RFC 6113
A Generalized Framework for Kerberos Pre-Authentication
5. Tools for Use in Pre-Authentication Mechanisms
This section describes common tools needed by multiple pre- authentication mechanisms. By using these tools, mechanism designers can use a modular approach to specify mechanism details and ease security analysis.5.1. Combining Keys
Frequently, a weak key needs to be combined with a stronger key before use. For example, passwords are typically limited in size and insufficiently random: therefore, it is desirable to increase the strength of the keys based on passwords by adding additional secrets. An additional source of secrecy may come from hardware tokens. This section provides standard ways to combine two keys into one. KRB-FX-CF1() is defined to combine two passphrases. KRB-FX-CF1(UTF-8 string, UTF-8 string) -> (UTF-8 string) KRB-FX-CF1(x, y) := x || y Where || denotes concatenation. The strength of the final key is roughly the total strength of the individual keys being combined, assuming that the string_to_key() function [RFC3961] uses all its input evenly.
An example usage of KRB-FX-CF1() is when a device provides random but short passwords, the password is often combined with a personal identification number (PIN). The password and the PIN can be combined using KRB-FX-CF1(). KRB-FX-CF2() combines two protocol keys based on the pseudo-random() function defined in [RFC3961]. Given two input keys, K1 and K2, where K1 and K2 can be of two different enctypes, the output key of KRB-FX-CF2(), K3, is derived as follows: KRB-FX-CF2(protocol key, protocol key, octet string, octet string) -> (protocol key) PRF+(K1, pepper1) -> octet-string-1 PRF+(K2, pepper2) -> octet-string-2 KRB-FX-CF2(K1, K2, pepper1, pepper2) := random-to-key(octet-string-1 ^ octet-string-2) Where ^ denotes the exclusive-OR operation. PRF+() is defined as follows: PRF+(protocol key, octet string) -> (octet string) PRF+(key, shared-info) := pseudo-random( key, 1 || shared-info ) || pseudo-random( key, 2 || shared-info ) || pseudo-random( key, 3 || shared-info ) || ... Here the counter value 1, 2, 3, and so on are encoded as a one-octet integer. The pseudo-random() operation is specified by the enctype of the protocol key. PRF+() uses the counter to generate enough bits as needed by the random-to-key() [RFC3961] function for the encryption type specified for the resulting key; unneeded bits are removed from the tail. Unless otherwise specified, the resulting enctype of KRB-FX-CF2 is the enctype of k1. The pseudo-random() operation is the RFC 3961 pseudo-random() operation for the corresponding input key; the random-to-key() operation is the RFC 3961 random-to-key operation for the resulting key. Mechanism designers MUST specify the values for the input parameter pepper1 and pepper2 when combining two keys using KRB-FX-CF2(). The pepper1 and pepper2 MUST be distinct so that if the two keys being combined are the same, the resulting key is not a trivial key.
5.2. Managing States for the KDC
Kerberos KDCs are stateless in that there is no requirement that clients will choose the same KDC for the second request in a conversation. Proxies or other intermediate nodes may also influence KDC selection. So, each request from a client to a KDC must include sufficient information that the KDC can regenerate any needed state. This is accomplished by giving the client a potentially long opaque cookie in responses to include in future requests in the same conversation. The KDC MAY respond that a conversation is too old and needs to restart by responding with a KDC_ERR_PREAUTH_EXPIRED error. KDC_ERR_PREAUTH_EXPIRED 90 When a client receives this error, the client SHOULD abort the existing conversation, and restart a new one. An example, where more than one message from the client is needed, is when the client is authenticated based on a challenge/response scheme. In that case, the KDC needs to keep track of the challenge issued for a client authentication request. The PA-FX-COOKIE padata type is defined in this section to facilitate state management in the AS exchange. These padata are sent by the KDC when the KDC requires state for a future transaction. The client includes this opaque token in the next message in the conversation. The token may be relatively large; clients MUST be prepared for tokens somewhat larger than the size of all messages in a conversation. PA-FX-COOKIE 133 -- Stateless cookie that is not tied to a specific KDC. The corresponding padata-value field [RFC4120] contains an opaque token that will be echoed by the client in its response to an error from the KDC. The cookie token is generated by the KDC and transmitted in a PA-FX- COOKIE pre-authentication data item of a KRB-ERROR message. The client MUST copy the exact cookie encapsulated in a PA-FX-COOKIE data element into the next message of the same conversation. The content of the cookie field is a local matter of the KDC. As a result, it is not generally possible to mix KDC implementations from different vendors in the same realm. However, the KDC MUST construct the cookie token in such a manner that a malicious client cannot subvert the authentication process by manipulating the token. The KDC implementation needs to consider expiration of tokens, key rollover, and other security issues in token design. The content of the cookie
