3. DKIM Key Generation, Storage, and Management
By itself, verification of a digital signature only allows the
verifier to conclude with a very high degree of certainty that the
signature was created by a party with access to the corresponding
private signing key. It follows that a verifier requires means to
(1) obtain the public key for the purpose of verification and (2)
infer useful attributes of the key holder.
In a traditional Public Key Infrastructure (PKI), the functions of
key distribution and key accreditation are separated. In DKIM
[RFC4871], these functions are both performed through the DNS.
In either case, the ability to infer semantics from a digital
signature depends on the assumption that the corresponding private
key is only accessible to a party with a particular set of
attributes. In a traditional PKI, a Trusted Third Party (TTP)
vouches that the key holder has been validated with respect to a
specified set of attributes. The range of attributes that can be
attested in such a scheme is thus limited only to the type of
attributes that a TTP can establish effective processes for
validating. In DKIM, TTPs are not employed and the functions of key
distribution and accreditation are combined.
Consequently, there are only two types of inference that a signer can
make from a key published in a DKIM key record:
1. That a party with the ability to control DNS records within a DNS
zone intends to claim responsibility for messages signed using
the corresponding private signature key.
2. That use of a specific key is restricted to the particular subset
of messages identified by the selector.
The ability to draw any useful conclusion from verification of a
digital signature relies on the assumption that the corresponding
private key is only accessible to a party with a particular set of
attributes. In the case of DKIM, this means that the party that
created the corresponding DKIM key record in the specific zone
intended to claim responsibility for the signed message.
Ideally, we would like to draw a stronger conclusion, that if we
obtain a DKIM key record from the DNS zone example.com, that the
legitimate holder of the DNS zone example.com claims responsibility
for the signed message. In order for this conclusion to be drawn, it
is necessary for the verifier to assume that the operational security
of the DNS zone and corresponding private key are adequate.
3.1. Private Key Management: Deployment and Ongoing Operations
Access to signing keys needs to be carefully managed to prevent use
by unauthorized parties and to minimize the consequences if a
compromise were to occur.
While a DKIM signing key is used to sign messages on behalf of many
mail users, the signing key itself needs to be under direct control
of as few key holders as possible. If a key holder were to leave the
organization, all signing keys held by that key holder need to be
withdrawn from service and, if appropriate, replaced.
If key management hardware support is available, it needs to be used.
If keys are stored in software, appropriate file control protections
need to be employed, and any location in which the private key is
stored in plaintext form needs to be excluded from regular backup
processes and is best not accessible through any form of network
including private local area networks. Auditing software needs to be
used periodically to verify that the permissions on the private key
files remain secure.
Wherever possible, a signature key needs to exist in exactly one
location and be erased when no longer used. Ideally, a signature key
pair needs to be generated as close to the signing point as possible,
and only the public key component transferred to another party. If
this is not possible, the private key needs to be transported in an
encrypted format that protects the confidentiality of the signing
key. A shared directory on a local file system does not provide
adequate security for distribution of signing keys in plaintext form.
Key escrow schemes are not necessary and are best not used. In the
unlikely event of a signing key becoming lost, a new signature key
pair can be generated as easily as recovery from a key escrow scheme.
To enable accountability and auditing:
o Responsibility for the security of a signing key needs to
ultimately vest in a single named individual.
o Where multiple parties are authorized to sign messages, each
signer needs to use a different key to enable accountability and
Best practices for management of cryptographic keying material
require keying material to be refreshed at regular intervals,
particularly where key management is achieved through software.
While this practice is highly desirable, it is of considerably less
importance than the requirement to maintain the secrecy of the
corresponding private key. An operational practice in which the
private key is stored in tamper-proof hardware and changed once a
year is considerably more desirable than one in which the signature
key is changed on an hourly basis but maintained in software.
