Network Working Group R. Van Rein
Internet-Draft ARPA2.net
Intended status: Standards Track April 03, 2016
Expires: October 5, 2016
TLS-KDH: Kerberos + Diffie-Hellman in TLS
draft-vanrein-tls-kdh-03
Abstract
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
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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 . . . . . . . . . . . . . 5
3.2. Authenticators as Signatures . . . . . . . . . . . . . . 5
3.3. Tickets in the TLS Certificate flow . . . . . . . . . . . 6
3.4. AuthorizationData for Backend Services . . . . . . . . . 6
4. Extending TLS to support Kerberos . . . . . . . . . . . . . . 7
4.1. Conceptual Data Model Extensions . . . . . . . . . . . . 8
4.2. Certificate Type KerberosTicket . . . . . . . . . . . . . 8
4.3. Signature Algorithms . . . . . . . . . . . . . . . . . . 8
4.4. KDH-only CipherSuites . . . . . . . . . . . . . . . . . . 8
4.5. TicketRequestFlags Extension . . . . . . . . . . . . . . 9
4.6. TLS Connection Expiration . . . . . . . . . . . . . . . . 11
4.7. Interaction with Applications . . . . . . . . . . . . . . 11
4.8. Kerberos-Only TLS Application Profile . . . . . . . . . . 12
5. The Message Flow of TLS-KDH . . . . . . . . . . . . . . . . . 13
5.1. ClientHello . . . . . . . . . . . . . . . . . . . . . . . 13
5.2. ServerHello . . . . . . . . . . . . . . . . . . . . . . . 14
5.3. Server Certificate . . . . . . . . . . . . . . . . . . . 14
5.4. ServerKeyExchange . . . . . . . . . . . . . . . . . . . . 15
5.5. CertificateRequest . . . . . . . . . . . . . . . . . . . 15
5.6. Client Certificate . . . . . . . . . . . . . . . . . . . 15
5.7. ClientKeyExhange . . . . . . . . . . . . . . . . . . . . 16
5.8. CertificateVerify . . . . . . . . . . . . . . . . . . . . 16
5.9. Finished . . . . . . . . . . . . . . . . . . . . . . . . 17
6. Comparison to Earlier Work . . . . . . . . . . . . . . . . . 17
7. Efficiency Considerations . . . . . . . . . . . . . . . . . . 18
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 18
9. Security Considerations . . . . . . . . . . . . . . . . . . . 19
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
11.1. Normative References . . . . . . . . . . . . . . . . . . 22
11.2. Informative References . . . . . . . . . . . . . . . . . 23
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 24
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 24
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
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its vulnerability to man-in-the-middle attacks must then be overcome
by cryptographically binding it to an authentication mechanism.
This specification describes how Kerberos data structures can be used
for TLS client authentication, by introducing a new certificate type
for use with TLS. The server can choose to provide a Certificate
with a traditional signing mechanism such as RSA for authentication,
in which case this specification speaks of a KDH-enhanced exchange;
even when presenting no server certificate at all, a client-side
Kerberos ticket 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-
KDH.
A variation of the KDH-only flow does incorporate a server-side
ticket; this can be used for user-to-user authentication, perhaps to
be used in peer-to-peer protocols that use TLS-KDH as their security
foundation.
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
CertificateVerify:
Client Server
ClientHello -------->
ServerHello
ServerCertificate*
ServerKeyExchange
CertificateRequest
<-------- ServerHelloDone
Certificate
ClientKeyExchange
CertificateVerify
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
Application Data <-------> Application Data
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* 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.
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 commonly 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.
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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
TLS.
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 7.4.1.4.1 of [RFC5246]]:
o SHA224
o SHA256
o SHA384
o SHA512
3.2. Authenticators as Signatures
Kerberos has a symmetric analogue to a signature, in the form of 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 Kerberos derives 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
time.
To avoid replay attacks, Kerberos solutions sometimes need to
remember received Authenticators, or their time stamps, until the
time window has passed. This complicates servers, especially for
redundant deployments. As used in TLS-KDH, with entropy from both
end in the hello exchange and with session-specific keys agreed
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through ephemeral ECDH, there is no need for such infrastructure to
avoid replay attacks.