field is likely specific to the pre-authentication mechanisms used to authenticate the client. If a client authentication response can be replayed to multiple KDCs via the PA-FX-COOKIE mechanism, an expiration in the cookie is RECOMMENDED to prevent the response being presented indefinitely. Implementations need to consider replay both of an entire conversation and of messages within a conversation when designing what information is stored in a cookie and how pre- authentication mechanisms are implemented. If at least one more message for a mechanism or a mechanism set is expected by the KDC, the KDC returns a KDC_ERR_MORE_PREAUTH_DATA_REQUIRED error with a PA-FX-COOKIE to identify the conversation with the client, according to Section 2.2. The cookie is not expected to stay constant for a conversation: the KDC is expected to generate a new cookie for each message. KDC_ERR_MORE_PREAUTH_DATA_REQUIRED 91 A client MAY throw away the state associated with a conversation and begin a new conversation by discarding its state and not including a cookie in the first message of a conversation. KDCs that comply with this specification MUST include a cookie in a response when the client can continue the conversation. In particular, a KDC MUST include a cookie in a KDC_ERR_PREAUTH_REQUIRED or KDC_ERR_MORE_PREAUTH_DATA_REQUIRED. KDCs SHOULD include a cookie in errors containing additional information allowing a client to retry. One reasonable strategy for meeting these requirements is to always include a cookie in KDC errors. A KDC MAY indicate that it is terminating a conversation by not including a cookie in a response. When FAST is used, clients can assume that the absence of a cookie means that the KDC is ending the conversation. Similarly, if a cookie is seen at all during a conversation, clients MAY assume that the absence of a cookie in a future message means that the KDC is ending the conversation. Clients also need to deal with KDCs, prior to this specification, that do not include cookies; if neither cookies nor FAST are used in a conversation, the absence of a cookie is not a strong indication that the KDC is terminating the conversation.5.3. Pre-Authentication Set
If all mechanisms in a group need to successfully complete in order to authenticate a client, the client and the KDC SHOULD use the PA- AUTHENTICATION-SET padata element. PA-AUTHENTICATION-SET 134
A PA-AUTHENTICATION-SET padata element contains the ASN.1 DER
encoding of the PA-AUTHENTICATION-SET structure:
PA-AUTHENTICATION-SET ::= SEQUENCE OF PA-AUTHENTICATION-SET-ELEM
PA-AUTHENTICATION-SET-ELEM ::= SEQUENCE {
pa-type [0] Int32,
-- same as padata-type.
pa-hint [1] OCTET STRING OPTIONAL,
pa-value [2] OCTET STRING OPTIONAL,
...
}
The pa-type field of the PA-AUTHENTICATION-SET-ELEM structure
contains the corresponding value of padata-type in PA-DATA [RFC4120].
Associated with the pa-type is a pa-hint, which is an octet string
specified by the pre-authentication mechanism. This hint may provide
information for the client that helps it determine whether the
mechanism can be used. For example, a public-key mechanism might
include the certificate authorities it trusts in the hint info. Most
mechanisms today do not specify hint info; if a mechanism does not
specify hint info, the KDC MUST NOT send a hint for that mechanism.
To allow future revisions of mechanism specifications to add hint
info, clients MUST ignore hint info received for mechanisms that the
client believes do not support hint info. The pa-value element of
the PA-AUTHENTICATION-SET-ELEM sequence is included to carry the
first padata-value from the KDC to the client. If the client chooses
this authentication set, then the client MUST process this pa-value.
The pa-value element MUST be absent for all but the first entry in
the authentication set. Clients MUST ignore the pa-value for the
second and following entries in the authentication set.
If the client chooses an authentication set, then its first AS-REQ
message MUST contain a PA-AUTH-SET-SELECTED padata element. This
element contains the encoding of the PA-AUTHENTICATION-SET sequence
received from the KDC corresponding to the authentication set that is
chosen. The client MUST use the same octet values received from the
KDC; it cannot re-encode the sequence. This allows KDCs to use bit-
wise comparison to identify the selected authentication set.
Permitting bit-wise comparison may limit the ability to use certain
pre-authentication mechanisms that generate a dynamic challenge in an
authentication set with optimistic selection of an authentication
set. As with other optimistic pre-authentication failures, the KDC
MAY return KDC_ERR_PREAUTH_FAILED with a new list of pre-
authentication mechanisms (including authentication sets) if
optimistic pre-authentication fails. The PA-AUTH-SET-SELECTED padata
element MUST come before any padata elements from the authentication
set in the padata sequence in the AS-REQ message. The client MAY
cache authentication sets from prior messages and use them to
construct an optimistic initial AS-REQ. If the KDC receives a PA-
AUTH-SET-SELECTED padata element that does not correspond to an
authentication set that it would offer, then the KDC returns the
KDC_ERR_PREAUTH_BAD_AUTHENTICATION_SET error. The e-data in this
error contains a sequence of padata just as for the
KDC_ERR_PREAUTH_REQUIRED error.
PA-AUTH-SET-SELECTED 135
KDC_ERR_PREAUTH_BAD_AUTHENTICATION_SET 92
The PA-AUTHENTICATION-SET appears only in the first message from the
KDC to the client. In particular, the client MAY fail if the
authentication mechanism sets change as the conversation progresses.