3.2. Storing Public Keys: DNS Server Software Considerations
In order to use DKIM, a DNS domain holder requires (1) the ability to
create the necessary DKIM DNS records and (2) sufficient operational
security controls to prevent insertion of spurious DNS records by an
DNS record management is often operated by an administrative staff
that is different from those who operate an organization's email
service. In order to ensure that DKIM DNS records are accurate, this
imposes a requirement for careful coordination between the two
operations groups. If the best practices for private key management
described above are observed, such deployment is not a one-time
event; DNS DKIM selectors will be changed over time as signing keys
are terminated and replaced.
At a minimum, a DNS server that handles queries for DKIM key records
needs to allow the server administrators to add free-form TXT
records. It would be better if the DKIM records could be entered
using a structured form, supporting the DKIM-specific fields.
Ideally, DNS Security (DNSSEC) [RFC4034] needs to be employed in a
configuration that provides protection against record insertion
attacks and zone enumeration. In the case that NextSECure version 3
(NSEC3) [RFC5155] records are employed to prevent insertion attack,
the OPT-OUT flag needs to be clear. (See [RFC5155] section 6 for
3.2.1. Assignment of Selectors
Selectors are assigned according to the administrative needs of the
signing domain, such as for rolling over to a new key or for the
delegation of the right to authenticate a portion of the namespace to
a TTP. Examples include:
It is intended that assessments of DKIM identities be based on the
domain name, and not include the selector. While past practice of a
signer can permit a verifier to infer additional properties of
particular messages from the structure DKIM key selector, unannounced
administrative changes such as a change of signing software can cause
such heuristics to fail at any time.
3.3. Per-User Signing Key Management Issues
While a signer can establish business rules, such as the issue of
individual signature keys for each end-user, DKIM makes no provision
for communicating these to other parties. Out-of-band distribution
of such business rules is outside the scope of DKIM. Consequently,
there is no means by which external parties can make use of such keys
to attribute messages with any greater granularity than a DNS domain.
If per-user signing keys are assigned for internal purposes (e.g.,
authenticating messages sent to an MTA (Mail Transfer Agent) for
distribution), the following issues need to be considered before
using such signatures as an alternative to traditional edge signing
at the outbound MTA:
External verifiers will be unable to make use of the additional
signature granularity without access to additional information
passed out of band with respect to [RFC4871].
If the number of user keys is large, the efficiency of local
caching of key records by verifiers will be lower.
A large number of end users is be less likely to do an adequate
job of managing private key data securely on their personal
computers than is an administrator running an edge MTA.
3.4. Third-Party Signer Key Management and Selector Administration
A DKIM key record only asserts that the holder of the corresponding
domain name makes a claim of responsibility for messages signed under
the corresponding key. In some applications, such as bulk mail
delivery, it is desirable to delegate use of the key. That is, to
allow a third party to sign on behalf of the domain holder. The
trust relationship is still established between the domain holder and
the verifier, but the private signature key is held by a third party.
Signature keys used by a third-party signer need to be kept entirely
separate from those used by the domain holder and other third-party
signers. To limit potential exposure of the private key, the
signature key pair needs to be generated by the third-party signer
and the public component of the key transmitted to the domain holder,
rather than have the domain holder generate the key pair and transmit
the private component to the third-party signer.
Domain holders needs to adopt a least-privilege approach and grant
third-party signers the minimum access necessary to perform the
desired function. Limiting the access granted to third-party signers
serves to protect the interests of both parties. The domain holder
minimizes its security risk and the TTP signer avoids unnecessary
In the most restrictive case, domain holders maintain full control
over the creation of key records. They can employ appropriate key
record restrictions to enforce limits on the messages for which the
third-party signer is able to sign. If such restrictions are
impractical, the domain holder needs to delegate a DNS subzone for
publishing key records to the third-party signer. It is best that
the domain holder NOT allow a third-party signer unrestricted access
to its DNS service for the purpose of publishing key records.
3.5. Key Pair / Selector Life Cycle Management
Deployments need to establish, document, and observe processes for
managing the entire life cycle of an asymmetric key pair.
