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UTA                                                            R. Mukhia
Internet-Draft                                              B. Rajendran
Intended status: Standards Track                       BS. Bindhumadhava
Expires: December 12, 2019                                CDAC Bangalore
                                                           June 10, 2019

     An approach for end-to-end Email Security with DANE and DMARC


   An end-to-end email security solution is proposed by implementing
   both DANE and DMARC protocols.  DMARC enables the recipient's mail
   server, with a method to verify the sender's ingenuity.  DANE intends
   to mitigate the MITM attack, by enabling the sender a method to
   authenticate the recipient's mail domain.  DANE and DMARC therefore
   complement each other by allowing the sender to verify the
   recipient's domain, and the recipient to verify the sender's address

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
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   This Internet-Draft will expire on December 12, 2019.

Copyright Notice

   Copyright (c) 2019 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
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   (https://trustee.ietf.org/license-info) in effect on the date of
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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Architecture of DANE with DAMRC for secure Email  . . . . . .   3
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  Normative References  . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   SMTP is a hop-by-hop mechanism.  For a long time now, email servers
   have had the option of using TLS to transparently encrypt the message
   transmission from one server to other.  Use of TLS with SMTP,when
   available ensures that the message content are secured during
   transmission between the servers.But not all servers support TLS.Some
   of the reasons many email providers doesn't support TLS are

   1.  Purchase of one or more SSL certificates is not done

   2.  Configuration of the email servers to use them (and keep these
       configurations updated)is not done

   3.  Allocation of additional computational resources on the email
       servers is not involved

   There are some issues from sending computers or servers also like,
   They never use TLS or They use TLS if receiver side is also using it
   otherwise sends insecurely or They use TLS otherwise doesn't deliver
   at all.

   Now comes the point that actually how secure is SMTP TLS.TLS protects
   the transmission of the content of the email messages,but it doesn't
   do anything for protecting the security of the message before it is
   sent or after it arrives at its destination .And for that, other
   encryption mechanisms are required.There are many reasons to say SMTP
   TLS doesn't provide end-to-end security.As there is no mandatory
   support for SSL/TLS in the email system.

   A receiver's support of the SMTP TLS can be removed by a Man-in-the-
   middle.  In such cases opportunistic TLS will deliver messages
   securely and forced TLS will not deliver the message.If any aspect of
   the TLS negotiation is garbled,then encryption is not used.  It is

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   very easy for a man-in-the-middle to inject garbage into the TLS
   handshake(which is done in clear text ) and have the connection
   downgraded to plain text(opportunistic TLS) or have the connection
   forced(forced TLS).Even when the SMTP TLS is offered and accepted,the
   certificate presented during the TLS handshake is usually not checked
   to see if it is really for the expected domain and unexpired.Most
   MTA's offer self signed certificates, therefore in many cases one has
   an encrypted channel to an unauthenticated MTA, which can only
   prevent passive eavesdropping.

   To mitigate the mentioned problems with SMTP TLS, DANE and DMARC can
   be used with SMTP.DANE prevents middle man by giving sender a method
   secured using DNSSEC,it ensures that message goes only to the
   receiver.This is done when key provided by receiver's mail exchanges
   matches with the key he has authorized in DNS to receive mail for his

   Phishing is a very common type of threat,it can be avoided if DMARC
   is implemented, as both DKIM and SPF are part of DMARC.It is job of
   DKIM to authenticate the domain that affixed the signature of the
   message.Therefore DMARC intends to mitigate the threat of arbitrary

   As we know,SMTP is not designed keeping sender in mind,attacker can
   easily connect to receiver's mail server and send him email appearing
   to be coming from sender.In this case,DMARC provides the solution by
   giving receiver mail server a method to verify that the sender is
   genuine and this is done via two methods either via cryptographic
   signature using DKIM or via IP ACL using SPF.So DANE and DMARC are
   complimentary to each other, DANE ensures that the correct receiver
   receives the message while the messages are correctly encrypted in
   the transit and DMARC makes sure that messages are coming from
   legitimate sender.

