path of email as well as facilitate per-user authentication. 1.1 Lack of authentication is damaging Internet email Authentication of email is not currently widespread. Not only is it difficult to prove your own identity, it is impossible to prevent others from abusing your identity. While most email exchanges do not intrinsically need authentication beyond context, it is the rampant abuse of identity by "spammers", "phishers", and their criminal ilk that makes proof necessary. In other words, authentication is as much about protection as proof. Importantly, the inability to authenticate email effectively delegates much of the control of the disposition of inbound email to the sender, since senders can trivially assume any email address. Creating email authentication is the first step to returning dispositional control of email to the recipient. For the purposes of this document, authentication is seen from a user perspective, and is intended to answer the question "who sent this email?" where "who" is the email address the recipient sees and "this email" is the content that the recipient sees. 1.2 Digitally signing email creates credible domain authentication DomainKeys combinespublic-keypublic key cryptography and the DNS to provide credible domain-level authentication for email. When an email claims to originate from a certain domain, DomainKeys provides a mechanism by which the recipient system can credibly determine that the email did in fact originate from a person or system authorized to send email for that domain. The authentication provided by DomainKeys works in a number of scenarios in which other authentication systems fail or create complex operational requirements. These include: o forwarded email o distributed sending systems Delany ExpiresSeptember,December, 2006 [Page 5] Internet-Draft DomainKeysMarchJune 2006 o authorized third-party sending This base definition of DomainKeys is intended to primarily enable domain-level authenticity; whether a given message is really sent by the purported user within the domain is outside the scope of the base definition. Having said that, this specification includes the possibility that some domains may wish to delegate fine-grained authentication to individual users. 1.3Public-keysPublic keys in the DNS DomainKeys differs from traditional hierarchicalpublic-keypublic key systems in that it leverages the DNS forpublic-keypublic key management, placing complete and direct control of key generation and management with the owner of the domain. That is, if you have control over the DNS for a given domain, you have control over your DomainKeys for that domain. The DNS is proposed as the initial mechanism for publishingpublic-keys.public keys. DomainKeys is specifically designed to be extensible to other key fetching services as they become available. 1.4 Initial deployment is likely at the border MTA For practical reasons, it is expected that initial implementations of DomainKeys will be deployed on MTAs that accept or relay email across administrative or organizational boundaries. There are numerous advantages to deployment at the border MTA, including: o a reduction in the number of MTAs that have to be changed to support an implementation of DomainKeys o a reduction in the number of MTAs involved in transmitting the email between a signing system and a verifying system, thus reducing the number of places that can make accidental changes to the contents o removing the need to implement DomainKeys within an internal email network. However there is no necessity to deploy DomainKeys at the border as signing and verifying can effectively occur anywhere from the border MTA right back to theUA.MUA. In particular the best place to sign an email for many domains is likely to be at the point of SUBMISSION where the sender is often authenticated through SMTP AUTH or other identifying mechanisms. 1.5 Conveying verification results toUAsMUAs Delany ExpiresSeptember,December, 2006 [Page 6] Internet-Draft DomainKeysMarchJune 2006 It follows that testing the authenticity of an email results in some action based on the results of the test. Oftentimes the action is to notify theUAMUA in some way - typically via a header line.As alluded toThe "Domainkey-Status:" header is defined inprevious versions ofthisdraft, a standard header to communicatespecification for recording authentication resultshas been definedin[AUTH-HEADER]. Henceforth, all implementations of DomainKeys should use the "Authentication-Results:" header instead ofthepreviously defined "Domainkey-Status:" header.email. 1.6 Technical minutiae are not completely covered The intent of this draft is to communicate the fundamental characteristics of DomainKeys for an implementor. However some aspects are derived from the functionality of the openssl command [OPENSSL] and, rather than duplicate that documentation, implementors are expected to understand the mechanics of the openssl command, sufficient to complete the implementation. 1.7 Motivation The motivation for DomainKeys is to define a simple, cheap, and "sufficiently effective" mechanism by which domain owners can control who has authority to send email using their domain. To this end, the designers of DomainKeys set out to build a framework which: o is transparent and compatible with the existing email infrastructure o requires no new infrastructure o can be implemented independently of clients in order to reduce deployment time o does not require the use of a central certificate authority which might impose fees for certificates or introduce delays to deployment o can be deployed incrementally While we believe that DomainKeys meets these criteria, it is by no means a perfect solution. The current Internet imposes considerable compromises on any similar scheme, and readers should be careful not to misinterpret the information provided in this document to imply that DomainKeys makes stronger credibility statements than it is able to do. 1.8 Benefits of DomainKeysDelany Expires September, 2006 [Page 7] Internet-Draft DomainKeys March 2006As the reader will discover, DomainKeys is solely an authentication system. It is not a magic bullet for spam, nor is it an authorization Delany Expires December, 2006 [Page 7] Internet-Draft DomainKeys June 2006 system, a reputation system, a certification system, or a trust system. However, a strong authentication system such as DomainKeys creates an unimpeachable framework within which comprehensive authorization systems, reputations systems and their ilk can be developed. 1.9 Definitions With reference to the following sample email: Line Data Number Bytes Content ---- --- -------------------------------------------- 01 46 From: "Joe SixPack" <joe@football.example.com> 02 40 To: "Suzie Q" <suzie@shopping.example.net> 03 25 Subject: Is dinner ready? 04 43 Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT) 05 40 Comment: This comment has a continuation 06 51 because this line begins with folding white space 07 60 Message-ID: <20030712040037.46341@football.example.com> 08 00 09 03 Hi. 10 00 11 37 We lost the game. Are you hungry yet? 12 00 13 04 Joe. 14 00 15 00 Line 01 is the first line of the email and the first line of the headers. Line 05 and 06 constitute the "Comment:" header. Line 06 is a continuation header line. Line 07 is the last line of the headers. Line 08 is the empty line that separates the header from the body. Line 09 is the first line of the body. Line 10, 12, 14 and 15 are empty lines. Line 13 is the last non-empty line of the email.Delany Expires September, 2006 [Page 8] Internet-Draft DomainKeys March 2006Line 15 is the last line of the body and the last line of the email. Line 01 to 15 constitute the complete email. Delany Expires December, 2006 [Page 8] Internet-Draft DomainKeys June 2006 Line 01 is earlier than line 02 and line 02 is later than line 01. 1.10 Requirements notation This document occasionally uses terms that appear in capital letters. When the terms "MUST", "SHOULD", "RECOMMENDED", "MUST NOT", "SHOULD NOT", and "MAY" appear capitalized, they are being used to indicate particular requirements of this specification. A discussion of the meanings of these terms appears in [RFC2119]. 2. DomainKeys overview Under DomainKeys, a domain owner generates one or more private/public key-pairs that will be used to sign messages originating from that domain. The domain owner places thepublic-keypublic key in his domain namespace (i.e., in a DNS record associated with that domain), and makes theprivate-keyprivate key available to the outbound email system. When an email is submitted by an authorized user of that domain, the email system uses theprivate-keyprivate key to digitally sign the email associated with the sending domain. The signature is added as a header to the email, and the message is transferred to its recipients in the usual way. When a message is received with a DomainKey signature header, the receiving system can verify the signature as follows: 1. Extract the signature and claimed sending domain from the email. 2. Fetch thepublic-keypublic key from the claimed sending domain namespace. 