3.5.1. Example Key Deployment Process
When it is determined that a new key pair is required:
1. A Key Pair is generated by the signing device.
2. A proposed key selector record is generated and transmitted to
the DNS administration infrastructure.
3. The DNS administration infrastructure verifies the authenticity
of the key selector registration request. If accepted:
1. A key selector is assigned.
2. The corresponding key record is published in the DNS.
3. Wait for DNS updates to propagate (if necessary).
4. Report assigned key selector to signing device.
4. The signer verifies correct registration of the key record.
5. The signer begins generating signatures using the new key pair.
6. The signer terminates any private keys that are no longer
required due to issue of replacement.
3.5.2. Example Key Termination Process
When it is determined that a private signature key is no longer
1. The signer stops using the private key for signature operations.
2. The signer deletes all records of the private key, including in-
memory copies at the signing device.
3. The signer notifies the DNS administration infrastructure that
the signing key is withdrawn from service and that the
corresponding key records can be withdrawn from service at a
specified future date.
4. The DNS administration infrastructure verifies the authenticity
of the key selector termination request. If accepted,
1. The key selector is scheduled for deletion at a future time
determined by site policy.
2. Wait for deletion time to arrive.
3. The signer either publishes a revocation key selector with an
empty public-key data (p=) field, or deletes the key selector
5. As far as the verifier is concerned, there is no functional
difference between verifying against a key selector with an empty
p= field, and verifying against a missing key selector: both
result in a failed signature and the signature needs to be
treated as if it had not been there. However, there is a minor
semantic difference: with the empty p= field, the signer is
explicitly stating that the key has been revoked. The empty p=
record provides a gravestone for an old selector, making it less
likely that the selector might be accidentally reused with a
different public key.
Creating messages that have one or more DKIM signatures requires
support in only two outbound email service components:
o A DNS Administrative interface that can create and maintain the
relevant DNS names -- including names with underscores -- and
resource records (RR).
o A trusted module, called the signing module, which is within the
organization's outbound email handling service and which creates
and adds the DKIM-Signature: header field(s) to the message.
If the module creates more than one signature, there needs to be the
appropriate means of telling it which one(s) to use. If a large
number of names are used for signing, it will help to have the
administrative tool support a batch-processing mode.
4.1. DNS Records
A receiver attempting to verify a DKIM signature obtains the public
key that is associated with the signature for that message. The
DKIM-Signature: header in the message contains the d= tag with the
basic domain name doing the signing and serving as output to the
Identity Assessor and the s= tag with the selector that is added to
the name, for finding the specific public key. Hence, the relevant
<selector>._domainkey.<domain-name> DNS record needs to contain a
DKIM-related RR that provides the public key information.
The administrator of the zone containing the relevant domain name
adds this information. Initial DKIM DNS information is contained
within TXT RRs. DNS administrative software varies considerably in
its abilities to support DKIM names, such as with underscores, and to
add new types of DNS information.
4.2. Signing Module
The module doing signing can be placed anywhere within an
organization's trusted Administrative Management Domain (ADMD);
obvious choices include department-level posting agents, as well as
outbound boundary MTAs to the open Internet. However, any other
module, including the author's MUA (Mail User Agent), is potentially
acceptable, as long as the signature survives any remaining handling
within the ADMD. Hence, the choice among the modules depends upon
software development, administrative overhead, security exposures,
and transit-handling tradeoffs. One perspective that helps to
resolve this choice is the difference between the increased
flexibility, from placement at (or close to) the MUA, versus the
streamlined administration and operation that is more easily obtained
by implementing the mechanism "deeper" into the organization's email
infrastructure, such as at its boundary MTA.
Note the discussion in Section 2.2 concerning the use of the i= tag.
