| Network Working Group | D. Crocker, Ed. |
| Internet-Draft | Brandenburg InternetWorking |
| Obsoletes: 4871 (if approved) | T. Hansen, Ed. |
| Intended status: Standards Track | AT&T Laboratories |
| Expires: August 20, 2011 | M. Kucherawy, Ed. |
| Cloudmark | |
| February 16, 2011 |
DomainKeys Identified Mail (DKIM) Signatures
draft-ietf-dkim-rfc4871bis-03
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 cryptographic signature and querying the signer's domain directly to retrieve the appropriate public key. Message transit from author to recipient is through relays that typically make no substantive change to the message content and thus preserve the DKIM signature.
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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]. 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 public key. Message transit from author to recipient is through relays that typically make no substantive change to the message content and thus preserve the DKIM signature. A message can contain multiple signatures, from the same or different organizations involved with the message.
The approach taken by DKIM differs from previous approaches to message signing (e.g., Secure/Multipurpose Internet Mail Extensions (S/MIME) [RFC1847], OpenPGP [RFC4880]) in that:
DKIM:
DKIM separates the question of the identity of the signer of the message from the purported author of the message. In particular, a signature includes the identity of the signer. Verifiers can use the signing information to decide how they want to process the message. The signing identity is included as part of the signature header field.
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.
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 of the DNS system. DKIM is designed to be extensible to other key fetching services as they become available.
A DKIM signature associates the d= name with the computed hash of some or all of the message (see Section 3.7) in order to prevent the re-use of the signature with different messages. Verifying the signature asserts that the hashed content has not changed since it was signed, and asserts nothing else about "protecting" the end-to-end integrity of the message.
This section defines terms used in the rest of the document.
DKIM is designed to operate within the Internet Mail service, as defined in [RFC5598]. Basic email terminology is taken from that specification.
Syntax descriptions use Augmented BNF (ABNF) [RFC5234].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
Elements in the mail system that sign messages on behalf of a domain are referred to as signers. These may be MUAs (Mail User Agents), MSAs (Mail Submission Agents), MTAs (Mail Transfer Agents), or other agents such as mailing list exploders. In general, any signer will be involved in the injection of a message into the message system in some way. The key issue is that a message must be signed before it leaves the administrative domain of the signer.
Elements in the mail system that verify signatures are referred to as verifiers. These may be MTAs, Mail Delivery Agents (MDAs), or MUAs. In most cases it is expected that verifiers will be close to an end user (reader) of the message or some consuming agent such as a mailing list exploder.
A person, role, or organization. In the context of DKIM, examples include author, author's organization, an ISP along the handling path, an independent trust assessment service, and a mailing list operator.
A label that refers to an identity.
A single domain name that is the mandatory payload output of DKIM and that refers to the identity claiming responsibility for introduction of a message into the mail stream. For DKIM processing, the name has only basic domain name semantics; any possible owner-specific semantics are outside the scope of DKIM. It is specified in Section 3.5.
A single identifier that refers to the agent or user on behalf of whom the Signing Domain Identifier (SDID) has taken responsibility. The AUID comprises a domain name and an optional <Local-part>. The domain name is the same as that used for the SDID or is a sub-domain 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 .
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.
There are three forms of whitespace:
The formal ABNF for these are (WSP and LWSP are given for information only):
WSP = SP / HTAB LWSP = *(WSP / CRLF WSP) FWS = [*WSP CRLF] 1*WSP
The definition of FWS is identical to that in [RFC5322] except for the exclusion of obs-FWS.
The following ABNF tokens are used elsewhere in this document:
hyphenated-word = ALPHA [ *(ALPHA / DIGIT / "-") (ALPHA / DIGIT) ]
ALPHADIGITPS = (ALPHA / DIGIT / "+" / "/")
base64string = ALPHADIGITPS *([FWS] ALPHADIGITPS)
[ [FWS] "=" [ [FWS] "=" ] ]
hdr-name = field-name
qp-hdr-value = dkim-quoted-printable ; with "|" encoded
The following tokens are imported from other RFCs as noted. Those RFCs should be considered definitive.
The following tokens are imported from [RFC5321]:
The following tokens are imported from [RFC5322]:
The following tokens are imported from [RFC2045]:
Other tokens not defined herein are imported from [RFC5234]. These are intuitive primitives such as SP, HTAB, WSP, ALPHA, DIGIT, CRLF, etc.
The DKIM-Quoted-Printable encoding syntax resembles that described in Quoted-Printable [RFC2045], Section 6.7: any character MAY be encoded as an "=" followed by two hexadecimal digits from the alphabet "0123456789ABCDEF" (no lowercase characters permitted) representing the hexadecimal-encoded integer value of that character. All control characters (those with values < %x20), 8-bit characters (values > %x7F), and the characters DEL (%x7F), SPACE (%x20), and semicolon (";", %x3B) MUST be encoded. Note that all whitespace, including SPACE, CR, and LF characters, MUST be encoded. After encoding, FWS MAY be added at arbitrary locations in order to avoid excessively long lines; such whitespace is NOT part of the value, and MUST be removed before decoding.
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.
Protocol Elements are conceptual parts of the protocol that are not specific to either signers or verifiers. The protocol descriptions for signers and verifiers are described in later sections (Signer Actions (Section 5) and Verifier Actions (Section 6)). NOTE: This section must be read in the context of those sections.
To support multiple concurrent public keys per signing domain, the key namespace is subdivided using "selectors". For example, selectors might indicate the names of office locations (e.g., "sanfrancisco", "coolumbeach", and "reykjavik"), the signing date (e.g., "january2005", "february2005", etc.), or even an individual user.
Selectors are needed to support some important use cases. For example:
Periods are allowed in selectors and are component separators. When keys are retrieved from the DNS, periods in selectors define DNS label boundaries in a manner similar to the conventional use in domain names. Selector components might be used to combine dates with locations, for example, "march2005.reykjavik". In a DNS 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 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 email servers are configured to sign with the "february2005" private key. At the end of the transition period, the "january2005" public key is removed from the public-key repository.
While some domains may wish to make selector values well known, others will want to take care not to allocate selector names in a way that allows harvesting of data by outside parties. For example, if per-user keys are issued, the domain owner will need to make the decision as to whether to associate this selector directly with the name of a registered end-user, or make it some unassociated random value, such as a fingerprint of the public key.
DKIM uses a simple "tag=value" syntax in several contexts, including in messages and domain signature records.