3. Usepublic-keypublic key to determine whether the signature of the email has been generated with the correspondingprivate-key,private key, and thus whether the email was sent with the authority of the claimed sending domain. In the event that an email arrives without a signature or when the signature verification fails, the receiving system retrieves the policy of the claimed sending domain to ascertain the preferred disposition of such email. Armed with this information, the recipient system can apply local policy based on the results of the signature test.Delany Expires September, 2006 [Page 9] Internet-Draft DomainKeys March 20063. DomainKeys detailed view This section discusses the specifics of DomainKeys that are needed to Delany Expires December, 2006 [Page 9] Internet-Draft DomainKeys June 2006 create interoperable implementations. This section answers the following questions: Given an email, how is the sending domain determined? How is thepublic-keypublic key retrieved for a sending domain? As email transits the email system, it can potentially go through a number of changes. Which parts of the email are included in the signature and how are they protected from such transformations? How is the signature represented in the email? If a signature is not present, or a verification fails, how does the recipient determine the policy intent of the sending domain? Finally, on verifying the authenticity of an email, how is that result conveyed to participatingUAs?MUAs? While there are many alternative design choices, most lead to comparable functionality. The overriding selection criteria used to choose amongst the alternatives are: o use deployed technology whenever possible o prefer ease of implementation o avoid trading risk for excessive flexibility or interoperability o include basic flexibility Adherence to these criteria implies that some existing email implementations will require changes to participate in DomainKeys. Ultimately some hard choices need to be made regarding which requirements are more important. 3.1 Determining the sending address of an email The goal of DomainKeys is to give the recipient confidence that the email originated from the claimed sender. As with much of Internet email, agreement over what constitutes the "sender" is no easy matter. Forwarding systems and mailing lists add serious complications to an overtly simple question. From the point of view of the recipient, the authenticity claim should be directed at the domain most visible to the recipient. In the first instance, the most visible address is clearly the RFC2822Delany Expires September, 2006 [Page 10] Internet-Draft DomainKeys March 2006"From:" address [RFC2822]. Therefore, a conforming email MUST contain a single "From:" header from which an email address with a domain name can be extracted. Delany Expires December, 2006 [Page 10] Internet-Draft DomainKeys June 2006 A conforming email MAY contain a single RFC2822 "Sender:" header from which an email address with a domain name can be extracted. If the email has a valid "From:" and a valid "Sender:" header, then the signer MUST use the sending address in the "Sender:" header. If the email has a valid "From:" and no "Sender:" header, then the signer MUST use the first sending address in the "From:" header. In all other cases, a signer MUST NOT sign the email. Implementors should note the an email with a "Sender:" header and no "From:" header MUST NOT be signed. The domain name in the sending address constitutes the "sending domain". 3.2 Retrieving thepublic-keypublic key given the sending domain To avoid namespace conflicts, it is proposed that the DNS namespace "_domainkey." be reserved within the sending domain for storingpublic-keys,public keys, e.g., if the sending domain is example.net, then thepublic-keyspublic keys for that domain are stored in the _domainkey.example.net namespace. 3.2.1 Introducing "selectors" To support multiple concurrentpublic-keyspublic keys per sending domain, the DNS namespace is further subdivided with "selectors". Selectors are arbitrary names below the "_domainkey." namespace. A selector value and length MUST be legal in the DNS namespace and in email headers with the additional provision that they cannot contain a semicolon. Examples of namespace using selectors are: "coolumbeach._domainkey.example.net" "healdsburg._domainkey.example.net" "reykjavik._domainkey.example.net" "default._domainkey.example.net" and "2005.pao._domainkey.example.net" "2005.sql._domainkey.example.net" "2005.rhv._domainkey.example.net"Delany Expires September, 2006 [Page 11] Internet-Draft DomainKeys March 2006Periods are allowed in selectors and are to be treated as component separators. In the case of DNS queries that means the period defines sub-domain boundaries. Delany Expires December, 2006 [Page 11] Internet-Draft DomainKeys June 2006 The number ofpublic-keyspublic keys and corresponding selectors for each domain are determined by the domain owner. Many domain owners will be satisfied with just one selector whereas administratively distributed organizations may choose to manage disparate selectors and key pairs in different regions or on different email servers. Beyond administrative convenience, selectors make it possible to seamlessly replacepublic-keyspublic keys on a routine basis. If a domain wishes to change from using apublic-keypublic key associated with selector "2005" to apublic-keypublic key associated with selector "2006", it merely makes sure that bothpublic-keyspublic keys are advertised in the DNS concurrently for the transition period during which email may be in transit prior to verification. At the start of the transition period, the outbound email servers are configured to sign with the "2006"private-key.private key. At the end of the transition period, the "2005"public-keypublic key is removed from the DNS. While some domains may wish to make selector values well known, others will want to take care not to allocate selector names in a way that allows harvesting of data by outside parties. E.g., if per-user keys are issued, the domain owner will need to make the decision as to whether to make this selector associated directly with the user name, or make it some unassociated random value, such as the fingerprint of thepublic-key.public key. 3.2.2Public-keyPublic key signing and verification algorithm The default signature is an RSA signed SHA1 digest of the complete email. For ease of explanation, the openssl command is used throughout this document to describe the mechanism by which keys and signatures are managed. One way to generate a 768 bitprivate-keyprivate key suitable for DomainKeys, is to use openssl like this: $ openssl genrsa -out rsa.private 768 Which results in the file rsa.private containing the key information similar to this: -----BEGIN RSA PRIVATE KEY----- MIIByQIBAAJhAKJ2lzDLZ8XlVambQfMXn3LRGKOD5o6lMIgulclWjZwP56LRqdg5 ZX15bhc/GsvW8xW/R5Sh1NnkJNyL/cqY1a+GzzL47t7EXzVc+nRLWT1kwTvFNGIo AUsFUq+J6+OprwIDAQABAmBOX0UaLdWWusYzNol++nNZ0RLAtr1/LKMX3tk1MkLH +Ug13EzB2RZjjDOWlUOY98yxW9/hX05Uc9V5MPo+q2Lzg8wBtyRLqlORd7pfxYCn Kapi2RPMcR1CxEJdXOkLCFECMQDTO0fzuShRvL8q0m5sitIHlLA/L+0+r9KaSRM/ 3WQrmUpV+fAC3C31XGjhHv2EuAkCMQDE5U2nP2ZWVlSbxOKBqX724amoL7rrkUew Delany ExpiresSeptember,December, 2006 [Page 12] Internet-Draft DomainKeysMarchJune 2006+Ug13EzB2RZjjDOWlUOY98yxW9/hX05Uc9V5MPo+q2Lzg8wBtyRLqlORd7pfxYCn Kapi2RPMcR1CxEJdXOkLCFECMQDTO0fzuShRvL8q0m5sitIHlLA/L+0+r9KaSRM/ 3WQrmUpV+fAC3C31XGjhHv2EuAkCMQDE5U2nP2ZWVlSbxOKBqX724amoL7rrkUewti9TEjfaBndGKF2yYF7/+g53ZowRkfcCME/xOJr58VN17pejSl1T8Icj88wGNHCs FDWGAH4EKNwDSMnfLMG4WMBqd9rzYpkvGQIwLhAHDq2CX4hq2tZAt1zT2yYH7tTb weiHAQxeHe0RK+x/UuZ2pRhuoSv63mwbMLEZAjAP2vy6Yn+f9SKw2mKuj1zLjEhG 6ppw+nKD50ncnPoP322UMxVNG4Eah0GYJ4DLP0U= -----END RSA PRIVATE KEY----- Once aprivate-keyprivate key has been generated, the openssl command can be used to sign an appropriately prepared email, like this: $ openssl dgst -sign rsa.private -sha1 <input.file Which results in signature data similar to this when represented in Base64[MIME][BASE64] format: aoiDeX42BB/gP4ScqTdIQJcpAObYr+54yvctqc4rSEFYby9+omKD3pJ/TVxATeTz msybuW3WZiamb+mvn7f3rhmnozHJ0yORQbnn4qJQhPbbPbWEQKW09AMJbyz/0lsl How this signature is added to the email is discussed later in this document. To extract thepublic-keypublic key component from theprivate-key,private key, use openssl like this: $ openssl rsa -in rsa.private -out rsa.public -pubout -outform PEM Which results in the file rsa.public containing the key information similar to this: -----BEGIN PUBLIC KEY----- MHwwDQYJKoZIhvcNAQEBBQADawAwaAJhAKJ2lzDLZ8XlVambQfMXn3LRGKOD5o6l MIgulclWjZwP56LRqdg5ZX15bhc/GsvW8xW/R5Sh1NnkJNyL/cqY1a+GzzL47t7E XzVc+nRLWT1kwTvFNGIoAUsFUq+J6+OprwIDAQAB -----END PUBLIC KEY----- Thispublic-keypublic key data is placed in the DNS. With the signature, canonical email contents andpublic-key,public key, a verifying system can test the validity of the signature. The openssl invocation to verify a signature looks like this: openssl dgst -verify rsa.public -sha1 -signature signature.file <input.file 3.2.3Public-keyPublic key representation in the DNSDelany Expires September, 2006 [Page 13] Internet-Draft DomainKeys March 2006There is currently no standard method defined for storingpublic-keyspublic keys in the DNS. As an interim measure, thepublic-keypublic key is stored as a TXT Delany Expires December, 2006 [Page 13] Internet-Draft DomainKeys June 2006 record derived from a PEM format [PEM], that is, as a Base64 representation of a DER encoded key. Here is an example of a 768 bit RSA key in PEM form: -----BEGIN PUBLIC KEY----- MHwwDQYJKoZIhvcNAQEBBQADawAwaAJhAKJ2lzDLZ8XlVambQfMXn3LRGKOD5o6l MIgulclWjZwP56LRqdg5ZX15bhc/GsvW8xW/R5Sh1NnkJNyL/cqY1a+GzzL47t7E XzVc+nRLWT1kwTvFNGIoAUsFUq+J6+OprwIDAQAB -----END PUBLIC KEY----- To save scarce DNS packet space and aid extensibility, the PEM