The signing module uses the appropriate private key to create one or
more signatures. (See Section 6.5 for a discussion of multiple
signatures.) The means by which the signing module obtains the
private key(s) is not specified by DKIM. Given that DKIM is intended
for use during email transit, rather than for long-term storage, it
is expected that keys will be changed regularly. For administrative
convenience, it is best not to hard-code key information into
4.3. Signing Policies and Practices
Every organization (ADMD) will have its own policies and practices
for deciding when to sign messages (message stream) and with what
domain name, selector, and key. Examples of particular message
streams include all mail sent from the ADMD versus mail from
particular types of user accounts versus mail having particular types
of content. Given this variability, and the likelihood that signing
practices will change over time, it will be useful to have these
decisions represented through run-time configuration information,
rather than being hard-coded into the signing software.
As noted in Section 2.3, the choice of signing name granularity
requires balancing administrative convenience and utility for
recipients. Too much granularity is higher administrative overhead
and might well attempt to impose more differential analysis on the
recipient than they wish to support. In such cases, they are likely
to use only a super-name -- right-hand substring -- of the signing
name. When this occurs, the signer will not know what portion is
being used; this then moves DKIM back to the non-deterministic world
of heuristics, rather than the mechanistic world of signer/recipient
collaboration that DKIM seeks.
A message recipient can verify a DKIM signature to determine if a
claim of responsibility has been made for the message by a trusted
Access control requires two components: authentication and
authorization. By design, verification of a DKIM signature only
provides the authentication component of an access control decision
and needs to be combined with additional sources of information such
as reputation data to arrive at an access control decision.
5.1. Intended Scope of Use
DKIM requires that a message with a signature that is found to be
invalid is to be treated as if the message had not been signed at
If a DKIM signature fails to verify, it is entirely possible that the
message is valid and that either there is a configuration error in
the signer's system (e.g., a missing key record) or that the message
was inadvertently modified in transit. It is thus undesirable for
mail infrastructure to treat messages with invalid signatures less
favorably than those with no signatures whatsoever. Contrariwise,
creation of an invalid signature requires a trivial amount of effort
on the part of an attacker. If messages with invalid signatures were
to be treated preferentially to messages with no signatures
whatsoever, attackers will simply add invalid signature blocks to
gain the preferential treatment. It follows that messages with
invalid signatures need to be treated no better and no worse than
those with no signature at all.
5.2. Signature Scope
As with any other digital signature scheme, verifiers need to
consider only the part of the message that is inside the scope of the
message as being authenticated by the signature.
For example, if the l= option is employed to specify a content length
for the scope of the signature, only the part of the message that is
within the scope of the content signature would be considered
5.3. Design Scope of Use
Public key cryptography provides an exceptionally high degree of
assurance, bordering on absolute certainty, that the party that
created a valid digital signature had access to the private key
corresponding to the public key indicated in the signature.
In order to make useful conclusions from the verification of a valid
digital signature, the verifier is obliged to make assumptions that
fall far short of absolute certainty. Consequently, mere validation
of a DKIM signature does not represent proof positive that a valid
claim of responsibility was made for it by the indicated party, that
the message is authentic, or that the message is not abusive. In
o The legitimate private key holder might have lost control of its
o The legitimate domain holder might have lost control of the DNS
server for the zone from which the key record was retrieved.
o The key record might not have been delivered from the legitimate
DNS server for the zone from which the key record was retrieved.
o Ownership of the DNS zone might have changed.
In practice, these limitations have little or no impact on the field
of use for which DKIM is designed, but they can have a bearing if use
is made of the DKIM message signature format or key retrieval
mechanism in other specifications.
In particular, the DKIM key retrieval mechanism is designed for ease
of use and deployment rather than to provide a high assurance Public
Key Infrastructure suitable for purposes that require robust non-
repudiation such as establishing legally binding contracts.
Developers seeking to extend DKIM beyond its design application need
to consider replacing or supplementing the DNS key retrieval
mechanism with one that is designed to meet the intended purposes.
5.4. Inbound Mail Filtering
DKIM is frequently employed in a mail filtering strategy to avoid
performing content analysis on email originating from trusted
sources. Messages that carry a valid DKIM signature from a trusted
source can be whitelisted, avoiding the need to perform computation
and hence energy-intensive content analysis to determine the
disposition of the message.
