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Network Working Group                                        R. Van Rein
Internet-Draft                                                 ARPA2.net
Intended status: Standards Track                          March 11, 2016
Expires: September 12, 2016

               TLS-KDH: Kerberos + Diffie-Hellman in TLS


   This specification defines a TLS message flow with Kerberos-based
   (mutual) authentication, binding in Elliptic-Curve Diffie-Hellman to
   achieve Forward Secrecy for the session.

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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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 12, 2016.

Copyright Notice

   Copyright (c) 2016 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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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Related Work  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Extending Kerberos to support TLS . . . . . . . . . . . . . .   4
     3.1.  Checksum Types for use with TLS . . . . . . . . . . . . .   4
     3.2.  Authenticators as Signatures  . . . . . . . . . . . . . .   5
     3.3.  Tickets in X.509 Certificates . . . . . . . . . . . . . .   5
     3.4.  AuthorizationData for Backend Services  . . . . . . . . .   7
   4.  Extending TLS to support Kerberos . . . . . . . . . . . . . .   8
     4.1.  Conceptual Data Model Extensions  . . . . . . . . . . . .   8
     4.2.  Client Certificate Type . . . . . . . . . . . . . . . . .   8
     4.3.  Signature Algorithms  . . . . . . . . . . . . . . . . . .   8
     4.4.  KDH-only CipherSuites . . . . . . . . . . . . . . . . . .   9
     4.5.  TicketRequestFlags Extension  . . . . . . . . . . . . . .  10
     4.6.  TLS Connection Expiration . . . . . . . . . . . . . . . .  12
     4.7.  Interaction with Applications . . . . . . . . . . . . . .  12
     4.8.  Kerberos-Only TLS Application Profile . . . . . . . . . .  13
   5.  The Message Flow of TLS-KDH . . . . . . . . . . . . . . . . .  13
     5.1.  ClientHello . . . . . . . . . . . . . . . . . . . . . . .  14
     5.2.  ServerHello . . . . . . . . . . . . . . . . . . . . . . .  14
     5.3.  Server Certificate  . . . . . . . . . . . . . . . . . . .  15
     5.4.  ServerKeyExchange . . . . . . . . . . . . . . . . . . . .  15
     5.5.  CertificateRequest  . . . . . . . . . . . . . . . . . . .  15
     5.6.  Client Certificate  . . . . . . . . . . . . . . . . . . .  15
     5.7.  ClientKeyExhange  . . . . . . . . . . . . . . . . . . . .  17
     5.8.  CertificateVerify . . . . . . . . . . . . . . . . . . . .  17
     5.9.  Finished  . . . . . . . . . . . . . . . . . . . . . . . .  17
   6.  Comparison to Earlier Work  . . . . . . . . . . . . . . . . .  17
   7.  Efficiency Considerations . . . . . . . . . . . . . . . . . .  18
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  19
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     11.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   Kerberos lends itself well to infrastructure-supported mutual
   authentication, and can even be used to crossover between realms.  A
   downside of this infrastructure is that a crack of one key can lead
   to a cascade of reverse-engineered keys.  Diffie-Hellman key
   exchange, nowadays primarily in its Elliptic-Curve variation, can be
   used to incorporate the desirable property of Forward Secrecy, but
   its vulnerability to man-in-the-middle attacks must then be overcome
   by cryptographically binding it to an authentication mechanism.

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   This specification describes how Kerberos data structures can be used
   for TLS client authentication, by embedding Kerberos data in X.509
   client certificates and signatures.  The server can still present a
   Certificate with a traditional signing mechanism such as RSA for
   authentication, in which case this specification speaks of a KDH-
   enhanced exchange; in absense of a server Certificate the Kerberos-
   based X.509 format can be used for mutual authentication in what will
   then be called a KDH-only exchange.  The KDH-enhanced variety uses
   existing CipherSuite, and KDH-only defines new CipherSuites.  Both
   KDH-enhanced and KDH-only message flows will be referred to as TLS-

   Both TLS-KDH variations form a cryptographic binding between Kerberos
   and Elliptic-Curve Diffie-Hellman (ECDH), leading to the combined
   advantages of infrastructure-supported mutual authentication and
   Forward Secrecy.

   The normal flow of TLS-KDH messages is basically a standard
   interaction with a modified form of client Certificate and

       Client                                               Server

       ClientHello                  -------->
                                    <--------      ServerHelloDone
       Finished                     -------->
                                    <--------             Finished
       Application Data             <------->     Application Data

   * Indicates that ServerCertificate is an optional message; it is
   present for KDH-enhanced message flows, and absent for KDH-only
   message flows.

   [] Indicates that ChangeCipherSpec is an independent TLS protocol
   content type; it is not actually a TLS handshake message.

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2.  Related Work

   Prior work exists for Kerberos authentication within TLS [RFC2712].
   This work has a few drawbacks that are addressed in this new
   specification.  Specifically, it is useful to combine Kerberos mutual
   authentication with the Forward Secrecy of Diffie-Hellman.