2.  Architecture of DANE with DAMRC for secure Email

   1.   The sender creates a message.

   2.   SHA-256 is used to generate a 256-bit message digest of the
        message.  The combination of SHA-256 and RSA provides an
        effective digital signature scheme.  Because of the strength of
        RSA, the recipient is assured that only the possessor of the
        matching private key could have generated the signature.
        Because of the strength of SHA-256, the recipient is assured
        that no one else could generate a new message that matches the
        hash code and, hence, the signature of the original message.

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   3.   The message digest is encrypted with RSA using the sender's
        private key, and the result is appended to the message.

   4.   DNS Based Authentication of Named Entities(DANE) offers the
        option to use the DNSSEC infrastructure to store and sign keys
        an certificates that are used by TLS.  This is to avoid a
        condition when many number of CA's are compromised then the
        attacker can obtain the private key of the CA, issues
        certificates under a false name, or introduce new bogus root
        certificates into a root certificate store.There is no
        limitation of scope for the global PKI, and a compromise of a
        single CA can damage the integrity of the entire PKI system.

   5.   Domain Keys Identified Mail (DKIM) permits a person,role,or
        organization that owns the signing domain to claim some
        responsibility for a message by associating the domain with the
        message.The domain can be an author's organization,an
        operational relay,or one of their agents.Responsibility is
        validated through a cryptographic signature and by querying the
        signer's domain directly to retrieve the appropriate public key
        which is provided to the receiving MTA.

   6.   DMARC works with SPF and DKIM.SPF enables senders to advise
        receivers, via DNS, whether mail purporting to come from the
        sender is valid, and whether it should be delivered, flagged, or
        discarded.  DKIM authenticates the domain that affixed a
        signature to the message.  SPF focuses on the SMTP envelope.
        DMARC requires that the From address match (be aligned with) an
        Authenticated Identifier from DKIM or SPF.  In the case of DKIM,
        the match is made between the DKIM signing domain and the From
        domain.  In the case of SPF, the match is between the SPF-
        authenticated domain and the From domain.

   7.   Signature is then passed onto the receiving MTA then to the MUA
        and following steps take place.

   8.   TLSA, DNS record type, which can be used for a secure method of
        authenticating Secure Sockets Layer/Transport Layer Security
        (SSL/TLS) certificates.  The TLSA provides for:

        1.  Specifying constraints on which CA can vouch for a
            certificate, or which specific PKI end-entity certificate is

        2.  Specifying that a service certificate or a CA can be
            directly authenticated in the DNS itself

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        The TLSA RR enables certificate issue and delivery to be tied to
        a given domain.  A server domain owner creates a TLSA resource
        record that identifies the certificate and its public key.  When
        a client receives an X.509 certificate in the TLS negotiation,
        it looks up the TLSA RR for that domain and matches the TLSA
        data against the certificate as part of the client's certificate
        validation procedure.

   9.   The receiver uses RSA with its private key to decrypt and
        recover the content-encryption key.

   10.  The content-encryption key is used to decrypt the message.

3.  IANA Considerations

   This memo includes no request to IANA.

4.  Security Considerations

   The security of the DNS RRtype relies on the security of DNSSEC to
   verify that the TLSA record has not been altered.  A better design
   for authenticating DNS would be to have the same level of
   authentication used for all DNS additions and changes for a
   particular domain name.DNSSEC forms certificates(the binding of an
   identity to a key) by combining a DNSKey,DS or DLV resource record
   with an associate RRSIG record.These records then form a signing
   chain extending from the clients trust anchors to the RR of interest.
   The risk that a given certificate that has a valid signing chaining
   fake is related to the number of keys that can contribute to the
   validation of the certificate the quality of protection each private
   key receives,the value of each key to an attacker and the value of
   falsifying the certificate.