Mail sources can be determined to be trusted by means of previously
observed behavior and/or reference to external reputation or
accreditation services. The precise means by which this is
accomplished is outside the scope of DKIM.
5.4.1. Non-Verifying Adaptive Spam Filtering Systems
Adaptive (or learning) spam filtering mechanisms that are not capable
of verifying DKIM signatures need to, at minimum, be configured to
ignore DKIM header data entirely.
5.5. Messages Sent through Mailing Lists and Other Intermediaries
Intermediaries, such as mailing lists, pose a particular challenge
for DKIM implementations, as the message processing steps performed
by the intermediary can cause the message content to change in ways
that prevent the signature passing verification.
Such intermediaries are strongly encouraged to deploy DKIM signing so
that a verifiable claim of responsibility remains available to
parties attempting to verify the modified message.
5.6. Generation, Transmission, and Use of Results Headers
In many deployments, it is desirable to separate signature
verification from the application relying on the verification. A
system can choose to relay information indicating the results of its
message authentication efforts using various means; adding a "results
header" to the message is one such mechanism [RFC5451]. For example,
consider the cases where:
o The application relying on DKIM signature verification is not
capable of performing the verification.
o The message can be modified after the signature verification is
o The signature key cannot be available by the time that the message
In such cases, it is important that the communication link between
the signature verifier and the relying application be sufficiently
secure to prevent insertion of a message that carries a bogus results
An intermediary that generates results headers need to ensure that
relying applications are able to distinguish valid results headers
issued by the intermediary from those introduced by an attacker. For
example, this can be accomplished by signing the results header. At
a minimum, results headers on incoming messages need to be removed if
they purport to have been issued by the intermediary but cannot be
verified as authentic.
Further discussion on trusting the results as relayed from a verifier
to something downstream can be found in [RFC5451].
6. Taxonomy of Signatures
As described in Section 2.1, a DKIM signature tells the signature
verifier that the owner of a particular domain name accepts some
responsibility for the message. It does not, in and of itself,
provide any information about the trustworthiness or behavior of that
identity. What it does provide is a verified identity to which such
behavioral information can be associated, so that those who collect
and use such information can be assured that it truly pertains to the
identity in question.
This section lays out a taxonomy of some of the different identities,
or combinations of identities, that might usefully be represented by
a DKIM signature.
6.1. Single Domain Signature
Perhaps the simplest case is when an organization signs its own
outbound email using its own domain in the SDID [RFC5672] of the
signature. For example, Company A would sign the outbound mail from
its employees with d=companyA.example.
In the most straightforward configuration, the addresses in the
RFC5322.From field would also be in the companyA.example domain, but
that direct correlation is not required.
A special case of the single domain signature is an author signature
as defined by the Author Domain Signing Practices specification
[RFC5617]. Author signatures are signatures from an author's
organization that have an SDID value that matches that of an
RFC5322.From address of the signed message.
Although an author signature might, in some cases, be proof against
spoofing the domain name of the RFC5322.From address, it is important
to note that the DKIM and ADSP validation apply only to the exact
address string and not to look-alike addresses or to the human-
friendly "display-name" or names and addresses used within the body
of the message. That is, it only protects against the misuse of a
precise address string within the RFC5322.From field and nothing
else. For example, a message from email@example.com with a valid
signature where d=d0main.example would fail an ADSP check because the
signature domain, however similar, is distinct; however, a message
from firstname.lastname@example.org with a valid signature where d=d0main.example
would pass an ADSP check, even though to a human it might be obvious
that d0main.example is likely a malicious attempt to spoof the domain
domain.example. This example highlights that ADSP, like DKIM, is
only able to validate a signing identifier: it still requires some
external process to attach a meaningful reputation to that
6.2. Parent Domain Signature
Another approach that might be taken by an organization with multiple
active subdomains is to apply the same (single) signature domain to
mail from all subdomains. In this case, the signature chosen would
usually be the signature of a parent domain common to all subdomains.