Clients MAY assume that the hints provided in the authentication set
contain enough information that the client knows what user interface
elements need to be displayed during the entire authentication
conversation. Exceptional circumstances, such as expired passwords
or expired accounts, may require that additional user interface be
displayed. Mechanism designers need to carefully consider the design
of their hints so that the client has this information. This way,
clients can construct necessary dialogue boxes or wizards based on
the authentication set and can present a coherent user interface.
Current standards for user interfaces do not provide an acceptable
experience when the client has to ask additional questions later in
the conversation.
When indicating which sets of pre-authentication mechanisms are
supported, the KDC includes a PA-AUTHENTICATION-SET padata element
for each pre-authentication mechanism set.
The client sends the padata-value for the first mechanism it picks in
the pre-authentication set, when the first mechanism completes, the
client and the KDC will proceed with the second mechanism, and so on
until all mechanisms complete successfully. The PA-FX-COOKIE, as
defined in Section 5.2, MUST be sent by the KDC. One reason for this
requirement is so that the conversation can continue if the
conversation involves multiple KDCs. KDCs MUST support clients that
do not include a cookie because they optimistically choose an
authentication set, although they MAY always return a
KDC_ERR_PREAUTH_BAD_AUTHENTICATION_SET and include a cookie in that
message. Clients that support PA-AUTHENTICATION-SET MUST support PA-
FX-COOKIE.
Before the authentication succeeds and a ticket is returned, the
message that the client sends is an AS-REQ and the message that the
KDC sends is a KRB-ERROR message. The error code in the KRB-ERROR
message from the KDC is KDC_ERR_MORE_PREAUTH_DATA_REQUIRED as defined
in Section 5.2 and the accompanying e-data contains the DER encoding of ASN.1 type METHOD-DATA. The KDC includes the padata elements in the METHOD-DATA. If there are no padata, the e-data field is absent in the KRB-ERROR message. If the client sends the last message for a given mechanism, then the KDC sends the first message for the next mechanism. If the next mechanism does not start with a KDC-side challenge, then the KDC includes a padata item with the appropriate pa-type and an empty pa- data. If the KDC sends the last message for a particular mechanism, the KDC also includes the first padata for the next mechanism.5.4. Definition of Kerberos FAST Padata
As described in [RFC4120], Kerberos is vulnerable to offline dictionary attacks. An attacker can request an AS-REP and try various passwords to see if they can decrypt the resulting ticket. RFC 4120 provides the encrypted timestamp pre-authentication method that ameliorates the situation somewhat by requiring that an attacker observe a successful authentication. However, stronger security is desired in many environments. The Kerberos FAST pre-authentication padata defined in this section provides a tool to significantly reduce vulnerability to offline dictionary attacks. When combined with encrypted challenge, FAST requires an attacker to mount a successful man-in-the-middle attack to observe ciphertext. When combined with host keys, FAST can even protect against active attacks. FAST also provides solutions to common problems for pre- authentication mechanisms such as binding of the request and the reply and freshness guarantee of the authentication. FAST itself, however, does not authenticate the client or the KDC; instead, it provides a typed hole to allow pre-authentication data be tunneled. A pre-authentication data element used within FAST is called a "FAST factor". A FAST factor captures the minimal work required for extending Kerberos to support a new pre-authentication scheme. A FAST factor MUST NOT be used outside of FAST unless its specification explicitly allows so. The typed holes in FAST messages can also be used as generic holes for other padata that are not intended to prove the client's identity, or establish the reply key. New pre-authentication mechanisms SHOULD be designed as FAST factors, instead of full-blown pre-authentication mechanisms. FAST factors that are pre-authentication mechanisms MUST meet the requirements in Section 4.
