--- 1/draft-ietf-dkim-rfc4871bis-13.txt 2011-07-03 06:16:27.000000000 +0200 +++ 2/draft-ietf-dkim-rfc4871bis-14.txt 2011-07-03 06:16:27.000000000 +0200 @@ -1,21 +1,21 @@ Network Working Group D. Crocker, Ed. Internet-Draft Brandenburg InternetWorking Obsoletes: 4871, 5672 T. Hansen, Ed. (if approved) AT&T Laboratories Intended status: Standards Track M. Kucherawy, Ed. -Expires: December 26, 2011 Cloudmark - June 24, 2011 +Expires: January 3, 2012 Cloudmark + July 2, 2011 DomainKeys Identified Mail (DKIM) Signatures - draft-ietf-dkim-rfc4871bis-13 + draft-ietf-dkim-rfc4871bis-14 Abstract DomainKeys Identified Mail (DKIM) permits a person, role, or organization that owns the signing domain to claim some responsibility for a message by associating the domain with the message. This can be an author's organization, an operational relay or one of their agents. DKIM separates the question of the identity of the signer of the message from the purported author of the message. Assertion of responsibility is validated through a @@ -34,21 +34,21 @@ 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 December 26, 2011. + This Internet-Draft will expire on January 3, 2012. Copyright Notice Copyright (c) 2011 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 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 @@ -89,86 +89,87 @@ 2.8. Whitespace . . . . . . . . . . . . . . . . . . . . . . . . 8 2.9. Imported ABNF Tokens . . . . . . . . . . . . . . . . . . . 9 2.10. Common ABNF Tokens . . . . . . . . . . . . . . . . . . . . 9 2.11. DKIM-Quoted-Printable . . . . . . . . . . . . . . . . . . 10 3. Protocol Elements . . . . . . . . . . . . . . . . . . . . . . 11 3.1. Selectors . . . . . . . . . . . . . . . . . . . . . . . . 11 3.2. Tag=Value Lists . . . . . . . . . . . . . . . . . . . . . 13 3.3. Signing and Verification Algorithms . . . . . . . . . . . 14 3.4. Canonicalization . . . . . . . . . . . . . . . . . . . . . 15 3.5. The DKIM-Signature Header Field . . . . . . . . . . . . . 19 - 3.6. Key Management and Representation . . . . . . . . . . . . 29 - 3.7. Computing the Message Hashes . . . . . . . . . . . . . . . 33 - 3.8. Input Requirements . . . . . . . . . . . . . . . . . . . . 36 - 3.9. Output Requirements . . . . . . . . . . . . . . . . . . . 36 - 3.10. Signing by Parent Domains . . . . . . . . . . . . . . . . 36 + 3.6. Key Management and Representation . . . . . . . . . . . . 28 + 3.7. Computing the Message Hashes . . . . . . . . . . . . . . . 32 + 3.8. Input Requirements . . . . . . . . . . . . . . . . . . . . 34 + 3.9. Output Requirements . . . . . . . . . . . . . . . . . . . 35 + 3.10. Signing by Parent Domains . . . . . . . . . . . . . . . . 35 3.11. Relationship between SDID and AUID . . . . . . . . . . . . 36 4. Semantics of Multiple Signatures . . . . . . . . . . . . . . . 37 4.1. Example Scenarios . . . . . . . . . . . . . . . . . . . . 37 - 4.2. Interpretation . . . . . . . . . . . . . . . . . . . . . . 39 - 5. Signer Actions . . . . . . . . . . . . . . . . . . . . . . . . 40 + 4.2. Interpretation . . . . . . . . . . . . . . . . . . . . . . 38 + 5. Signer Actions . . . . . . . . . . . . . . . . . . . . . . . . 39 5.1. Determine Whether the Email Should Be Signed and by - Whom . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 + Whom . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 5.2. Select a Private Key and Corresponding Selector - Information . . . . . . . . . . . . . . . . . . . . . . . 40 - 5.3. Normalize the Message to Prevent Transport Conversions . . 41 - 5.4. Determine the Header Fields to Sign . . . . . . . . . . . 42 - 5.5. Recommended Signature Content . . . . . . . . . . . . . . 44 - 5.6. Compute the Message Hash and Signature . . . . . . . . . . 45 - 5.7. Insert the DKIM-Signature Header Field . . . . . . . . . . 46 + Information . . . . . . . . . . . . . . . . . . . . . . . 39 + 5.3. Normalize the Message to Prevent Transport Conversions . . 40 + 5.4. Determine the Header Fields to Sign . . . . . . . . . . . 41 + 5.5. Compute the Message Hash and Signature . . . . . . . . . . 45 + 5.6. Insert the DKIM-Signature Header Field . . . . . . . . . . 45 6. Verifier Actions . . . . . . . . . . . . . . . . . . . . . . . 46 - 6.1. Extract Signatures from the Message . . . . . . . . . . . 47 - 6.2. Communicate Verification Results . . . . . . . . . . . . . 52 + 6.1. Extract Signatures from the Message . . . . . . . . . . . 46 + 6.2. Communicate Verification Results . . . . . . . . . . . . . 51 6.3. Interpret Results/Apply Local Policy . . . . . . . . . . . 52 - 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 54 - 7.1. DKIM-Signature Tag Specifications . . . . . . . . . . . . 54 - 7.2. DKIM-Signature Query Method Registry . . . . . . . . . . . 55 - 7.3. DKIM-Signature Canonicalization Registry . . . . . . . . . 55 - 7.4. _domainkey DNS TXT Resource Record Tag Specifications . . 56 - 7.5. DKIM Key Type Registry . . . . . . . . . . . . . . . . . . 56 - 7.6. DKIM Hash Algorithms Registry . . . . . . . . . . . . . . 57 - 7.7. DKIM Service Types Registry . . . . . . . . . . . . . . . 57 - 7.8. DKIM Selector Flags Registry . . . . . . . . . . . . . . . 57 - 7.9. DKIM-Signature Header Field . . . . . . . . . . . . . . . 58 - 8. Security Considerations . . . . . . . . . . . . . . . . . . . 58 - 8.1. Misuse of Body Length Limits ("l=" Tag) . . . . . . . . . 58 - 8.2. Misappropriated Private Key . . . . . . . . . . . . . . . 58 - 8.3. Key Server Denial-of-Service Attacks . . . . . . . . . . . 59 - 8.4. Attacks Against the DNS . . . . . . . . . . . . . . . . . 59 - 8.5. Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 60 - 8.6. Limits on Revoking Keys . . . . . . . . . . . . . . . . . 60 - 8.7. Intentionally Malformed Key Records . . . . . . . . . . . 61 - 8.8. Intentionally Malformed DKIM-Signature Header Fields . . . 61 - 8.9. Information Leakage . . . . . . . . . . . . . . . . . . . 61 - 8.10. Remote Timing Attacks . . . . . . . . . . . . . . . . . . 61 - 8.11. Reordered Header Fields . . . . . . . . . . . . . . . . . 61 - 8.12. RSA Attacks . . . . . . . . . . . . . . . . . . . . . . . 62 - 8.13. Inappropriate Signing by Parent Domains . . . . . . . . . 62 - 8.14. Attacks Involving Addition of Header Fields . . . . . . . 62 - 8.15. Malformed Inputs . . . . . . . . . . . . . . . . . . . . . 63 - 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 64 - 9.1. Normative References . . . . . . . . . . . . . . . . . . . 64 - 9.2. Informative References . . . . . . . . . . . . . . . . . . 65 - Appendix A. Example of Use (INFORMATIVE) . . . . . . . . . . . . 66 - A.1. The User Composes an Email . . . . . . . . . . . . . . . . 66 - A.2. The Email is Signed . . . . . . . . . . . . . . . . . . . 67 - A.3. The Email Signature is Verified . . . . . . . . . . . . . 68 - Appendix B. Usage Examples (INFORMATIVE) . . . . . . . . . . . . 69 - B.1. Alternate Submission Scenarios . . . . . . . . . . . . . . 69 - B.2. Alternate Delivery Scenarios . . . . . . . . . . . . . . . 71 - Appendix C. Creating a Public Key (INFORMATIVE) . . . . . . . . . 73 - C.1. Compatibility with DomainKeys Key Records . . . . . . . . 74 - C.2. RFC4871 Compatibility . . . . . . . . . . . . . . . . . . 74 - Appendix D. MUA Considerations (INFORMATIVE) . . . . . . . . . . 74 - Appendix E. Changes since RFC4871 . . . . . . . . . . . . . . . . 75 - Appendix F. Acknowledgements . . . . . . . . . . . . . . . . . . 77 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 77 + + 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 53 + 7.1. Email Authentication Methods Registry . . . . . . . . . . 53 + 7.2. DKIM-Signature Tag Specifications . . . . . . . . . . . . 53 + 7.3. DKIM-Signature Query Method Registry . . . . . . . . . . . 54 + 7.4. DKIM-Signature Canonicalization Registry . . . . . . . . . 54 + 7.5. _domainkey DNS TXT Resource Record Tag Specifications . . 55 + 7.6. DKIM Key Type Registry . . . . . . . . . . . . . . . . . . 56 + 7.7. DKIM Hash Algorithms Registry . . . . . . . . . . . . . . 56 + 7.8. DKIM Service Types Registry . . . . . . . . . . . . . . . 56 + 7.9. DKIM Selector Flags Registry . . . . . . . . . . . . . . . 57 + 7.10. DKIM-Signature Header Field . . . . . . . . . . . . . . . 57 + 8. Security Considerations . . . . . . . . . . . . . . . . . . . 57 + 8.1. ASCII Art Attacks . . . . . . . . . . . . . . . . . . . . 57 + 8.2. Misuse of Body Length Limits ("l=" Tag) . . . . . . . . . 58 + 8.3. Misappropriated Private Key . . . . . . . . . . . . . . . 58 + 8.4. Key Server Denial-of-Service Attacks . . . . . . . . . . . 59 + 8.5. Attacks Against the DNS . . . . . . . . . . . . . . . . . 59 + 8.6. Replay/Spam Attacks . . . . . . . . . . . . . . . . . . . 60 + 8.7. Limits on Revoking Keys . . . . . . . . . . . . . . . . . 60 + 8.8. Intentionally Malformed Key Records . . . . . . . . . . . 60 + 8.9. Intentionally Malformed DKIM-Signature Header Fields . . . 61 + 8.10. Information Leakage . . . . . . . . . . . . . . . . . . . 61 + 8.11. Remote Timing Attacks . . . . . . . . . . . . . . . . . . 61 + 8.12. Reordered Header Fields . . . . . . . . . . . . . . . . . 61 + 8.13. RSA Attacks . . . . . . . . . . . . . . . . . . . . . . . 61 + 8.14. Inappropriate Signing by Parent Domains . . . . . . . . . 61 + 8.15. Attacks Involving Addition of Header Fields . . . . . . . 62 + 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 63 + 9.1. Normative References . . . . . . . . . . . . . . . . . . . 63 + 9.2. Informative References . . . . . . . . . . . . . . . . . . 64 + Appendix A. Example of Use (INFORMATIVE) . . . . . . . . . . . . 65 + A.1. The User Composes an Email . . . . . . . . . . . . . . . . 65 + A.2. The Email is Signed . . . . . . . . . . . . . . . . . . . 66 + A.3. The Email Signature is Verified . . . . . . . . . . . . . 67 + Appendix B. Usage Examples (INFORMATIVE) . . . . . . . . . . . . 68 + B.1. Alternate Submission Scenarios . . . . . . . . . . . . . . 68 + B.2. Alternate Delivery Scenarios . . . . . . . . . . . . . . . 70 + Appendix C. Creating a Public Key (INFORMATIVE) . . . . . . . . . 72 + C.1. Compatibility with DomainKeys Key Records . . . . . . . . 73 + C.2. RFC4871 Compatibility . . . . . . . . . . . . . . . . . . 73 + Appendix D. MUA Considerations (INFORMATIVE) . . . . . . . . . . 73 + Appendix E. Changes since RFC4871 . . . . . . . . . . . . . . . . 74 + Appendix F. Acknowledgements . . . . . . . . . . . . . . . . . . 76 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 76 1. Introduction DomainKeys Identified Mail (DKIM) permits a person, role, or organization to claim some responsibility for a message by associating a domain name [RFC1034] with the message [RFC5322], which they are authorized to use. This can be an author's organization, an operational relay or one of their agents. Assertion of responsibility is validated through a cryptographic signature and querying the signer's domain directly to retrieve the appropriate @@ -231,25 +232,25 @@ field. INFORMATIVE RATIONALE: The signing identity specified by a DKIM signature is not required to match an address in any particular header field because of the broad methods of interpretation by recipient mail systems, including MUAs. 1.3. Scalability DKIM is designed to support the extreme scalability requirements that - characterize the email identification problem. There are currently - over 70 million domains and a much larger number of individual - addresses. DKIM seeks to preserve the positive aspects of the - current email infrastructure, such as the ability for anyone to - communicate with anyone else without introduction. + characterize the email identification problem. There are many + millions of domains and a much larger number of individual addresses. + DKIM seeks to preserve the positive aspects of the current email + infrastructure, such as the ability for anyone to communicate with + anyone else without introduction. 