   Specifically for the HTTP and HTTPS protocols, the Negotiate header
   [RFC4559] can provide Kerberos authentication, but its use is not
   considered a strong security practice.  Applications that currently
   rely on this mechanism can strengthen their security if they migrate
   to HTTP over TLS-KDH.  Note that this provides an alternative for
   Kerberos, not to SPNEGO and not for general GSS-API protocols.  This
   restriction of TLS-KDH to Kerberos, rather than a more general GSS-
   API protocol, is a result of the fixed number of message exchanges
   available within TLS.

   Many other protocols incorporate Kerberos through GSS-API, usually
   via SASL.  This is considered secure, but has an aguable disadvantage
   of separating encryption and authentication layers, and quite
   possibly also the identities involved in these layers.  Furthermore,
   encryption through SASL is not commonly used.  In situations where
   Kerberos is used for GSS-API over SASL, TLS-KDH offers a comparable
   but more efficient and tighter-coupled mechanism for encryption and
   mutual authentication, in a way that also lends itself to non-SASL
   applications.  Specifically useful in this respect is that there is
   no longer a requirement to setup X.509 certificates plus
   infrastructure and validation mechanisms, just to satisfy encryption
   requirements with their own authentication infrastructure.  In
   applications that use SASL, the EXTERNAL mechanism [RFC4422] can use
   the client identity in a Kerberos ticket, and make it available to
   the application layer; SASL EXTERNAL is also used when TLS
   authenticates peers through X.509 certificates.

3.  Extending Kerberos to support TLS

   This section specifies individual extensions to Kerberos that make it
   possible to use TLS.

3.1.  Checksum Types for use with TLS

   The IANA registry of Kerberos Parameters defines a number of Checksum
   Types.  This includes keyed and non-keyed checksums.  We introduce
   checksum types to match the secure hash algorithms that are used in

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   There already are two values to represent SHA1.  An implementation
   that processes Checksum Types MAY send either and MUST accept both as
   equivalent indications.

   The following additional Checksum Types are introduced for use with
   TLS [Section of [RFC5246]]:

   o  SHA224

   o  SHA256

   o  SHA384

   o  SHA512

3.2.  Authenticators as Signatures

   Kerberos has a symmetric analogon to a signature, namely an
   Authenticator [Section 5.5.1 of [RFC4120]].  When used in TLS-KDH,
   the Authenticator MUST have a secure hash embedded in the cksum
   field.  The checksum type used in the context of TLS MUST be taken to
   match one of the entries in IANA's TLS HashAlgorithm Registry.

   The Authenticator is not sent in the plain, but encrypted with a
   Kerberos session key as EncryptedData [Section 5.2.9 of [RFC4120]]
   and this is how it provides authenticity: only the client and the
   service can unpack the EncryptedData and process the Authenticator.

   A standard part of an Authenticator is a timestamp with microsecond
   accuracy.  This is validated within a small window around the
   independently sycnchronised time of the TLS client and server.  It is
   customary to allow a time window of about 5 minutes around the server

   To avoid replay attacks, Kerberos solutions need to remember received
   Authenticators, or their time stamps, until the time window has
   passed.  This can complicate servers, especially for redundant
   deployments.  When combined with ephemeral ECDH, there is no need for
   such infrastructure to avoid replay attacks.

   Other fields in the Authenticator than specified above MUST be
   ignored by the recipient.

3.3.  Tickets in X.509 Certificates

   This specification leaves the certification possibilities for servers
   in tact, even if it extends the client certification side.  This is
   why it is not desirable to define a new certificate type [RFC5801]

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   for Kerberos, as that would impact the server certificate.  Instead,
   we define a method to encapsulate Kerberos credentials in the general
   X.509 certificate format on the client side.

   Kerberos is based on symmetric keys, but its infrastructure is
   similar to a public-key cryptosystem.  The main difference is that
   its cryptographic claims can only be verified by a targeted service,
   but within the scope of TLS this does not present any problems.
   Embedding a Kerberos Ticket into an X.509 certificate is akin to
   tunneling, where a message is packed into a message with a similar

   The X.509 client certificate is constructed from Kerberos by building
   a TBSCertificate [Section 4.1 of [RFC5280]] as follows:

   o  The certificate is self-signed.  Its issuer and subject are set to
      the distinguishedName CN=Kerberos.

   o  The certificate MUST NOT claim validity before its contained
      Ticket is valid, and MUST NOT claim validity after the Ticket
      expires.  The certificate MAY be short lived, lasting for example
      from 2 minutes before the client's current time to 3 minutes after
      the current time.

   o  The signature algorithm is set to an OID that signifies an
      Authenticator as specified in Section 3.2 and based on a TLS-
      accepted hash algorithm.  The Authenticator MUST NOT contain a
      subkey field.

   o  The subjectPublicKeyInfo is set to an algorithm OID that signifies
      a Kerberos Ticket; the subjectPublicKey is filled with a Kerberos
      Ticket [Section 5.3 of [RFC4120]] that the client wants to use to
      access a service.

   The Certificate built from the TBSCertificate adds the following

   o  The signatureAlgorithm contains the same value as the signature
      field in the TBSCertificate.

   o  The signatureValue field is filled with the EncryptedData
      Section 3.2 holding an Authenticator whose cksum is computed over
      the TBSCertficate using the secure hash algorithm specified in the

   The OIDs referred to above are defined below Section 4.3.