wrapping MUST be removed and the remainingpublic-keypublic key data along with other attributes relevant to DomainKeys functionality are stored as tag=value pairs separated by semicolons, e.g.: brisbane._domainkey IN TXT "g=; k=rsa; p=MHww ... IDAQAB" Verifiers MUST support key sizes of 512, 768, 1024, 1536 and 2048 bits. Signers MUST support at least one of the verifier supported key sizes. The current valid tags are: g = granularity of the key. If present with a non-zero length value, this value MUST exactly match the local part of the sending address. This tag is optional. The intent of this tag is to constrain which sending address can legitimately use this selector. An email with a sending address that does not match the value of this tag constitutes a failed verification. k = key type (rsa is the default). Signers and verifiers MUST support the 'rsa' key type. This tag is optional. n = Notes that may be of interest to a human. No interpretation is made by any program. This tag is optional. p =public-keypublic key data, encoded as a Base64 string. An empty value means that thispublic-keypublic key has been revoked. This tag MUST be present. t = a set of flags that define boolean attributes. Valid attributes are: y = testing mode. This domain is testing DomainKeys and unverified email MUST NOT be treated differently from verified email. Recipient systems MAY wish to trackDelany Expires September, 2006 [Page 14] Internet-Draft DomainKeys March 2006testing mode results to assist the sender.) This tag is optional. Delany Expires December, 2006 [Page 14] Internet-Draft DomainKeys June 2006 (Syntax rules for the tag=value format are discussed in Appendix A). Keeping the size of the TXT record to a minimum is important as some implementations of content and caching DNS servers are reported to have problems supporting large TXT records. In the example above, the encoding generates a 182 byte TXT record. That this encoding is less than 512 bytes is of particular significance as it fits within a single UDP response packet. With careful selection of query values, a TXT record can accommodate a 2048 bit key. For the same size restriction reason, the "n" tag SHOULD be used sparingly. The most likely use of this tag is to convey a reason why apublic-keypublic key might have been revoked. In this case set the "n" tag to the explanation and remove thepublic-keypublic key value from the "p" tag. 3.2.4 Key sizes Selecting appropriate key sizes is a trade-off between cost, performance and risk. This specification does not define either minimum or maximum keys sizes - that decision is a matter for each domain owner. Factors that should influence this decision include: o the practical constraint that a 2048 bit key is the largest key that fits within a 512 byte DNS UDP response packet o Larger keys impose higher CPU costs to verify and sign email o Keys can be replaced on a regular basis, thus their lifetime can be relatively short o The security goals of this specification are modest compared to typical goals of public key systems In general, it is expected that most domain owners will use keys that are no larger than 1024 bits. 3.3 Storing the signature in the email header The signature of the email is stored in the "DomainKey-Signature:" header. This header contains all of the signature and key-fetching data.Delany Expires September, 2006 [Page 15] Internet-Draft DomainKeys March 2006When generating the signed email, the "DomainKey-Signature:" header MUST precede the original email headers presented to the signature algorithm. Delany Expires December, 2006 [Page 15] Internet-Draft DomainKeys June 2006 The "DomainKey-Signature:" header is not included in the signature calculation. For extensibility, the "DomainKey-Signature:" header contains tag=value pairs separated by semicolons, e.g.: DomainKey-Signature: a=rsa-sha1; s=brisbane; d=example.net; q=dns; c=simple The current valid tags are: a = The algorithm used to generate the signature. The default is "rsa-sha1", an RSA signed SHA1 digest. Signers and verifiers MUST support "rsa-sha1". b = The signature data, encoded as a Base64 string. This tag MUST be present. Whitespace is ignored in this value and MUST be removed when re-assembling the original signature. This is another way of saying that the signing process can safely insert folding whitespace in this value to conform to line-length limits. c = Canonicalization algorithm. The method by which the headers and content are prepared for presentation to the signing algorithm. This tag MUST be present. Verifiers MUST support "simple" and "nofws". Signers MUST support at least one of the verifier supported algorithms. d = The domain name of the signing domain. This tag MUST be present. In conjunction with the selector tag, this domain forms the basis of thepublic-keypublic key query. The value in this tag MUST match the domain of the sending email address or MUST be one of the parent domains of the sending email address. Domain name comparison is case insensitive. The matching process for this tag is called sub-domain matching. As the sending email address must be the domain or sub-domain of the value. h = A colon separated list of header field names that identify the headers presented to the signing algorithm. If present, the value MUST contain the complete list of headers in the order presented to the signing algorithm. If present, this tag MUST include the header that was used to identify the sending domain, ie, the "From:" or "Sender:" header, thus this tag can never contain an empty value.Delany Expires September, 2006 [Page 16] Internet-Draft DomainKeys March 2006If this tag is not present, all headers subsequent to the signature header are included in the order found in the email. Delany Expires December, 2006 [Page 16] Internet-Draft DomainKeys June 2006 A verifier MUST support this tag. A signer MAY support this tag. If a signer generates this tag it MUST include all email headers in the original email as a verifier MAY remove or render suspicious, lines that are not included in the signature. In the presence of duplicate headers, a signer may include duplicate entries in the list of headers in this tag. If a header is included in this list, a verifier must include all occurrences of that header, subsequent to the "DomainKey-Signature:" header in the verification. If a header identified in this list is not found after the "DomainKey-Signature:" header in the verification process, a verifier may "look" for a matching header prior to the "DomainKey-Signature:" header, however signers should not rely on this as early experience suggests that most verifiers do not try to "look" back before the "DomainKey-Signature:" header. Whitespace is ignored in this value and header comparisons are case insensitive. q = The query method used to retrieve thepublic-key.public key. This tag MUST be present. Currently the only valid value is "dns" which defines the DNS lookup algorithm described in this document. Verifiers and signers MUST support "dns". s = The selector used to form the query for thepublic-key.public key. This tag MUST be present. In the DNS query type, this value is prepended to the "_domainkey." namespace of the sending domain. (Syntax rules for the tag=value format are discussed in Appendix A). Here is an example of a signature header spread across multiple continuation lines: DomainKey-Signature: a=rsa-sha1; s=brisbane; d=example.net; c=simple; q=dns; b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ VoG4ZHRNiYzR; Extreme care must be taken to ensure that any new tags added to this header are defined and used solely for the purpose of fetching and verifying the signature. Any semantics beyond verification cannot beDelany Expires September, 2006 [Page 17] Internet-Draft DomainKeys March 2006trusted as this header is not protected by the signature. If additional semantics not pertaining directly to signature Delany Expires December, 2006 [Page 17] Internet-Draft DomainKeys June 2006 verification are required, they must only be added as subsequent headers protected by the signature. Semantic additions might include audit information describing the initial submission. 3.4 Preparation of email for transit and signing The fundamental purpose of a cryptographic signature is to ensure that the signed content matches the contents presented for verification. However, unlike just about every other Internet protocol, the email content is routinely modified as it enters and transits the email system. Fortunately most of the modifications typically made to email can be predicted and consequently accounted for when signing and verifying. To maximize the chance of a successful verification, submitted email should be prepared for transport prior to signing and the data presented to the signing algorithm is canonicalized to excluded the most common and minor changes made to email. 3.4.1 Preparation for transit The SMTP protocol defines a number of potential limitations to email transport, particularly pertaining to line lengths and eight bit content. While the editor has observed that most modern SMTP implementations accept eight bit email and long lines, some implementations still do not. Consequently a DomainKeys implementation SHOULD prepare an email to be suitable for the lowest common denominator of SMTP prior to presenting the email for signing. 