FAST employs an armoring scheme. The armor can be a Ticket Granting Ticket (TGT) obtained by the client's machine using the host keys to pre-authenticate with the KDC, or an anonymous TGT obtained based on anonymous PKINIT [RFC6112] [RFC4556]. The rest of this section describes the types of armors and the syntax of the messages used by FAST. Conforming implementations MUST support Kerberos FAST padata. Any FAST armor scheme MUST provide a fresh armor key for each conversation. Clients and KDCs can assume that if a message is encrypted and integrity protected with a given armor key, then it is part of the conversation using that armor key. All KDCs in a realm MUST support FAST if FAST is offered by any KDC as a pre-authentication mechanism.5.4.1. FAST Armors
An armor key is used to encrypt pre-authentication data in the FAST request and the response. The KrbFastArmor structure is defined to identify the armor key. This structure contains the following two fields: the armor-type identifies the type of armors and the armor- value is an OCTET STRING that contains the description of the armor scheme and the armor key. KrbFastArmor ::= SEQUENCE { armor-type [0] Int32, -- Type of the armor. armor-value [1] OCTET STRING, -- Value of the armor. ... } The value of the armor key is a matter of the armor type specification. Only one armor type is defined in this document. FX_FAST_ARMOR_AP_REQUEST 1 The FX_FAST_ARMOR_AP_REQUEST armor is based on Kerberos tickets. Conforming implementations MUST implement the FX_FAST_ARMOR_AP_REQUEST armor type. If a FAST KDC receives an unknown armor type it MUST respond with KDC_ERR_PREAUTH_FAILED. An armor type may be appropriate for use in armoring AS requests, armoring TGS requests, or both. TGS armor types MUST authenticate the client to the KDC, typically by binding the TGT sub-session key
to the armor key. As discussed below, it is desirable for AS armor types to authenticate the KDC to the client, but this is not required. FAST implementations MUST maintain state about whether the armor mechanism authenticates the KDC. If it does not, then a FAST factor that authenticates the KDC MUST be used if the reply key is replaced.5.4.1.1. Ticket-Based Armors
This is a ticket-based armoring scheme. The armor-type is FX_FAST_ARMOR_AP_REQUEST, the armor-value contains an ASN.1 DER encoded AP-REQ. The ticket in the AP-REQ is called an armor ticket or an armor TGT. The subkey field in the AP-REQ MUST be present. The armor key is defined by the following function: armor_key = KRB-FX-CF2( subkey, ticket_session_key, "subkeyarmor", "ticketarmor" ) The 'ticket_session_key' is the session key from the ticket in the ap-req. The 'subkey' is the ap-req subkey. This construction guarantees that both the KDC (through the session key) and the client (through the subkey) contribute to the armor key. The server name field of the armor ticket MUST identify the TGS of the target realm. Here are three common ways in the decreasing preference order how an armor TGT SHOULD be obtained: 1. If the client is authenticating from a host machine whose Kerberos realm has an authentication path to the client's realm, the host machine obtains a TGT by using the host keys. If the client's realm is different than the realm of the local host, the machine then obtains a cross-realm TGT to the client's realm as the armor ticket. Otherwise, the host's primary TGT is the armor ticket. 2. If the client's host machine cannot obtain a host ticket strictly based on RFC 4120, but the KDC has an asymmetric signing key whose binding with the expected KDC can be verified by the client, the client can use anonymous PKINIT [RFC6112] [RFC4556] to authenticate the KDC and obtain an anonymous TGT as the armor ticket. The armor ticket can also be a cross-realm TGT obtained based on the initial primary TGT obtained using anonymous PKINIT with KDC authentication. 3. Otherwise, the client uses anonymous PKINIT to get an anonymous TGT without KDC authentication and that TGT is the armor ticket. Note that this mode of operation is vulnerable to man-in-the-
middle attacks at the time of obtaining the initial anonymous
armor TGT.
If anonymous PKINIT is used to obtain the armor ticket, the KDC
cannot know whether its signing key can be verified by the client;
hence, the KDC MUST be marked as unverified from the KDC's point of
view while the client could be able to authenticate the KDC by
verifying the KDC's signing key is bound with the expected KDC. The
client needs to carefully consider the risk and benefit tradeoffs
associated with active attacks before exposing cipher text encrypted
using the user's long-term secrets when the armor does not
authenticate the KDC.
The TGS MUST reject a request if there is an AD-fx-fast-armor (71)
element in the authenticator of the pa-tgs-req padata or if the
ticket in the authenticator of a pa-tgs-req contains the AD-fx-fast-
armor authorization data element. These tickets and authenticators
MAY be used as FAST armor tickets but not to obtain a ticket via the
TGS. This authorization data is used in a system where the
encryption of the user's pre-authentication data is performed in an
unprivileged user process. A privileged process can provide to the
user process a host ticket, an authenticator for use with that
ticket, and the sub-session key contained in the authenticator. In
order for the host process to ensure that the host ticket is not
accidentally or intentionally misused, (i.e., the user process might
use the host ticket to authenticate as the host), it MUST include a
critical authorization data element of the type AD-fx-fast-armor when
providing the authenticator or in the enc-authorization-data field of
the TGS request used to obtain the TGT. The corresponding ad-data
field of the AD-fx-fast-armor element is empty.
This armor type is only valid for AS requests; implicit armor,
described below in TGS processing, is the only supported way to
establish an armor key for the TGS at this time.
5.4.2. FAST Request
A padata type PA-FX-FAST is defined for the Kerberos FAST pre-
authentication padata. The corresponding padata-value field
[RFC4120] contains the DER encoding of the ASN.1 type PA-FX-FAST-
REQUEST. As with all pre-authentication types, the KDC SHOULD
advertise PA-FX-FAST in a PREAUTH_REQUIRED error. KDCs MUST send the
advertisement of PA-FX-FAST with an empty pa-value. Clients MUST
ignore the pa-value of PA-FX-FAST in an initial PREAUTH_REQUIRED
error. FAST is not expected to be used in an authentication set:
clients will typically use FAST padata if available and this decision
should not depend on what other pre-authentication methods are
available. As such, no pa-hint is defined for FAST at this time.
