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RFC 5863

DomainKeys Identified Mail (DKIM) Development, Deployment, and Operations

Pages: 51
Informational
Part 2 of 3 – Pages 15 to 31
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Top   ToC   RFC5863 - Page 15   prevText

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
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   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.
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   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
      auditing.

   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 attacker. 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 details.)
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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: jun2005.eng._domainkey.example.com widget.promotion._domainkey.example.com 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.
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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 liability. 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.
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   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 required: 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 record entirely. 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
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       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.

4. Signing

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
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   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
   software.

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.
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5. Verifying

A message recipient can verify a DKIM signature to determine if a claim of responsibility has been made for the message by a trusted domain. 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 all. 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 authentic.
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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 particular: o The legitimate private key holder might have lost control of its private key. 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.
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   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 performed. o The signature key cannot be available by the time that the message is read. 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 header. 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
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   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 bob@domain.example with a valid
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   signature where d=d0main.example would fail an ADSP check because the
   signature domain, however similar, is distinct; however, a message
   from bob@d0main.example 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
   identifier.

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 i=@marketing.domain.example and d=domain.example. 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
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   described above would be considered a third-party signature because
   it would not be an exact match for the domain in the RFC5322.From
   address.

   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
   third-party signature.

   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).
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   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
      subdomains.

   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 controlling organization. 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 DNS. 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 use d=benefits.companyA.example.
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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 message. 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
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      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
      originator signature.

   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.



(page 31 continued on part 3)

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