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3.4.  AuthorizationData for Backend Services

   DISCUSSION: This is an alternative to S4U2Proxy, which relies on the
   server realm to care for the provisioned rights.  If TLS-KDH is to
   scale up to more loose collaborations, without implied mutual trust,
   then it needs a new method for backend service support.  This is
   defined below.

   The TLS server may depend on additional Kerberos-protected services,
   generally referred to as "backend services".  As an example, a
   webmail service may need to access IMAP and SMTP backend services,
   possibly under independent administrative control.  This section
   describes an OPTIONAL Kerberos mechanism in support of such backend

   In addition to the main Ticket supplied in the client Certificate's
   public key field, the TLS server would need Tickets to gain access to
   any backend services, and in fact these Tickets can help to define
   where these backend services are located and under what client
   identity they are accessed.  The client needs to provide these
   additional Tickets in an AuthorizationData element whose ad-type is
   AD-BACKEND-TICKETS (TBD) and whose ad-data holds a KRB-CRED message
   [Section 5.8 of [RFC4120]] with an enc-part that uses NULL encryption
   [Section 6.3.1 of [RFC1510]].  The element may be carried in the
   authorization-data field of either the service Ticket or an

   Additional Tickets MUST NOT be renewable, but the main Ticket MAY be;
   when this first Ticket is renewed it SHOULD be resent with the
   backend extension as they are setup at that time.  Additional Tickets
   SHOULD have neither the FORWARDABLE nor the PROXIABLE flag set.

   Additional Tickets should be supplied every time the main Ticket is
   supplied for TLS-KDH.  As a result, both the main and additional
   Tickets MAY be forgotten by the server whenever a TLS-KDH session
   ends.  However, when needed for longer-lasting or deferred backend
   processing, the server MAY hold the Tickets longer.

   It is possible for backend services to have backend services
   themselves; this can be facilitated by a an AD-BACKEND-TICKETS
   element contained in the respective backend service Ticket.

   The generation of AD-BACKEND-TICKETS at the client falls outside the
   scope of this document, but it usually involves requesting the TLS-
   KDH service Ticket from the client's KDC.  To facilitate that, the
   server desiring additional Tickets SHOULD set the LocalRealmService
   flag Section 4.5; without this flag, the client MAY choose not to
   supply additional Tickets.  The use of this flag may imply that the

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   server needs to be flexible in the identity that the client uses for
   its service.

   The client MUST NOT take hints from the server or any non-local KDC
   about required backend Tickets; this might make the client's identity

4.  Extending TLS to support Kerberos

   This section describes changes to TLS in support of Kerberos.

4.1.  Conceptual Data Model Extensions

   The following conceptual data should be available while the TLS-KDH
   message flows evolve:

   o  A flag that is initially set, indicating that the connection could
      be a KDH-enhanced connection;

   o  A flag that is initially set, indicating that the connection could
      be a KDH-only connection;

   o  A series of TicketRequestFlags that can be requested for a Ticket
      used in an X.509 client certificate.

4.2.  Client Certificate Type

   This specification adds a new entry named kerberos_sign in IANA's TLS
   ClientCertificateType Identifiers Registry, with the value TBD.

4.3.  Signature Algorithms

   This specification introduces a mechanism for signatures under
   Kerberos Section 3.2.  This mechanism is represented in two places.

   In TLS, a new SignatureAlgorithm named kerberos is allocated with
   value TBD in IANA's TLS Parameters Registry.  This Kerberos
   SignatureAlgorithm is usually combined with a HashAlgorithm that is
   in common use with TLS, to for a SignatureAndHashAlgorithm.  The
   digitally-signed structure [Section 4.7 of [RFC5246]] uses this
   structure, followed by a variable-sized opaque byte sequence, which
   should hold the EncryptedData holding an Authenticator Section 3.2.

   In X.509 certificates, an AlgorithmIdentifier is used to represent
   the combination of a signature and hash algorithm.  This consists of
   an OID and optional parameters.  For the Kerberos signature
   algorithm, the parameters are absent, and the OID is (TBD -- for now, concatenated with the OID of a

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   secure hash algorithm from IANA's Hash Function Textual Names
   registry.  For example, the SHA-1 hash with OID would
   combine to form (TBD -- now it is

4.4.  KDH-only CipherSuites

   KDH-enhanced message flows can use existing ECDHE CipherSuites using
   server certificates that may be signed with RSA or other common
   algorithms.  In addition, this specification introduces a number of
   KDH-only CipherSuites with names that start with TLS_ECDHE_KDH_.
   These new CipherSuites rely on Kerberos' mutual authentication plus
   ECDHE but not on a server Certificate.  They may be used starting
   from TLS 1.2.  They default to a higher verify_data_length than the
   default 12.

   The following Kerberos-only CipherSuites are entered into the IANA
   TLS Cipher Suite Registry; the list below provides their names and
   their desired verify_data_lengths between brackets:

   o  TLS_ECDHE_KDH_WITH_AES_128_GCM_SHA256 (32)

   o  TLS_ECDHE_KDH_WITH_AES_256_GCM_SHA384 (48)



   o  TLS_ECDHE_KDH_WITH_AES_128_CCM_8 (32)

   o  TLS_ECDHE_KDH_WITH_AES_256_CCM_8 (48)

      TLS 1.3?)