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 2.6). 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.
Formally, the ABNF syntax rules are as follows:
tag-list = tag-spec 0*( ";" 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 ) ]
; WSP and FWS prohibited 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.
Tags MUST be interpreted in a case-sensitive manner. Values MUST be processed as case sensitive unless the specific tag description of semantics specifies case insensitivity.
Tags with duplicate names MUST NOT occur within a single tag-list; if a tag name does occur more than once, the entire tag-list is invalid.
Whitespace within a value MUST be retained unless explicitly excluded by the specific tag description.
Tag=value pairs that represent the default value MAY be included to aid legibility.
Unrecognized tags MUST be ignored.
Tags that have an empty value are not the same as omitted tags. An omitted tag is treated as having the default value; a tag with an empty value explicitly designates the empty string as the value.
DKIM supports multiple digital signature algorithms. Two algorithms are defined by this specification at this time: rsa-sha1 and rsa-sha256. Signers MUST implement and SHOULD sign using rsa-sha256. Verifiers MUST implement rsa-sha256.
The rsa-sha1 Signing Algorithm computes a message hash as described in Section 3.7 below using SHA-1 [FIPS-180-2-2002] as the hash-alg. That hash is then signed by the signer using the RSA algorithm (defined in PKCS#1 version 1.5 [RFC3447]) as the crypt-alg and the signer's private key. The hash MUST NOT be truncated or converted into any form other than the native binary form before being signed. The signing algorithm SHOULD use a public exponent of 65537.
The rsa-sha256 Signing Algorithm computes a message hash as described in Section 3.7 below using SHA-256 [FIPS-180-2-2002] as the hash-alg. That hash is then signed by the signer using the RSA algorithm (defined in PKCS#1 version 1.5 [RFC3447]) as the crypt-alg and the signer's private key. The hash MUST NOT be truncated or converted into any form other than the native binary form before being signed.
Selecting appropriate key sizes is a trade-off between cost, performance, and risk. Since short RSA keys more easily succumb to 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:
See [RFC3766] for further discussion on selecting key sizes.
Other algorithms MAY be defined in the future. Verifiers MUST ignore any signatures using algorithms that they do not implement.
Some mail systems modify email in transit, potentially invalidating a signature. For most signers, mild modification of email is immaterial to validation of the DKIM domain name's use. For such signers, a canonicalization algorithm that survives modest in-transit modification is preferred.
Other signers demand that any modification of the email, however minor, result in a signature verification failure. These signers prefer a canonicalization algorithm that does not tolerate in-transit modification of the signed email.
Some signers may be willing to accept modifications to header fields that are within the bounds of email standards such as [RFC5322], but are unwilling to accept any modification to the body of messages.
To satisfy all requirements, two canonicalization algorithms are defined for each of the header and the body: a "simple" algorithm that tolerates almost no modification and a "relaxed" algorithm that tolerates common modifications such as whitespace replacement and header field line rewrapping. A signer MAY specify either algorithm for header or body when signing an email. If no canonicalization algorithm is specified by the signer, the "simple" algorithm defaults for both header and body. Verifiers MUST implement both canonicalization algorithms. Note that the header and body may use different canonicalization algorithms. Further canonicalization algorithms MAY be defined in the future; verifiers MUST ignore any signatures that use unrecognized canonicalization algorithms.
Canonicalization simply prepares the email for presentation to the signing or verification algorithm. It MUST NOT change the transmitted data in any way. Canonicalization of header fields and body are described below.
NOTE: This section assumes that the message is already in "network normal" format (text is ASCII encoded, lines are separated with CRLF characters, etc.). See also Section 5.3 for information about normalizing the message.
The "simple" header canonicalization algorithm does not change header fields in any way. Header fields MUST be presented to the signing or verification algorithm exactly as they are in the message being signed or verified. In particular, header field names MUST NOT be case folded and whitespace MUST NOT be changed.
The "relaxed" header canonicalization algorithm MUST apply the following steps in order:
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 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.
uoq1oCgLlTqpdDX/iUbLy7J1Wic=
frcCV1k9oG9oKj3dpUqdJg1PxRT2RSN/XKdLCPjaYaY=
2jmj7l5rSw0yVb/vlWAYkK/YBwk=
47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=
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.
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. Signers of MIME messages that include a body length count SHOULD be sure that the length extends to the closing MIME boundary string.
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.
In the following examples, actual whitespace is used only for clarity. The actual input and output text is designated using bracketed descriptors: "<SP>" for a space character, "<HTAB>" for a tab character, and "<CRLF>" for a carriage-return/line-feed sequence. For example, "X <SP> Y" and "X<SP>Y" represent the same three characters.
Example 1: A message reading:
A: <SP> X <CRLF>
B <SP> : <SP> Y <HTAB><CRLF>
<HTAB> Z <SP><SP><CRLF>
<CRLF>
<SP> C <SP><CRLF>
D <SP><HTAB><SP> E <CRLF>
<CRLF>
<CRLF>
when canonicalized using relaxed canonicalization for both header and body results in a header reading:
a:X <CRLF> b:Y <SP> Z <CRLF>
and a body reading:
<SP> C <CRLF> D <SP> E <CRLF>
Example 2: The same message canonicalized using simple canonicalization for both header and body results in a header reading:
A: <SP> X <CRLF>
B <SP> : <SP> Y <HTAB><CRLF>
<HTAB> Z <SP><SP><CRLF>
and a body reading:
<SP> C <SP><CRLF> D <SP><HTAB><SP> E <CRLF>
Example 3: When processed using relaxed header canonicalization and simple body canonicalization, the canonicalized version has a header of:
a:X <CRLF> b:Y <SP> Z <CRLF>
and a body reading:
<SP> C <SP><CRLF> D <SP><HTAB><SP> E <CRLF>
The signature of the email is stored in the DKIM-Signature header field. This header field contains all of the signature and key- fetching data. The DKIM-Signature value is a tag-list as described in Section 3.2.
The DKIM-Signature header field SHOULD be treated as though it were a trace header field as defined in Section 3.6 of [RFC5322], and hence SHOULD NOT be reordered and SHOULD be prepended to the message.
The DKIM-Signature header field being created or verified is always included in the signature calculation, after the rest of the header fields being signed; however, when calculating or verifying the signature, the value of the "b=" tag (signature value) of that DKIM- Signature header field MUST be treated as though it were an empty string. Unknown tags in the DKIM-Signature header field MUST be included in the signature calculation but MUST be otherwise ignored by verifiers. Other DKIM-Signature header fields that are included in the signature should be treated as normal header fields; in particular, the "b=" tag is not treated specially.