PA-FX-FAST 136
-- Padata type for Kerberos FAST
PA-FX-FAST-REQUEST ::= CHOICE {
armored-data [0] KrbFastArmoredReq,
...
}
KrbFastArmoredReq ::= SEQUENCE {
armor [0] KrbFastArmor OPTIONAL,
-- Contains the armor that identifies the armor key.
-- MUST be present in AS-REQ.
req-checksum [1] Checksum,
-- For AS, contains the checksum performed over the type
-- KDC-REQ-BODY for the req-body field of the KDC-REQ
-- structure;
-- For TGS, contains the checksum performed over the type
-- AP-REQ in the PA-TGS-REQ padata.
-- The checksum key is the armor key, the checksum
-- type is the required checksum type for the enctype of
-- the armor key, and the key usage number is
-- KEY_USAGE_FAST_REQ_CHKSUM.
enc-fast-req [2] EncryptedData, -- KrbFastReq --
-- The encryption key is the armor key, and the key usage
-- number is KEY_USAGE_FAST_ENC.
...
}
KEY_USAGE_FAST_REQ_CHKSUM 50
KEY_USAGE_FAST_ENC 51
The PA-FX-FAST-REQUEST structure contains a KrbFastArmoredReq type.
The KrbFastArmoredReq encapsulates the encrypted padata.
The enc-fast-req field contains an encrypted KrbFastReq structure.
The armor key is used to encrypt the KrbFastReq structure, and the
key usage number for that encryption is KEY_USAGE_FAST_ENC.
The armor key is selected as follows:
o In an AS request, the armor field in the KrbFastArmoredReq
structure MUST be present and the armor key is identified
according to the specification of the armor type.
o There are two possibilities for armor for a TGS request. If the
ticket presented in the PA-TGS-REQ authenticator is a TGT, then
the client SHOULD NOT include the armor field in the Krbfastreq
and a subkey MUST be included in the PA-TGS-REQ authenticator. In
this case, the armor key is the same armor key that would be
computed if the TGS-REQ authenticator was used in an
FX_FAST_ARMOR_AP_REQUEST armor. Clients MAY present a non-TGT in
the PA-TGS-REQ authenticator and omit the armor field, in which
case the armor key is the same that would be computed if the
authenticator were used in an FX_FAST_ARMOR_AP_REQUEST armor.
This is the only case where a ticket other than a TGT can be used
to establish an armor key; even though the armor key is computed
the same as an FX_FAST_ARMOR_AP_REQUEST, a non-TGT cannot be used
as an armor ticket in FX_FAST_ARMOR_AP_REQUEST. Alternatively, a
client MAY use an armor type defined in the future for use with
the TGS request.
The req-checksum field contains a checksum computed differently for
AS and TGS. For an AS-REQ, it is performed over the type KDC-REQ-
BODY for the req-body field of the KDC-REQ structure of the
containing message; for a TGS-REQ, it is performed over the type AP-
REQ in the PA-TGS-REQ padata of the TGS request. The checksum key is
the armor key, and the checksum type is the required checksum type
for the enctype of the armor key per [RFC3961]. This checksum MUST
be a keyed checksum and it is included in order to bind the FAST
padata to the outer request. A KDC that implements FAST will ignore
the outer request, but including a checksum is relatively cheap and
may prevent confusing behavior.
The KrbFastReq structure contains the following information:
KrbFastReq ::= SEQUENCE {
fast-options [0] FastOptions,
-- Additional options.
padata [1] SEQUENCE OF PA-DATA,
-- padata typed holes.
req-body [2] KDC-REQ-BODY,
-- Contains the KDC request body as defined in Section
-- 5.4.1 of [RFC4120].
-- This req-body field is preferred over the outer field
-- in the KDC request.
...
}
The fast-options field indicates various options that are to modify
the behavior of the KDC. The following options are defined:
FastOptions ::= KerberosFlags
-- reserved(0),
-- hide-client-names(1),
Bits Name Description
-----------------------------------------------------------------
0 RESERVED Reserved for future expansion of this
field.
1 hide-client-names Requesting the KDC to hide client
names in the KDC response, as
described next in this section.
16 kdc-follow-referrals reserved [REFERRALS].
Bits 1 through 15 inclusive (with bit 1 and bit 15 included) are
critical options. If the KDC does not support a critical option, it
MUST fail the request with KDC_ERR_UNKNOWN_CRITICAL_FAST_OPTIONS, and
there is no accompanying e-data defined in this document for this
error code. Bit 16 and onward (with bit 16 included) are non-
critical options. KDCs conforming to this specification ignore
unknown non-critical options.
KDC_ERR_UNKNOWN_CRITICAL_FAST_OPTIONS 93
The hide-client-names Option
The Kerberos response defined in [RFC4120] contains the client
identity in cleartext. This makes traffic analysis
straightforward. The hide-client-names option is designed to
complicate traffic analysis. If the hide-client-names option is
set, the KDC implementing PA-FX-FAST MUST identify the client as
the anonymous principal [RFC6112] in the KDC reply and the error
response. Hence, this option is set by the client if it wishes to
conceal the client identity in the KDC response. A conforming KDC
ignores the client principal name in the outer KDC-REQ-BODY field,
and identifies the client using the cname and crealm fields in the
req-body field of the KrbFastReq structure.