      TLS 1.3REALLY?)

      CAMELLIA IN TLS 1.3?)

      CAMELLIA IN TLS 1.3?)

   Neither server nor client should accept lower values for
   verify_data_length than given here.  TODO: the list follows
   http://www.keylength.com/en/4/ and the hash algorithm sizes -- is
   this agreed?

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   The premaster secret for KDH-only CipherSuites is composed from an
   ECDHE shared secret and a client-sent, connection-specific Kerberos
   key.  Use S to refer to the DER representation of the EncryptionKey
   [Section 5.2.9 of [RFC4120]] in the key contained in the Ticket in
   the client's Certificate.  Use E to refer to the DER representation
   of the EncryptionKey in the subkey field of the Authentication in the
   ClientVerify message, which MUST be present.  Perform the ECDH
   computation in the common manner [RFC4492] and let Z be the value
   produced by this computation (with leading zero bytes kept as they
   are).  The premaster secret is the concatenation of an uint16
   containing the length of Z (in octets), Z itself, an uint16
   containing the lenght of S (in octets), S itself, the length of E (in
   octets) and E itself.  TODO: DER already has lengths, why prefix with
   uint16t (adds no entropy, but adds info relations)

   The master secret is derived from the premaster secret using the
   extended master secret computation [Section 4 of [RFC7627].

4.5.  TicketRequestFlags Extension

   Some clients may be able to offer more facilities in Tickets than
   others, and some servers need more than others.  To communicate this,
   a TLS Extension known as TicketRequestFlags is hereby defined.  This
   extension is optional for TLS in general, but it is required for TLS-
   KDH.  The client uses the Extension to specify the flags that it
   understands and may be able to fulfil.  The server uses the Extension
   to indicate the flags that it requires to be fulfilled.

   The Extension's structure is an extensible list of flag values that
   indicate constraints on the ticket that the client should supply.
   These should be seen as hints how the client should present its
   identity, as the server can always decide to reject a client on
   grounds that are or are not expressible in this form.

   Flag values defined in this specification are:

   TicketFlags (flags number 0..31)  are taken from Kerberos'
         TicketFlags definitions [[RFC4120] and updates]; clients MUST
         NOT accept requested TicketFlags without scrutinising their
         security impact; servers SHOULD NOT assume that their requested
         TicketFlags will actually be provided.  Only TicketFlags 0
         through 31 are included in this definition; when Kerberos is
         extended with more TicketFlags then they will be assigned a new
         range of values as TicketRequestFlags.

   CrossoverRealm (flag number 32)  indicates that the client SHOULD be
         inquisitive about redirections to other realm than its own;
         this MAY be defined in locally trusted domain-to-realm

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         mappings; and the client SHOULD request a service ticket under
         canonicalization [RFC6806] and be open to server referrals.
         When this flag is not set, the server MUST accept the client
         realm on the tickets received, and its setup must make the
         effort of connecting to the realm of the clients that it wants
         to support.

   VisibleClientRealm (flag number 33)  requests that the client's realm
         name is revealed in the service ticket.  With the flag not set,
         the server MUST NOT reject the well-known anonymous realm name
         WELLKNOWN:ANONYMOUS [Section 3 of [RFC6112]] in the client
         realm name.

   UniqueClientIdentity (flag number 34)  requests that the client
         presents a unique identity, even if it is a pseudonym that is
         specific to this service.  Some services can make good use of
         identities that are also presented over other protocols, which
         is why the choice to share such an identity SHOULD be made
         during an interaction with the user, if possible.  The user MAY
         determine to use only a short-lived identity.  When this flag
         is not set, the server MUST NOT reject the client principal
         name WELLKNOWN/ANONYMOUS of type KRB_NT_WELLKNOWN [Section 3 of
         [RFC6112]].  Regardless of this flag, it is RECOMMENDED for the
         server to be open to as many forms of client principal name
         [Section 6.2 of [RFC4120]] as possible.

   requests that the client presents an identity that it will use on
   recurring visits.  Client software is advised to confer with their
   users on this, and so this request should only be used for
   subscription services that would be agreeable to their users.
   Without this flag, the client is free to use a short-lived identity
   that is unlikely to survive to its across its next Authentication
   Service request.

   The flags are chosen such that their default values may be set to 0
   as a safe default; safe in the sense that they do not lead to privacy
   problems, do not impair the peer and do not offer something that
   could block progress of TLS at a later point.  Servers MUST NOT
   respond with TicketRequestFlags set that the client left cleared.
   Senders MUST NOT include flags that they don't know and recipients
   MUST NOT accept flags they cannot interpret.

   Flag values are registered by IANA in a newly created "TLS-KDH Ticket
   Request Flag Registry", whose initial values are as defined above.
   Future specifications of flag values may state that a flag is an
   alternative to another flag, including to the ones specified above.
   When flag A is an alternative to flag B then the fulfillment of the
   requirements for A suffices to ignore flag B.  It is possible for

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   flags to cyclically refer to each other as alternatives; since being-
   an-alternative is not defined as a transitive property, this need not
   distract from this definition.  This is explicitly permitted to
   enhance expressiveness of this principle.

