MARF Working Group M. Kucherawy
Internet-Draft Cloudmark
Intended status: Standards Track February 28, 2012
Expires: August 31, 2012
Extensions to DKIM for Failure Reporting
draft-ietf-marf-dkim-reporting-11
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.
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This Internet-Draft will expire on August 31, 2012.
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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 . . . . . . . . . . . . . . . . . 7
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). The RR is made up of a sequence of tag-value objects (much
like DKIM key records, as defined in Section 3.6.1 of [DKIM]), but it
is entirely independent of those key records and is found at a
different name. In the case a record advertising the desire for
authentication failure reports, 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.
Section 3.6.2.2 of [DKIM] provides guidance with respect to handling
of a TXT RR that comprises multiple distinct strings ("character-
strings" in the parlance of [DNS]). The same process MUST be applied
here.
Any tag found in the content of this record that is not registered
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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:
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.
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3.3. DKIM Reporting Algorithm
Report generators MUST apply the following algorithm, or one
semantically equivalent to it, for each DKIM-Signature header field
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 RCODE 0 (NOERROR),
or if multiple TXT records are returned, terminate.
4. If the resultant TXT is in several string fragments, concatenate
them as described in Section 3.6.2.2 of [DKIM].
5. If the TXT content is syntactically invalid, 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
not lower 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
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SMTP reply to the DATA command.
This algorithm has the following advantages over previous pre-
standardization 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 the mechanism does not
support it. To offer such support, a Verifier would have to violate
the first step above and continue even in the absence of an "r=" tag.
Although that would enable the desired report, it would also create a
possible denial-of-service attack: such Verifiers would always look
for the reporting TXT record, so a generator of fraudulent messages
could simply send a large volume of messages without an "r=" tag to a
number of destinations. To avoid that outcome, reports of fraudulent
DKIM-Signature header fields are not possible using this mechanism.
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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. An exception to this
might be some out-of-band arrangement between two parties to
override it with some mutually agreed value. 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:
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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 )
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.
Negative caching offers some protection against this pattern of
abuse, although it will work only as long as the negative time-to-
live on the relevant SOA record in the DNS.
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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,
Scott Kitterman, and Andrew Sullivan.
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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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