The kdc-follow-referrals Option
This option is reserved for [REFERRALS].
The padata field contains a list of PA-DATA structures as described in Section 5.2.7 of [RFC4120]. These PA-DATA structures can contain FAST factors. They can also be used as generic typed-holes to contain data not intended for proving the client's identity or establishing a reply key, but for protocol extensibility. If the KDC supports the PA-FX-FAST-REQUEST padata, unless otherwise specified, the client MUST place any padata that is otherwise in the outer KDC request body into this field. In a TGS request, PA-TGS-REQ padata is not included in this field and it is present in the outer KDC request body. The KDC-REQ-BODY in the FAST structure is used in preference to the KDC-REQ-BODY outside of the FAST pre-authentication. The outer KDC- REQ-BODY structure SHOULD be filled in for backwards compatibility with KDCs that do not support FAST. A conforming KDC ignores the outer KDC-REQ-BODY field in the KDC request. Pre-authentication data methods such as [RFC4556] that include a checksum of the KDC-REQ-BODY should checksum the KDC-REQ-BODY in the FAST structure. In a TGS request, a client MAY include the AD-fx-fast-used authdata either in the pa-tgs-req authenticator or in the authorization data in the pa-tgs-req ticket. If the KDC receives this authorization data but does not find a FAST padata, then it MUST return KRB_APP_ERR_MODIFIED.5.4.3. FAST Response
The KDC that supports the PA-FX-FAST padata MUST include a PA-FX-FAST padata element in the KDC reply. In the case of an error, the PA-FX- FAST padata is included in the KDC responses according to Section 5.4.4. The corresponding padata-value field [RFC4120] for the PA-FX-FAST in the KDC response contains the DER encoding of the ASN.1 type PA-FX- FAST-REPLY. PA-FX-FAST-REPLY ::= CHOICE { armored-data [0] KrbFastArmoredRep, ... } KrbFastArmoredRep ::= SEQUENCE { enc-fast-rep [0] EncryptedData, -- KrbFastResponse -- -- The encryption key is the armor key in the request, and -- the key usage number is KEY_USAGE_FAST_REP. ... } KEY_USAGE_FAST_REP 52
The PA-FX-FAST-REPLY structure contains a KrbFastArmoredRep
structure. The KrbFastArmoredRep structure encapsulates the padata
in the KDC reply in the encrypted form. The KrbFastResponse is
encrypted with the armor key used in the corresponding request, and
the key usage number is KEY_USAGE_FAST_REP.
The Kerberos client MUST support a local policy that rejects the
response if PA-FX-FAST-REPLY is not included in the response.
Clients MAY also support policies that fall back to other mechanisms
or that do not use pre-authentication when FAST is unavailable. It
is important to consider the potential downgrade attacks when
deploying such a policy.
The KrbFastResponse structure contains the following information:
KrbFastResponse ::= SEQUENCE {
padata [0] SEQUENCE OF PA-DATA,
-- padata typed holes.
strengthen-key [1] EncryptionKey OPTIONAL,
-- This, if present, strengthens the reply key for AS and
-- TGS. MUST be present for TGS.
-- MUST be absent in KRB-ERROR.
finished [2] KrbFastFinished OPTIONAL,
-- Present in AS or TGS reply; absent otherwise.
nonce [3] UInt32,
-- Nonce from the client request.
...
}
The padata field in the KrbFastResponse structure contains a list of
PA-DATA structures as described in Section 5.2.7 of [RFC4120]. These
PA-DATA structures are used to carry data advancing the exchange
specific for the FAST factors. They can also be used as generic
typed-holes for protocol extensibility. Unless otherwise specified,
the KDC MUST include any padata that are otherwise in the outer KDC-
REP or KDC-ERROR structure into this field. The padata field in the
KDC reply structure outside of the PA-FX-FAST-REPLY structure
typically includes only the PA-FX-FAST-REPLY padata.
The strengthen-key field provides a mechanism for the KDC to
strengthen the reply key. If set, the strengthen-key value MUST be
randomly generated to have the same etype as that of the reply key
before being strengthened, and then the reply key is strengthened
after all padata items are processed. Let padata-reply-key be the
reply key after padata processing.
reply-key = KRB-FX-CF2(strengthen-key, padata-reply-key,
"strengthenkey", "replykey")
The strengthen-key field MAY be set in an AS reply; it MUST be set in
a TGS reply; it must be absent in an error reply. The strengthen key
is required in a TGS reply so that an attacker cannot remove the FAST
PADATA from a TGS reply, causing the KDC to appear not to support
FAST.
The finished field contains a KrbFastFinished structure. It is
filled by the KDC in the final message in the conversation. This
field is present in an AS-REP or a TGS-REP when a ticket is returned,
and it is not present in an error reply.