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.
ABNF:
sig-v-tag = %x76 [FWS] "=" [FWS] "1"
ABNF:
sig-a-tag = %x61 [FWS] "=" [FWS] sig-a-tag-alg
sig-a-tag-alg = sig-a-tag-k "-" sig-a-tag-h
sig-a-tag-k = "rsa" / x-sig-a-tag-k
sig-a-tag-h = "sha1" / "sha256" / x-sig-a-tag-h
x-sig-a-tag-k = ALPHA *(ALPHA / DIGIT)
; for later extension
x-sig-a-tag-h = ALPHA *(ALPHA / DIGIT)
; for later extension
ABNF:
sig-b-tag = %x62 [FWS] "=" [FWS] sig-b-tag-data sig-b-tag-data = base64string
ABNF:
sig-bh-tag = %x62 %x68 [FWS] "=" [FWS] sig-bh-tag-data sig-bh-tag-data = base64string
ABNF:
sig-c-tag = %x63 [FWS] "=" [FWS] sig-c-tag-alg
["/" sig-c-tag-alg]
sig-c-tag-alg = "simple" / "relaxed" / x-sig-c-tag-alg
x-sig-c-tag-alg = hyphenated-word ; for later extension
ABNF:
sig-d-tag = %x64 [FWS] "=" [FWS] domain-name
domain-name = sub-domain 1*("." sub-domain)
; from RFC 5321 Domain, but excluding address-literal
ABNF:
sig-h-tag = %x68 [FWS] "=" [FWS] hdr-name
0*( [FWS] ":" [FWS] hdr-name )
ABNF:
sig-i-tag = %x69 [FWS] "=" [FWS] [ Local-part ]
"@" domain-name
ABNF:
sig-l-tag = %x6c [FWS] "=" [FWS]
1*76DIGIT
ABNF:
sig-q-tag = %x71 [FWS] "=" [FWS] sig-q-tag-method
*([FWS] ":" [FWS] sig-q-tag-method)
sig-q-tag-method = "dns/txt" / x-sig-q-tag-type
["/" x-sig-q-tag-args]
x-sig-q-tag-type = hyphenated-word ; for future extension
x-sig-q-tag-args = qp-hdr-value
ABNF:
sig-s-tag = %x73 [FWS] "=" [FWS] selector
ABNF:
sig-t-tag = %x74 [FWS] "=" [FWS] 1*12DIGIT
ABNF:
sig-x-tag = %x78 [FWS] "=" [FWS]
1*12DIGIT
ABNF:
sig-z-tag = %x7A [FWS] "=" [FWS] sig-z-tag-copy
*( "|" [FWS] sig-z-tag-copy )
sig-z-tag-copy = hdr-name [FWS] ":" qp-hdr-value
INFORMATIVE EXAMPLE of a signature header field spread across multiple continuation lines:
DKIM-Signature: v=1; a=rsa-sha256; d=example.net; s=brisbane;
c=simple; q=dns/txt; i=@eng.example.net;
t=1117574938; x=1118006938;
h=from:to:subject:date;
z=From:foo@eng.example.net|To:joe@example.com|
Subject:demo=20run|Date:July=205,=202005=203:44:08=20PM=20-0700;
bh=MTIzNDU2Nzg5MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTI=;
b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZVoG4ZHRNiYzR
Signature applications require some level of assurance that the verification public key is associated with the claimed signer. Many applications achieve this by using public key certificates issued by a trusted third party. However, DKIM can achieve a sufficient level of security, with significantly enhanced scalability, by simply having the verifier query the purported signer's DNS entry (or some security-equivalent) in order to retrieve the public key.
DKIM keys can potentially be stored in multiple types of key servers and in multiple formats. The storage and format of keys are irrelevant to the remainder of the DKIM algorithm.
public_key = dkim_find_key(q_val, d_val, s_val)
Parameters to the key lookup algorithm are the type of the lookup (the "q=" tag), the domain of the signer (the "d=" tag of the DKIM- Signature header field), and the selector (the "s=" tag).
This document defines a single binding, using DNS TXT records to distribute the keys. Other bindings may be defined in the future.
It is expected that many key servers will choose to present the keys in an otherwise unstructured text format (for example, an XML form would not be considered to be unstructured text for this purpose). The following definition MUST be used for any DKIM key represented in an otherwise unstructured textual form.
The overall syntax is a tag-list as described in Section 3.2. The current valid tags are described below. Other tags MAY be present and MUST be ignored by any implementation that does not understand them.
key-v-tag = %x76 [FWS] "=" [FWS] %x44 %x4B %x49 %x4D %x31
ABNF:
key-h-tag = %x68 [FWS] "=" [FWS] key-h-tag-alg
0*( [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
ABNF:
key-k-tag = %x76 [FWS] "=" [FWS] key-k-tag-type key-k-tag-type = "rsa" / x-key-k-tag-type x-key-k-tag-type = hyphenated-word ; for future extension
ABNF:
key-n-tag = %x6e [FWS] "=" [FWS] qp-section
ABNF:
key-p-tag = %x70 [FWS] "=" [ [FWS] base64string]
ABNF:
key-s-tag = %x73 [FWS] "=" [FWS] key-s-tag-type
0*( [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
ABNF:
key-t-tag = %x74 [FWS] "=" [FWS] key-t-tag-flag
0*( [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
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.
A binding using DNS TXT records as a key service is hereby defined. All implementations MUST support this binding.
All DKIM keys are stored in a subdomain named "_domainkey". Given a DKIM-Signature field with a "d=" tag of "example.com" and an "s=" tag of "foo.bar", the DNS query will be for "foo.bar._domainkey.example.com".
The DNS Resource Record type used is specified by an option to the query-type ("q=") tag. The only option defined in this base specification is "txt", indicating the use of a TXT Resource Record (RR). A later extension of this standard may define another RR type.
Strings in a TXT RR MUST be concatenated together before use with no intervening whitespace. TXT RRs MUST be unique for a particular selector name; that is, if there are multiple records in an RRset, the results are undefined.
TXT RRs are encoded as described in Section 3.6.1
Both signing and verifying message signatures start with a step of computing two cryptographic hashes over the message. Signers will choose the parameters of the signature as described in Signer Actions Section 5; verifiers will use the parameters specified in the DKIM- Signature header field being verified. In the following discussion, the names of the tags in the DKIM-Signature header field that either exists (when verifying) or will be created (when signing) are used. Note that canonicalization (Section 3.4) is only used to prepare the email for signing or verifying; it does not affect the transmitted email in any way.