1.4. Simple Key Management DKIM differs from traditional hierarchical public-key systems in that no Certificate Authority infrastructure is required; the verifier requests the public key from a repository in the domain of the claimed signer directly rather than from a third party. The DNS is proposed as the initial mechanism for the public keys. Thus, DKIM currently depends on DNS administration and the security @@ -325,27 +326,27 @@ of it. For DKIM processing, the domain name portion of the AUID has only basic domain name semantics; any possible owner-specific semantics are outside the scope of DKIM. It is specified in Section 3.5. Note that acceptable values for the AUID may be constrained via a flag in the public key record. (See Section 3.6.1.) 2.7. Identity Assessor - A module that consumes DKIM's mandatory payload, which is the - responsible Signing Domain Identifier (SDID). The module is - dedicated to the assessment of the delivered identifier. Other DKIM - (and non-DKIM) values can also be delivered to this module as well as - to a more general message evaluation filtering engine. However, this - additional activity is outside the scope of the DKIM signature - specification. + An element in the mail system that consumes DKIM's payload, which is + the responsible Signing Domain Identifier (SDID). The Identity + Assessor is dedicated to the assessment of the delivered identifier. + Other DKIM (and non-DKIM) values can also be used by the Identity + Assessor (if they are available) to provide a more general message + evaluation filtering engine. However, this additional activity is + outside the scope of the DKIM signature specification. 2.8. Whitespace There are three forms of whitespace: o WSP represents simple whitespace, i.e., a space or a tab character (formal definition in [RFC5234]). o LWSP is linear whitespace, defined as WSP plus CRLF (formal definition in [RFC5234]). @@ -417,22 +418,20 @@ long lines; such whitespace is NOT part of the value, and MUST be removed before decoding. Use of characters not listed as "mail-safe" in [RFC2049] is NOT RECOMMENDED. ABNF: dkim-quoted-printable = *(FWS / hex-octet / dkim-safe-char) ; hex-octet is from RFC2045 dkim-safe-char = %x21-3A / %x3C / %x3E-7E ; '!' - ':', '<', '>' - '~' - ; Characters not listed as "mail-safe" in - ; [RFC2049] are also NOT RECOMMENDED. INFORMATIVE NOTE: DKIM-Quoted-Printable differs from Quoted- Printable as defined in [RFC2045] in several important ways: 1. Whitespace in the input text, including CR and LF, must be encoded. [RFC2045] does not require such encoding, and does not permit encoding of CR or LF characters that are part of a CRLF line break. 2. Whitespace in the encoded text is ignored. This is to allow @@ -488,21 +487,21 @@ implementation, this can be used to allow delegation of a portion of the selector namespace. ABNF: selector = sub-domain *( "." sub-domain ) The number of public keys and corresponding selectors for each domain is 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 + distributed organizations can 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 replace public keys on a routine basis. If a domain wishes to change from using a public key associated with selector "january2005" to a public key associated with selector "february2005", it merely makes sure that both public keys are advertised in the public-key repository concurrently for the transition period during which email may be in transit prior to verification. At the start of the transition period, the outbound @@ -543,28 +542,28 @@ Values are a series of strings containing either plain text, "base64" text (as defined in [RFC2045], Section 6.8), "qp-section" (ibid, Section 6.7), or "dkim-quoted-printable" (as defined in Section 2.11). The name of the tag will determine the encoding of each value. Unencoded semicolon (";") characters MUST NOT occur in the tag value, since that separates tag-specs. INFORMATIVE IMPLEMENTATION NOTE: Although the "plain text" defined below (as "tag-value") only includes 7-bit characters, an implementation that wished to anticipate future standards would be - advised not to preclude the use of UTF8-encoded text in tag=value - lists. + advised not to preclude the use of UTF8-encoded ([RFC3629]) text + in tag=value lists. Formally, the ABNF syntax rules are as follows: - tag-list = tag-spec 0*( ";" tag-spec ) [ ";" ] + tag-list = tag-spec *( ";" tag-spec ) [ ";" ] tag-spec = [FWS] tag-name [FWS] "=" [FWS] tag-value [FWS] - tag-name = ALPHA 0*ALNUMPUNC - tag-value = [ tval 0*( 1*(WSP / FWS) tval ) ] + tag-name = ALPHA *ALNUMPUNC + tag-value = [ tval *( 1*(WSP / FWS) tval ) ] ; Prohibits WSP and FWS at beginning and end tval = 1*VALCHAR VALCHAR = %x21-3A / %x3C-7E ; EXCLAMATION to TILDE except SEMICOLON ALNUMPUNC = ALPHA / DIGIT / "_" Note that WSP is allowed anywhere around tags. In particular, any WSP after the "=" and any WSP before the terminating ";" is not part of the value; however, WSP inside the value is significant. @@ -627,22 +626,22 @@ off-line attacks, signers MUST use RSA keys of at least 1024 bits for long-lived keys. Verifiers MUST be able to validate signatures with keys ranging from 512 bits to 2048 bits, and they MAY be able to validate signatures with larger keys. Verifier policies may use the length of the signing key as one metric for determining whether a signature is acceptable. Factors that should influence the key size choice include the following: - o The practical constraint that large (e.g., 4096 bit) keys may not - fit within a 512-byte DNS UDP response packet + o The practical constraint that large (e.g., 4096 bit) keys might + not fit within a 512-byte DNS UDP response packet o The security constraint that keys smaller than 1024 bits are subject to off-line attacks 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 @@ -728,21 +727,21 @@ separating the header field name from the header field value. The colon separator MUST be retained. 3.4.3. The "simple" Body Canonicalization Algorithm The "simple" body canonicalization algorithm ignores all empty lines at the end of the message body. An empty line is a line of zero length after removal of the line terminator. If there is no body or no trailing CRLF on the message body, a CRLF is added. It makes no other changes to the message body. In more formal terms, the - "simple" body canonicalization algorithm converts "0*CRLF" at the end + "simple" body canonicalization algorithm converts "*CRLF" at the end of the body to a single "CRLF". Note that a completely empty or missing body is canonicalized as a single "CRLF"; that is, the canonicalized length will be 2 octets. The SHA-1 value (in base64) for an empty body (canonicalized to a "CRLF") is: uoq1oCgLlTqpdDX/iUbLy7J1Wic= The SHA-256 value is: frcCV1k9oG9oKj3dpUqdJg1PxRT2RSN/XKdLCPjaYaY= @@ -764,64 +763,22 @@ line" is defined in Section 3.4.3. If the body is non-empty, but does not end with a CRLF, a CRLF is added. (For email, this is only possible when using extensions to SMTP or non-SMTP transport mechanisms.) The SHA-1 value (in base64) for an empty body (canonicalized to a null input) is: 2jmj7l5rSw0yVb/vlWAYkK/YBwk= The SHA-256 value is: 47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU= - INFORMATIVE NOTE: It should be noted that the relaxed body - canonicalization algorithm may enable certain types of extremely - crude "ASCII Art" attacks where a message may be conveyed by - adjusting the spacing between words. If this is a concern, the - "simple" body canonicalization algorithm should be used instead. - -3.4.5. Body Length Limits - A body length count MAY be specified to limit the signature - calculation to an initial prefix of the body text, measured in - octets. If the body length count is not specified, the entire - message body is signed. - - INFORMATIVE RATIONALE: This capability is provided because it is - very common for mailing lists to add trailers to messages (e.g., - instructions how to get off the list). Until those messages are - also signed, the body length count is a useful tool for the - verifier since it may as a matter of policy accept messages having - valid signatures with extraneous data. - - INFORMATIVE IMPLEMENTATION NOTE: Using body length limits enables - an attack in which an attacker modifies a message to include - content that solely benefits the attacker. It is possible for the - appended content to completely replace the original content in the - end recipient's eyes, such as via alterations to the MIME - structure or exploiting lax HTML parsing in the MUA, and to defeat - duplicate message detection algorithms. To avoid this attack, - signers should be wary of using this tag, and verifiers might wish - to ignore the tag, perhaps based on other criteria. - - The body length count allows the signer of a message to permit data - to be appended to the end of the body of a signed message. The body - length count MUST be calculated following the canonicalization - algorithm; for example, any whitespace ignored by a canonicalization - algorithm is not included as part of the body length count. - - A body length count of zero means that the body is completely - unsigned. - - Signers wishing to ensure that no modification of any sort can occur - should specify the "simple" canonicalization algorithm for both - header and body and omit the body length count. - -3.4.6. Canonicalization Examples (INFORMATIVE) +3.4.5. Canonicalization Examples (INFORMATIVE) In the following examples, actual whitespace is used only for clarity. The actual input and output text is designated using bracketed descriptors: "" for a space character, "" for a tab character, and "" for a carriage-return/line-feed sequence. For example, "X Y" and "XY" represent the same three characters. Example 1: A message reading: A: X @@ -888,29 +844,28 @@ The encodings for each field type are listed below. Tags described as qp-section are encoded as described in Section 6.7 of MIME Part One [RFC2045], with the additional conversion of semicolon characters to "=3B"; intuitively, this is one line of quoted-printable encoded text. The dkim-quoted-printable syntax is defined in Section 2.11. Tags on the DKIM-Signature header field along with their type and requirement status are shown below. Unrecognized tags MUST be ignored. - v= Version (MUST be included). This tag defines the version of this - specification that applies to the signature record. It MUST have - the value "1". Note that verifiers must do a string comparison on - this value; for example, "1" is not the same as "1.0". + v= Version (plain-text; REQUIRED). This tag defines the version of + this specification that applies to the signature record. It MUST + have the value "1" for implementations compliant with this version + of DKIM. ABNF: - sig-v-tag = %x76 [FWS] "=" [FWS] "1" - + sig-v-tag = %x76 [FWS] "=" [FWS] 1*DIGIT INFORMATIVE NOTE: DKIM-Signature version numbers may increase arithmetically as new versions of this specification are released. a= The algorithm used to generate the signature (plain-text; REQUIRED). Verifiers MUST support "rsa-sha1" and "rsa-sha256"; signers SHOULD sign using "rsa-sha256". See Section 3.3 for a description of the algorithms. ABNF: @@ -986,46 +941,47 @@ h= Signed header fields (plain-text, but see description; REQUIRED). A colon-separated list of header field names that identify the header fields presented to the