   The wire format representing TicketRequestFlags is a sequence of
   bytes, where the byte at index i (starting from 0) represents the
   flags numbered 8*i (in its least-significat bit) through 8*i+7 (in
   its most-significant bit).  The last byte MUST NOT be 0, meaning that
   it is possible for the TicketRequestFlags to be a sequence of no
   bytes if all flags are cleared.

4.6.  TLS Connection Expiration

   TLS-KDH connections expire when their authenticating Kerberos tickets
   expire.  This is not a reason for termination of the TLS connection,
   but instead it is a trigger for refreshing the ticket.  Such a
   refresh should be executed by the TLS-KDH client, where it may
   trigger user interaction.  Note that Kerberos' facility of ticket
   renewal [Section 2.3 of [RFC4120]] may provide some relief from such
   user interaction.

   When the TLS-KDH connection expires, neither side will send any
   further data records, and both sides will, upon receiving any data
   records, trigger a TLS Alert.  The other records are still accepted,
   to permit a TLS handshake for re-issuance of session keys.
   Implementations MAY choose to initiate and permit re-authentication
   some time before the actual expiration.  This can remedy clock skew
   between the TLS-KDH client and server, which might otherwise lead to
   undesired connection reset.

4.7.  Interaction with Applications

   To be able to use Kerberos, application protocols that run over TLS
   must exchange some configuration information with the TLS stack.
   This includes communication about Kerberos properties such as service
   name and realm, requested/accepted TicketRequestFlags, and a key for
   use with the local identity.

   When a SASL EXTERNAL mechanism is used to communicate an identity
   between the application and the TLS stack, then a good alignment with
   X.509 certificates is possible when both aim to derive or match a
   Network Access Identifier [RFC7542] and/or a domain name.  In the
   case of a Kerberos principal name, this would involve translation
   between case-sensitive realm names to DNS names whose case is not
   reliably reproduced [Section 4.1 of [RFC4343]]; this may be handled
   by ignoring or lowering the case of the realm name while upholding

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   the requirement that no two realm names may differ only in their case
   [Section of [RFC4120]].

4.8.  Kerberos-Only TLS Application Profile

   TLS and Kerberos have long been independent infrastructures for
   secure connectivity; with the introduction of the KDH-only
   CipherSuites in this specification, the worlds can merge elegantly.
   The newly introduced CipherSuites are expected to integrate
   relatively straightforwardly with any TLS stack.

   Just like the TLS-KDH CipherSuites are optimal to implement in TLS
   stacks, TLS-KDH should not force all Kerberos applications to process
   the full potential of TLS, especially not public key cryptography and
   the complexity of proper validation of X.509 certificates.  Some
   applications simply want to use Kerberos in a standardised protocol,
   without any added CipherSuites.  For such applications, we hereby
   introduce a TLS application profile under which such applications can
   stand on their own:

   o  Based on TLS 1.2 or newer;

   o  Setting a default verify_data_size dependent on the CipherSuite;

   o  Supporting the TLS-KDH CipherSuite

   o  Not necessarily supporting the TLS_RSA_WITH_AES_128_CBC_SHA
      CipherSuite that is mandatory in the default TLS application
      profile [Section 9 of [RFC5246]];

   o  This application profile will be known as the "Kerberos-Only TLS
      Application Profile".

   The Kerberos-only CipherSuites can be used with any TLS application
   profile; that includes, but is not limited to, the one specified
   above and the default application profile.

5.  The Message Flow of TLS-KDH

   This specification introduces the name TLS-KDH to certain message
   flows within the TLS framework.  There are two distinct variations,
   called KDH-only and KDH-enhanced.  This section defines constraints
   to the message flow for it to be a TLS-KDH message flow.  This,
   together with the flags in the conceptual data model [Section 4.1],
   guides the use of the extensions defined in this specification.

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   TLS endpoints that find that the other side of the TLS connection
   only implements some of the TLS-KDH constraints MUST NOT continue the
   connection with the TLS-KDH extensions of this specification (unless
   future specifications assign a meaningful procedure for such
   situations).  If the remote endpoint does not implement all
   requirements for TLS-KDH but also enforces it, for instance by
   sending required information that can only be interpreted under this
   specification, then it MUST send a suitable TLS Alert and close the

5.1.  ClientHello

   To support TLS-KDH, a client's ClientHello message MUST include the
   TicketRequestFlags extension, and it MUST mention the kerberos
   SignatureAlgorithm in at least one of the

   In addition, the ClientHello MAY include one or more KDH-only
   CipherSuites in the list of cipher_suites, but if that is done the
   protocol used MUST be TLS 1.2 or later, indicated on the record layer
   with ProtocolVersion 3,3 or later.  Without at least one of the KDH-
   only CipherSuites, the connection cannot be KDH-only, but it may
   still proceed as KDH-enhanced.

   The client MUST NOT send TicketRequestFlags that it does not
   understand and it MUST NOT offer all the TicketFlags that are defined
   for Kerberos, but instead SHOULD limit itself to what would be
   acceptable from a security perspective.