3.4.2 Canonicalization for signing DomainKeys is initially expected to be deployed at, or close to, the email borders of an organization rather than inUAsMUAs or SUBMISSION servers. In other words, the signing and verifying algorithms normally apply after an email has been packaged, transmogrified and generally prepared for transmission across the Internet via SMTP and thus the likelihood of the email being subsequently modified is reduced. Nonetheless, empirical evidence suggests that some mail servers and relay systems modify email in transit, potentially invalidating a signature.Delany Expires September, 2006 [Page 18] Internet-Draft DomainKeys March 2006There are two competing perspectives on such modifications. For most senders, mild modification of email is immaterial to the authentication status of the email. For such senders a Delany Expires December, 2006 [Page 18] Internet-Draft DomainKeys June 2006 canonicalization algorithm that survives modest in-transit modification is preferred. For other senders however, any modification of the email - however minor - results in a desire for the authentication to fail. In other words, such senders do not want a modified email to be seen as being authorized by them. These senders prefer a canonicalization algorithm that does not tolerate in-transit modification of the signed email. To satisfy both requirements, two canonicalization algorithms are defined. A "simple" algorithm that tolerates almost no modification and a "nofws" algorithm that tolerates common modifications as white-space replacement and header line re-wrapping. A sender may choose either algorithm when signing an email. A verifier MUST be able to process email using either algorithm. Either algorithm can be used in conjunction with the "h" tag in the "DomainKey-Signature:" header. Canonicalization simply prepares the email for the signing or verification algorithm. It does not change the transmitted data in any way. 3.4.2.1 The "simple" canonicalization algorithm o Each line of the email is presented to the signing algorithm in the order it occurs in the complete email, from the first line of the headers to the last line of the body. o If the "h" tag is used, only those header lines (and their continuation lines if any) added to the "h" tag list are included. o The "h" tag only constrains header lines. It has no bearing on body lines, which are always included. o Remove any local line terminator. o Append CRLF to the resulting line. o All trailing empty lines are ignored. An empty line is a line of zero length after removal of the local line terminator. If the body consists entirely of empty lines, then the header/body line is similarly ignored. 3.4.2.2 The "nofws" canonicalization algorithm Delany ExpiresSeptember,December, 2006 [Page 19] Internet-Draft DomainKeysMarchJune 20063.4.2.2 The "nofws" canonicalization algorithmThe "No Folding White Space" algorithm (nofws) is more complicated than the "simple" algorithm for two reasons; folding white space is removed from all lines and header continuation lines are unwrapped. o Each line of the email is presented to the signing algorithm in the order it occurs in the complete email, from the first line of the headers to the last line of the body. o Header continuation lines are unwrapped so that header lines are processed as a single line. o If the "h" tag is used, only those header lines (and their continuation lines if any) added to the "h" tag list are included. o The "h" tag only constrains header lines. It has no bearing on body lines, which are are always included. o For each line in the email, remove all folding white space characters. Folding white space is defined in RFC2822 as being the decimal ASCII values 9 (HTAB), 10 (NL), 13 (CR) and 32 (SP). o Append CRLF to the resulting line. o Trailing empty lines are ignored. An empty line is a line of zero length after removal of the local line terminator. Note that the test for an empty line occurs after removing all folding white characters. If the body consists entirely of empty lines, then the header/body line is similarly ignored. 3.5 The signing process The previous sections defined the various components and mechanisms needed to sign an email. This section brings those together to define the complete process of signing an email. A signer MUST only sign email from submissions that are authorized to use the sending address. Once authorization of the submission has been determined, the signing process consists of the following steps: o identifying the sending domain o determining if an email should be signed o selecting a private key and corresponding selector information Delany ExpiresSeptember,December, 2006 [Page 20] Internet-Draft DomainKeysMarchJune 2006 oselecting a private-key and corresponding selector information ocalculating the signature value o prepending the "DomainKey-Signature:" header If an email cannot be signed for some reason, it is a local policy decision as to what to do with that email. 3.5.1 Identifying the sending domain The sending domain is determined by finding the email address in the "Sender:" header, or, if the "Sender:" header is not present, the first email address of the "From:" header is used to determine the sending domain. If the email has an invalid "From:" or an invalid "Sender:" header, it MUST NOT be signed. If the signer adds the "h" tag to the "DomainKey-Signature:" header, that tag MUST include the header that was used to determine the sending domain. 3.5.2 Determining if an email should be signed A signer can obviously only sign email for domains for which it has aprivate-keyprivate key and the necessary knowledge of the correspondingpublic-keypublic key and selector information, however there are a number of other reasons why a signer may choose not to sign an email. A signer MUST NOT sign an email if the email submission is not authorized to use the sending domain. A signer MUST NOT sign an email that already contains a "DomainKey-Signature:" header unless a "Sender:" header has been added that was not included in the original signature. The most obvious case where this occurs is with mailing lists. A signer SHOULD NOT remove an existing "DomainKey-Signature:" header. 3.5.3 Selecting aprivate-keyprivate key and corresponding selector information This specification does not define the basis by which a signer should choose whichprivate-keyprivate key and selector information to use. Currently, all selectors are equal as far as this specification is concerned, so the decision should largely be a matter of administrative convenience. 3.5.4 Calculating the signature value Delany ExpiresSeptember,December, 2006 [Page 21] Internet-Draft DomainKeysMarchJune 20063.5.4 Calculating the signature valueThe signer MUST use one of the defined canonicalization algorithms to present the email to the signing algorithm. Canonicalization is only used to prepare the email for signing. It does not affect the transmitted email in any way. To avoid possible ambiguity, asigner MAYsigning server may choose to remove any pre-existing"Authentication-Results:""DomainKey-Status:" headers from the email prior to preparation for signing and transmission.This action MUST only be taken if the signer is certain of the authenticity of the email submission as the presence of a "DomainKey-Signature:" header normally indicates an externally submitted email on which verification has been attempted.3.5.5 Prepending the "DomainKey-Signature:" header The final step in the signing process is that the signer MUST prepend the "DomainKey-Signature:" header prior to continuing with the process of transmitting the email. 3.6 Policy statement of sending domain While the disposition of inbound email is ultimately a matter for the receiving system, the introduction of authentication in email creates a need for the sender domain to indicate their signing policy and preferred disposition of unsigned email. In particular, whether a domain is participating in DomainKeys, whether it is testing and whether it signs all outbound email.3.6.1 DomainThe sending domain policy isnascent As yet therevery simple and isno standard for a domain to indicate sending policy to recipient MTAs. Indeedexpressed in thewhole notion of sender policy is relatively new to email. To complicate matters, there are currently a number_domainkey TXT record in the DNS ofincompatible draft proposalsthe sending domain. E.g., in the example.com domain thatattempt to define comprehensive sender policy mechanisms.record is called _domainkey.example.com. Theprevious version of this draft expressed some hope that the IETF MARID Working Group would develop such a standard, however that group has been disbanded. Until a unified standard exists, this specification proposes a minimalist and interim sender policy mechanism. The sole purpose of this policy is to define the level of participation in DomainKeys and consequently how unverified email should be treated relative to verified email from that domain. Delany Expires September, 2006 [Page 22] Internet-Draft DomainKeys March 2006 To re-iterate this last point, we strongly take the view that sending domain policy will ultimately encompass much more than just the dispositional needs of DomainKeys. To that end, implementors are encouraged to completely separate their DomainKeys verification process from the determination and application of sender domain policy. 