For example, mail from marketing.domain.example,
sales.domain.example, and engineering.domain.example might all use a
signature where d=domain.example.
This approach has the virtue of simplicity, but it is important to
consider the implications of such a choice. As discussed in
Section 2.3, if the type of mail sent from the different subdomains
is significantly different or if there is reason to believe that the
reputation of the subdomains would differ, then it can be a good idea
to acknowledge this and provide distinct signatures for each of the
subdomains (d=marketing.domain.example, sales.domain.example, etc.).
However, if the mail and reputations are likely to be similar, then
the simpler approach of using a single common parent domain in the
signature can work well.
Another approach to distinguishing the streams using a single DKIM
key would be to leverage the AUID [RFC5672] (i= tag) in the DKIM
signature to differentiate the mail streams. For example, marketing
email would be signed with email@example.com and
It's important to remember, however, that under core DKIM semantics,
the AUID is opaque to receivers. That means that it will only be an
effective differentiator if there is an out-of-band agreement about
the i= semantics.
6.3. Third-Party Signature
A signature whose domain does not match the domain of the
RFC5322.From address is sometimes referred to as a third-party
signature. In certain cases, even the parent domain signature
described above would be considered a third-party signature because
it would not be an exact match for the domain in the RFC5322.From
Although there is often heated debate about the value of third party
signatures, it is important to note that the DKIM specification
attaches no particular significance to the identity in a DKIM
signature ([RFC4871], [RFC5672]). The identity specified within the
signature is the identity that is taking responsibility for the
message, and it is only the interpretation of a given receiver that
gives one identity more or less significance than another. In
particular, most independent reputation services assign trust based
on the specific identifier string, not its "role": in general they
make no distinction between, for example, an author signature and a
For some, a signature unrelated to the author domain (the domain in
the RFC5322.From address) is less valuable because there is an
assumption that the presence of an author signature guarantees that
the use of the address in the RFC5322.From header is authorized.
For others, that relevance is tied strictly to the recorded
behavioral data assigned to the identity in question, i.e., its trust
assessment or reputation. The reasoning here is that an identity
with a good reputation is unlikely to maintain that good reputation
if it is in the habit of vouching for messages that are unwanted or
abusive; in fact, doing so will rapidly degrade its reputation so
that future messages will no longer benefit from it. It is therefore
low risk to facilitate the delivery of messages that contain a valid
signature of a domain with a strong positive reputation, independent
of whether or not that domain is associated with the address in the
RFC5322.From header field of the message.
Third-party signatures encompass a wide range of identities. Some of
the more common are:
Service Provider: In cases where email is outsourced to an Email
Service Provider (ESP), Internet Service Provider (ISP), or other
type of service provider, that service provider can choose to
DKIM-sign outbound mail with either its own identifier -- relying
on its own, aggregate reputation -- or with a subdomain of the
provider that is unique to the message author but still part of
the provider's aggregate reputation. Such service providers can
also encompass delegated business functions such as benefit
management, although these will more often be treated as trusted
third-party senders (see below).
Parent Domain: As discussed above, organizations choosing to apply a
parent-domain signature to mail originating from subdomains can
have their signatures treated as third party by some verifiers,
depending on whether or not the "t=s" tag is used to constrain the
parent signature to apply only to its own specific domain. The
default is to consider a parent-domain signature valid for its
Reputation Provider: Another possible category of third-party
signature would be the identity of a third-party reputation
provider. Such a signature would indicate to receivers that the
message was being vouched for by that third party.
6.4. Using Trusted Third-Party Senders
For most of the cases described so far, there has been an assumption
that the signing agent was responsible for creating and maintaining
its own DKIM signing infrastructure, including its own keys, and
signing with its own identity.
A different model arises when an organization uses a trusted third-
party sender for certain key business functions, but still wants that
email to benefit from the organization's own identity and reputation.