5.2.  ServerHello

   To support TLS-KDH, a ServerHello message MUST include the
   TicketRequestFlags extension, and it MUST mention the kerberos
   SignatureAlgorithm in at least one of the
   supported_signature_algorithms.  To continue the message flow as TLS-
   KDH, the server MUST select a cipher_suite with ephemeral ECDH key
   exchange; aside from generally available CipherSuites, the server MAY
   select a KDH-only cipher_suite.  When it does not select a KDH-only
   CipherSuite, the connection cannot be a KDH-only connection, but it
   may still proceed as KDH-enhanced.

   The server MUST ignore TicketRequestFlags from the client that it
   does not understand; it MUST NOT send TicketRequestFlags that it does
   not understand and it MUST NOT set TicketRequestFlags that were not
   set in the ClientHello.

   Note that none of the Anonymous CipherSuites can be made to work with
   TLS-KDH, because then it is not permitted [Section 2.5 of [RFC4492]]

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   to send a CertificateRequest, client Certificate or CertificateVerify
   message.  Although the KDH-only CipherSuites do not use a server
   Certificate this does not constitute Anonymous server authentication,
   because Kerberos provides mutual authentication.

5.3.  Server Certificate

   When a KDH-only CipherSuite was selected in the ServerHello, then the
   server MUST NOT send a Certificate message.  For KDH-enhanced
   CipherSuites, the server Certificate message MUST be sent, following
   the definitions of the server-selected cipher_suite.

5.4.  ServerKeyExchange

   All TLS-KDH connections MUST use ephemeral ECDH.  Under KDH-enhanced
   CipherSuites, this implies the case ec_diffie_hellman [Section 5.4 of
   [RFC4492]].  Under KDH-only CipherSuites, the same case is used, but
   without signatures, formatted in the same way as for ECDH_anon

5.5.  CertificateRequest

   Under TLS-KDH, a CertificateRequest MUST be sent by the server, and
   it MUST include at least one SignatureAndHashAlgorithm based on the
   kerberos SignatureAlgorithm, and it MUST list the kerberos_sign
   CertificateType.  For KDH-only CipherSuites, no other
   CertificateTypes and SignatureAndHashAlgorithms are permitted.  The
   list of certificate_authorities is not used by TLS-KDH and MUST be
   empty for KDH-only CipherSuites.

   When sending a CertificateRequest under TLS-KDH, the TLS server MUST
   be prepared to accept the X.509 certificate form for Kerberos
   [Section 3.3] using any of the corresponding signature algorithm OIDs
   [Section 4.3].

5.6.  Client Certificate

   A client MAY choose any of the certificate and signature types to
   fulfil the CertificateRequest.  In order to continue the TLS-KDH
   message flow, it MUST select the kerberos_sign CertificateType and a
   SignatureAndHashAlgorithm with the kerberos SignatureAlgorithm.
   Under TLS-KDH the client MUST supply a Certificate based on these

   When continuing with the TLS-KDH flow, the client SHOULD take the
   TicketRequestFlags into consideration; it is not required however to
   implement all, because some flags may be forbidden by policy that was
   concealed from the ClientHello-supplied TicketRequestFlags, or that

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   turn out to be unavailable while requesting the Ticket.  In such
   cases, the client MAY simply continue without fulfilling the request
   flags, or it MAY choose to divert from a KDH-enhanced message flow by
   authenticating with another kind of Certificate, or not to present
   one at all.

   When the server has not set the UniqueClientIdentity flag, then the
   RECOMMENDED client Certificate under TLS-KDH would be based on an
   anonymous Ticket [RFC6112]; however, when the server has set the
   UniqueClientIdentity flag, then an anonymous Ticket that uses the
   anonymous realm MUST NOT be sent.  A Ticket that is not anonymous may
   still be pseudonymous, including names based on NT-UID principal
   names [Section 6.2 of [RFC4120]] when the server has sent the
   UniqueClientIdentity flag; the LastingClientIdentity flag indicates
   the server's perspecitve on longevity of any such pseudonyms, but the
   client MAY choose to ignore that wish.

   Under the present specification, the kerberos_sign CertificateType
   and the kerberos SignatureAlgorithm SHOULD NOT be used independently,
   but always in combination [Section 3.3].  If any of the messages
   preceding the client's Certificate deviated from the specifications
   for TLS-KDH, then these choices MUST NOT be made.  Only when these
   two choices are made do we continue to speak of a TLS-KDH message

   The impact of using an anonymous ticket is that the server cannot
   establish the identity of the client, except perhaps that the same
   service ticket may be used repeatedly during its short period of
   validity.  This means that the ability to trace the client is limited
   for both server and client.  Under customary X.509 authentication,
   the interpretation of not sending the CertificateRequest is that the
   server should not care for the client identity; anonymous tickets
   provide a mechanism for achieving a similar pattern under TLS-KDH,
   although it has some benefits of short-term session protection.