signing algorithm. The field MUST contain the complete list of header fields in the order presented to the signing algorithm. The field MAY contain names of header fields that do not exist when signed; nonexistent header fields do not contribute to the signature computation (that is, they are treated as the null input, including the header field name, the separating colon, the header field value, and any CRLF - terminator). The field MUST NOT include the DKIM-Signature header - field that is being created or verified, but may include others. - Folding whitespace (FWS) MAY be included on either side of the - colon separator. Header field names MUST be compared against - actual header field names in a case-insensitive manner. This list - MUST NOT be empty. See Section 5.4 for a discussion of choosing - header fields to sign. + terminator). The field MAY contain multiple instances of a header + field name, meaning multiple occurrences of the corresponding + header field are included in the header hash. The field MUST NOT + include the DKIM-Signature header field that is being created or + verified, but may include others. Folding whitespace (FWS) MAY be + included on either side of the colon separator. Header field + names MUST be compared against actual header field names in a + case-insensitive manner. This list MUST NOT be empty. See + Section 5.4 for a discussion of choosing header fields to sign, + and Section 5.4.2 for requirements when signing multiple instances + of a single field. ABNF: sig-h-tag = %x68 [FWS] "=" [FWS] hdr-name - 0*( [FWS] ":" [FWS] hdr-name ) + *( [FWS] ":" [FWS] hdr-name ) INFORMATIVE EXPLANATION: By "signing" header fields that do not actually exist, a signer can allow a verifier to detect insertion of those header fields after signing. However, since a signer cannot possibly know what header fields might be - created in the future, and that some MUAs might present header - fields that are embedded inside a message (e.g., as a message/ - rfc822 content type), the security of this solution is not - total. + defined in the future, this mechanism can't be used to prevent + the addition of any possible unknown header fields. - INFORMATIVE EXPLANATION: The exclusion of the header field name - and colon as well as the header field value for non-existent - header fields allows detection of an attacker that inserts an - actual header field with a null value. + INFORMATIVE NOTE: "Signing" fields that are not present at the + time of signing not only prevents fields and values from being + added, but also prevents adding fields with no values. i= The Agent or User Identifier (AUID) on behalf of which the SDID is taking responsibility (dkim-quoted-printable; OPTIONAL, default is an empty Local-part followed by an "@" followed by the domain from the "d=" tag). The syntax is a standard email address where the Local-part MAY be omitted. The domain part of the address MUST be the same as, or a subdomain of, the value of the "d=" tag. @@ -1078,36 +1034,21 @@ what extent a typical end-user recipient can rely on any assurances that might be made by successful use of the "i=" options. l= Body length count (plain-text unsigned decimal integer; OPTIONAL, default is entire body). This tag informs the verifier of the number of octets in the body of the email after canonicalization included in the cryptographic hash, starting from 0 immediately following the CRLF preceding the body. This value MUST NOT be larger than the actual number of octets in the canonicalized - message body. - - INFORMATIVE IMPLEMENTATION WARNING: Use of the "l=" tag might - allow display of fraudulent content without appropriate warning - to end users. The "l=" tag is intended for increasing - signature robustness when sending to mailing lists that both - modify their content and do not sign their messages. However, - using the "l=" tag enables attacks in which an intermediary - with malicious intent modifies a message to include content - that solely benefits the attacker. It is possible for the - appended content to completely replace the original content in - the end recipient's eyes and to defeat duplicate message - detection algorithms. Examples are described in Security - Considerations (Section 8). To avoid this attack, signers - should be extremely wary of using this tag, and assessors might - wish to ignore signatures that use the tag. + message body. See further discussion in Section 8.2. INFORMATIVE NOTE: The value of the "l=" tag is constrained to 76 decimal digits. This constraint is not intended to predict the size of future messages or to require implementations to use an integer representation large enough to represent the maximum possible value, but is intended to remind the implementer to check the length of this and all other tags during verification and to test for integer overflow when decoding the value. Implementers may need to limit the actual value expressed to a value smaller than 10^76, e.g., to allow a @@ -1283,21 +1225,21 @@ allowing all algorithms). A colon-separated list of hash algorithms that might be used. Unrecognized algorithms MUST be ignored. Refer to Section 3.3 for a discussion of the hash algorithms implemented by Signers and Verifiers. The set of algorithms listed in this tag in each record is an operational choice made by the Signer. ABNF: key-h-tag = %x68 [FWS] "=" [FWS] key-h-tag-alg - 0*( [FWS] ":" [FWS] key-h-tag-alg ) + *( [FWS] ":" [FWS] key-h-tag-alg ) key-h-tag-alg = "sha1" / "sha256" / x-key-h-tag-alg x-key-h-tag-alg = hyphenated-word ; for future extension k= Key type (plain-text; OPTIONAL, default is "rsa"). Signers and verifiers MUST support the "rsa" key type. The "rsa" key type indicates that an ASN.1 DER-encoded [ITU-X660-1997] RSAPublicKey [RFC3447] (see Sections Section 3.1 and A.1.1) is being used in the "p=" tag. (Note: the "p=" tag further encodes the value using the base64 algorithm.) Unrecognized key types MUST be ignored. @@ -1349,21 +1292,21 @@ email electronic mail (not necessarily limited to SMTP) This tag is intended to constrain the use of keys for other purposes, should use of DKIM be defined by other services in the future. ABNF: key-s-tag = %x73 [FWS] "=" [FWS] key-s-tag-type - 0*( [FWS] ":" [FWS] key-s-tag-type ) + *( [FWS] ":" [FWS] key-s-tag-type ) key-s-tag-type = "email" / "*" / x-key-s-tag-type x-key-s-tag-type = hyphenated-word ; for future extension t= Flags, represented as a colon-separated list of names (plain- text; OPTIONAL, default is no flags set). Unrecognized flags MUST be ignored. The defined flags are as follows: y This domain is testing DKIM. Verifiers MUST NOT treat messages from signers in testing mode differently from unsigned email, even should the signature fail to verify. Verifiers MAY wish to track @@ -1371,21 +1314,21 @@ s Any DKIM-Signature header fields using the "i=" tag MUST have the same domain value on the right-hand side of the "@" in the "i=" tag and the value of the "d=" tag. That is, the "i=" domain MUST NOT be a subdomain of "d=". Use of this flag is RECOMMENDED unless subdomaining is required. ABNF: key-t-tag = %x74 [FWS] "=" [FWS] key-t-tag-flag - 0*( [FWS] ":" [FWS] key-t-tag-flag ) + *( [FWS] ":" [FWS] key-t-tag-flag ) key-t-tag-flag = "y" / "s" / x-key-t-tag-flag x-key-t-tag-flag = hyphenated-word ; for future extension 3.6.2. DNS Binding A binding using DNS TXT RRs as a key service is hereby defined. All implementations MUST support this binding. 3.6.2.1. Namespace @@ -1473,21 +1416,21 @@ With the exception of the canonicalization procedure described in Section 3.4, the DKIM signing process treats the body of messages as simply a string of octets. DKIM messages MAY be either in plain-text or in MIME format; no special treatment is afforded to MIME content. Message attachments in MIME format MUST be included in the content that is signed. More formally, pseudo-code for the signature algorithm is: body-hash = hash-alg (canon-body, l-param) - data-hash = hash-alg (h-headers, D-SIG, content-hash) + data-hash = hash-alg (h-headers, D-SIG, body-hash) signature = sig-alg (d-domain, selector, data-hash) where: body-hash: is the output from hashing the body, using hash-alg. hash-alg: is the hashing algorithm specified in the "a" parameter. canon-body: is a canonicalized representation of the body, @@ -1519,30 +1462,48 @@ selector: is the selector value specified in the "s" parameter. NOTE: Many digital signature APIs provide both hashing and application of the RSA private key using a single "sign()" primitive. When using such an API, the last two steps in the algorithm would probably be combined into a single call that would perform both the "a-hash-alg" and the "sig-alg". 3.8. Input Requirements - DKIM's design is predicated on valid input. Therefore, signers and - verifiers SHOULD take reasonable steps to ensure that the messages - they are processing are valid according to [RFC5322], [RFC2045], and - any other relevant message format standards. See Section 8.14 and - Section 8.15 for additional discussion and references. + A message that is not compliant with RFC5322, RFC2045 and RFC2047 can + be subject to attempts by intermediaries to correct or interpret such + content. See Section 8 of [RFC4409] for examples of changes that are + commonly made. Such "corrections" may invalidate DKIM signatures or + have other undesirable effects, including some that involve changes + to the way a message is presented to an end user. + + Accordingly, DKIM's design is predicated on valid input. Therefore, + signers and verifiers SHOULD take reasonable steps to ensure that the + messages they are processing are valid according to [RFC5322], + [RFC2045], and any other relevant message format standards. + + See Section 8.15 for additional discussion. 3.9. Output Requirements + The evaluation of each signature ends in one of three states, which + this document refers to as follows: + + SUCCESS: a successful verification + + PERMFAIL: a permanent, non-recoverable error such as a signature + verification failure + + TEMPFAIL: a temporary, recoverable error such as a DNS query timeout + For each signature that verifies successfully or produces a TEMPFAIL - result, the output of a DKIM verifier module MUST include the set of: + result, output of the DKIM algorithm MUST include the set of: o The domain name, taken from the "d=" signature tag; and o The result of the verification attempt for that signature. The output MAY include other signature properties or result meta- data, including PERMFAILed or otherwise ignored signatures, for use by modules that consume those results. See Section 6.1 for discussion of signature validation result codes. @@ -1605,88 +1566,76 @@ be strictly limited. In particular, it is not at all clear to what extent a typical end-user recipient can rely on any assurances that might be made by successful use of the SDID or AUID. 4. Semantics of Multiple Signatures 4.1. Example Scenarios There are many reasons why a message might have multiple signatures. - For example, a given signer might sign multiple times, perhaps with - different hashing or signing algorithms during a transition phase. - - INFORMATIVE EXAMPLE: Suppose SHA-256 is in the future found to be + For example, suppose SHA-256 is in the future found to be insufficiently strong, and DKIM usage transitions to SHA-1024. A - signer might immediately sign using the newer algorithm, but - continue to sign using the older algorithm for interoperability - with verifiers that had not yet upgraded. The signer would do - this by adding two DKIM-Signature header fields, one using each - algorithm. Older verifiers that did