3.6.2 Interim sending domain policy The interim sending domain policy is very simple and is expressed in the _domainkey TXT record in the DNS of the sending domain. E.g., in the example.com domain that record is called _domainkey.example.com The contentscontents of this TXT record are stored as tag=value pairs separated by semicolons, e.g.: _domainkey IN TXT "t=y; o=-; n=notes; r=emailAddress" All tags areoptionaloptional. The current valid tags are: n = Notes that may be of interest to a human. No interpretation is made by any program. o = Outbound Signing policy ("-" means that this domain signs all email, "~" is the default and means that this domain may sign some email with DomainKeys). r = A reporting email address. If present, this defines the email address where invalid verification results are reported. This tag is primarily intended for early implementors - the content and frequency of the reports will be defined in a separate document. Delany Expires December, 2006 [Page 22] Internet-Draft DomainKeys June 2006 t = a set of flags that define boolean attributes. Valid attributes are: y = testing mode. This domain is testing DomainKeys and unverified email MUST NOT be treated differently from verified email. Recipient systems MAY wish to track testing mode results to assist the sender.) Note that testing mode cannot be turned off by this tag - thus policy cannot revert the testing mode setting of a Selector. This tag is optional. (Syntax rules for the tag=value format are discussed in Appendix A). Recipient systems SHOULD only retrieve this policy TXT record toDelany Expires September, 2006 [Page 23] Internet-Draft DomainKeys March 2006determine policy when an email fails to verify or does not include a signature. Recipient systems SHOULD not retrieve this policy TXT record for email that successfully verifies. Note that "testing mode" SHOULD also be in the Selector TXT record if the domain owner is running a DomainKeys test. If the policy TXT record does not exist, recipient systems MUST assume the default values. There is an important implication when a domain states that it signs all email with the "o=-" setting. Namely that the sending domain prefers that the recipient system treat unsigned mail with a great deal of suspicion. Such suspicion could reasonably extend to rejecting such email. A verifying system MAY reject unverified email if a domain policy indicates that it signs all email. Of course nothing compels a recipient MTA to abide by the policy of the sender. In fact, during the trial a sending domain would want to be very certain about setting this policy, as processing by recipient MTAs may be unpredictable. Nonetheless, a domain that states that it signs all email MUST expect that unverified email may be rejected by some receiving MTAs. 3.7 The verification process There is no defined or recommended limit on the lifetime of a selector and correspondingpublic-key,public key, however it is recommended that verification occur in a timely manner with the most timely place being during acceptance or local delivery by the MTA. Verifying a signature consists of the following three steps: o Extract the signature information from the headers Delany Expires December, 2006 [Page 23] Internet-Draft DomainKeys June 2006 o Retrieve thepublic-keypublic key based on the signature information o Check that the signature verifies against the contents In the event that any of these steps fails, the sending domain policy is ascertained to assist in applying local policy. 3.7.1 Presumption that headers are notre-orderedreordered Indications from deployment of previous versions of this specification suggest that the canonicalization algorithms in conjunction with the "h" tag in the "DomainKey-Signature:" header allows most email to cryptographically survive intact between signing and verifying. The one assumption that most of the early deployments make, is thatDelany Expires September, 2006 [Page 24] Internet-Draft DomainKeys March 2006the headers included in the signature are notre-orderedreordered prior to verification. While nothing in this specification precludes a verifier from "looking" for a header that may have beenre-ordered,reordered, including being moved to a position prior to the "DomainKey-Signature:" header, suchre-orderedreordered email is unlikely to be successfully verified by most implementations. A second consequence of this assumption - particularly in the presence of multiple "Domainkey-Signature:" headers - is that the first DomainKey-Signature: header in the email was the last signature added to the email and thus is the one to be verified. 3.7.2 Verification should render a binary result While the symptoms of a failed verification are obvious - the signature doesn't verify - establishing the exact cause can be more difficult. If a selector cannot be found, is that because the selector has been removed or was the value changed somehow in transit? If the signature line is missing is that because it was never there, or was it removed by an over-zealous filter? For diagnostic purposes, the exact reason why the verification fails SHOULD be recorded, however in terms of presentation to the end user, the result SHOULD be presented as a simple binary result: either the email is verified or it is not. If the email cannot be verified, then it SHOULD be rendered the same as all unverified email regardless of whether it looks like it was signed or not. 3.7.3 Selecting the most appropriate "DomainKey-Signature:" header Delany Expires December, 2006 [Page 24] Internet-Draft DomainKeys June 2006 In most cases a signed email is expected to have just one signature - that is, one "DomainKey-Signature:" header. However it is entirely possible that an email can contain multiple signatures. In such cases, a verifier MUST find the most appropriate signature to use and SHOULD ignore all other signatures. The process of finding the most appropriate signature consists of the following "best match" rules. The rules are to be evaluated in order. 1. Selecting the sending domain If the email contains a "Sender:" header, the sending domain is extracted from the "Sender:" address. If this extraction fails, the email SHALL fail verification. If no "Sender:" header is present, the sending domain is extracted from the first address of the "From:" header. If thisDelany Expires September, 2006 [Page 25] Internet-Draft DomainKeys March 2006extraction fails, the email SHALL fail verification. 2. Domain matching A signature can only match if the sending domain matches the "d" tag domain - according to the "d" tag sub-domain matching rules. 3. "h" tag matching If the signature contains the "h" tag list of headers, that list must include the header used to extract the sending domain in rule 1, above. 4. Most secure signing algorithm While it is not yet the case, in the event that additional algorithms are added to this specification, a verifier MUST use the signature that contains the most secure algorithm - as defined by the future specification. For current implementations that means verifiers MUST ignore signatures that are coded with an unrecognized signing algorithm. 5. Earlier signatures are preferred If multiple signatures are equal as far as these rules apply, the signature from the earlier header MUST be used in preference to later signature headers. Implementors MUST meticulously validate the format and values in the "DomainKey-Signature:" header; any inconsistency or unexpected values MUST result in ignoring that header. Being "liberal in what you accept" is definitely a bad strategy in this security context. Delany Expires December, 2006 [Page 25] Internet-Draft DomainKeys June 2006 In all cases, if a verification fails, the"Authenticated-Results:""DomainKey-Status:" header SHOULD be generated and include a message to help explain the reason for failure. 3.7.4 Retrieve thepublic-keypublic key based on the signature information Thepublic-keypublic key is needed to complete the verification process. The process of retrieving thepublic-keypublic key depends on the query type as defined by the "q" tag in the "DomainKey-Signature:" header line. Obviously, apublic-keypublic key should only be retrieved if the process of extracting the signature information is completely successful. Currently the only valid query type is "dns". Thepublic-keypublic key retrieval process for this type is: 1. Using the selector name defined by the "s" tag, theDelany Expires September, 2006 [Page 26] Internet-Draft DomainKeys March 2006"_domainkey" namespace and the domain name defined by the "d" tag, construct and issue the DNS TXT record query string. E.g., if s=brisbane and d=example.net, the query string is "brisbane._domainkey.example.net". 2. If the query for thepublic-keypublic key fails to respond, the verifier SHOULD defer acceptance of this email (normally this will be achieved with a 4XX SMTP response code). 3. If the query for thepublic-keypublic key fails because the corresponding data does not exist, the verifier MUST treat the email as unverified. 4. If the result returned from the query does not adhere to the format defined in this specification, the verifier MUST treat the email as unverified. 5. If thepublic-keypublic key data is not suitable for use with the algorithm type defined by the "a" tag in the "DomainKey-Signature:" header, the verifier MUST treat the email as unverified. Implementors MUST meticulously validate the format and values returned by thepublic-keypublic key query. Any inconsistency or unexpected values MUST result in an unverified email. Being "liberal in what you accept" is definitely a bad strategy in this security context. Latency critical implementations may wish to initiate the public key query in parallel with calculating the SHA-1 hash as the public key is not needed until the the final RSA is calculated. 