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Versions: (draft-kucherawy-dkim-reporting) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 RFC 6651

MARF Working Group                                          M. Kucherawy
Internet-Draft                                                 Cloudmark
Intended status: Standards Track                       February 12, 2012
Expires: August 15, 2012


                Extensions to DKIM for Failure Reporting
                   draft-ietf-marf-dkim-reporting-10

Abstract

   This memo presents extensions to the DomainKeys Identified Mail
   (DKIM) specification to allow for detailed reporting of message
   authentication failures in an on-demand fashion.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 15, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.





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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Keywords . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  Imported Definitions . . . . . . . . . . . . . . . . . . .  4
     2.3.  Other Definitions  . . . . . . . . . . . . . . . . . . . .  4
   3.  Optional Reporting for DKIM  . . . . . . . . . . . . . . . . .  5
     3.1.  Extension DKIM Signature Tag . . . . . . . . . . . . . . .  5
     3.2.  DKIM Reporting TXT Record  . . . . . . . . . . . . . . . .  5
     3.3.  DKIM Reporting Algorithm . . . . . . . . . . . . . . . . .  6
   4.  Optional Reporting Address for DKIM-ADSP . . . . . . . . . . .  9
   5.  Requested Reports  . . . . . . . . . . . . . . . . . . . . . . 11
     5.1.  Requested Reports for DKIM Failures  . . . . . . . . . . . 11
     5.2.  Requested Reports for DKIM ADSP Failures . . . . . . . . . 11
   6.  Report Generation  . . . . . . . . . . . . . . . . . . . . . . 13
     6.1.  Report Format  . . . . . . . . . . . . . . . . . . . . . . 13
     6.2.  Other Guidance . . . . . . . . . . . . . . . . . . . . . . 13
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
     7.1.  DKIM Signature Tag Registration  . . . . . . . . . . . . . 14
     7.2.  DKIM ADSP Tag Registration . . . . . . . . . . . . . . . . 14
     7.3.  DKIM Reporting Tag Registry  . . . . . . . . . . . . . . . 14
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
     8.1.  Inherited Considerations . . . . . . . . . . . . . . . . . 16
     8.2.  Deliberate Misuse  . . . . . . . . . . . . . . . . . . . . 16
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 17
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 18
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 19
   Appendix B.  Examples  . . . . . . . . . . . . . . . . . . . . . . 20
     B.1.  Example Use of DKIM Signature Extension Tag  . . . . . . . 20
     B.2.  Example DKIM Reporting TXT Record  . . . . . . . . . . . . 20
     B.3.  Example Use of DKIM ADSP Extension Tags  . . . . . . . . . 21
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 22

















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1.  Introduction

   [DKIM] introduced a mechanism for message signing and authentication.
   It uses digital signing to associate a domain name with a message in
   a reliable (i.e. not forgeable) manner.  The output is a verified
   domain name that can then be subjected to some sort of evaluation
   process (e.g., advertised sender policy, comparison to a known-good
   list, submission to a reputation service, etc.).

   Deployers of message authentication technologies are increasingly
   seeking visibility into DKIM verification failures and conformance
   failures involving the published signing practices (e.g., [ADSP]) of
   an Administrative Management Domain (ADMD; see [EMAIL-ARCH]).

   This document extends [DKIM] and [ADSP] to add an optional reporting
   address and some reporting parameters.  Reports are generated using
   the format defined in [I-D.IETF-MARF-AUTHFAILURE-REPORT].


































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2.  Definitions

2.1.  Keywords

   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 [KEYWORDS].

2.2.  Imported Definitions

   The ABNF token "qp-section" is imported from [MIME].

   Numerous DKIM-specific terms used here are defined in [DKIM].  The
   definition of the ABNF token "domain-name" can also be found there.