In other words, the mail would come out of the trusted third party's
mail servers, but the signature applied would be that of the
This can be done by having the third party generate a key pair that
is designated uniquely for use by that trusted third party and
publishing the public key in the controlling organization's DNS
domain, thus enabling the third party to sign mail using the
signature of the controlling organization. For example, if Company A
outsources its employee benefits to a third party, it can use a
special key pair that enables the benefits company to sign mail as
"companyA.example". Because the key pair is unique to that trusted
third party, it is easy for Company A to revoke the authorization if
necessary by simply removing the public key from the companyA.example
A more cautious approach would be to create a dedicated subdomain
(e.g., benefits.companyA.example) to segment the outsourced mail
stream, and to publish the public key there; the signature would then
6.4.1. DNS Delegation
Another possibility for configuring trusted third-party access, as
discussed in Section 3.4, is to have Company A use DNS delegation and
have the designated subdomain managed directly by the trusted third
party. In this case, Company A would create a subdomain
benefits.companya.example, and delegate the DNS management of that
subdomain to the benefits company so it could maintain its own key
records. When revocation becomes necessary, Company A could simply
remove the DNS delegation record.
6.5. Multiple Signatures
A simple configuration for DKIM-signed mail is to have a single
signature on a given message. This works well for domains that
manage and send all of their own email from single sources, or for
cases where multiple email streams exist but each has its own unique
key pair. It also represents the case in which only one of the
participants in an email sequence is able to sign, no matter whether
it represents the author or one of the operators.
The examples thus far have considered the implications of using
different identities in DKIM signatures, but have used only one such
identity for any given message. In some cases, it can make sense to
have more than one identity claiming responsibility for the same
There are a number of situations where applying more than one DKIM
signature to the same message might make sense. A few examples are:
Companies with multiple subdomain identities: A company that has
multiple subdomains sending distinct categories of mail might
choose to sign with distinct subdomain identities to enable each
subdomain to manage its own identity. However, it might also want
to provide a common identity that cuts across all of the distinct
subdomains. For example, Company A can sign mail for its sales
department with a signature where d=sales.companya.example and a
second signature where d=companya.example
Service Providers: A service provider can, as described above,
choose to sign outbound messages with either its own identity or
an identity unique to each of its clients (possibly delegated).
However, it can also do both: sign each outbound message with its
own identity as well as with the identity of each individual
client. For example, ESP A might sign mail for its client Company
B with its service provider signature d=espa.example, and a second
client-specific signature where d= either companyb.example or
companyb.espa.example. The existence of the service provider
signature could, for example, help cover a new client while it
establishes its own reputation, or help a very small volume client
who might never reach a volume threshold sufficient to establish
an individual reputation.
Forwarders: Forwarded mail poses a number of challenges to email
authentication. DKIM is relatively robust in the presence of
forwarders as long as the signature is designed to avoid message
parts that are likely to be modified; however, some forwarders do
make modifications that can invalidate a DKIM signature.
Some forwarders such as mailing lists or "forward article to a
friend" services might choose to add their own signatures to
outbound messages to vouch for them having legitimately originated
from the designated service. In this case, the signature would be
added even in the presence of a preexisting signature, and both
signatures would be relevant to the verifier.
Any forwarder that modifies messages in ways that will break
preexisting DKIM signatures needs to sign its forwarded messages.
Reputation Providers: Although third-party reputation providers
today use a variety of protocols to communicate their information
to receivers, it is possible that they, or other organizations
willing to put their "seal of approval" on an email stream, might
choose to use a DKIM signature to do it. In nearly all cases,
this "reputation" signature would be in addition to the author or
One important caveat to the use of multiple signatures is that there
is currently no clear consensus among receivers on how they plan to
handle them. The opinions range from ignoring all but one signature
(and the specification of which of them is verified differs from
receiver to receiver), to verifying all signatures present and
applying a weighted blend of the trust assessments for those
identifiers, to verifying all signatures present and simply using the
identifier that represents the most positive trust assessment. It is
likely that the industry will evolve to accept multiple signatures
using either the second or third of these, but it can take some time
before one approach becomes pervasive.