The signer/verifier MUST compute two hashes, one over the body of the message and one over the selected header fields of the message.
Signers MUST compute them in the order shown. Verifiers MAY compute them in any order convenient to the verifier, provided that the result is semantically identical to the semantics that would be the case had they been computed in this order.
In hash step 1, the signer/verifier MUST hash the message body, canonicalized using the body canonicalization algorithm specified in the "c=" tag and then truncated to the length specified in the "l=" tag. That hash value is then converted to base64 form and inserted into (signers) or compared to (verifiers) the "bh=" tag of the DKIM- Signature header field.
In hash step 2, the signer/verifier MUST pass the following to the hash algorithm in the indicated order.
All tags and their values in the DKIM-Signature header field are included in the cryptographic hash with the sole exception of the value portion of the "b=" (signature) tag, which MUST be treated as the null string. All tags MUST be included even if they might not be understood by the verifier. The header field MUST be presented to the hash algorithm after the body of the message rather than with the rest of the header fields and MUST be canonicalized as specified in the "c=" (canonicalization) tag. The DKIM-Signature header field MUST NOT be included in its own h= tag, although other DKIM-Signature header fields MAY be signed (see Section 4).
When calculating the hash on messages that will be transmitted using base64 or quoted-printable encoding, signers MUST compute the hash after the encoding. Likewise, the verifier MUST incorporate the values into the hash before decoding the base64 or quoted-printable text. However, the hash MUST be computed before transport level encodings such as SMTP "dot-stuffing" (the modification of lines beginning with a "." to avoid confusion with the SMTP end-of-message marker, as specified in [RFC5321]).
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, the ABNF of the algorithm for the signature is as follows:
body-hash = bh-hash-alg (canon-body, l-param) data-hash = a-hash-alg (h-headers, D-SIG, content-hash) signature = sig-alg (d-domain, selector, data-hash)
In some circumstances, it is desirable for a domain to apply a signature on behalf of any of its subdomains without the need to maintain separate selectors (key records) in each subdomain. By default, private keys corresponding to key records can be used to sign messages for any subdomain of the domain in which they reside; for example, a key record for the domain example.com can be used to verify messages where the AUID ("i=" tag of the signature) is sub.example.com, or even sub1.sub2.example.com. In order to limit the capability of such keys when this is not intended, the "s" flag MAY be set in the "t=" tag of the key record, to constrain the validity of the domain of the AUID. If the referenced key record contains the "s" flag as part of the "t=" tag, the domain of the AUID ("i=" flag) MUST be the same as that of the SDID (d=) domain. If this flag is absent, the domain of the AUID MUST be the same as, or a subdomain of, the SDID.
DKIM's primary task is to communicate from the Signer to a recipient-side Identity Assessor a single Signing Domain Identifier (SDID) that refers to a responsible identity. DKIM MAY optionally provide a single responsible Agent or User Identifier (AUID).
Hence, DKIM's mandatory output to a receive-side Identity Assessor is a single domain name. Within the scope of its use as DKIM output, the name has only basic domain name semantics; any possible owner-specific semantics are outside the scope of DKIM. That is, within its role as a DKIM identifier, additional semantics cannot be assumed by an Identity Assessor.
Upon successfully verifying the signature, a receive-side DKIM verifier MUST communicate the Signing Domain Identifier (d=) to a consuming Identity Assessor module and MAY communicate the Agent or User Identifier (i=) if present.
To the extent that a receiver attempts to intuit any structured semantics for either of the identifiers, this is a heuristic function that is outside the scope of DKIM's specification and semantics. Hence, it is relegated to a higher-level service, such as a delivery handling filter that integrates a variety of inputs and performs heuristic analysis of them.
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.
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.
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.
Another related example of multiple signers might be forwarding services, such as those commonly associated with academic alumni sites.
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.
A signer MAY add more than one DKIM-Signature header field using different parameters. For example, during a transition period a signer might want to produce signatures using two different hash algorithms.
Signers SHOULD NOT remove any DKIM-Signature header fields from messages they are signing, even if they know that the signatures cannot be verified.
When evaluating a message with multiple signatures, a verifier SHOULD evaluate signatures independently and on their own merits. For example, a verifier that by policy chooses not to accept signatures with deprecated cryptographic algorithms would consider such signatures invalid. Verifiers MAY process signatures in any order of their choice; for example, some verifiers might choose to process signatures corresponding to the From field in the message header before other signatures. See Section 6.1 for more information about signature choices.
Verifiers SHOULD ignore failed signatures as though they were not present in the message. Verifiers SHOULD continue to check signatures until a signature successfully verifies to the satisfaction of the verifier. To limit potential denial-of-service attacks, verifiers MAY limit the total number of signatures they will attempt to verify.
The following steps are performed in order by signers.
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.
If an email cannot be signed for some reason, it is a local policy decision as to what to do with that email.
This specification does not define the basis by which a signer should 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.
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.
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.
The DKIM-Signature header field is always implicitly signed and MUST NOT be included in the "h=" tag except to indicate that other preexisting signatures are also signed.
Signers MAY claim to have signed header fields that do not exist (that is, signers MAY include the header field name in the "h=" tag even if that header field does not exist in the message). When computing the signature, the non-existing header field MUST be treated as the null string (including the header field name, header field value, all punctuation, and the trailing CRLF).
Received: <A> Received: <B> Received: <c>
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.
then the resulting DKIM-Signature header field should read:
DKIM-Signature: ... h=Received : Received :...
Signers should 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.
In order to maximize compatibility with a variety of verifiers, it is recommended that signers follow the practices outlined in this section when signing a message. However, these are generic recommendations applying to the general case; specific senders may wish to modify these guidelines as required by their unique situations. Verifiers MUST be capable of verifying signatures even if one or more of the recommended header fields is not signed (with the exception of From, which must always be signed) or if one or more of the dis-recommended header fields is signed. Note that verifiers do have the option of ignoring signatures that do not cover a sufficient portion of the header or body, just as they may ignore signatures from an identity they do not trust.