2.3.  Other Definitions

   report generator:  A report generator is an entitiy that generates
      and sends reports.  For the scope of this memo, the term refers to
      Verifiers, as defined in Section 2.2 of [DKIM], designed also to
      generate authentication failure reports according to this
      specification.





























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3.  Optional Reporting for DKIM

   A domain name owner employing [DKIM] for email signing and
   authentication might want to know when signatures in use by specific
   keys are not successfully verifying.  Currently there is no such
   mechanism defined.

   This document adds optional "tags" (as defined in [DKIM]) to the
   DKIM-Signature header field and the DKIM key record in the DNS, using
   the formats defined in that specification.

3.1.  Extension DKIM Signature Tag

   The following tag is added to DKIM-Signature header fields when a
   Signer wishes to request that reports of failed verifications be
   generated by a Verifier:

   r= Reporting Requested (plain-text; OPTIONAL; no default).  If
      present, this tag indicates that the Signer requests that
      Verifiers generate a report when verification of the DKIM
      signature fails.  At present, the only legal value is the single
      character "y" (in either upper or lower case).  A complete
      description and illustration of how this is applied can be found
      in Section 3.3.

   ABNF:

       sig-r-tag = %x72 *WSP "=" *WSP "y"

3.2.  DKIM Reporting TXT Record

   When a Signer wishes to advertise that it wants to receive failed
   verification reports, it also places in the DNS a TXT resource record
   (RR) whose content follows the same general syntax as DKIM key
   records, as defined in Section 3.6.1 of [DKIM], in that it is made up
   of a sequence of tag-value objects.  In this case, the tags and
   values comprise the parameters to be used when generating the
   reports.  A report generator will request the content of this record
   when it sees an "r=" tag in a DKIM-Signature header field.

   The reassembly rules of Section 3.6.2.2 of [DKIM] also apply here if
   the reporting TXT record consists of several string fragments.

   Any tag found in the content of this record that is not registered
   with IANA as described in Section 7.3 MUST be ignored.

   The initial list of tags supported for the reporting TXT record is as
   follows:



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   ra=  Reporting Address (plain-text; OPTIONAL).  A dkim-quoted-
      printable string (see Section 2.11 of [DKIM]) containing the
      local-part of an email address to which a report SHOULD be sent
      when mail fails DKIM verification for one of the reasons
      enumerated below.  The value MUST be interpreted as a local-part
      only.  To construct the actual address to which the report is
      sent, the Verifier simply appends to this value an "@" followed by
      the domain name found in the "d=" tag of the DKIM-Signature header
      field.  Therefore, an ADMD making use of this specification MUST
      ensure that an email address thus constructed can receive reports
      generated as described in Section 6.  ABNF:

       rep-ra-tag = %x72.61 *WSP "=" *WSP qp-section

   rp=  Requested Report Percentage (plain-text; OPTIONAL; default is
      "100").  The value is an integer from 0 to 100 inclusive that
      indicates what percentage of incidents of signature authentication
      failures, selected at random, are to cause reports to be
      generated.  The report generator SHOULD NOT issue reports for more
      than the requested percentage of incidents.  Report generators MAY
      make use of the "Incidents:" field in [ARF] to indicate that there
      are more reportable incidents than there are reports.  ABNF:

       rep-rp-tag = %x72.70 *WSP "=" *WSP 1*3DIGIT

   rr=  Requested Reports (plain-text; OPTIONAL; default is "all").  The
      value MUST be a colon-separated list of tokens representing those
      conditions under which a report is desired.  See Section 5.1 for a
      list of valid tags.  ABNF:

       rep-rr-type = ( "all" / "d" / "o" / "p"/ "s" / "v" / "x" )
       rep-rr-tag = %x72.72 *WSP "=" *WSP rep-rr-type
                    *WSP 0* ( ":" *WSP rep-rr-type )

   rs=  Requested SMTP Error String (text; OPTIONAL; no default).  The
      value is a dkim-quoted-printable string that the publishing ADMD
      requests be included in [SMTP] error strings if messages are
      rejected during the delivery SMTP session.  ABNF:

       rep-rs-tag = %x72.73 *WSP "=" qp-section

   In the absence of an "ra=" tag, the "rp=" and "rr=" tags MUST be
   ignored, and the report generator MUST NOT issue a report.