3.7.5 Verify the signature Delany Expires December, 2006 [Page 26] Internet-Draft DomainKeys June 2006 Armed with the signature information from the "DomainKey-Signature:" header and thepublic-keypublic key information returned by the query, the signature of the email can now be verified. The canonicalization algorithm defined by the "c" tag in the "DomainKey-Signature:" header defines how the data is prepared for the verification algorithm and the "a" tag in the same header defines which verification algorithm to use. 3.7.6 Retrieving sending domain policy In the event that an email fails to verify, the policy of the sending domain MUST be consulted. For now that means consulting the _domainkey TXT record in the DNS of the domain in the sending domain as defined in 3.5.1. Eg, if example.net is the sending domain, the TXT query is:Delany Expires September, 2006 [Page 27] Internet-Draft DomainKeys March 2006_domainkey.example.net A verifier SHOULD consider the sending domain policy statement and act accordingly. The range of possibilities is up to the receiver, but it MAY include rejecting the email. 3.7.7 Applying local policy After all verification processes are complete the recipient system has authentication information that can help it decide what to do with the email. It is beyond the scope of this specification to describe what actions a recipient system should make, but an authenticated email presents an opportunity to a receiving system that unauthenticated email cannot. Specifically, an authenticated email creates a predictable identifier by which other decisions can reliably be managed, such as trust and reputation. Conversely, unauthenticated email lacks a reliable identifier that can be used to assign trust and reputation. It is not unreasonable to treat unauthenticated email as lacking any trust and having no positive reputation. 3.8 Conveying verification results toUAsMUAs Apart from the application of automated policy, the result of a signature verification should be conveyed to the user reading the email. Most email clients can be configured to recognize specific headers andapply simple rules -Delany Expires December, 2006 [Page 27] Internet-Draft DomainKeys June 2006 apply simple rules - e.g., filing into a particular folder. Since DomainKey signatures are expected to be initially verified at the border MTA, the results of the verification need to be conveyed to the email client. This is done with the"Authentication-Results:""DomainKey-Status:" header line prepended to the email.This header is described in [AUTH-HEADER].The"Authentication-Results:""DomainKey-Status:" headerprovidesstarts with alocation for additional clarifying information. Suggested textstring thata DomainKeys implementation might use, include: "Sender not parsable" The sending domain could not be extracted from an existing "Sender:" header. "From not parsable" The sending domain couldindicate the result of the verification. Valid values are: "good" - the signature was verified at the time of testing "bad" - the signature failed the verification "no key" - the public key query failed as the key does notbe extracted from an existing "From:" header. Delany Expires September, 2006 [Page 28] Internet-Draft DomainKeys March 2006 "Selector revoked" The public-keyexist "revoked" - the public key query failed as the key has beenrevoked.revoked "no signature"The- this email has no "DomainKey-Signature:"header.header "badsignatureformat"The- the signature or the public key contains unexpected data"bad Selector format" The Selector contains unexpected"non-participant" - this sending domain has indicated that it does not participate in DomainKeys. Verifiers may append additional data that expands on the reason for the particular status value. A client SHOULD just look for "good" and assume that all other values imply that the verification could not be performed for some reason. Policy action as a consequence of this header is entirely a local matter. Here are some examples: DomainKey-Status: good DomainKey-Status: bad format Although it is expected that MTAs will be DomainKey aware before MUAs, it is nonetheless possible that a DomainKey aware MUA can be fooled by a spoofed "DomainKey-Status:" header that passes through a non-DomainKey aware MTA. If this is perceived to be a serious problem, then it may make sense to preclude the "good" value and only have values that effectively demote the email as far as the UA is concerned. That way successful spoofing attempts can only serve to demote themselves. 4. Example of use This section shows the complete flow of an email from submission to final delivery, demonstrating how the various components fit together. 4.1 The user composes an email From: "Joe SixPack" <joe@football.example.com> Delany Expires December, 2006 [Page 28] Internet-Draft DomainKeys June 2006 To: "Suzie Q" <suzie@shopping.example.net> Subject: Is dinner ready? Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT) Message-ID: <20030712040037.46341.5F8J@football.example.com> Hi. We lost the game. Are you hungry yet? Joe. 4.2 The email is signed This email is signed by the football.example.com outbound email server and now looks like this: DomainKey-Signature: a=rsa-sha1; s=brisbane; d=football.example.com; c=simple; q=dns; b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ VoG4ZHRNiYzR; Received: from dsl-10.2.3.4.football.example.com [10.2.3.4] by submitserver.football.example.com with SUBMISSION; Fri, 11 Jul 2003 21:01:54 -0700 (PDT) From: "Joe SixPack" <joe@football.example.com> To: "Suzie Q" <suzie@shopping.example.net> Subject: Is dinner ready? Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT) Message-ID: <20030712040037.46341.5F8J@football.example.com> Hi.Delany Expires September, 2006 [Page 29] Internet-Draft DomainKeys March 2006We lost the game. Are you hungry yet? Joe. The signing email server requires access to theprivate-keyprivate key associated with the "brisbane" selector to generate this signature. Distribution and management ofprivate-keysprivate keys is outside the scope of this document. 4.3 The email signature is verified The signature is normally verified by an inbound SMTP server or possibly the final delivery agent. However, intervening MTAs can also perform this verification if they choose to do so. The verification process uses the domain "football.example.com" extracted from the "From:" header and the selector "brisbane" from the "DomainKey-Signature:" header to form the DNS TXT query for: Delany Expires December, 2006 [Page 29] Internet-Draft DomainKeys June 2006 brisbane._domainkey.football.example.com Since there is no "h" tag in the "DomainKey-Signature:" header, signature verification starts with the line following the "DomainKey-Signature:" line. The email is canonically prepared for verifying with the "simple" method. The result of the query and subsequent verification of the signature is stored in the"Authentication-Results:""DomainKey-Status:" header line. After successful verification, the email looks like this:Authentication-Results: auth-checker.example.comDomainKey-Status: good from=joe@football.example.com; domainkeys=pass Received: from mout23.brisbane.football.example.com (192.168.1.1) by shopping.example.net with SMTP; Fri, 11 Jul 2003 21:01:59 -0700 (PDT) DomainKey-Signature: a=rsa-sha1; s=brisbane; d=football.example.com; c=simple; q=dns; b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ VoG4ZHRNiYzR; Received: from dsl-10.2.3.4.network.example.com [10.2.3.4] by submitserver.example.com with SUBMISSION; Fri, 11 Jul 2003 21:01:54 -0700 (PDT) From: "Joe SixPack" <joe@football.example.com> To: "Suzie Q" <suzie@shopping.example.net> Subject: Is dinner ready? Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT) Message-ID: <20030712040037.46341.5F8J@football.example.com> Hi.Delany Expires September, 2006 [Page 30] Internet-Draft DomainKeys March 2006We lost the game. Are you hungry yet? Joe. 5. Association with a Certificate Authority A fundamental aspect of DomainKeys is thatpublic-keyspublic keys are generated and advertised by each domain at no additional cost. This accessibility markedly differs from traditional Public Key Infrastructures where there is typically a Certificate Authority (CA) who validates an applicant and issues a signed certificate - containing theirpublic-keypublic key - often for a recurring fee. While CAs do impose costs, they also have the potential to provide additional value as part of their certification process. Consider financial institutions, public utilities, law enforcement agencies and the like. In many cases, such entities justifiably need to discriminate themselves above and beyond the authentication that DomainKeys offers. Delany Expires December, 2006 [Page 30] Internet-Draft DomainKeys June 2006 Creating a link between DomainKeys and CA issued certificates has the potential to access additional authentication mechanisms that are more authoritative than domain owner issued authentication. It is well beyond the scope of this specification to describe such authorities apart from defining how the linkage could be achieved with the "DomainKey-X509:" header. 