An Authenticator MAY contain a subkey field, used to provide keying
material to the other side. Under KDH-only, such a field MUST be
supplied as input to the premaster secret computation.
Other fields in the Authenticator than specified above SHOULD NOT be
produced and MUST be ignored by the recipient.
3.3. Tickets in the TLS Certificate flow
This specification defines a new certificate type [RFC7250] named
KerberosTicket, to be negotiated for the client and, though only in
special circumstances, for the server.
The Kerberos Ticket is included in certificate messages following
this syntax:
struct {
select(certificate_type){
// certificate type defined in this document.
case KerberosTicket: opaque Ticket<1..2^24-1>;
// Additional certificate type based on
// "TLS Certificate Types" subregistry
};
} Certificate;
In this syntax, Ticket holds the DER-encoded form of a Kerberos
Ticket [Section 5.3 of [RFC4120]].
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
services.
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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
Authenticator.
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
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
vulnerable.
4. Extending TLS to support Kerberos
This section describes changes to TLS in support of Kerberos.
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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. Certificate Type KerberosTicket
This specification adds a new entry named kerberos_sign in IANA's TLS
ClientCertificateType Identifiers Registry, with the value TBD.
This specification also adds a new entry named KerberosTicket in
IANA's TLS Certificate Types" subregistry of the "Transport Layer
Security (TLS) Extensions" registry.
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.
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.
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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 (32)
o TLS_ECDHE_KDH_WITH_AES_256_CCM (48)
o TLS_ECDHE_KDH_WITH_AES_128_CCM_8 (32)
o TLS_ECDHE_KDH_WITH_AES_256_CCM_8 (48)
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?
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. 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; when absent, all flags are considered to be
cleared. 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 would like to be fulfilled.
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The Extension's structure is an extensible list of flag values that
indicate constraints on the ticket that the client should try to
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.
VisibleClientRealm (flag number 32) 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 33) 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.
LastingClientIdentity (flag number 34) 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 after the ticket's endtime or renew-till
time.
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
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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
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
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name and realm, offered/requested 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
the requirement that no two realm names may differ only in their case
[Section 7.2.3.1 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
TLS_ECDHE_KDH_WITH_AES_256_GCM_SHA384;
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.
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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.
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
connection.
5.1. ClientHello
To support TLS-KDH, a client's ClientHello MUST mention the kerberos
SignatureAlgorithm in at least one of the
supported_signature_algorithms, and it also MUST include
KerberosTicket in a client_certificate_type extension [RFC7250]. The
client MAY include the TicketRequestFlags extension if it is
interested on providing more than a generally available ticket to the
service.
Furthermore, the client MAY offer to process KerberosTicket in a
server_certificate_type extension [RFC7250], and the client MAY
include the TicketRequestFlags extension.
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. With at least one of the KDH-only
CipherSuites, the client MAY offer to process KerberosTicket in a
server_certificate_type extension [RFC7250] through the ENC-TKT-IN-
SKEY [Section 2.9.2 of [RFC4120]] Kerberos extension.
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.
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5.2. ServerHello
To support TLS-KDH, a ServerHello message MUST mention the kerberos
SignatureAlgorithm in at least one of the
supported_signature_algorithms, and it MUST include the
KerberosTicket value in the client_certificate_type extension
[RFC7250]. Furthermore, 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.
When the server selects a KDH-enhanced CipherSuite, it MUST choose
another means of authenticating its identity than through Kerberos,
following the customary flow of TLS and only using Kerberos for
client authentication.
When the server selects a KDH-only CipherSuite and when the
ClientHello message includes the KerberosTicket in a
server_certificate_type extension [RFC7250], then the server MAY
choose to send back a server_certificate_type extension selecting
KerberosTicket, in which case it MUST send the corresponding Server
Certificate later in the process. This may be used to facilitate
user-to-user negotiation of TLS.
The server MAY include the TicketRequestFlags extension in the
ServerHello message if the client included it in the ClientHello
message. 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]]
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
For KDH-enhanced CipherSuites, the server Certificate message MUST be
sent, following the definitions of the server-selected cipher_suite.