The following header fields SHOULD be included in the signature, if they are present in the message being signed:
The following header fields SHOULD NOT be included in the signature:
Optional header fields (those not mentioned above) normally SHOULD NOT be included in the signature, because of the potential for additional header fields of the same name to be legitimately added or reordered prior to verification. There are likely to be legitimate exceptions to this rule, because of the wide variety of application- specific header fields that may be applied to a message, some of which are unlikely to be duplicated, modified, or reordered.
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.
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.
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
The DKIM-Signature header field MUST be inserted before any other DKIM-Signature fields in the header block.
Since a signer MAY remove or revoke a public key at any time, it is recommended that verification occur in a timely manner. In many configurations, the most timely place is during acceptance by the border MTA or shortly thereafter. In particular, deferring verification until the message is accessed by the end user is discouraged.
A border or intermediate MTA MAY verify the message signature(s). An MTA who has performed verification MAY communicate the result of that verification by adding a verification header field to incoming messages. This considerably simplifies things for the user, who can now use an existing mail user agent. Most MUAs have the ability to filter messages based on message header fields or content; these filters would be used to implement whatever policy the user wishes with respect to unsigned mail.
A verifying MTA MAY implement a policy with respect to unverifiable mail, regardless of whether or not it applies the verification header field to signed messages.
Verifiers MUST produce a result that is semantically equivalent to applying the following steps in the order listed. In practice, several of these steps can be performed in parallel in order to improve performance.
The order in which verifiers try DKIM-Signature header fields is not defined; verifiers MAY try signatures in any order they like. For example, one implementation might try the signatures in textual order, whereas another might try signatures by identities that match the contents of the From header field before trying other signatures. Verifiers MUST NOT attribute ultimate meaning to the order of multiple DKIM-Signature header fields. In particular, there is reason to believe that some relays will reorder the header fields in potentially arbitrary ways.
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.
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.
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 defer the message for later processing, perhaps by queueing it locally or issuing a 451/4.7.5 SMTP reply, or try another signature; if no good signature is found and any of the signatures resulted in a TEMPFAIL status, the verifier MAY save the message for later processing. The "(explanation)" is not normative text; it is provided solely for clarification.
Verifiers SHOULD ignore any DKIM-Signature header fields where the signature does not validate. Verifiers that are prepared to validate multiple signature header fields SHOULD proceed to the next signature header field, should it exist. However, verifiers MAY make note of the fact that an invalid signature was present for consideration at a later step.
For each signature to be validated, the following steps should be performed in such a manner as to produce a result that is semantically equivalent to performing them in the indicated order.
Implementers MUST meticulously validate the format and values in the DKIM-Signature header field; any inconsistency or unexpected values MUST cause the header field to be completely ignored and the verifier to return PERMFAIL (signature syntax error). Being "liberal in what you accept" is definitely a bad strategy in this security context. Note however that this does not include the existence of unknown tags in a DKIM-Signature header field, which are explicitly permitted. Verifiers MUST ignore DKIM-Signature header fields with a "v=" tag that is inconsistent with this specification and return PERMFAIL (incompatible version).
If any tag listed as "required" in Section 3.5 is omitted from the DKIM-Signature header field, the verifier MUST ignore the DKIM- Signature header field and return PERMFAIL (signature missing required tag).
If the DKIM-Signature header field does not contain the "i=" tag, the verifier MUST behave as though the value of that tag were "@d", where "d" is the value from the "d=" tag.
Verifiers MUST confirm that the domain specified in the "d=" tag is the same as or a parent domain of the domain part of the "i=" tag. If not, the DKIM-Signature header field MUST be ignored and the verifier should return PERMFAIL (domain mismatch).
If the "h=" tag does not include the From header field, the verifier 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 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.
The public key for a signature is needed to complete the verification process. The process of retrieving the public key depends on the query type as defined by the "q=" tag in the DKIM-Signature header field. Obviously, a public key need only be retrieved if the process of extracting the signature information is completely successful. Details of key management and representation are described in Section 3.6. The verifier MUST validate the key record and MUST ignore any public key records that are malformed.
When validating a message, a verifier MUST perform the following steps in a manner that is semantically the same as performing them in the order indicated -- in some cases the implementation may parallelize or reorder these steps, as long as the semantics remain unchanged:
Given a signer and a public key, verifying a signature consists of actions semantically equivalent to the following steps.
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 truncating the message at the indicated body length, declaring the signature invalid (e.g., by returning PERMFAIL (unsigned content)), or conveying the partial verification to the policy module.
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 purpose.
It is beyond the scope of this specification to describe what actions an Identity Assessor can make, but mail carrying a validated SDID presents an opportunity to an Identity Assessor that unauthenticated email does not. Specifically, an authenticated email creates a predictable identifier by which other decisions can reliably be managed, such as trust and reputation. Conversely, unauthenticated email lacks a reliable identifier that can be used to assign trust and reputation. It is reasonable to treat unauthenticated email as lacking any trust and having no positive reputation.
In general, verifiers SHOULD NOT reject messages solely on the basis of a lack of signature or an unverifiable signature; such rejection would cause severe interoperability problems. However, if the verifier does opt to reject such messages (for example, when communicating with a peer who, by prior agreement, agrees to only send signed messages), and the verifier runs synchronously with the SMTP session and a signature is missing or does not verify, the MTA SHOULD use a 550/5.7.x reply code.
If it is not possible to fetch the public key, perhaps because the key server is not available, a temporary failure message MAY be generated using a 451/4.7.5 reply code, such as:
451 4.7.5 Unable to verify signature - key server unavailable
Temporary failures such as inability to access the key server or other external service are the only conditions that SHOULD use a 4xx SMTP reply code. In particular, cryptographic signature verification failures MUST NOT return 4xx SMTP replies.
Once the signature has been verified, that information MUST be conveyed to the Identity Assessor (such as an explicit allow/ whitelist and reputation system) and/or to the end user. If the SDID is not the same as the address in the From: header field, the mail system SHOULD take pains to ensure that the actual SDID is clear to the reader.
The verifier MAY treat unsigned header fields with extreme skepticism, including marking them as untrusted or even deleting them before display to the end user.
While the symptoms of a failed verification are obvious -- the signature doesn't verify -- establishing the exact cause can be more difficult. If a selector cannot be found, is that because the selector has been removed, or was the value changed somehow in transit? If the signature line is missing, is that because it was never there, or was it removed by an overzealous filter? For diagnostic purposes, the exact reason why the verification fails SHOULD be made available to the policy module and possibly recorded in the system logs. If the email cannot be verified, then it SHOULD be rendered the same as all unverified email regardless of whether or not it looks like it was signed.