   The presence of a Ticket enables the server to conclude that the
   client has procured a Ticket through the formal pathways of Kerberos,
   ending in the server-side realm; the reason this can be assumed is
   that the ticket holds an encrypted part that the server can decrypt
   and thereby validate with its own key, as setup in its KDC for
   sharing in service tickets.  In other words, even an anonymous Ticket
   establishes that the server may trust that the client was checked
   along the way to the service.  As a result, the ECDH key exchange is
   known to be protected from a man-in-the-middle attack.

   Briefly put, we can speak of mutual authentication in this
   specification, even when the client uses an anonymous ticket.  The

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   only thing that is missing under an anonymous ticket is the
   visibility of the client's validated identity.

5.7.  ClientKeyExhange

   Every TLS-KDH message flow MUST use ECDH, and since the keys MUST be
   ephemeral, the explicit form of the ClientECDiffieHellmanPublic for
   the case ec_diffie_hellman [Section 5.7 of [RFC4492]] MUST be used.

5.8.  CertificateVerify

   If CertificateVerify was preceded by a client Certificate with the
   kerberos SignatureAlgorithm, then the CertificateVerify MUST follow
   up accordingly.  This means that the CertificateVerify message MUST
   follow the descriptions of a Kerberos Authenticator Section 3.2 for
   use with TLS.

   In KDH-only message flows, the Authenticator MUST introduce a new
   client-generated key in the subkey field, to be used in the formation
   of the premaster secret.  The subkey field SHOULD NOT be included in
   KDH-enhanced message flows.

5.9.  Finished

   For KDH-enhanced flows, the server can be authenticated through its
   Certificate, so that the usual Finished messages suffice, and TLS
   versions preceding 1.2 may still suffice.

   For KDH-only flows, the Finished message is the first place where the
   server identity can be validated, prior to reporting successful
   authentication to the application running atop TLS.  As a result, the
   KDH-only CipherSuites have been defined with an elongated Finished
   message, for improved security.  This is possible since TLS 1.2.  The
   desired minimum length is defined with the introduction of the KDH-
   only CipherSuite.

6.  Comparison to Earlier Work

   An older specification [RFC2712] introduces Kerberos into TLS.  That
   specification is hereby deprecated because this new specification
   improves on it work in a number of ways:

   o  The premaster secret is no longer sent to the server under
      encryption with the KDC-provided session key; instead, Forward
      Secrecy is supported through ECDHE;

   o  The authenticator following the Kerberos ticket is made
      obligatory, as an intrinsic part of replay protection and the

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      mutual authentication between TLS client and TLS server to protect
      all in-transit application data;

   o  There is no need to implement a replay cache, which means that
      more efficient implementation is possible, certainly on highly
      active and/or replicated TLS-KDH server systems;

   o  The mutual authentication of TLS client and TLS server is
      established with Kerberos-only CipherSuites that define a stronger
      Finished message size;

   o  The service name is not statically set to the literal "host", but
      both the client and server TLS stacks assume an application
      context to provide the service name to be used;

   o  The KDH-enhanced variation can be used with another mode of server

   o  Support for modern TLS CipherSuites is added, and support for ones
      that are currently considered deprecated or insecure have been

7.  Efficiency Considerations

   The efficiency of the mechanism described here compares favourably
   with the more common approach of authentication through X.509
   certificates based on public-key algorithms.

   The Kerberos architecture is founded on symmetric cryptography, which
   makes it more efficient than the asymmetric architectures around
   X.509 public-key certificates.  Furthermore, Kerberos' identity
   statements are short-lived, which is generally accepted to evade the
   need for withdrawal mechanisms based on chains of trust, CRLs
   [RFC3280], OCSP [RFC6960], DANE [RFC6698] and perhaps other
   mechanisms.  As a result, the validity of a Kerberos ticket can be
   checked with relatively modest computational effort.

   The inclusion of ephemeral ECDH is a relatively expensive asymmetric
   operation, but the same introduction is needed when Forward Secrecy
   is introduced alongside public-key authentication.

   The one thing that is costly about Kerberos is its reliance on a
   replay cache.  Such caches store recent authentication attempts to
   avoid that they are being replayed; an accurate clock helps to
   release entries, but some care for clock skew between TLS-KDH client
   and server must be resolved with these caches.  Their volatile nature
   makes them a particularly difficult problem in highly active and/or
   replicated and/or distributed Kerberos services.

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   A replay cache is not required for any of the TLS-KDH protocol flows,
   because this specification requires an ephemeral ECDH key exchange.
   This is of particular use to redundant (and possibly distributed)
   server farms, where sharing the time-critical information of the
   replay cache is a performance bottle neck.  Since this is a new
   specification, there is no need to implement backward compatibility
   with older mechanisms for which a replay cache might be needed.

8.  Privacy Considerations

   The information that is publicly shown in the TLS-KDH protocol flows
   consists of:

   o  Supported protocol versions, TLS extensions and CipherSuites

   o  For other than Kerberos-only CipherSuites, the server's

   o  The server's principal name, host name and service name

   A Kerberos ticket transmits less information in plaintext than a
   public-key X.509 client certificate; furthermore, DNS may have to
   reveal the realm name(s) of server-trusted KDC(s) but neither the
   TLS-KDH server nor any KDC publishes long-lasting key material for
   TLS or Kerberos, so parties looking for a cracking challenge are
   constrained to a brief period of attack on keys.