Under KDH-only, when the server has not selected the KerberosTicket
as the server_certificate_type, then it MUST NOT send a Certificate
message.
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Under KDH-only, when the server did select the KerberosTicket as the
server_certificate_type, then it MUST include the corresponding
ticket in the Server Certificate.
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
CipherSuites.
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. Having selected the KerberosTicket value for the
client_certificate_type, other CertificateTypes and
SignatureAndHashAlgorithms SHOULD NOT be sent. The list of
certificate_authorities is not used by TLS-KDH and MUST be empty.
5.6. Client Certificate
TLS-KDH clients MUST fill the ClientCertificate message with a
KerberosTicket. In doing so, it SHOULD take any negotiated
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 their
implementation may 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.
The impact of using an anonymous ticket is that the server cannot
establish the identity of the client, except perhaps that the same
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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.
When the server has selected the KerberosTicket as the
server_certificate_type, then the client SHOULD pass it along with
its attempt to obtain a ticket for the server name, using the KDC
option ENC-TKT-IN-SKEY [Section 2.9.2 of [RFC4120]] and supplying an
additional ticket in the TGS request [Section 3.3.1 of [RFC4120]];
this Ticket is supplied completely by the KerberosTicket certificate-
type, and although it includes a server-side realm name, this MUST
NOT be trusted by the client before it has been confirmed by the
client's attempts to reach the server by its host name; this is
because a man-in-the-middle attack is still possible through a forged
service realm. Customary processing rules for finding a server's
realm MUST therefore be followed by the client.
The presence of the client's Ticket in the ClientCertificate message
enables the server to ascertain 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 supplies an anonymous ticket.
The 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
Under TLS-KDH, the CertificateVerify was preceded by a client's
Ticket, and the CertificateVerify MUST follow as a proof of posession
of the corresponding key material on the client. This means that the
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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
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;
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o The KDH-enhanced variation can be used with another mode of server
authentication.
o Support for modern TLS CipherSuites is added, and support for ones
that are currently considered deprecated or insecure have been
removed;
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.
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
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o For other than Kerberos-only CipherSuites, the server's
Certificate
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
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
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to combine Kerberos with Diffie-Hellman for Forward Secrecy. TLS-KDH
implies this desirable property in all its CipherSuites.
10. IANA Considerations
IANA adds the following Kerberos Checksum Type Numbers to the
Kerberos Parameters registry, to match the hash algorithms available
to TLS:
+---------------+------------------+---------------+
| sumtype value | Checksum type | checksum size |
+---------------+------------------+---------------+
| TBD | sha224 (unkeyed) | 28 |
| TBD | sha256 (unkeyed) | 32 |
| TBD | sha384 (unkeyed) | 48 |
| TBD | sha512 (unkeyed) | 64 |
+---------------+------------------+---------------+
Through the IETF Kitten WG, the Kerberos community assigns the
following value for the AuthorizationData type for backend tickets,
as defined in this specification:
+-------+--------------------+
| Value | Name |
+-------+--------------------+
| TBD | AD-BACKEND-TICKETS |
+-------+--------------------+
IANA adds the following entries to the TLS Cipher Suite Registry
underneath the TLS Paramters registry, to enable KDH-only negotiation
and the Kerberos-Only profile that relies on it:
+-------+--------------------------------------------+---------+
| Value | Description (verify_data_length) | DTLS-OK |
+-------+--------------------------------------------+---------+
| TBD | TLS_ECDHE_KDH_WITH_AES_128_GCM_SHA256 (32) | Y |
| TBD | TLS_ECDHE_KDH_WITH_AES_256_GCM_SHA384 (48) | Y |
| TBD | TLS_ECDHE_KDH_WITH_AES_128_CCM (32) | Y |
| TBD | TLS_ECDHE_KDH_WITH_AES_256_CCM (48) | Y |