DKIM has registered new 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].
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 initial entries in the registry comprise:
| TYPE | REFERENCE |
|---|---|
| v | (this document) |
| a | (this document) |
| b | (this document) |
| bh | (this document) |
| c | (this document) |
| d | (this document) |
| h | (this document) |
| i | (this document) |
| l | (this document) |
| q | (this document) |
| s | (this document) |
| t | (this document) |
| x | (this document) |
| z | (this document) |
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 initial entry in the registry comprises:
| TYPE | OPTION | REFERENCE |
|---|---|---|
| dns | txt | (this document) |
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 initial entries in the body registry comprise:
| TYPE | REFERENCE |
|---|---|
| simple | (this document) |
| relaxed | (this document) |
A _domainkey DNS TXT record 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 Records.
The initial entries in the registry comprise:
| TYPE | REFERENCE |
|---|---|
| v | (this document) |
| g | (this document) |
| h | (this document) |
| k | (this document) |
| n | (this document) |
| p | (this document) |
| s | (this document) |
| t | (this document) |
The initial entries in the body registry comprise:
| TYPE | REFERENCE |
|---|---|
| simple | (this document) |
| relaxed | (this document) |
The "k=" <key-k-tag> (specified in Section 3.6.1) and the "a=" <sig- a-tag-k> (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 initial entry in the registry comprises:
| TYPE | REFERENCE |
|---|---|
| rsa | [RFC3447] |
The "h=" <key-h-tag> (specified in Section 3.6.1) and the "a=" <sig- a-tag-h> (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 initial entries in the registry comprise:
| TYPE | REFERENCE |
|---|---|
| sha1 | [FIPS-180-2-2002] |
| sha256 | [FIPS-180-2-2002] |
The "s=" <key-s-tag> 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 initial entries in the registry comprise:
| TYPE | REFERENCE |
|---|---|
| (this document) | |
| * | (this document) |
The "t=" <key-t-tag> 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 initial entries in the registry comprise:
| TYPE | REFERENCE |
|---|---|
| y | (this document) |
| s | (this document) |
IANA has added DKIM-Signature to the "Permanent Message Header Fields" registry (see [RFC3864]) for the "mail" protocol, using this document as the reference.
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].
Body length limits (in the form of the "l=" tag) are subject to several potential attacks.
If the body length limit does not cover a closing MIME multipart section (including the trailing "--CRLF" portion), then it is possible for an attacker to intercept a properly signed multipart message and add a new body part. Depending on the details of the MIME type and the implementation of the verifying MTA and the receiving MUA, this could allow an attacker to change the information displayed to an end user from an apparently trusted source.
For example, if attackers can append information to a "text/html" body part, they may be able to exploit a bug in some MUAs that continue to read after a "</html>" marker, and thus display HTML text on top of already displayed text. If a message has a "multipart/alternative" body part, they might be able to add a new body part that is preferred by the displaying MUA.
<div style="position: relative; bottom: 350px; z-index: 2;">
<img src="http://www.ietf.org/images/ietflogo2e.gif" width=578 height=370> </div>
Several receiving MUA implementations do not cease display after a ""</html>"" tag. In particular, this allows attacks involving overlaying images on top of existing text.
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.
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.
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.
A somewhat more effective countermeasure is to send 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.
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.
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 proof about authorship or contents. Other technologies such as OpenPGP [RFC4880] and S/MIME [RFC5751] address those requirements.
A second security issue related to the DNS revolves around the increased DNS traffic as a consequence of fetching selector-based data as well as fetching signing domain policy. Widespread deployment of DKIM will result in a significant increase in DNS queries to the claimed signing domain. In the case of forgeries on a large scale, DNS servers could see a substantial increase in queries.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
The trust relationship described in Section 3.8 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.
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 last, 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.)
To resist this specific attack the signed header field list can include an additional reference for each field that was present at signing. For example, a proper message with one From: field could be signed using "h=From:From:..." Due to the way header fields are canonicalized for input to the hash function, the extra field references will prevent instances of the cited fields from being added after signing, as doing so would render the signature invalid.
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 malformations, or is careless about identifying which parts of a message were authenticated, is open to exploitation.
This section shows the complete flow of an email from submission to final delivery, demonstrating how the various components fit together. The key used in this example is shown in Appendix Appendix C.
From: Joe SixPack <joe@football.example.com> To: Suzie Q <suzie@shopping.example.net> Subject: Is dinner ready? Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT) Message-ID: <;20030712040037.46341.5F8J@football.example.com> Hi. We lost the game. Are you hungry yet? Joe.
This email is signed by the example.com outbound email server and now looks like this:
DKIM-Signature: v=1; a=rsa-sha256; s=brisbane; d=example.com;
c=simple/simple; q=dns/txt; i=joe@football.example.com;
h=Received : From : To : Subject : Date : Message-ID;
bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=;
b=AuUoFEfDxTDkHlLXSZEpZj79LICEps6eda7W3deTVFOk4yAUoqOB
4nujc7YopdG5dWLSdNg6xNAZpOPr+kHxt1IrE+NahM6L/LbvaHut
KVdkLLkpVaVVQPzeRDI009SO2Il5Lu7rDNH6mZckBdrIx0orEtZV
4bmp/YzhwvcubU4=;
Received: from client1.football.example.com [192.0.2.1]
by submitserver.example.com with SUBMISSION;
Fri, 11 Jul 2003 21:01:54 -0700 (PDT)
From: Joe SixPack <joe@football.example.com>
To: Suzie Q <suzie@shopping.example.net>
Subject: Is dinner ready?
Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
Message-ID: <20030712040037.46341.5F8J@football.example.com>
Hi.
We lost the game. Are you hungry yet?
Joe.
The signing email server requires access to the private key associated with the "brisbane" selector to generate this signature.