The KrbFastFinished structure contains the following information:
KrbFastFinished ::= SEQUENCE {
timestamp [0] KerberosTime,
usec [1] Microseconds,
-- timestamp and usec represent the time on the KDC when
-- the reply was generated.
crealm [2] Realm,
cname [3] PrincipalName,
-- Contains the client realm and the client name.
ticket-checksum [4] Checksum,
-- checksum of the ticket in the KDC-REP using the armor
-- and the key usage is KEY_USAGE_FAST_FINISH.
-- The checksum type is the required checksum type
-- of the armor key.
...
}
KEY_USAGE_FAST_FINISHED 53
The timestamp and usec fields represent the time on the KDC when the
reply ticket was generated, these fields have the same semantics as
the corresponding identically named fields in Section 5.6.1 of
[RFC4120]. The client MUST use the KDC's time in these fields
thereafter when using the returned ticket. The client need not
confirm that the timestamp returned is within allowable clock skew:
the armor key guarantees that the reply is fresh. The client MAY
trust the timestamp returned.
The cname and crealm fields identify the authenticated client. If
facilities described in [REFERRALS] are used, the authenticated
client may differ from the client in the FAST request.
The ticket-checksum is a checksum of the issued ticket. The checksum
key is the armor key, and the checksum type is the required checksum
type of the enctype of that key, and the key usage number is
KEY_USAGE_FAST_FINISHED.
When FAST padata is included, the PA-FX-COOKIE padata as defined in Section 5.2 MUST be included in the padata sequence in the KrbFastResponse sequence if the KDC expects at least one more message from the client in order to complete the authentication. The nonce field in the KrbFastResponse contains the value of the nonce field in the KDC-REQ of the corresponding client request and it binds the KDC response with the client request. The client MUST verify that this nonce value in the reply matches with that of the request and reject the KDC reply otherwise. To prevent the response from one message in a conversation from being replayed to a request in another message, clients SHOULD use a new nonce for each message in a conversation.5.4.4. Authenticated Kerberos Error Messages Using Kerberos FAST
If the Kerberos FAST padata was included in the request, unless otherwise specified, the e-data field of the KRB-ERROR message [RFC4120] contains the ASN.1 DER encoding of the type METHOD-DATA [RFC4120] and a PA-FX-FAST is included in the METHOD-DATA. The KDC MUST include all the padata elements such as PA-ETYPE-INFO2 and padata elements that indicate acceptable pre-authentication mechanisms [RFC4120] in the KrbFastResponse structure. The KDC MUST also include a PA-FX-ERROR padata item in the KRBFastResponse structure. The padata-value element of this sequence is the ASN.1 DER encoding of the type KRB-ERROR. The e-data field MUST be absent in the PA-FX-ERROR padata. All other fields should be the same as the outer KRB-ERROR. The client ignores the outer error and uses the combination of the padata in the KRBFastResponse and the error information in the PA-FX-ERROR. PA-FX-ERROR 137 If the Kerberos FAST padata is included in the request but not included in the error reply, it is a matter of the local policy on the client to accept the information in the error message without integrity protection. However, the client SHOULD process the KDC errors as the result of the KDC's inability to accept the AP_REQ armor and potentially retry another request with a different armor when applicable. The Kerberos client MAY process an error message without a PA-FX-FAST-REPLY, if that is only intended to return better error information to the application, typically for trouble-shooting purposes. In the cases where the e-data field of the KRB-ERROR message is expected to carry a TYPED-DATA [RFC4120] element, that information should be transmitted in a pa-data element within the KRBFastResponse
structure. The padata-type is the same as the data-type would be in the typed data element and the padata-value is the same as the data- value. As discussed in Section 7, data-types and padata-types are drawn from the same namespace. For example, the TD_TRUSTED_CERTIFIERS structure is expected to be in the KRB-ERROR message when the error code is KDC_ERR_CANT_VERIFY_CERTIFICATE [RFC4556].5.4.5. Outer and Inner Requests
Typically, a client will know that FAST is being used before a request containing PA-FX-FAST is sent. So, the outer AS request typically only includes one pa-data item: PA-FX-FAST. The client MAY include additional pa-data, but the KDC MUST ignore the outer request body and any padata besides PA-FX-FAST if and only if PA-FX-FAST is processed. In the case of the TGS request, the outer request should include PA-FX-FAST and PA-TGS-REQ. When an AS generates a response, all padata besides PA-FX-FAST should be included in PA-FX-FAST. The client MUST ignore other padata outside of PA-FX-FAST.5.4.6. The Encrypted Challenge FAST Factor
The encrypted challenge FAST factor authenticates a client using the client's long-term key. This factor works similarly to the encrypted timestamp pre-authentication option described in [RFC4120]. The word "challenge" is used instead of "timestamp" because while the timestamp is used as an initial challenge, if the KDC and client do not have synchronized time, then the KDC can provide updated time to the client to use as a challenge. The client encrypts a structure containing a timestamp in the challenge key. The challenge key used by the client to send a message to the KDC is KRB-FX- CF2(armor_key,long_term_key, "clientchallengearmor", "challengelongterm"). The challenge key used by the KDC encrypting to the client is KRB-FX-CF2(armor_key, long_term_key, "kdcchallengearmor", "challengelongterm"). Because the armor key is fresh and random, the challenge key is fresh and random. The only purpose of the timestamp is to limit the validity of the authentication so that a request cannot be replayed. A client MAY base the timestamp on the KDC time in a KDC error and need not maintain accurate time synchronization itself. If a client bases its time on an untrusted source, an attacker may trick the client into producing an authentication request that is valid at some future time. The attacker may be able to use this authentication request to make it appear that a client has authenticated at that future time. If ticket-based armor is used, then the lifetime of the ticket will limit the window in which an attacker can make the client appear to
have authenticated. For many situations, the ability of an attacker
to cause a client to appear to have authenticated is not a
significant concern; the ability to avoid requiring time
synchronization on clients is more valuable.