3.3.  DKIM Reporting Algorithm

   Report generators MUST apply the following algorithm, or one
   semantically equivalent to it, for each DKIM-Signature header field



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   whose verification fails for some reason.  Note that this processing
   is done as a reporting extension only; the outcome of the specified
   DKIM evaluation MUST be otherwise unaffected.

   1.   If the DKIM-Signature field did not contain a valid "r=" tag,
        terminate.

   2.   Issue a [DNS] TXT query to the name that results from appending
        the value of the "d=" tag in the DKIM-Signature field to the
        string "_report._domainkey.".  For example, if the DKIM-
        Signature header field contains "d=example.com", issue a DNS TXT
        query to "_report._domainkey.example.com".

   3.   If the DNS query returns anything other than a success status
        code (0), also known as NOERROR, or if multiple TXT records are
        returned, terminate.

   4.   If the resultant TXT is in several string fragments, reassemble
        it as described in Section 3.6.2.2 of [DKIM].

   5.   If the TXT content is syntactically invalid, the implementation,
        terminate.

   6.   If the reason for the signature evaluation failure does not
        match one of the report requests found in the "rr=" tag (or its
        default value), terminate.

   7.   If a report percentage ("rp=") tag was present, select a random
        number between 0 and 99, inclusive; if the selected number is
        higher than the tag's value, terminate.

   8.   If no "ra=" tag was present, skip this step and the next one.
        Otherwise, determine the reporting address by extracting the
        value of the "ra=" tag and appending to it "@" followed by the
        domain name found in the "d=" tag of the DKIM-Signature header
        field.

   9.   Construct and send a report in compliance with Section 6 of this
        memo that includes as its intended recipient the address
        constructed in the previous step.

   10.  If the [SMTP] session during which the DKIM signautre was
        evaluated is still active and the SMTP server has not already
        given its response to the DATA command that relayed the message,
        and an "rs=" tag was present in the TXT record, the SMTP server
        SHOULD include the decoded string found in the "rs=" tag in its
        SMTP reply to the DATA command.




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   This algorithm has the following advantages over previous
   implementations:

   a.  If the DKIM signature fails to verify, no additional DNS check is
       made to see if reporting is requested; the request is active in
       that it is included in the DKIM-Signature header field.
       (Previous implementations included the reporting address in the
       DKIM key record, which is not queried for certain failure cases.
       This meant, for full reporting, that the key record had to be
       retrieved even when it was not otherwise necessary.)

   b.  The request is confirmed by the presence of a corresponding TXT
       record in the DNS, since the Signer thus provides the parameters
       required to construct and send the report.  This means a
       malicious Signer cannot falsely assert that someone else wants
       failure reports and cause unwanted mail to be generated.  It can
       cause additional DNS traffic against the domain listed in the
       "d=" signature tag, but negative caching of the requested DNS
       record will help to mitigate this issue.

   c.  It is not possible for a Signer to direct reports to an email
       address outside of its own domain, preventing distributed email-
       based denial-of-service attacks.

   The above procedure does not permit the detection and reporting of
   messages including a fraudulent DKIM-Signature header field, where
   such signature did not include an "r=" tag.  It might be useful to
   some Signers to receive such reports, but this use case is not
   supported.  To support this, a Verifier would have to violate the
   first step above and continue even in the absence of an "r=" tag.
   Although this satisfies this reporting requirement (which is expected
   to be unusual), it also creates a possible denial-of-service attack
   as such Verifiers will always look for the reporting TXT record, so
   the generator of fraudulent messages could simply send a large volume
   of such messages to a number of destinations.  Thus, the specified
   algorithm does not accommodate this case.