The signature is normally verified by an inbound SMTP server or possibly the final delivery agent. However, intervening MTAs can also perform this verification if they choose to do so. The verification process uses the domain "example.com" extracted from the "d=" tag and the selector "brisbane" from the "s=" tag in the DKIM- Signature header field to form the DNS DKIM query for: brisbane._domainkey.example.com
X-Authentication-Results: shopping.example.net
header.from=joe@football.example.com; dkim=pass
Received: from mout23.football.example.com (192.168.1.1)
by shopping.example.net with SMTP;
Fri, 11 Jul 2003 21:01:59 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; s=brisbane; d=example.com;
c=simple/simple; q=dns/txt; i=joe@football.example.com;
h=Received : From : To : Subject : Date : Message-ID;
bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=;
b=AuUoFEfDxTDkHlLXSZEpZj79LICEps6eda7W3deTVFOk4yAUoqOB
4nujc7YopdG5dWLSdNg6xNAZpOPr+kHxt1IrE+NahM6L/LbvaHut
KVdkLLkpVaVVQPzeRDI009SO2Il5Lu7rDNH6mZckBdrIx0orEtZV
4bmp/YzhwvcubU4=;
Received: from client1.football.example.com [192.0.2.1]
by submitserver.example.com with SUBMISSION;
Fri, 11 Jul 2003 21:01:54 -0700 (PDT)
From: Joe SixPack <joe@football.example.com>
To: Suzie Q <suzie@shopping.example.net>
Subject: Is dinner ready?
Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
Message-ID: <20030712040037.46341.5F8J@football.example.com>
Hi.
We lost the game. Are you hungry yet?
Joe.
Signature verification starts with the physically last Received header field, the From header field, and so forth, in the order listed in the "h=" tag. Verification follows with a single CRLF followed by the body (starting with "Hi."). The email is canonically prepared for verifying with the "simple" method. The result of the query and subsequent verification of the signature is stored (in this example) in the X-Authentication-Results header field line. After successful verification, the email looks like this:
DKIM signing and validating can be used in different ways, for different operational scenarios. This Appendix discusses some common examples.
In the most simple scenario, a user's MUA, MSA, and Internet (boundary) MTA are all within the same administrative environment, using the same domain name. Therefore, all of the components involved in submission and initial transfer are related. However, it is common for two or more of the components to be under independent administrative control. This creates challenges for choosing and administering the domain name to use for signing, and for its relationship to common email identity header fields.
Some organizations assign specific business functions to discrete groups, inside or outside the organization. The goal, then, is to authorize that group to sign some mail, but to constrain what signatures they can generate. DKIM selectors (the "s=" signature tag) facilitate this kind of restricted authorization. Examples of these outsourced business functions are legitimate email marketing providers and corporate benefits providers.
Here, the delegated group needs to be able to send messages that are signed, using the email domain of the client company. At the same time, the client often is reluctant to register a key for the provider that grants the ability to send messages for arbitrary addresses in the domain.
There are multiple ways to administer these usage scenarios. In one case, the client organization provides all of the public query service (for example, DNS) administration, and in another it uses DNS delegation to enable all ongoing administration of the DKIM key record by the delegated group.
If the client organization retains responsibility for all of the DNS administration, the outsourcing company can generate a key pair, supplying the public key to the client company, which then registers it in the query service, using a unique selector. The client company retains control over the use of the delegated key because it retains the ability to revoke the key at any time.
If the client wants the delegated group to do the DNS administration, it can have the domain name that is specified with the selector point to the provider's DNS server. The provider then creates and maintains all of the DKIM signature information for that selector. Hence, the client cannot provide constraints on the Local-part of addresses that get signed, but it can revoke the provider's signing rights by removing the DNS delegation record.
PDAs demonstrate the need for using multiple keys per domain. Suppose that John Doe wanted to be able to send messages using his corporate email address, jdoe@example.com, and his email device did not have the ability to make a Virtual Private Network (VPN) connection to the corporate network, either because the device is limited or because there are restrictions enforced by his Internet access provider. If the device was equipped with a private key registered for jdoe@example.com by the administrator of the example.com domain, and appropriate software to sign messages, John could sign the message on the device itself before transmission through the outgoing network of the access service provider.
Roaming users often find themselves in circumstances where it is convenient or necessary to use an SMTP server other than their home server; examples are conferences and many hotels. In such circumstances, a signature that is added by the submission service will use an identity that is different from the user's home system.
Ideally, roaming users would connect back to their home server using either a VPN or a SUBMISSION server running with SMTP AUTHentication on port 587. If the signing can be performed on the roaming user's laptop, then they can sign before submission, although the risk of further modification is high. If neither of these are possible, these roaming users will not be able to send mail signed using their own domain key.
Stand-alone services, such as walk-up kiosks and web-based information services, have no enduring email service relationship with the user, but users occasionally request that mail be sent on their behalf. For example, a website providing news often allows the reader to forward a copy of the article to a friend. This is typically done using the reader's own email address, to indicate who the author is. This is sometimes referred to as the "Evite problem", named after the website of the same name that allows a user to send invitations to friends.
A common way this is handled is to continue to put the reader's email address in the From header field of the message, but put an address owned by the email posting site into the Sender header field. The posting site can then sign the message, using the domain that is in 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 <jdoe@example.com>. (Note that such rewriting will break a signature, unless it is done after the verification pass is complete.)
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.
"Affinity addresses" allow a user to have an email address that remains stable, even as the user moves among different email providers. They are typically associated with college alumni associations, professional organizations, and recreational organizations with which they expect to have a long-term relationship. These domains usually provide forwarding of incoming email, and they often have an associated Web application that authenticates the user and allows the forwarding address to be changed. However, these services usually depend on users sending outgoing messages through their own service providers' MTAs. Hence, mail that is signed with the domain of the affinity address is not signed by an entity that is administered by the organization owning that domain.
With DKIM, affinity domains could use the Web application to allow users to register per-user keys to be used to sign messages on behalf of their affinity address. The user would take away the secret half of the key pair for signing, and the affinity domain would publish the public half in DNS for access by verifiers.
This is another application that takes advantage of user-level keying, and domains used for affinity addresses would typically have a very large number of user-level keys. Alternatively, the affinity domain could handle outgoing mail, operating a mail submission agent that authenticates users before accepting and signing messages for them. This is of course dependent on the user's service provider not blocking the relevant TCP ports used for mail submission.
In some cases a recipient is allowed to configure an email address to cause automatic redirection of email messages from the original address to another, such as through the use of a Unix .forward file. In this case, messages are typically redirected by the mail handling service of the recipient's domain, without modification, except for the addition of a Received header field to the message and a change in the envelope recipient address. In this case, the recipient at the final address' mailbox is likely to be able to verify the original signature since the signed content has not changed, and DKIM is able to validate the message signature.
There is a wide range of behaviors in services that take delivery of a message and then resubmit it. A primary example is with mailing lists (collectively called "forwarders" below), ranging from those that make no modification to the message itself, other than to add a Received header field and change the envelope information, to those that add header fields, change the Subject header field, add content to the body (typically at the end), or reformat the body in some manner. The simple ones produce messages that are quite similar to the automated alias services. More elaborate systems essentially create a new message.