| TBD | TLS_ECDHE_KDH_WITH_AES_128_CCM_8 (32) | Y |
| TBD | TLS_ECDHE_KDH_WITH_AES_256_CCM_8 (48) | Y |
+-------+--------------------------------------------+---------+
IANA adds the following ClientCertificateType Identifier to the TLS
Parameters registry, to be able to formule certificate requests under
TLS-KDH as described in this document:
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+-------+---------------+---------+
| Value | Description | DTLS-OK |
+-------+---------------+---------+
| TBD | kerberos_sign | Y |
+-------+---------------+---------+
IANA adds the following TLS SignatureAlgorithm to the TLS Parameters
registry, to permit the Kerberos-based signatures described in this
document:
+-------+-------------+---------+
| Value | Description | DTLS-OK |
+-------+-------------+---------+
| TBD | kerberos | Y |
+-------+-------------+---------+
IANA adds the following TLS Certificate Type to the TLS Extensions
registry, to support negotiating it as a client_certificate_type and
possibly as a server_certificate_type:
+-------+----------------+---------+
| Value | Description | DTLS-OK |
+-------+----------------+---------+
| TBD | KerberosTicket | Y |
+-------+----------------+---------+
IANA adds the following ExtensionType Value to the TLS Extensions
registry, to support the negotiation of TicketRequestFlags as
described in this specification:
+-------+--------------------+
| Value | Extension Name |
+-------+--------------------+
| TBD | TicketRequestFlags |
+-------+--------------------+
IANA appends a new "TLS-KDH Ticket Request Flags" sub-register to the
TLS Extensions register, to support the allocation of
TicketRequestFlags by other specifications. New entries to this
table can be made under the named Assignment Policy [RFC5226]. The
initial entries to this registry are:
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+----------+-----------------------+--------------+-----------------+
| Value(s) | Name | Reference | Assignment |
| | | | Policy |
+----------+-----------------------+--------------+-----------------+
| 0..31 | TicketFlags | TBD:THISSPEC | RFC Required |
| 32 | VisibleClientRealm | TBD:THISSPEC | RFC Required |
| 33 | UniqueClientIdentity | TBD:THISSPEC | RFC Required |
| 34 | LastingClientIdentity | TBD:THISSPEC | RFC Required |
| 35..63 | Unassigned | | RFC Required |
| 64..79 | Private Use | N/A | Private Use |
| 80..95 | Experimental | N/A | Experimental |
| | | | Use |
| 96.. | Reserved | | RFC Required |
+----------+-----------------------+--------------+-----------------+
11. References
11.1. Normative References
[RFC1510] Kohl, J. and C. Neuman, "The Kerberos Network
Authentication Service (V5)", RFC 1510,
DOI 10.17487/RFC1510, September 1993,
<http://www.rfc-editor.org/info/rfc1510>.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
DOI 10.17487/RFC4120, July 2005,
<http://www.rfc-editor.org/info/rfc4120>.
[RFC4343] Eastlake 3rd, D., "Domain Name System (DNS) Case
Insensitivity Clarification", RFC 4343,
DOI 10.17487/RFC4343, January 2006,
<http://www.rfc-editor.org/info/rfc4343>.
[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,
<http://www.rfc-editor.org/info/rfc4492>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
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[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC6112] Zhu, L., Leach, P., and S. Hartman, "Anonymity Support for
Kerberos", RFC 6112, DOI 10.17487/RFC6112, April 2011,
<http://www.rfc-editor.org/info/rfc6112>.
[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
June 2014, <http://www.rfc-editor.org/info/rfc7250>.
[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,
<http://www.rfc-editor.org/info/rfc7627>.
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,
<http://www.rfc-editor.org/info/rfc2712>.
[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,
<http://www.rfc-editor.org/info/rfc3280>.
[RFC4422] Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
Authentication and Security Layer (SASL)", RFC 4422,
DOI 10.17487/RFC4422, June 2006,
<http://www.rfc-editor.org/info/rfc4422>.
[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,
<http://www.rfc-editor.org/info/rfc4559>.
[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>.
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[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,
<http://www.rfc-editor.org/info/rfc6960>.
[RFC7542] DeKok, A., "The Network Access Identifier", RFC 7542,
DOI 10.17487/RFC7542, May 2015,
<http://www.rfc-editor.org/info/rfc7542>.
Appendix A. Acknowledgements
TODO
Author's Address
Rick van Rein
ARPA2.net
Haarlebrink 5
Enschede, Overijssel 7544 WP
The Netherlands
Email: rick@openfortress.nl
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