not recognize SHA-1024 as an - acceptable algorithm would skip that signature and use the older - algorithm; newer verifiers could use either signature at their - option, and all other things being equal might not even attempt to - verify the other signature. - - Similarly, a signer might sign a message including all headers and no - "l=" tag (to satisfy strict verifiers) and a second time with a - limited set of headers and an "l=" tag (in anticipation of possible - message modifications in route to other verifiers). Verifiers could - then choose which signature they preferred. + signer might immediately sign using the newer algorithm, but also + continue to sign using the older algorithm for interoperability with + verifiers that had not yet upgraded. The signer would do this by + adding two DKIM-Signature header fields, one using each algorithm. + Older verifiers that did not recognize SHA-1024 as an acceptable + algorithm would skip that signature and use the older algorithm; + newer verifiers could use either signature at their option, and all + other things being equal might not even attempt to verify the other + signature. - INFORMATIVE EXAMPLE: A verifier might receive a message with two - signatures, one covering more of the message than the other. If - the signature covering more of the message verified, then the - verifier could make one set of policy decisions; if that signature - failed but the signature covering less of the message verified, - the verifier might make a different set of policy decisions. + Similarly, a signer might sign a message including all header fields + and no "l=" tag (to satisfy strict verifiers) and a second time with + a limited set of header fields and an "l=" tag (in anticipation of + possible message modifications en route to other verifiers). + Verifiers could then choose which signature they preferred. Of course, a message might also have multiple signatures because it passed through multiple signers. A common case is expected to be that of a signed message that passes through a mailing list that also signs all messages. Assuming both of those signatures verify, a recipient might choose to accept the message if either of those signatures were known to come from trusted sources. - INFORMATIVE EXAMPLE: Recipients might choose to whitelist mailing - lists to which they have subscribed and that have acceptable anti- - abuse policies so as to accept messages sent to that list even - from unknown authors. They might also subscribe to less trusted - mailing lists (e.g., those without anti-abuse protection) and be - willing to accept all messages from specific authors, but insist - on doing additional abuse scanning for other messages. + In particular, recipients might choose to whitelist mailing lists to + which they have subscribed and that have acceptable anti-abuse + policies so as to accept messages sent to that list even from unknown + authors. They might also subscribe to less trusted mailing lists + (e.g., those without anti-abuse protection) and be willing to accept + all messages from specific authors, but insist on doing additional + abuse scanning for other messages. Another related example of multiple signers might be forwarding services, such as those commonly associated with academic alumni - sites. - - INFORMATIVE EXAMPLE: A recipient might have an address at + sites. For example, a recipient might have an address at members.example.org, a site that has anti-abuse protection that is somewhat less effective than the recipient would prefer. Such a - recipient might have specific authors whose messages would be - trusted absolutely, but messages from unknown authors that had - passed the forwarder's scrutiny would have only medium trust. + recipient might have specific authors whose messages would be trusted + absolutely, but messages from unknown authors that had passed the + forwarder's scrutiny would have only medium trust. 4.2. Interpretation A signer that is adding a signature to a message merely creates a new DKIM-Signature header, using the usual semantics of the h= option. A signer MAY sign previously existing DKIM-Signature header fields using the method described in Section 5.4 to sign trace header fields. - INFORMATIVE NOTE: Signers should be cognizant that signing DKIM- - Signature header fields may result in signature failures with - intermediaries that do not recognize that DKIM-Signature header - fields are trace header fields and unwittingly reorder them, thus - breaking such signatures. For this reason, signing existing DKIM- - Signature header fields is unadvised, albeit legal. + Note that signers should be cognizant that signing DKIM-Signature + header fields may result in signature failures with intermediaries + that do not recognize that DKIM-Signature header fields are trace + header fields and unwittingly reorder them, thus breaking such + signatures. For this reason, signing existing DKIM-Signature header + fields is unadvised, albeit legal. INFORMATIVE NOTE: If a header field with multiple instances is signed, those header fields are always signed from the bottom up. Thus, it is not possible to sign only specific DKIM-Signature header fields. For example, if the message being signed already contains three DKIM-Signature header fields A, B, and C, it is possible to sign all of them, B and C only, or C only, but not A only, B only, A and B only, or A and C only. A signer MAY add more than one DKIM-Signature header field using @@ -1729,26 +1678,26 @@ The following steps are performed in order by signers. 5.1. Determine Whether the Email Should Be Signed and by Whom A signer can obviously only sign email for domains for which it has a private key and the necessary knowledge of the corresponding public key and selector information. However, there are a number of other reasons beyond the lack of a private key why a signer could choose not to sign an email. - INFORMATIVE NOTE: Signing modules may be incorporated into any + INFORMATIVE NOTE: A signer can be implemented as part of any portion of the mail system as deemed appropriate, including an MUA, a SUBMISSION server, or an MTA. Wherever implemented, signers should beware of signing (and thereby asserting responsibility for) messages that may be problematic. In - particular, within a trusted enclave the signing address might be + particular, within a trusted enclave the signing domain might be derived from the header according to local policy; SUBMISSION servers might only sign messages from users that are properly authenticated and authorized. INFORMATIVE IMPLEMENTER ADVICE: SUBMISSION servers should not sign Received header fields if the outgoing gateway MTA obfuscates Received header fields, for example, to hide the details of internal topology. If an email cannot be signed for some reason, it is a local policy @@ -1760,60 +1709,84 @@ choose which private 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. Distribution and management of private keys is also outside the scope of this document. INFORMATIVE OPERATIONS ADVICE: A signer should not sign with a private key when the selector containing the corresponding public key is expected to be revoked or removed before the verifier has an opportunity to validate the signature. The signer should - anticipate that verifiers may choose to defer validation, perhaps + anticipate that verifiers can choose to defer validation, perhaps until the message is actually read by the final recipient. In particular, when rotating to a new key pair, signing should immediately commence with the new private key and the old public key should be retained for a reasonable validation interval before being removed from the key server. 5.3. Normalize the Message to Prevent Transport Conversions Some messages, particularly those using 8-bit characters, are subject to modification during transit, notably conversion to 7-bit form. Such conversions will break DKIM signatures. In order to minimize the chances of such breakage, signers SHOULD convert the message to a suitable MIME content transfer encoding such as quoted-printable or base64 as described in [RFC2045] before signing. Such conversion is outside the scope of DKIM; the actual message SHOULD be converted to 7-bit MIME by an MUA or MSA prior to presentation to the DKIM algorithm. - Similarly, a message that is not compliant with RFC5322, RFC2045 and - RFC2047 can be subject to attempts by intermediaries to correct or - interpret such content. See Section 8 of [RFC4409] for examples of - changes that are commonly made. Such "corrections" may break DKIM - signatures or have other undesirable effects. Therefore, a verifier - SHOULD NOT validate a message that is not compliant with those - specifications. - If the message is submitted to the signer with any local encoding that will be modified before transmission, that modification to canonical [RFC5322] form MUST be done before signing. In particular, bare CR or LF characters (used by some systems as a local line separator convention) MUST be converted to the SMTP-standard CRLF sequence before the message is signed. Any conversion of this sort SHOULD be applied to the message actually sent to the recipient(s), not just to the version presented to the signing algorithm. More generally, the signer MUST sign the message as it is expected to be received by the verifier rather than in some local or internal form. +5.3.1. Body Length Limits + + A body length count MAY be specified to limit the signature + calculation to an initial prefix of the body text, measured in + octets. If the body length count is not specified, the entire + message body is signed. + + INFORMATIVE RATIONALE: This capability is provided because it is + very common for mailing lists to add trailers to messages (e.g., + instructions how to get off the list). Until those messages are + also signed, the body length count is a useful tool for the + verifier since it may as a matter of policy accept messages having + valid signatures with extraneous data. + + The length actually hashed should be inserted in the "l=" tag of the + DKIM-Signature header field. (See Section 3.5.) + + The body length count allows the signer of a message to permit data + to be appended to the end of the body of a signed message. The body + length count MUST be calculated following the canonicalization + algorithm; for example, any whitespace ignored by a canonicalization + algorithm is not included as part of the body length count. + + A body length count of zero means that the body is completely + unsigned. + + Signers wishing to ensure that no modification of any sort can occur + should specify the "simple" canonicalization algorithm for both + header and body and omit the body length count. + + See Section 8.2 for further discussion. + 5.4. Determine the Header Fields to Sign The From header field MUST be signed (that is, included in the "h=" tag of the resulting DKIM-Signature header field). Signers SHOULD NOT sign an existing header field likely to be legitimately modified or removed in transit. In particular, [RFC5321] explicitly permits modification or removal of the Return-Path header field in transit. Signers MAY include any other header fields present at the time of signing at the discretion of the signer. @@ -1847,68 +1820,47 @@ INFORMATIVE NOTE: A header field name need only be listed once more than the actual number of that header field in a message at the time of signing in order to prevent any further additions. For example, if there is a single Comments header field at the time of signing, listing Comments twice in the "h=" tag is sufficient to prevent any