   The TicketRequestFlags may provide information about Tickets present
   in the client, but that would take the risk of leaking information
   prior to authentication of the server, and in plaintext.

9.  Security Considerations

   For KDH-enhanced message flows, the server can be authenticated
   through its public-key X.509 Certificate.  For KDH-onnly message
   flows this is not possible, which is why a longer verify_data_size in
   the Finished messages is required; the ability to generate these
   messages properly proves that the other side has succeeded in
   decrypting the Kerberos-encrypted materials, and so, that it is the
   intended remote party.

   In Kerberos, all key material is supplied by the KDC.  This is a
   central point in each realm that is usually guarded well enough, but
   it is nonetheless a critical point in any infrastructure founded on
   Kerberos.  When client and server are in different realms, but have
   cross-signed directly or through a chain of KDC's, then all
   intermediate KDC's are potential places where the session key could

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   be detected.  The weakest KDC in the chain then defines the security
   of the entire chain.

   Kerberos requires accurate clocks in order to operate securely;
   without them, once-used and since-forgotten credentials could be
   replayed by an attacker that has been able to recover an old service
   ticket's session key.  This problem is worsened in cross-realm
   scenario's where clock synchronisation is hard to realise.  This is
   however resolved in all TLS-KDH flows by using ephemeral Elliptic-
   Curve Diffie-Hellman keys, thus forcing new master secrets on each
   connection and removing the need for a replay buffer.  Note however,
   that ticket validity times must still be checked.

   Basic Kerberos security hinges on the secrecy of the user's password;
   if this password is guessed, then all captured traffic can be
   decoded, even in retrospect.  This means that it is highly advisable
   to combine Kerberos with Diffie-Hellman for Forward Secrecy.  TLS-KDH
   implies this desirable property in all its CipherSuites.

10.  IANA Considerations

   TODO - for Kerberos:

   o  ChecksumTypes SHA-xxx



   TODO - for TLS:

   o  ClientCertificateType "kerberos_sign"

   o  SignatureAlgorithm "kerberos"

   o  CipherSuites TLS_ECDHE_KDH_xxx

   o  ExtensionType for TicketRequestFlags

   o  TicketRequestFlags registry

11.  References

11.1.  Normative References

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   [RFC1510]  Kohl, J. and C. Neuman, "The Kerberos Network
              Authentication Service (V5)", RFC 1510,
              DOI 10.17487/RFC1510, September 1993,

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              DOI 10.17487/RFC4120, July 2005,

   [RFC4343]  Eastlake 3rd, D., "Domain Name System (DNS) Case
              Insensitivity Clarification", RFC 4343,
              DOI 10.17487/RFC4343, January 2006,

   [RFC4492]  Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
              Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
              for Transport Layer Security (TLS)", RFC 4492,
              DOI 10.17487/RFC4492, May 2006,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,

   [RFC6806]  Hartman, S., Ed., Raeburn, K., and L. Zhu, "Kerberos
              Principal Name Canonicalization and Cross-Realm
              Referrals", RFC 6806, DOI 10.17487/RFC6806, November 2012,

   [RFC6112]  Zhu, L., Leach, P., and S. Hartman, "Anonymity Support for
              Kerberos", RFC 6112, DOI 10.17487/RFC6112, April 2011,

   [RFC7627]  Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
              Langley, A., and M. Ray, "Transport Layer Security (TLS)
              Session Hash and Extended Master Secret Extension",
              RFC 7627, DOI 10.17487/RFC7627, September 2015,

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11.2.  Informative References

   [RFC2712]  Medvinsky, A. and M. Hur, "Addition of Kerberos Cipher
              Suites to Transport Layer Security (TLS)", RFC 2712,
              DOI 10.17487/RFC2712, October 1999,

   [RFC3280]  Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              DOI 10.17487/RFC3280, April 2002,

   [RFC4422]  Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
              Authentication and Security Layer (SASL)", RFC 4422,
              DOI 10.17487/RFC4422, June 2006,

   [RFC4559]  Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based
              Kerberos and NTLM HTTP Authentication in Microsoft
              Windows", RFC 4559, DOI 10.17487/RFC4559, June 2006,

   [RFC5801]  Josefsson, S. and N. Williams, "Using Generic Security
              Service Application Program Interface (GSS-API) Mechanisms
              in Simple Authentication and Security Layer (SASL): The
              GS2 Mechanism Family", RFC 5801, DOI 10.17487/RFC5801,
              July 2010, <http://www.rfc-editor.org/info/rfc5801>.

   [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, <http://www.rfc-editor.org/info/rfc6698>.

   [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
              Galperin, S., and C. Adams, "X.509 Internet Public Key
              Infrastructure Online Certificate Status Protocol - OCSP",
              RFC 6960, DOI 10.17487/RFC6960, June 2013,

   [RFC7542]  DeKok, A., "The Network Access Identifier", RFC 7542,
              DOI 10.17487/RFC7542, May 2015,

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Author's Address

   Rick van Rein
   Haarlebrink 5
   Enschede, Overijssel  7544 WP
   The Netherlands

   Email: rick@openfortress.nl

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