The client sends a padata of type PA-ENCRYPTED-CHALLENGE. The
corresponding padata-value contains the DER encoding of ASN.1 type
EncryptedChallenge.
EncryptedChallenge ::= EncryptedData
-- Encrypted PA-ENC-TS-ENC, encrypted in the challenge key
-- using key usage KEY_USAGE_ENC_CHALLENGE_CLIENT for the
-- client and KEY_USAGE_ENC_CHALLENGE_KDC for the KDC.
PA-ENCRYPTED-CHALLENGE 138
KEY_USAGE_ENC_CHALLENGE_CLIENT 54
KEY_USAGE_ENC_CHALLENGE_KDC 55
The client includes some timestamp reasonably close to the KDC's
current time and encrypts it in the challenge key in a PA-ENC-TS-ENC
structure (see Section 5.2.7.2 in RFC 4120). Clients MAY use the
current time; doing so prevents the exposure where an attacker can
cause a client to appear to authenticate in the future. The client
sends the request including this factor.
On receiving an AS-REQ containing the PA-ENCRYPTED-CHALLENGE FAST
factor, the KDC decrypts the timestamp. If the decryption fails the
KDC SHOULD return KDC_ERR_PREAUTH_FAILED, including PA-ETYPE-INFO2 in
the KRBFastResponse in the error. The KDC confirms that the
timestamp falls within its current clock skew returning
KRB_APP_ERR_SKEW if not. The KDC then SHOULD check to see if the
encrypted challenge is a replay. The KDC MUST NOT consider two
encrypted challenges replays simply because the timestamps are the
same; to be a replay, the ciphertext MUST be identical. Allowing
clients to reuse timestamps avoids requiring that clients maintain
state about which timestamps have been used.
If the KDC accepts the encrypted challenge, it MUST include a padata
element of type PA-ENCRYPTED-CHALLENGE. The KDC encrypts its current
time in the challenge key. The KDC MUST strengthen the reply key
before issuing a ticket. The client MUST check that the timestamp
decrypts properly. The client MAY check that the timestamp is within
the window of acceptable clock skew for the client. The client MUST
NOT require that the timestamp be identical to the timestamp in the
issued credentials or the returned message.
The encrypted challenge FAST factor provides the following facilities: Client Authentication and KDC Authentication. This FAST factor also takes advantage of the FAST facility to strengthen the reply key. It does not provide the Replace Reply Key facility. The Security Considerations section of this document provides an explanation why the security requirements are met. The encrypted challenge FAST factor can be useful in an authentication set. No pa-hint is defined because the only information needed by this mechanism is information contained in the PA-ETYPE-INFO2 pre-authentication data. KDCs are already required to send PA-ETYPE-INFO2. If KDCs were not required to send PA-ETYPE- INFO2 then that information would need to be part of a hint for encrypted challenge. Conforming implementations MUST support the encrypted challenge FAST factor.5.5. Authentication Strength Indication
Implementations that have pre-authentication mechanisms offering significantly different strengths of client authentication MAY choose to keep track of the strength of the authentication used as an input into policy decisions. For example, some principals might require strong pre-authentication, while less sensitive principals can use relatively weak forms of pre-authentication like encrypted timestamp. An AuthorizationData data type AD-Authentication-Strength is defined for this purpose. AD-authentication-strength 70 The corresponding ad-data field contains the DER encoding of the pre- authentication data set as defined in Section 5.3. This set contains all the pre-authentication mechanisms that were used to authenticate the client. If only one pre-authentication mechanism was used to authenticate the client, the pre-authentication set contains one element. Unless otherwise specified, the hint and value fields of the members of this sequence MUST be empty. In order to permit mechanisms to carry additional information about strength in these fields in the future, clients and application servers MUST ignore non-empty hint and value fields for mechanisms unless the implementation is updated with the interpretation of these fields for a given pre-authentication mechanism in this authorization element. The AD-authentication-strength element MUST be included in the AD- KDC-ISSUED container so that the KDC integrity protects its contents. This element can be ignored if it is unknown to the receiver.
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