number of Comments header fields from being appended; it is not necessary (but is legal) to list Comments three or more times in the "h=" tag. - Signers choosing to sign an existing header field that occurs more - than once in the message (such as Received) MUST sign the physically - last instance of that header field in the header block. Signers - wishing to sign multiple instances of such a header field MUST - include the header field name multiple times in the h= tag of the - DKIM-Signature header field, and MUST sign such header fields in - order from the bottom of the header field block to the top. The - signer MAY include more instances of a header field name in h= than - there are actual corresponding header fields to indicate that - additional header fields of that name SHOULD NOT be added. - - INFORMATIVE EXAMPLE: - - If the signer wishes to sign two existing Received header fields, - and the existing header contains: - Received: - Received: - Received: - - then the resulting DKIM-Signature header field should read: - - DKIM-Signature: ... h=Received : Received :... - and Received header fields and will be signed in that - order. + Refer to Section 5.4.2 for a discussion of the procedure to be + followed when canonicalizing a header with more than one instance of + a particular header field name. - Signers should be careful of signing header fields that might have + Signers need to be careful of signing header fields that might have additional instances added later in the delivery process, since such header fields might be inserted after the signed instance or otherwise reordered. Trace header fields (such as Received) and Resent-* blocks are the only fields prohibited by [RFC5322] from being reordered. In particular, since DKIM-Signature header fields may be reordered by some intermediate MTAs, signing existing DKIM- Signature header fields is error-prone. - INFORMATIVE ADMONITION: Despite the fact that [RFC5322] permits - header fields to be reordered (with the exception of Received - header fields), reordering of signed header fields with multiple - instances by intermediate MTAs will cause DKIM signatures to be - broken; such anti-social behavior should be avoided. + INFORMATIVE ADMONITION: Despite the fact that [RFC5322] does not + prohibit the reordering of header fields, reordering of signed + header fields with multiple instances by intermediate MTAs will + cause DKIM signatures to be broken; such anti-social behavior + should be avoided. INFORMATIVE IMPLEMENTER'S NOTE: Although not required by this specification, all end-user visible header fields should be signed to avoid possible "indirect spamming". For example, if the Subject header field is not signed, a spammer can resend a previously signed mail, replacing the legitimate subject with a one-line spam. -5.5. Recommended Signature Content +5.4.1. Recommended Signature Content The purpose of the DKIM cryptographic algorithm is to affix an identifier to the message in a way that is both robust against normal transit-related changes and resistant to kinds of replay attacks. An essential aspect of satisfying these requirements is choosing what header fields to include in the hash and what fields to exclude. The basic rule for choosing fields to include is to select those fields that constitute the "core" of the message content. Hence, any replay attack will have to include these in order to have the @@ -1975,49 +1927,72 @@ Signers SHOULD choose canonicalization algorithms based on the types of messages they process and their aversion to risk. For example, e-commerce sites sending primarily purchase receipts, which are not expected to be processed by mailing lists or other software likely to modify messages, will generally prefer "simple" canonicalization. Sites sending primarily person-to-person email will likely prefer to be more resilient to modification during transport by using "relaxed" canonicalization. - Signers SHOULD NOT use "l=" unless they intend to accommodate - intermediate mail processors that append text to a message. For - example, many mailing list processors append "unsubscribe" - information to message bodies. If signers use "l=", they SHOULD - include the entire message body existing at the time of signing in - computing the count. In particular, signers SHOULD NOT specify a - body length of 0 since this may be interpreted as a meaningless - signature by some verifiers. + Unless mail is processed through intermediaries, such as mailing + lists that might add "unsubscribe" instructions to the bottom of the + message body, the "l=" tag is likely to convey no additional benefit + while providing an avenue for unauthorized addition of text to a + message. The use of "l=0" takes this to the extreme, allowing + complete alteration of the text of the message without invalidating + the signature. Moreover, a verifier would be within its rights to + consider a partly-signed message body as unacceptable. Judicious use + is advised. -5.6. Compute the Message Hash and Signature +5.4.2. Signatures Involving Multiple Instances of a Field + + Signers choosing to sign an existing header field that occurs more + than once in the message (such as Received) MUST sign the physically + last instance of that header field in the header block. Signers + wishing to sign multiple instances of such a header field MUST + include the header field name multiple times in the h= tag of the + DKIM-Signature header field, and MUST sign such header fields in + order from the bottom of the header field block to the top. The + signer MAY include more instances of a header field name in h= than + there are actual corresponding header fields to indicate that + additional header fields of that name SHOULD NOT be added. + + INFORMATIVE EXAMPLE: + + If the signer wishes to sign two existing Received header fields, + and the existing header contains: + Received: + Received: + Received: + + then the resulting DKIM-Signature header field should read: + + DKIM-Signature: ... h=Received : Received :... + and Received header fields and will be signed in that + order. + +5.5. Compute the Message Hash and Signature The signer MUST compute the message hash as described in Section 3.7 and then sign it using the selected public-key algorithm. This will result in a DKIM-Signature header field that will include the body hash and a signature of the header hash, where that header includes the DKIM-Signature header field itself. Entities such as mailing list managers that implement DKIM and that modify the message or a header field (for example, inserting unsubscribe information) before retransmitting the message SHOULD check any existing signature on input and MUST make such modifications before re-signing the message. - The signer MAY elect to limit the number of bytes of the body that - will be included in the hash and hence signed. The length actually - hashed should be inserted in the "l=" tag of the DKIM-Signature - header field. - -5.7. Insert the DKIM-Signature Header Field +5.6. Insert the DKIM-Signature Header Field Finally, the signer MUST insert the DKIM-Signature header field created in the previous step prior to transmitting the email. The DKIM-Signature header field MUST be the same as used to compute the hash as described above, except that the value of the "b=" tag MUST be the appropriately signed hash computed in the previous step, signed using the algorithm specified in the "a=" tag of the DKIM- Signature header field and using the private key corresponding to the selector given in the "s=" tag of the DKIM-Signature header field, as chosen above in Section 5.2 @@ -2069,39 +2043,31 @@ multiple DKIM-Signature header fields. In particular, there is reason to believe that some relays will reorder the header fields in potentially arbitrary ways. INFORMATIVE IMPLEMENTATION NOTE: Verifiers might use the order as a clue to signing order in the absence of any other information. However, other clues as to the semantics of multiple signatures (such as correlating the signing host with Received header fields) might also be considered. - A verifier SHOULD NOT treat a message that has one or more bad - signatures and no good signatures differently from a message with no - signature at all; such treatment is a matter of local policy and is - beyond the scope of this document. + Survivability of signatures after transit is not guaranteed, and + signatures can fail to verify through no fault of the signer. + Therefore, a verifier SHOULD NOT treat a message that has one or more + bad signatures and no good signatures differently from a message with + no signature at all. When a signature successfully verifies, a verifier will either stop processing or attempt to verify any other signatures, at the discretion of the implementation. A verifier MAY limit the number of - signatures it tries to avoid denial-of-service attacks. - - INFORMATIVE NOTE: An attacker could send messages with large - numbers of faulty signatures, each of which would require a DNS - lookup and corresponding CPU time to verify the message. This - could be an attack on the domain that receives the message, by - slowing down the verifier by requiring it to do a large number of - DNS lookups and/or signature verifications. It could also be an - attack against the domains listed in the signatures, essentially - by enlisting innocent verifiers in launching an attack against the - DNS servers of the actual victim. + signatures it tries, in order to avoid denial-of-service attacks (see + Section 8.4 for further discussion). In the following description, text reading "return status (explanation)" (where "status" is one of "PERMFAIL" or "TEMPFAIL") means that the verifier MUST immediately cease processing that signature. The verifier SHOULD proceed to the next signature, if any is present, and completely ignore the bad signature. If the status is "PERMFAIL", the signature failed and should not be reconsidered. If the status is "TEMPFAIL", the signature could not be verified at this time but may be tried again later. A verifier MAY either arrange to defer the message for later processing, or try another @@ -2168,21 +2134,21 @@ MUST ignore the DKIM-Signature header field and return PERMFAIL (From field not signed). Verifiers MAY ignore the DKIM-Signature header field and return PERMFAIL (signature expired) if it contains an "x=" tag and the signature has expired. Verifiers MAY ignore the DKIM-Signature header field if the domain used by the signer in the "d=" tag is not associated with a valid signing entity. For example, signatures with "d=" values such as - "com" and "co.uk" may be ignored. The list of unacceptable domains + "com" and "co.uk" could be ignored. The list of unacceptable domains SHOULD be configurable. Verifiers MAY ignore the DKIM-Signature header field and return PERMFAIL (unacceptable signature header) for any other reason, for example, if the signature does not sign header fields that the verifier views to be essential. As a case in point, if MIME header fields are not signed, certain attacks may be possible that the verifier would prefer to avoid. 6.1.2. Get the Public Key @@ -2214,21 +2180,21 @@ 2. If the query for the public key fails to respond, the verifier MAY seek a later verification attempt by returning TEMPFAIL (key unavailable). 3. If the query for the public key fails because the corresponding key record does not exist, the verifier MUST immediately return PERMFAIL (no key for signature). 