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4.  Optional Reporting Address for DKIM-ADSP

   There also exist cases in which a domain name owner employing [ADSP]
   for announcing signing practises with DKIM may want to know when
   messages are received without valid author domain signatures.
   Currently there is no such mechanism defined.

   This document adds the following optional "tags" (as defined in
   [ADSP]) to the DKIM ADSP records, using the form defined in that
   specification:

   ra=  Reporting Address (plain-text; OPTIONAL; no default).  The value
      MUST be a dkim-quoted-printable string containing the local-part
      of an email address to which a report SHOULD be sent when mail
      claiming to be from this domain failed the verification algorithm
      described in [ADSP], in particular because a message arrived
      without a signature that validates, which contradicts what the
      ADSP record claims.  The value MUST be interpreted as a local-part
      only.  To construct the actual address to which the report is
      sent, the Verifier simply appends to this value an "@" followed by
      the domain whose policy was queried in order to evaluate the
      sender's ADSP, i.e., the one taken from the RFC5322.From domain of
      the message under evaluation.  Therefore, a signer making use of
      this extension tag MUST ensure that an email address thus
      constructed can receive reports generated as described in
      Section 6.  ABNF:

       adsp-ra-tag = %x72.61 *WSP "=" qp-section

   rp=  Requested Report Percentage (plain-text; OPTIONAL; default is
      "100").  The value is a single integer from 0 to 100 inclusive
      that indicates what percentage of incidents of ADSP evaluation
      failures, selected at random, should cause reports to be
      generated.  The report generator SHOULD NOT issue reports for more
      than the requested percentage of incidents.  Report generators MAY
      make use of the "Incidents:" field in [ARF] to indicate that there
      are more reportable incidents than there are reports.  ABNF:

       adsp-rp-tag = %x72.70 *WSP "=" *WSP 1*3DIGIT

   rr=  Requested Reports (plain-text; OPTIONAL; default is "all").  The
      value MUST be a colon-separated list of tokens representing those
      conditions under which a report is desired.  See Section 5.2 for a
      list of valid tags.  ABNF:

       adsp-rr-type = ( "all" / "d" / "o" / "p" / "s" / "u" )
       adsp-rr-tag = %x72.72 *WSP "=" *WSP adsp-rr-type
                     *WSP 0* ( ":" *WSP adsp-rr-type )



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   rs=  Requested SMTP Error String (plain-text; OPTIONAL; no default).
      The value is a string the signing domain requests be included in
      [SMTP] error strings when messages are rejected during a single
      SMTP session.  ABNF:

       adsp-rs-tag = %x72.73 *WSP "=" qp-section

   In the absence of an "ra=" tag, the "rp=" and "rr=" tags MUST be
   ignored, and the report generator MUST NOT issue a report.










































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5.  Requested Reports

   This memo also includes, as the "ro" tags defined above, the means by
   which the signer can request reports for specific circumstances of
   interest.  Verifiers MUST NOT generate reports for incidents that do
   not match a requested report, and MUST ignore requests for reports
   not included in this list.

5.1.  Requested Reports for DKIM Failures

   The following report requests are defined for DKIM keys:

   all  All reports are requested.

   d  Reports are requested for signature evaluation errors that
      resulted from DNS issues (e.g., key retrieval problems).

   o  Reports are requested for any reason related to DKIM signature
      evaluation not covered by other report requests listed here.

   p  Reports are requested for signatures that are rejected for local
      policy reasons at the Verifier that are related to DKIM signature
      evaluation.

   s  Reports are requested for signature or key syntax errors.

   v  Reports are requested for signature verification failures or body
      hash mismatches.

   x  Reports are requested for signatures rejected by the Verifier
      because the expiration time has passed.