5.1 The "DomainKey-X509:" header The "DomainKey-X509:" header provides a link between thepublic-keypublic key used to sign the email and the certificate issued by a CA. The exact content, syntax and semantics of this header are yet to be resolved. One possibility is that this header contains an encoding of the certificate issued by a CA. Another possibility is that this header contains a URL that points to a certificate issued by a CA. In either case, this header can only be consulted if the signature verifies and MUST be part of the content signed by the corresponding "DomainKey-Signature:" header. Furthermore, it is likely that MUAs rather than MTAs will confirm that the link to the CA issued certificate is valid. In part this is because many MUAs already have built-in capabilities as a consequence of S/MIME[RFC1847][SMIME] and SSL [SSL] support. The proof of linkage is made by testing that thepublic-keypublic key in the certificate matches thepublic-keypublic key used to sign the email. An example of a email containing the "DomainKey-X509:" header is:Delany Expires September, 2006 [Page 31] Internet-Draft DomainKeys March 2006DomainKey-Signature: a=rsa-sha1; s=statements; d=largebank.example.com; c=simple; q=dns; b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ VoG4ZHRNiYzR; DomainKey-X509: https://ca.example.net/largebank.example.com From: "Large Bank" <statements@largebank.example.com> To: "Suzie Q" <suzie@shopping.example.net> Subject: Statement for Account: 1234-5678 ... The format of the retrieved value from the URL is not yet defined, nor is the determination of valid CAs. The whole matter of linkage to CA issued certificates is one aspect of DomainKeys that needs to be resolved with relevant CA and certificate issuing entities. The primary point is that a link is possible to a higher authority. Delany Expires December, 2006 [Page 31] Internet-Draft DomainKeys June 2006 6. Topics for discussion 6.1 The benefits of selectors Selectors are at the heart of the flexibility of DomainKeys. A domain administrator is free to use a single DomainKey for all outbound mail. Alternatively, they may use many DomainKeys differentiated by selector and assign each key to different servers. For example, a large outbound email farm might have a unique DomainKey for each server, and thus their DNS will advertise potentially hundreds of keys via their unique selectors. Another example is a corporate email administrator who might generate a separate DomainKey for each regional office email server. In essence, selectors allow a domain owner to distribute authority to send on behalf of that domain. Combined with the ability to revoke by removal or TTL expiration, a domain owner has coarse-grained control over the duration of the distributed authority. Selectors are particularly useful for domain owners who want to contract a third-party mailing system to send a particular set of mail. The domain owner can generate a special key pair and selector just for this mail-out. The domain owner has to provide the Private Key and selector to the third party for the life of the mail-out. However, as soon as the mail-out is completely delivered, the domain owner can revoke thepublic-keypublic key by the simple expedient of removing the entry from the DNS.Delany Expires September, 2006 [Page 32] Internet-Draft DomainKeys March 20066.2 Canonicalization of email It is an unfortunate fact that some email software routinely (and often unnecessarily) transforms email as it transits through the network. Such transformations conflict with the fundamental purpose of cryptographic signatures - to detect modifications. While two canonicalization algorithms are defined in this specification, the primary goal of "nofws" is to provide a transition path to "simple". With a mixture of "simple" and "nofws" email, a receiver can determine which systems are modifying email in ways that cause the signature to fail and thus provide feedback to the modifying system. 6.3 Mailing lists Integrating existing Mailing List Managers (MLMs) into the DomainKeys authentication system is a complicated areas as the behavior of MLMs is Delany Expires December, 2006 [Page 32] Internet-Draft DomainKeys June 2006 highly variable. Essentially there are two types of MLMs under consideration. Those that modify email to such an extent that verification of the original content is not possible, and MLMs that make minimal or no modifications to an email. MLMs that modify email in a way that causes verification to fail, MUST pre-pend either a "Sender:" header and preferably a "List-ID:" header and arrange for the email to be resigned as it is distributed to the list recipients. A participating SUBMISSION server can deduce the need to resign such an email by the presence of a "Sender:" or "List-ID:" header from an authorized submission. MLMs that do not modify email in a way that causes verification to fail, MAY perform the same actions as a modifying MLM. 6.4 Roving users One scenario that presents a particular problem with any form of email authentication, including DomainKeys, is the roving user. A user who is obliged to use a third-party SUBMISSION service when unable to connect to their own SUBMISSION service. The classic example cited is a traveling salesperson being redirected to a hotel email server to send email. As far as DomainKeys is concerned, email of this nature clearly originates from an email server that does not have authority to send on behalf of the domain of the salesperson and is therefore indistinguishable from a forgery. While DomainKeys does not prescribeDelany Expires September, 2006 [Page 33] Internet-Draft DomainKeys March 2006any specific action for such email it is likely that over time, such email will be treated as second class email. The typical solution offered to roving users is to submit email via an authorized server for their domain - perhaps via a VPN or a web interface or even SMTP AUTH back to a SUBMISSION server. While these are perfectly acceptable solutions for many, they are not necessarily solutions that are available or possible for all such users. One possible way to address the needs of this contingent of potentially disenfranchised users, is for the domain to issue per-user DomainKeys. Per-user DomainKeys are identified by a non-empty "g" tag value in the corresponding DNS record. 7. Security Considerations Delany Expires December, 2006 [Page 33] Internet-Draft DomainKeys June 2006 7.1 DNS DomainKeys is primarily a security mechanism. Its core purpose is to make claims about email authentication in a credible way. However, DomainKeys, like virtually all Internet applications, relies on the DNS which has well-known security flaws [RFC3833]. 7.1.1 The DNS is not currently secure While the DNS is currently insecure, it is expected that the security problems should and will be solved by DNSSEC [DNSSEC], and all users of the DNS will reap the benefit of that work. Secondly, the types of DNS attacks relevant to DomainKeys are very costly and are far less rewarding than DNS attacks on other Internet applications. To systematically thwart the intent of DomainKeys, an attacker must conduct a very costly and very extensive attack on many parts of the DNS over an extended period. No one knows for sure how attackers will respond, however the cost/benefit of conducting prolonged DNS attacks of this nature is expected to be uneconomical. Finally, DomainKeys is only intended as a "sufficient" method of proving authenticity. It is not intended to provide strong cryptographic proof about authorship or contents. Other technologies such as GnuPG and S/MIME address those requirements. 7.1.2 DomainKeys creates additional DNS loadDelany Expires September, 2006 [Page 34] Internet-Draft DomainKeys March 2006A second security issue related to the DNS revolves around the increased DNS traffic as a consequence of fetching Selector-based data as well as fetching sending domain policy. Widespread deployment of DomainKeys will result in a significant increase in DNS queries to the claimed sending domain. In the case of forgeries on a large scale, DNS servers could see a substantial increase in queries. 7.2 Key Management Allpublic-keypublic key systems require management of key pairs.Private-keysPrivate keys in particular need to be securely distributed to each signing mail server and protected on those servers. For those familiar with SSL, the key management issues are similar to those of managing SSL certificates. Poor key management may result in unauthorized access toprivate-keys,private keys, which in essence gives unauthorized access to your identity. Delany Expires December, 2006 [Page 34] Internet-Draft DomainKeys June 2006 7.3 Implementation Risks It is well recognized in cryptographic circles that many security failures are caused by poor implementations rather than poor algorithms. For example, early SSL implementations were vulnerable because the implementors used predictable "random numbers". While some MTA software already supports various cryptographic techniques, such as TLS, many do not. This proposal introduces cryptographic requirements into MTA software which implies a much higher duty of care to manage the increased risk. There are numerous articles, books, courses, and consultants that help programming security applications. Potential implementors are strongly encouraged to avail themselves of all possible resources to ensure secure implementations. 