4. If the query for the public key returns multiple key records, the - verifier may choose one of the key records or may cycle through + verifier can choose one of the key records or may cycle through the key records performing the remainder of these steps on each record at the discretion of the implementer. The order of the key records is unspecified. If the verifier chooses to cycle through the key records, then the "return ..." wording in the remainder of this section means "try the next key record, if any; if none, return to try another signature in the usual way". 5. If the result returned from the query does not adhere to the format defined in this specification, the verifier MUST ignore the key record and return PERMFAIL (key syntax error). Verifiers @@ -2254,24 +2220,24 @@ PERMFAIL (inappropriate key algorithm). 6.1.3. Compute the Verification Given a signer and a public key, verifying a signature consists of actions semantically equivalent to the following steps. 1. Based on the algorithm defined in the "c=" tag, the body length specified in the "l=" tag, and the header field names in the "h=" tag, prepare a canonicalized version of the message as is - described in Section 3.7 (note that this version does not - actually need to be instantiated). When matching header field - names in the "h=" tag against the actual message header field, - comparisons MUST be case-insensitive. + described in Section 3.7 (note that this canonicalized version + does not actually replace the original content). When matching + header field names in the "h=" tag against the actual message + header field, comparisons MUST be case-insensitive. 2. Based on the algorithm indicated in the "a=" tag, compute the message hashes from the canonical copy as described in Section 3.7. 3. Verify that the hash of the canonicalized message body computed in the previous step matches the hash value conveyed in the "bh=" tag. If the hash does not match, the verifier SHOULD ignore the signature and return PERMFAIL (body hash did not verify). @@ -2289,23 +2255,26 @@ calculated. Implementations may also verify the signature on the message header before validating that the message hash listed in the "bh=" tag in the DKIM-Signature header field matches that of the actual message body; however, if the body hash does not match, the entire signature must be considered to have failed. A body length specified in the "l=" tag of the signature limits the number of bytes of the body passed to the verification algorithm. All data beyond that limit is not validated by DKIM. Hence, verifiers might treat a message that contains bytes beyond the - indicated body length with suspicion, such as by declaring the - signature invalid (e.g., by returning PERMFAIL (unsigned content)), - or conveying the partial verification to the policy module. + indicated body length with suspicion, and can choose to treat the + signature as if it were invalid (e.g., by returning PERMFAIL + (unsigned content)). + + Should the algorithm reach this point, the verification has + succeeded, and DKIM reports SUCCESS for this signature. 6.2. Communicate Verification Results Verifiers wishing to communicate the results of verification to other parts of the mail system may do so in whatever manner they see fit. For example, implementations might choose to add an email header field to the message before passing it on. Any such header field SHOULD be inserted before any existing DKIM-Signature or preexisting authentication status header fields in the header field block. The Authentication-Results: header field ([RFC5451]) MAY be used for this @@ -2366,40 +2335,47 @@ 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 overzealous filter? For diagnostic purposes, the exact reason why the verification fails SHOULD be made available and possibly recorded in the system logs. If the email cannot be verified, then it SHOULD be treated the same as all unverified email regardless of whether or not it looks like it was signed. - See Section 8.14 and Section 8.15 for additional discussion and - references. + See Section 8.15 for additional discussion. 7. IANA Considerations DKIM has registered namespaces with IANA. In all cases, new values are assigned only for values that have been documented in a published RFC that has IETF Consensus [RFC5226]. This memo updates these registries as described below. Of note is the addition of a new "status" column. All registrations into these namespaces MUST include the name being registered, the document in which it was registered or updated, and an indication of its current status which MUST be one of "active" (in current use) or "historic" (no longer in current use). No new tags are defined in this specification compared to [RFC4871], - but one has been obsoleted. + but one has been designated as "historic". -7.1. DKIM-Signature Tag Specifications + Also, the Email Authentication Methods Registry is revised to refer + to this update. + +7.1. Email Authentication Methods Registry + + The Email Authentication Methods registry is updated to indicate that + "dkim" is defined in this memo. + +7.2. DKIM-Signature Tag Specifications A DKIM-Signature provides for a list of tag specifications. IANA has established the DKIM-Signature Tag Specification Registry for tag specifications that can be used in DKIM-Signature fields. The updated entries in the registry comprise: +------+-----------------+--------+ | TYPE | REFERENCE | STATUS | +------+-----------------+--------+ @@ -2414,40 +2390,40 @@ | l | (this document) | active | | q | (this document) | active | | s | (this document) | active | | t | (this document) | active | | x | (this document) | active | | z | (this document) | active | +------+-----------------+--------+ Table 1: DKIM-Signature Tag Specification Registry Updated Values -7.2. DKIM-Signature Query Method Registry +7.3. DKIM-Signature Query Method Registry The "q=" tag-spec (specified in Section 3.5) provides for a list of query methods. IANA has established the DKIM-Signature Query Method Registry for mechanisms that can be used to retrieve the key that will permit validation processing of a message signed using DKIM. The updated entry in the registry comprises: +------+--------+-----------------+--------+ | TYPE | OPTION | REFERENCE | STATUS | +------+--------+-----------------+--------+ | dns | txt | (this document) | active | +------+--------+-----------------+--------+ DKIM-Signature Query Method Registry Updated Values -7.3. DKIM-Signature Canonicalization Registry +7.4. DKIM-Signature Canonicalization Registry The "c=" tag-spec (specified in Section 3.5) provides for a specifier for canonicalization algorithms for the header and body of the message. IANA has established the DKIM-Signature Canonicalization Algorithm Registry for algorithms for converting a message into a canonical form before signing or verifying using DKIM. The updated entries in the header registry comprise: @@ -2467,21 +2443,21 @@ +---------+-----------------+--------+ | TYPE | REFERENCE | STATUS | +---------+-----------------+--------+ | simple | (this document) | active | | relaxed | (this document) | active | +---------+-----------------+--------+ DKIM-Signature Body Canonicalization Algorithm Registry Updated Values -7.4. _domainkey DNS TXT Resource Record Tag Specifications +7.5. _domainkey DNS TXT Resource Record Tag Specifications A _domainkey DNS TXT RR provides for a list of tag specifications. IANA has established the DKIM _domainkey DNS TXT Tag Specification Registry for tag specifications that can be used in DNS TXT resource records. The updated entries in the registry comprise: +------+-----------------+----------+ | TYPE | REFERENCE | STATUS | @@ -2492,174 +2468,178 @@ | k | (this document) | active | | n | (this document) | active | | p | (this document) | active | | s | (this document) | active | | t | (this document) | active | +------+-----------------+----------+ DKIM _domainkey DNS TXT Tag Specification Registry Updated Values -7.5. DKIM Key Type Registry +7.6. DKIM Key Type Registry The "k=" (specified in Section 3.6.1) and the "a=" (specified in Section 3.5) tags provide for a list of mechanisms that can be used to decode a DKIM signature. IANA has established the DKIM Key Type Registry for such mechanisms. The updated entry in the registry comprises: +------+-----------+--------+ | TYPE | REFERENCE | STATUS | +------+-----------+--------+ | rsa | [RFC3447] | active | +------+-----------+--------+ DKIM Key Type Updated Values -7.6. DKIM Hash Algorithms Registry +7.7. DKIM Hash Algorithms Registry The "h=" (specified in Section 3.6.1) and the "a=" (specified in Section 3.5) tags provide for a list of mechanisms that can be used to produce a digest of message data. IANA has established the DKIM Hash Algorithms Registry for such mechanisms. The updated entries in the registry comprise: +--------+-------------------+--------+ | TYPE | REFERENCE | STATUS | +--------+-------------------+--------+ | sha1 | [FIPS-180-3-2008] | active | | sha256 | [FIPS-180-3-2008] | active | +--------+-------------------+--------+ DKIM Hash Algorithms Updated Values -7.7. DKIM Service Types Registry +7.8. DKIM Service Types Registry The "s=" tag (specified in Section 3.6.1) provides for a list of service types to which this selector may apply. IANA has established the DKIM Service Types Registry for service types. The updated entries in the registry comprise: +-------+-----------------+--------+ | TYPE | REFERENCE | STATUS | +-------+-----------------+--------+ | email | (this document) | active | | * | (this document) | active | +-------+-----------------+--------+ DKIM Service Types Registry Updated Values -7.8. DKIM Selector Flags Registry +7.9. DKIM Selector Flags Registry The "t=" tag (specified in Section 3.6.1) provides for a list of flags to modify interpretation of the selector. IANA has established the DKIM Selector Flags Registry for additional flags. The updated entries in the registry comprise: +------+-----------------+--------+ | TYPE | REFERENCE | STATUS | +------+-----------------+--------+ | y | (this document) | active | | s | (this document) | active | +------+-----------------+--------+ DKIM Selector Flags Registry Updated Values -7.9. DKIM-Signature Header Field +7.10. DKIM-Signature Header Field IANA has added DKIM-Signature to the "Permanent Message Header Fields" registry (see [RFC3864]) for the "mail" protocol, using this document as the reference. 