5.2.  Requested Reports for DKIM ADSP Failures

   The following report requests are defined for ADSP records:

   all  All reports are requested.

   d  Reports are requested for messages that could not have [ADSP]
      evaluated due to DNS (policy retrieval) issues.

   o  Reports are requested for any [ADSP]-related failure reason not
      covered by other report requests listed here.

   p  Reports are requested for messages that are rejected for local
      policy reasons at the Verifier that are related to [ADSP].





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   s  Reports are requested for messages that have a valid [DKIM]
      signature but do not match the published [ADSP] policy.

   u  Reports are requested for messages that have no valid [DKIM]
      signature and do not match the published [ADSP] policy.














































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6.  Report Generation

   This section describes the process for generating and sending reports
   in accordance with the request of the signer and/or sender as
   described above.

6.1.  Report Format

   All reports generated as a result of requests contained in these
   extension parameters MUST be generated in compliance with [ARF] and
   its extension specific to this work,
   [I-D.IETF-MARF-AUTHFAILURE-REPORT].

6.2.  Other Guidance

   Additional guidance about the generation of these reports can be
   found in [I-D.IETF-MARF-AS], especially Section 9.


































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7.  IANA Considerations

   As required by [IANA-CONSIDERATIONS], this section contains registry
   information for the new [DKIM] signature tags, and the new [ADSP]
   tags.  It also creates a DKIM reporting tag registry.

7.1.  DKIM Signature Tag Registration

   IANA is requested to update the DKIM Signature Tag Specification
   Registry to include the following new items:

                 +------+-----------------+--------+
                 | TYPE | REFERENCE       | STATUS |
                 +------+-----------------+--------+
                 | r    | (this document) | active |
                 +------+-----------------+--------+

7.2.  DKIM ADSP Tag Registration

   IANA is requested to update the DKIM ADSP Specification Tag Registry
   to include the following new items:

                 +------+-----------------+
                 | TYPE | REFERENCE       |
                 +------+-----------------+
                 | ra   | (this document) |
                 | rp   | (this document) |
                 | rr   | (this document) |
                 | rs   | (this document) |
                 +------+-----------------+

7.3.  DKIM Reporting Tag Registry

   IANA is requested to create a sub-registry of the DKIM Parameters
   registry called "DKIM Reporting Tags".  Additions to this registry
   follow the "Specification Required" rules, with the following columns
   required for all registrations:

   Type:  The name of the tag being used in reporting records

   Reference:  The document that specifies the tag being defined

   Status:  The status of the tag's current use, either "active"
      indicating active use, or "historic" indicating discontinued or
      deprecated use

   The initial registry entries are as follows:




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                 +------+-----------------+--------+
                 | TYPE | REFERENCE       | STATUS |
                 +------+-----------------+--------+
                 | ra   | (this document) | active |
                 | rp   | (this document) | active |
                 | rr   | (this document) | active |
                 | rs   | (this document) | active |
                 +------+-----------------+--------+











































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8.  Security Considerations

   Security issues with respect to these reports are similar to those
   found in [DSN].

8.1.  Inherited Considerations

   Implementers are advised to consider the Security Considerations
   sections of [DKIM], [ADSP], [I-D.IETF-MARF-AS], and
   [I-D.IETF-MARF-AUTHFAILURE-REPORT].  Many security issues related to
   this draft are already covered in those documents.

8.2.  Deliberate Misuse

   Some threats caused by deliberate misuse of this mechanism are
   discussed in Section 3.3.



































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9.  References

9.1.  Normative References

   [ADSP]     Allman, E., Delany, M., Fenton, J., and J. Levine, "DKIM
              Sender Signing Practises", RFC 5617, August 2009.

   [ARF]      Shafranovich, Y., Levine, J., and M. Kucherawy, "An
              Extensible Format for Email Feedback Reports", RFC 5965,
              August 2010.

   [DKIM]     Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
              "DomainKeys Identified Mail (DKIM) Signatures", RFC 6376,
              September 2011.