   DNSSEC allows any set of domains to be configured as trust anchors
   and/or DLVs, but most clients are likely to use the root zone as
   their only trust anchor.Also because a given DNSKey can only sign
   resources record for that zone,the number of private keys capable of
   compromising a given TLSA resource record and the nearest trust
   anchor,plus any configured DLV Domains.Typically this will be six
   keys,half of which will be KSKs.  KSK is stored off-line and
   protected more carefully than the ZSK,but not all the domains do so.
   The Security applied to a zone's DNSKey should be proportional to the
   value of domain,but that is difficult to estimate.For Example the
   root DNSKey has protections and controls comparable to or exceeding
   those of public CAs.On the other hand,small domain might provide no
   more protection to their keys than they do to their other data.DNSKey
   are limited in what they can sign ,so a compromise of the DNSKey
   for"example.com" provides no avenue of attack against

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   "example.org".Therefore the impact of a compromise of.Com's DNSKey
   ,while considerable would be limited t .com domains.

   Public CAs are not typically constrained in what names they can sign
   and therefore a compromise of even one CA allows the attacker to
   generate a certificate for any name in the DNS.  Since TLSA
   certificate association is constrained to it's associated
   name,protocol and port,the PKIX certificate is similarly
   constrained,even if it's public CAs signing the certificate(if any)
   or not.If public CA is compromised,only the victim will see the
   fraudulent certificate.Implementation of DANE rely heavily on the DNS
   ,and therefore is prone to security attacks based on the deli berate
   mis-association of TLSA records and DNS names.The connection between
   TLSA records and DNS name should rely on DNS resolver,rather than
   depending on caching result of previous domain name lookups ,also it
   should depend on the TTL of that lookup,if it is more then only the
   information will be useful otherwise not.If this part is not taken
   care of then it can fall the victim of spoofing,having access denied
   when a previously accessed servers TLSA record changes,such as during
   a certificate rollover.Even with secure communications between a host
   and the external validating resolver,there is a risk that the
   external validator could become compromised.Nothing prevents a
   compromised external DNSSEC validator from claiming that all the
   records it provides are secure,even is the data is falsified unless
   the client checks the DNSSEC data itself.For this reason DNSSEC
   validation is best performed, on-host even when a secure path to an
   external validator is available.

   In DMARC, URI is a format by which a domain owner specifies the
   destination for the two report types that are supported.Receivers may
   impose a limit on the number of URIs to which they will send
   reports,they must support the ability to send to at least two.DMARC
   and it's underlying techniques SPF and DKIM depend on the security of
   the DNS.To avois DNS-based exploits,the deployment of DNSSEC should
   be done parallel with the deployment of DMARC by both domain owners
   and mail receivers.  A common attack in messaging abuse is the
   presentation of false information in the display name portion of the
   "FROM" field.This takes place when it is possible for the email
   address in that field to be an arbitrary address or domain name,while
   containing a well known name( a celebrity,company,eole etc.) in the
   display name to fool th receiver.  This attack is based on the habit
   of common MUAs that they show the display name and not the email
   address when both are available.  If email address is found with
   display name,execute the DMARC mechanism of the domain name found
   there rather than the domain name discovered before.but spoofers can
   cause the attack by simply not using an email address in the display
   name ,So this doesn't solve the problem.In the MUA display name
   should be shown only if the DMARC mechanism succeeds.  This is also

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   easily defeated,the attacker can use another domain name in the
   display name to pass the DMARC Test.In the MUA,the display name
   should be shown if the DMARC mechanism passes and the email address
   thus validated matches one found in the receiving user's list of
   known addresses.

5.  Normative References

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <https://www.rfc-editor.org/info/rfc6698>.

   [RFC7489]  Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
              Message Authentication, Reporting, and Conformance
              (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,

Authors' Addresses

   CDAC Bangalore

   Email: ranjana@cdac.in

   Balaji Rajendran
   CDAC Bangalore

   Email: balaji@cdac.in

   Bindhumadhava BS
   CDAC Bangalore

   Email: bindhu@cdac.in

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