A Forwarder that does not modify the body or signed header fields of a message is likely to maintain the validity of the existing signature. It also could choose to add its own signature to the message.
Forwarders which modify a message in a way that could make an existing signature invalid are particularly good candidates for adding their own signatures (e.g., mailing-list-name@example.net). Since (re-)signing is taking responsibility for the content of the message, these signing forwarders are likely to be selective, and forward or re-sign a message only if it is received with a valid signature or if they have some other basis for knowing that the message is not spoofed.
A common practice among systems that are primarily redistributors of mail is to add a Sender header field to the message, to identify the address being used to sign the message. This practice will remove any preexisting Sender header field as required by [RFC5322]. The forwarder applies a new DKIM-Signature header field with the signature, public key, and related information of the forwarder.
$ openssl genrsa -out rsa.private 1024
The default signature is an RSA signed SHA256 digest of the complete email. For ease of explanation, the openssl command is used to describe the mechanism by which keys and signatures are managed. One way to generate a 1024-bit, unencrypted private key suitable for DKIM is to use openssl like this:
The "genrsa" step results in the file rsa.private containing the key information similar to this:
-----BEGIN RSA PRIVATE KEY----- MIICXwIBAAKBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkMoGeLnQg1fWn7/zYtIxN2SnFC jxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v/RtdC2UzJ1lWT947qR+Rcac2gb to/NMqJ0fzfVjH4OuKhitdY9tf6mcwGjaNBcWToIMmPSPDdQPNUYckcQ2QIDAQAB AoGBALmn+XwWk7akvkUlqb+dOxyLB9i5VBVfje89Teolwc9YJT36BGN/l4e0l6QX /1//6DWUTB3KI6wFcm7TWJcxbS0tcKZX7FsJvUz1SbQnkS54DJck1EZO/BLa5ckJ gAYIaqlA9C0ZwM6i58lLlPadX/rtHb7pWzeNcZHjKrjM461ZAkEA+itss2nRlmyO n1/5yDyCluST4dQfO8kAB3toSEVc7DeFeDhnC1mZdjASZNvdHS4gbLIA1hUGEF9m 3hKsGUMMPwJBAPW5v/U+AWTADFCS22t72NUurgzeAbzb1HWMqO4y4+9Hpjk5wvL/ eVYizyuce3/fGke7aRYw/ADKygMJdW8H/OcCQQDz5OQb4j2QDpPZc0Nc4QlbvMsj 7p7otWRO5xRa6SzXqqV3+F0VpqvDmshEBkoCydaYwc2o6WQ5EBmExeV8124XAkEA qZzGsIxVP+sEVRWZmW6KNFSdVUpk3qzK0Tz/WjQMe5z0UunY9Ax9/4PVhp/j61bf eAYXunajbBSOLlx4D+TunwJBANkPI5S9iylsbLs6NkaMHV6k5ioHBBmgCak95JGX GMot/L2x0IYyMLAz6oLWh2hm7zwtb0CgOrPo1ke44hFYnfc= -----END RSA PRIVATE KEY-----
To extract the public-key component from the private key, use openssl like this:
$ openssl rsa -in rsa.private -out rsa.public -pubout -outform PEM
This results in the file rsa.public containing the key information similar to this:
-----BEGIN PUBLIC KEY----- MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkM oGeLnQg1fWn7/zYtIxN2SnFCjxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v/R tdC2UzJ1lWT947qR+Rcac2gbto/NMqJ0fzfVjH4OuKhitdY9tf6mcwGjaNBcWToI MmPSPDdQPNUYckcQ2QIDAQAB -----END PUBLIC KEY-----
This public-key data (without the BEGIN and END tags) is placed in the DNS:
brisbane IN TXT ("v=DKIM1; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQ"
"KBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkMoGeLnQg1fWn7/zYt"
"IxN2SnFCjxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v"
"/RtdC2UzJ1lWT947qR+Rcac2gbto/NMqJ0fzfVjH4OuKhi"
"tdY9tf6mcwGjaNBcWToIMmPSPDdQPNUYckcQ2QIDAQAB")
When a DKIM signature is verified, the processing system sometimes makes the result available to the recipient user's MUA. How to present this information to the user in a way that helps them is a matter of continuing human factors usability research. The tendency is to have the MUA highlight the SDID, in an attempt to show the user the identity that is claiming responsibility for the message. An MUA might do this with visual cues such as graphics, or it might include the address in an alternate view, or it might even rewrite the original From address using the verified information. Some MUAs might indicate which header fields were protected by the validated DKIM signature. This could be done with a positive indication on the 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 any other information that may, in the opinion of the MUA author, be deemed important to the end user.
The previous IETF version of DKIM [RFC4871] was edited by: Eric Allman, Jon Callas, Mark Delany, Miles Libbey, Jim Fenton and Michael Thomas.
That specification was the result of an extended, collaborative effort, including participation by: Russ Allbery, Edwin Aoki, Claus Assmann, Steve Atkins, Rob Austein, Fred Baker, Mark Baugher, Steve Bellovin, Nathaniel Borenstein, Dave Crocker, Michael Cudahy, Dennis Dayman, Jutta Degener, Frank Ellermann, Patrik Faeltstroem, Mark Fanto, Stephen Farrell, Duncan Findlay, Elliot Gillum, Olafur Gu[eth]mundsson, Phillip Hallam-Baker, Tony Hansen, Sam Hartman, Arvel Hathcock, Amir Herzberg, Paul Hoffman, Russ Housley, Craig Hughes, Cullen Jennings, Don Johnsen, Harry Katz, Murray S. Kucherawy, Barry Leiba, John Levine, Charles Lindsey, Simon Longsdale, David Margrave, Justin Mason, David Mayne, Thierry Moreau, Steve Murphy, Russell Nelson, Dave Oran, Doug Otis, Shamim Pirzada, Juan Altmayer Pizzorno, Sanjay Pol, Blake Ramsdell, Christian Renaud, Scott Renfro, Neil Rerup, Eric Rescorla, Dave Rossetti, Hector Santos, Jim Schaad, the Spamhaus.org team, Malte S. Stretz, Robert Sanders, Rand Wacker, Sam Weiler, and Dan Wing.
The earlier DomainKeys was a primary source from which DKIM was derived. Further information about DomainKeys is at [RFC4870].