7.4 Privacy assumptions with forwarding addresses Some people believe that they can achieve anonymity by using an email forwarding service. While this has never been particularly true as bounces, over-quota messages, vacation messages and web bugs all conspire to expose IP addresses and domain names associated with the delivery path, the DNS queries that are required to verify DomainKeys signature can provide additional information to the sender. In particular, as mail is forwarded through the mail network, the DNS queries for the selector will typically identify the DNS cache used by the forwarding and delivery MTAs.Delany Expires September, 2006 [Page 35] Internet-Draft DomainKeys March 20067.5 Cryptographic processing is computationally intensive Verifying a signature is computationally significant. Early indications are that a typical mail server can expect to increase CPU demands by 8-15 percent. While this increased demand is modest compared to other common mail processing costs - such as Bayesian filtering - any increase in resource requirements can make a denial-of-service attack more effective against a mail system. A constraining factor of such attacks is that the net computational cost of verifying is bounded by the maximum key size allowed by this specification and is essentially linear to the rate at which mail is accepted by the verifying system. Consequently, the additional computational cost may augment a denial-of-service attack, but it does not add a non-linear component to such attacks. 8. The trial Delany Expires December, 2006 [Page 35] Internet-Draft DomainKeys June 2006 Since the publication of the last version of this specification, a considerable number of both small and large organizations have developed and deployed DomainKeys to authenticate email. Additionally, Open Source implementations are available at various places, particularly Source Forge [SOURCEFORGE] which has links to numerous implementations, both Open Source and commercial. Interested parties are encouraged to participate in this trial and help evolve this specification based on those experiences. 8.1 Goals The primary goals of the trial are to: o understand the operational implications of running a DNS-basedpublic-keypublic key system for email o measure the effectiveness of the canonicalization algorithms o experiment with possible per-user key deployment models o fully define the semantics of the "DomainKey-X509:" header 8.2 Constraints on participation Participants should understand that this specification has evolved over the last year and will likely change again as requirements are further refined. While dramatic changes are not expected and every effort is being made to minimize gratuitous changes, participantsDelany Expires September, 2006 [Page 36] Internet-Draft DomainKeys March 2006should be prepared to adopt such changes as they arise. Since early DomainKey implementations may be less than perfect and that senders may be merely testing their implementations, recipient systems should be reticent about applying strict policy to unverified email. Particularly if the sending domain policy or the selector information has the testing mode set. 8.3 Results of trial thus far The DomainKeys trial has run for approximately 18 months in which time numerous large ISPs and many thousands of smaller domains participated in signing or verifying with DomainKeys. The low order numbers are that approximately 1 billion DomainKey signed emails transit the Internet each day between some 10,000 participating domains. The operational and development experience of that trial is being applied to DKIM. Delany Expires December, 2006 [Page 36] Internet-Draft DomainKeys June 2006 9.NotesNote to Implementors9.1regarding TXT records The DNS is very flexible in that it is possible to have multiple TXT records for a single name and for those TXT records to contain multiple strings. In all cases, implementors of DomainKeys should expect a single TXT record for any particular name. If multiple TXT records are returned, the implementation is free to pick any single TXT record as the authoritative data. In other words, if a name server returns different TXT records for the same name, it can expect unpredictable results. Within a single TXT record, implementors should concatenate multiple strings in the order presented and ignore string boundaries. Note that a number of popular DNS command-line tools render multiple strings as separately quoted strings which can be misleading to a novice implementor. 10. References 10.1 Normative References[AUTH-HEADER] http://www.ietf.org/internet-drafts/draft-kucherawy-sender-auth-header [MIME] Borenstein, N., Freed, N., "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Delany Expires September, 2006 [Page 37] Internet-Draft DomainKeys March 2006 Message Bodies ",[BASE64] S. Josefsson, Ed., "The Base16, Base32, and Base64 Data Encodings", RFC2045, November, 1996. [OPENSSL] http://www.openssl.org3548, July, 2003. [PEM] Linn, J., "Privacy Enhancement for Internet Electronic Mail: Part I: Message Encryption and Authentication Procedures", RFC 1421 February, 1993. 10.2 Informative References [DKIM] http://www.ietf.org/html.charters/dkim-charter.html [DNSSEC] http://www.ietf.org/html.charters/dnsext-charter.html[RFC1847] Galvin, J., Murphy, S., Crocker, S., Freed, N., "Security Multiparts for MIME", RFC 1847, October, 1995.[OPENSSL] http://www.openssl.org [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [RFC2822] Resnick, P., Editor, "Internet Message Format", RFC 2822, April 2001. [RFC3833] Atkins, D., Austein, R., "Threat Analysis of the Domain Name System (DNS)", RFC3833, August,3833, August, 2004. [SMIME] B. Ramsdell, Editor, "Secure/Multipurpose Internet Delany Expires December, 2006 [Page 37] Internet-Draft DomainKeys June 2006 Mail Extensions (S/MIME) Version 3.1 Message Specification", RFC 3851, July 2004. [SOURCEFORGE] http://domainkeys.sourceforge.net [SSL] http://wp.netscape.com/security/techbriefs/ssl.html Delany ExpiresSeptember,December, 2006 [Page 38] Internet-Draft DomainKeysMarchJune 2006 Appendix A - Syntax rules for the tag=value format A simple tag=value syntax is used to encode data in the response values for DNS queries as well as headers embedded in emails. In a later draft, a formal grammar will be used to precisely define the rules, for now, this section summarized the most salient syntactic rules for this encoding: o A tag=value pair consists of three tokens, a "tag", the "=" character and the "value" o A tag MUST be one character long and MUST be a lower-case alphabetic character o Duplicate tags are not allowed o A value MUST only consist of characters that are valid in RFC2822 headers, DNS TXT records and are within the ASCII range of characters from SPACE (0x20) to TILDE (0x7E) inclusive. Values MUST NOT contain a semicolon but they may contain "=" characters. o A tag=value pair MUST be terminated by a semicolon or the end of the data o Values MUST be processed as case sensitive unless the specific tag description of semantics imply case insensitivity. o Values MAY be zero bytes long o Whitespace MAY surround any of the tokens, however whitespace within a value MUST be retained unless explicitly excluded by the specific tag description. Currently the only tags that specifically ignores embedded whitespace are the "b" and "h" tag in the "DomainKey-Signature:" header. o Tag=value pairs that represent the default value MAY be included to aid legibility. o Unrecognized tags MUST be ignored Delany ExpiresSeptember,December, 2006 [Page 39] Internet-Draft DomainKeysMarchJune 2006 Acknowledgments The editor wishes to thank Russ Allbery, Eric Allman, Edwin Aoki, Claus Asmann, Steve Atkins, Jon Callas, Dave Crocker, Michael Cudahy, Jutta Degener, Timothy Der, Jim Fenton, Duncan Findlay, Phillip Hallam-Baker, Murray S. Kucherawy, John Levine, Miles Libbey, David Margrave, Justin Mason, David Mayne, Russell Nelson, Juan Altmayer Pizzorno, Blake Ramsdell, Scott Renfro, the Spamhaus.org team, Malte S. Stretz, Robert Sanders, Bradley Taylor, and Rand Wacker for their valuable suggestions and constructive criticism.Change History This document is a revision of the previous version named draft-delany-domainkeys-base-02.txt. For the convenience of those familiar with the previous version, the major changes are summarized here: 1. Cleaned up typographical and grammatical errors 2. Clarify that d= and h= tag values are case insensitiveEditor's Address Mark Delany Yahoo! Inc 701 First Avenue Sunnyvale, CA 95087 USA Email: markd+domainkeys@yahoo-inc.com Delany ExpiresSeptember,December, 2006 [Page 40] Internet-Draft DomainKeysMarchJune 2006Copyright Notice Copyright (C)Intellectual Property Statement TheInternet Society (2006). ThisIETF 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 documentis subjector 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 therights, licenses and restrictions containedprocedures with respect to rights in RFC documents can be found in BCP78,78 andexcept as set forth therein,BCP 79. Copies of IPR disclosures made to theauthors retainIETF 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. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. The IETF has been notified of intellectual property rights claimed in regard to some or alltheirof the specification contained in this document. For more information consult the online list of claimed rights. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.RCS: $Id: draft-delany-domainkeys-base-04.txt,v 1.4 2006/03/29 20:48:52 markd Exp $Copyright Statement Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Delany ExpiresSeptember,December, 2006 [Page 41] ----