8. Security Considerations It has been observed that any mechanism that is introduced that attempts to stem the flow of spam is subject to intensive attack. DKIM needs to be carefully scrutinized to identify potential attack vectors and the vulnerability to each. See also [RFC4686]. -8.1. Misuse of Body Length Limits ("l=" Tag) +8.1. ASCII Art Attacks - As noted in Section 3.4.5, use of the "l=" signature tag enables a - variety of attacks in which added content can partially or completely - change the recipient's view of the message. + The relaxed body canonicalization algorithm may enable certain types + of extremely crude "ASCII Art" attacks where a message may be + conveyed by adjusting the spacing between words. If this is a + concern, the "simple" body canonicalization algorithm should be used + instead. -8.2. Misappropriated Private Key +8.2. Misuse of Body Length Limits ("l=" Tag) - If the private key for a user is resident on their computer and is - not protected by an appropriately secure mechanism, it is possible - for malware to send mail as that user and any other user sharing the - same private key. The malware would not, however, be able to - generate signed spoofs of other signers' addresses, which would aid - in identification of the infected user and would limit the - possibilities for certain types of attacks involving socially - engineered messages. This threat applies mainly to MUA-based - implementations; protection of private keys on servers can be easily - achieved through the use of specialized cryptographic hardware. + Use of the "l=" tag might allow display of fraudulent content without + appropriate warning to end users. The "l=" tag is intended for + increasing signature robustness when sending to mailing lists that + both modify their content and do not sign their modified messages. + However, using the "l=" tag enables attacks in which an intermediary + with malicious intent modifies a message to include content that + solely benefits the attacker. It is possible for the appended + content to completely replace the original content in the end + recipient's eyes and to defeat duplicate message detection + algorithms. - A larger problem occurs if malware on many users' computers obtains - the private keys for those users and transmits them via a covert - channel to a site where they can be shared. The compromised users - would likely not know of the misappropriation until they receive - "bounce" messages from messages they are purported to have sent. - Many users might not understand the significance of these bounce - messages and would not take action. + An example of such an attack includes alterations to the MIME + structure or exploiting lax HTML parsing in the MUA, and to defeat + duplicate message detection algorithms. - One countermeasure is to use a user-entered passphrase to encrypt the - private key, although users tend to choose weak passphrases and often - reuse them for different purposes, possibly allowing an attack - against DKIM to be extended into other domains. Nevertheless, the - decoded private key might be briefly available to compromise by - malware when it is entered, or might be discovered via keystroke - logging. The added complexity of entering a passphrase each time one - sends a message would also tend to discourage the use of a secure - passphrase. + To avoid this attack, signers should be extremely wary of using this + tag, and assessors might wish to ignore signatures that use the tag. - A somewhat more effective countermeasure is to send messages through - an outgoing MTA that can authenticate the submitter using existing +8.3. Misappropriated Private Key + + As with any other security application that uses private/public key + pairs, DKIM requires caution around the handling and protection of + keys. A compromised private key or access to one means an intruder + or malware can send mail signed by the domain that advertises the + matching public key. + + Thus, private keys issued to users, rather than one used by an ADMD + itself, create the usual problem of securing data stored on personal + resources that can affect the ADMD. + + A more secure architecture involves sending messages through an + outgoing MTA that can authenticate the submitter using existing techniques (e.g., SMTP Authentication), possibly validate the message itself (e.g., verify that the header is legitimate and that the content passes a spam content check), and sign the message using a key appropriate for the submitter address. Such an MTA can also apply controls on the volume of outgoing mail each user is permitted to originate in order to further limit the ability of malware to generate bulk email. -8.3. Key Server Denial-of-Service Attacks +8.4. Key Server Denial-of-Service Attacks Since the key servers are distributed (potentially separate for each domain), the number of servers that would need to be attacked to defeat this mechanism on an Internet-wide basis is very large. Nevertheless, key servers for individual domains could be attacked, impeding the verification of messages from that domain. This is not significantly different from the ability of an attacker to deny service to the mail exchangers for a given domain, although it affects outgoing, not incoming, mail. - A variation on this attack is that if a very large amount of mail - were to be sent using spoofed addresses from a given domain, the key - servers for that domain could be overwhelmed with requests. However, - given the low overhead of verification compared with handling of the - email message itself, such an attack would be difficult to mount. + A variation on this attack involves a very large amount of mail being + sent using spoofed signatures from a given domain, the key servers + for that domain could be overwhelmed with requests in a denial-of- + service attack (see [RFC4732]). However, given the low overhead of + verification compared with handling of the email message itself, such + an attack would be difficult to mount. -8.4. Attacks Against the DNS +8.5. Attacks Against the DNS Since the DNS is a required binding for key services, specific attacks against the DNS must be considered. While the DNS is currently insecure [RFC3833], these security problems are the motivation behind DNS Security (DNSSEC) [RFC4033], and all users of the DNS will reap the benefit of that work. DKIM is only intended as a "sufficient" method of proving authenticity. It is not intended to provide strong cryptographic @@ -2675,210 +2655,172 @@ A specific DNS security issue that should be considered by DKIM verifiers is the name chaining attack described in Section 2.3 of [RFC3833]. A DKIM verifier, while verifying a DKIM-Signature header field, could be prompted to retrieve a key record of an attacker's choosing. This threat can be minimized by ensuring that name servers, including recursive name servers, used by the verifier enforce strict checking of "glue" and other additional information in DNS responses and are therefore not vulnerable to this attack. -8.5. Replay Attacks +8.6. Replay/Spam Attacks - In this attack, a spammer sends a message to be spammed to an - accomplice, which results in the message being signed by the - originating MTA. The accomplice resends the message, including the - original signature, to a large number of recipients, possibly by - sending the message to many compromised machines that act as MTAs. - The messages, not having been modified by the accomplice, have valid - signatures. + In this attack, a spammer sends a piece of spam through an MTA that + signs it, banking on the reputation of the signing domain (e.g., a + large popular mailbox provider) rather than its own, and then re- + sends that message to a large number of intended recipients. The + recipients observe the valid signature from the well-known domain, + elevating their trust in the message and increasing the likelihood of + delivery and presentation to the user. Partial solutions to this problem involve the use of reputation services to convey the fact that the specific email address is being used for spam and that messages from that signer are likely to be spam. This requires a real-time detection mechanism in order to react quickly enough. However, such measures might be prone to abuse, if for example an attacker resent a large number of messages received from a victim in order to make them appear to be a spammer. Large verifiers might be able to detect unusually large volumes of mails with the same signature in a short time period. Smaller verifiers can get substantially the same volume of information via existing collaborative systems. -8.6. Limits on Revoking Keys +8.7. Limits on Revoking Keys When a large domain detects undesirable behavior on the part of one of its users, it might wish to revoke the key used to sign that user's messages in order to disavow responsibility for messages that have not yet been verified or that are the subject of a replay attack. However, the ability of the domain to do so can be limited if the same key, for scalability reasons, is used to sign messages for many other users. Mechanisms for explicitly revoking keys on a per-address basis have been proposed but require further study as to their utility and the DNS load they represent. -8.7. Intentionally Malformed Key Records +8.8. Intentionally Malformed Key Records It is possible for an attacker to publish key records in DNS that are intentionally malformed, with the intent of causing a denial-of- service attack on a non-robust verifier implementation. The attacker could then cause a verifier to read the malformed key record by sending a message to one of its users referencing the malformed record in a (not necessarily valid) signature. Verifiers MUST thoroughly verify all key records retrieved from the DNS and be robust against intentionally as well as unintentionally malformed key records. -8.8. Intentionally Malformed DKIM-Signature Header Fields +8.9. Intentionally Malformed DKIM-Signature Header Fields Verifiers MUST be prepared to receive messages with malformed DKIM- Signature header fields, and thoroughly verify the header field before depending on any of its contents. -8.9. Information Leakage +8.10. Information Leakage An attacker could determine when a particular signature was verified by using a per-message selector and then monitoring their DNS traffic for the key lookup. This would act as the equivalent of a "web bug" for verification time rather than when the message was read. -8.10. Remote Timing Attacks +8.11. Remote Timing Attacks In some cases it may be possible to extract private keys using a remote timing attack [BONEH03]. Implementations should consider obfuscating the timing to prevent such attacks. -8.11. Reordered Header Fields +8.12. Reordered Header Fields Existing standards allow intermediate MTAs to reorder header fields. If a signer signs two or more header fields of the same name, this can cause spurious verification errors on otherwise legitimate messages. In particular, signers that sign any existing DKIM- Signature fields run the risk of having messages incorrectly fail to verify. -8.12. RSA Attacks +8.13. RSA Attacks An attacker could create a large RSA signing key with a small exponent, thus requiring that the verification key have a large exponent. This will force verifiers to use considerable computing resources to verify the signature. Verifiers might avoid this attack by refusing to verify signatures that reference selectors with public keys having unreasonable exponents. In general, an attacker might try to overwhelm a verifier by flooding it with messages requiring verification. This is similar to other MTA denial-of-service attacks and should be dealt with in a similar fashion. -8.13. Inappropriate Signing by Parent Domains +8.14. Inappropriate Signing by Parent Domains The trust relationship described in Section 3.10 could conceivably be used by a parent domain to sign messages with identities in a subdomain not administratively related to the parent. For example, the ".com" registry could create messages with signatures using an "i=" value in the example.com domain. There is no general solution to this problem, since the administrative cut could occur anywhere in the domain name. For example, in the domain "example.podunk.ca.us" there are three administrative cuts (podunk.ca.us, ca.us, and us), any of which could create messages with an identity in the full domain. INFORMATIVE NOTE: This is considered an acceptable risk for the same reason that it is acceptable for domain delegation. For example, in the example above any of the domains could potentially simply delegate "example.podunk.ca.us" to a server of their choice and completely replace all DNS-served information. Note that a verifier MAY ignore signatures that come from an unlikely domain such as ".com", as discussed in Section 6.1.1. -8.14. Attacks Involving Addition of Header Fields - - Many email