   [DNS]      Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [EMAIL-ARCH]
              Crocker, D., "Internet Mail Architecture", RFC 5598,
              October 2008.

   [I-D.IETF-MARF-AS]
              Falk, J. and M. Kucherawy, Ed., "Creation and Use of Email
              Feedback Reports: An Applicability Statement for the Abuse
              Reporting Format (ARF)", I-D draft-ietf-marf-as,
              January 2012.

   [I-D.IETF-MARF-AUTHFAILURE-REPORT]
              Fontana, H., "Authentication Failure Reporting using the
              Abuse Report Format",
              I-D draft-ietf-marf-authfailure-report, January 2012.

   [IANA-CONSIDERATIONS]
              Alvestrand, H. and T. Narten, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 5226, May 2008.

   [KEYWORDS]
              Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", RFC 2119, March 1997.

   [MIME]     Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

   [SMTP]     Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.




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9.2.  Informative References

   [DSN]      Moore, K. and G. Vaudreuil, "An Extensible Message Format
              for Delivery Status Notifications", RFC 3464,
              January 2003.














































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Appendix A.  Acknowledgements

   The authors wish to acknowledge the following for their review and
   constructive criticism of this proposal: Steve Atkins, Monica Chew,
   Dave Crocker, Tim Draegen, Frank Ellermann, JD Falk, John Levine, and
   Scott Kitterman.













































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Appendix B.  Examples

   This section contains examples of the use of each of the extensions
   defined by this memo.

B.1.  Example Use of DKIM Signature Extension Tag

   A DKIM-Signature field including use of the extension tag defined by
   this memo:

       DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple;
               d=example.com; s=jan2012; r=y;
               h=from:to:subject:date:message-id;
               bh=YJAYwiNdc3wMh6TD8FjVhtmxaHYHo7Z/06kHQYvQ4tQ=;
               b=jHF3tpgqr6nH/icHKIqFK2IJPtCLF0CRJaz2Hj1Y8yNwTJ
                 IMYIZtLccho3ymGF2GYqvTl2nP/cn4dH+55rH5pqkWNnuJ
                 R9z54CFcanoKKcl9wOZzK9i5KxM0DTzfs0r8

   Example 1: DKIM-Signature field using this extension

   This example DKIM-Signature field contains the "r=" tag that
   indicates reports are requested on verification failure.

   If this signature fails to verify, a TXT query will be sent to
   "_report._domainkey.example.com" to retrieve a reporting address and
   other report parameters.

B.2.  Example DKIM Reporting TXT Record

   An example DKIM Reporting TXT Record as defined by this memo:

       ra=dkim-errors; rp=100; rr=v:x

   Example 2: Example DKIM Reporting TXT Record

   This example, continuing from the previous one, shows a message that
   might be found at "_report._domainkey.example.com" in a TXT record.
   It makes the following requests:

   o  Reports about signature evaluation failures should be send to the
      address "dkim-errors" at the signer's domain;

   o  All (100%) incidents should be reported;

   o  Only reports about signature verification failures and expired
      signatures should be generated.





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B.3.  Example Use of DKIM ADSP Extension Tags

   A DKIM ADSP record including use of the extensions defined by this
   memo:

       dkim=all; ra=dkim-adsp-errors; rr=u

   Example 3: DKIM ADSP record using these extensions

   This example ADSP record makes the following assertions:

   o  The sending domain (i.e. the one that is advertising this policy)
      signs all mail it sends;

   o  Reports about ADSP evaluation failures should be send to the
      address "dkim-adsp-errors" at the Author's domain;

   o  Only reports about unsigned messages should be generated.

































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Author's Address

   Murray S. Kucherawy
   Cloudmark
   128 King St., 2nd Floor
   San Francisco, CA  94107
   US

   Phone: +1 415 946 3800
   Email: msk@cloudmark.com









































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