implementations do not enforce [RFC5322] with strictness. - As discussed in Section 5.3, DKIM processing is predicated on a valid - mail message as its input. However, DKIM implementers should be - aware of the potential effect of having loose enforcement by email - components interacting with DKIM modules. - - For example, a message with multiple From: header fields violates - Section 3.6 of [RFC5322]. With the intent of providing a better user - experience, many agents tolerate these violations and deliver the - message anyway. An MUA then might elect to render to the user the - value of the first, or "top", From: field. This may also be done - simply out of the expectation that there is only one, where a "find - first" algorithm would have the same result. Such code in an MUA can - be exploited to fool the user if it is also known that the other - From: field is the one checked by arriving message filters. Such is - the case with DKIM; although the From: field must be signed, a - malformed message bearing more than one From: field might only have - the first ("bottom") one signed, in an attempt to show the message - with some "DKIM passed" annotation while also rendering the From: - field that was not authenticated. (This can also be taken as a - demonstration that DKIM is not designed to support author - validation.) - - Note that the technique for using "h=...:from:from:...", described in - Section 8.15 below, is of no effect in the case of an attacker that - is also the signer. - - The From: field is used above to illustrate this issue, but it is - only one of several fields that Section 3.6 of [RFC5322] constrains - in this way. In reality any agent that forgives such malformations, - or is careless about identifying which parts of a message were - authenticated, is open to exploitation. +8.15. Attacks Involving Addition of Header Fields -8.15. Malformed Inputs + DKIM is able to sign and validate many types of messages that might + cause problems elsewhere in the message system. The message might + violate some part of [RFC5322], such as having multiple From: fields. + Equally, it might contain data that constitutes an attack on the + recipient, such as falsely indicating the name of the author. These + can represent serious attacks, but they have nothing to do with DKIM; + they are attacks on the recipient. - DKIM allows additional header fields to be added to a signed message - without breaking the signature. This tolerance can be abused, for - example in a replay attack or a man-in-the-middle attack. The attack - is accomplished by creating additional instances of header fields to - an already signed message, without breaking the signature. These - then might be displayed to the end user or are used as filtering - input. Applicable fields might include From: and Subject:. + Many email components, including MTAs, MSAs, MUAs and filtering + modules, implement message format checks only loosely. This is done + out of years of industry pressure to be liberal in what is accepted + into the mail stream for the sake of reducing support costs; + improperly formed messages are often silently fixed in transit or + even delivered unrepaired. - The resulting message violates section 3.6 of [RFC5322]. The way - such input will be handled and displayed by an MUA is unpredictable, - but it will commonly display the newly added header fields rather - than those that are part of the originally signed message alongside - some "valid DKIM signature" annotation. This might allow an attacker - to replay a previously sent, signed message with a different - Subject:, From: or To: field. + DKIM signs and validates the data it is told to and works correctly. + So in this case, DKIM has done its job of delivering a validated + domain (the "d=" value) and, given the semantics of a DKIM signature, + essentially the signer has taken some responsibility for a + problematic message. The verifier or receiver is able to act on this + information as needed, such as degrading the trust of the message + (or, indeed, of the signer). - However, [RFC5322] also tolerates obsolete message syntax, which does - allow things like multiple From: fields on messages. The - implementation of DKIM thus potentially creates a more stringent - layer of expectation regarding the meaning of an identity, while that - additional meaning is either obscured from or incorrectly presented - to an end user in this context. + An agent consuming DKIM results needs to be aware that the validity + of any header field, signed or otherwise, is not guaranteed by DKIM. - Implementers need to consider this possibility when designing their - input handling functions. Outright rejection of messages that - violate the relevant standards such as [RFC5322], [RFC2045], etc. - will interfere with delivery of legacy formats. Instead, given such - input, a signing module could return an error rather than generate a - signature; a verifying module might return a syntax error code or - arrange not to return a positive result even if the signature - technically validates. + At the same time, DKIM can aid in detecting addition of specific + fields in transit. This is done by having the signer list the field + name(s) in the "h=" tag an extra time (e.g., "h=from:from:..." for a + message with one From field), so that addition of an instance of that + field downstream will render the signature unable to be verified. - Senders concerned that their messages might be particularly - vulnerable to this sort of attack and who do not wish to rely on - receiver filtering of invalid messages can ensure that adding - additional header fields will break the DKIM signature by including - two copies of the header fields about which they are concerned in the - signature (e.g. "h= ... from:from:to:to:subject:subject ..."). See - Sections 3.5 and 5.4 for further discussion of this mechanism. + (See Section 3.5 for details.) This in essence is an explicit + indication that the signer does not wish to take any responsibility + for such a malformed message. - Specific validity rules for all known header fields can be gleaned - from the IANA "Permanent Header Field Registry" and the reference - documents it identifies. + Components of the mail system that perform loose enforcement of other + mail standards will need to revisit that posture when incorporating + DKIM, especially when considering matters of potential attacks on + receivers. 9. References 9.1. Normative References [FIPS-180-3-2008] U.S. Department of Commerce, "Secure Hash Standard", FIPS PUB 180-3, October 2008. [ITU-X660-1997] @@ -2923,20 +2865,23 @@ 9.2. Informative References [BONEH03] "Remote Timing Attacks are Practical", Proceedings 12th USENIX Security Symposium, 2003. [I-D.DKIM-MAILINGLISTS] Kucherawy, M., "DKIM And Mailing Lists", I-D draft-ietf-dkim-mailinglists, June 2011. + [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO + 10646", RFC 3629, June 2011. + [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For Public Keys Used For Exchanging Symmetric Keys", BCP 86, RFC 3766, April 2004. [RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain Name System (DNS)", RFC 3833, August 2004. [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, September 2004. @@ -2944,20 +2889,24 @@ [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, March 2005. [RFC4409] Gellens, R. and J. Klensin, "Message Submission for Mail", RFC 4409, April 2006. [RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys Identified Mail (DKIM)", RFC 4686, September 2006. + [RFC4732] Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet + Denial-of-Service Considerations", RFC 4732, + November 2006. + [RFC4870] Delany, M., "Domain-Based Email Authentication Using Public Keys Advertised in the DNS (DomainKeys)", RFC 4870, May 2007. [RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, "DomainKeys Identified Mail (DKIM) Signatures", RFC 4871, May 2007. [RFC4880] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer, "OpenPGP Message Format", RFC 4880, November 2007. @@ -3195,21 +3144,21 @@ the Sender field. This provides useful information to the receiving email site, which is able to correlate the signing domain with the initial submission email role. Receiving sites often wish to provide their end users with information about mail that is mediated in this fashion. Although the real efficacy of different approaches is a subject for human factors usability research, one technique that is used is for the verifying system to rewrite the From header field, to indicate the address that was verified. For example: From: John Doe via - news@news-site.com . (Note that such rewriting + news@news-site.example . (Note that such rewriting will break a signature, unless it is done after the verification pass is complete.) B.2. Alternate Delivery Scenarios Email is often received at a mailbox that has an address different from the one used during initial submission. In these cases, an intermediary mechanism operates at the address originally used and it then passes the message on to the final destination. This mediation process presents some challenges for DKIM signatures. @@ -3350,23 +3299,23 @@ C.1. Compatibility with DomainKeys Key Records DKIM key records were designed to be backwards-compatible in many cases with key records used by DomainKeys [RFC4870] (sometimes referred to as "selector records" in the DomainKeys context). One area of incompatibility warrants particular attention. The "g=" tag/ value may be used in DomainKeys and [RFC4871] key records to provide finer granularity of the validity of the key record to a specific local-part. A null "g=" value in DomainKeys is valid for all addresses in the domain. This differs from the usage in the original - DKIM specification, where a null "g=" value is not valid for any - address. In particular, the example public key record in Section - 3.2.3 of [RFC4870] with DKIM. + DKIM specification ([RFC4871]), where a null "g=" value is not valid + for any address. In particular, see the example public key record in + Section 3.2.3 of [RFC4870]. C.2. RFC4871 Compatibility Although the "g=" tag has been deprecated in this version of the DKIM specification (and thus MUST now be ignored), signers are advised not to include the "g=" tag in key records because some [RFC4871]- compliant verifiers will be in use for a considerable period to come. Appendix D. MUA Considerations (INFORMATIVE) @@ -3384,31 +3333,32 @@ signed header fields, with a negative indication on the unsigned header fields, by visually hiding the unsigned header fields, or some combination of these. If an MUA uses visual indications for signed header fields, the MUA probably needs to be careful not to display unsigned header fields in a way that might be construed by the end user as having been signed. If the message has an l= tag whose value does not extend to the end of the message, the MUA might also hide or mark the portion of the message body that was not signed. The aforementioned information is not intended to be exhaustive. The - MUA may choose to highlight, accentuate, hide, or otherwise display + MUA can choose to highlight, accentuate, hide, or otherwise display any other information that may, in the opinion of the MUA author, be deemed important to the end user. Appendix E. Changes since RFC4871 o Abstract and introduction refined based on accumulated experience. o Various references updated. - o Several errata resolved: + o Several errata resolved (see + http://www.rfc-editor.org/errata_search.php?rfc=4871): * 1376 applied * 1377 applied * 1378 applied * 1379 applied * 1380 applied