Network Working Group Wilkinson
Internet-Draft YFS
Intended status: Informational Kaduk
Expires: September 22, 2013 MIT
March 21, 2013
rxgk: GSSAPI based security class for RX
draft-wilkinson-afs3-rxgk-04
Abstract
rxgk is a security class for the RX RPC protocol. It uses the GSSAPI
framework to provide an authentication service that provides
authentication, confidentiality and integrity protection for the rxgk
security class. This document provides a general description of
rxgk. A further document will provide details of integration with
specific RX applications.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 22, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Time Representation . . . . . . . . . . . . . . . . . . . . . 3
3. Encryption Framework . . . . . . . . . . . . . . . . . . . . 4
3.1. Key Usage Values . . . . . . . . . . . . . . . . . . . . 4
4. Security Levels . . . . . . . . . . . . . . . . . . . . . . . 4
5. Token Format . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Key Negotiation . . . . . . . . . . . . . . . . . . . . . . . 5
7. Combining Tokens . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 11
7.2. Key Combination Algorithm . . . . . . . . . . . . . . . . 12
7.3. RPC Definition . . . . . . . . . . . . . . . . . . . . . 12
7.4. Server Operation . . . . . . . . . . . . . . . . . . . . 12
7.5. Client Operation . . . . . . . . . . . . . . . . . . . . 14
8. The rxgk Security Class . . . . . . . . . . . . . . . . . . . 14
8.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 14
8.2. Rekeying . . . . . . . . . . . . . . . . . . . . . . . . 15
8.3. Key Derivation . . . . . . . . . . . . . . . . . . . . . 15
8.4. The Challenge . . . . . . . . . . . . . . . . . . . . . . 16
8.5. The Response . . . . . . . . . . . . . . . . . . . . . . 16
8.5.1. The Authenticator . . . . . . . . . . . . . . . . . . 16
8.6. Checking the Response . . . . . . . . . . . . . . . . . . 17
8.7. Packet Handling . . . . . . . . . . . . . . . . . . . . . 17
8.7.1. Authentication Only . . . . . . . . . . . . . . . . . 17
8.7.2. Integrity Protection . . . . . . . . . . . . . . . . 17
8.7.3. Encryption . . . . . . . . . . . . . . . . . . . . . 19
9. RXGK protocol error codes . . . . . . . . . . . . . . . . . . 20
10. AFS-3 Registry Considerations . . . . . . . . . . . . . . . . 21
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
12. Security Considerations . . . . . . . . . . . . . . . . . . . 22
12.1. Abort Packets . . . . . . . . . . . . . . . . . . . . . 22
12.2. Token Expiry . . . . . . . . . . . . . . . . . . . . . . 22
12.3. Nonce Lengths . . . . . . . . . . . . . . . . . . . . . 22
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
13.1. Informational References . . . . . . . . . . . . . . . . 23
13.2. Normative References . . . . . . . . . . . . . . . . . . 23
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 24
Appendix B. Changes . . . . . . . . . . . . . . . . . . . . . . 24
B.1. Since 00 . . . . . . . . . . . . . . . . . . . . . . . . 24
B.2. Since 01 . . . . . . . . . . . . . . . . . . . . . . . . 25
B.3. Since 02 . . . . . . . . . . . . . . . . . . . . . . . . 25
B.4. Since 03 . . . . . . . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
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rxgk is a GSSAPI [RFC2743] based security class for the rx [RX]
protocol. It provides authentication, confidentiality and integrity
protection for rx RPC calls, using a security context established
using any GSSAPI mechanism with confidentiality, mutual
authentication, and PRF [RFC4401] support. The External Data
Representation Standardard, XDR [RFC4506], is used to represent data
structures on the wire and in the code fragments contained within
this document.
Architecturally, rxgk is split into two parts. The rxgk rx security
class provides strong encryption using previously negotiated ciphers
and keys. It builds on the Kerberos crypto framework for its
encryption requirements, but is authentication mechanism independent
-- the class itself does not require the use of either Kerberos, or
GSSAPI. The security class simply uses a previously negotiated
encryption type, and master key. The master key is never directly
used, but instead a per-connection key is derived for each new secure
connection that is established.
The second portion of rxgk is a service which permits the negotiation
of an encryption algorithm, and the establishment of a master key.
This is done via a separate RPC exchange with a server, prior to the
setup of any rxgk connections. The exchange establishes an rxgk
token, and a master key shared between client and server. This
exchange is protected within a GSSAPI security context.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Time Representation
rxgk expresses absolute time as a 64-bit integer. This contains the
time relative to midnight, or 0 hour, January 1, 1970 UTC,
represented in increments of 100 nanoseconds, excluding any leap
seconds. Negative times, whilst permitted by the representation,
MUST NOT be used within rxgk.
typedef hyper rxgkTime;
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3. Encryption Framework
Bulk data encryption within rxgk is performed using the encryption
framework defined by RFC3961 [RFC3961]. Any algorithm which is
defined using this framework and supported by both client and server
may be used.
3.1. Key Usage Values
In order to avoid using the same key for multiple tasks, key
derivation is employed. To avoid any conflicts with other users of
these keys, key usage numbers are allocated within the application
space documented in section 4 of RFC4120 [RFC4120].
const RXGK_CLIENT_ENC_PACKET = 1026;
const RXGK_CLIENT_MIC_PACKET = 1027;
const RXGK_SERVER_ENC_PACKET = 1028;
const RXGK_SERVER_MIC_PACKET = 1029;
const RXGK_CLIENT_ENC_RESPONSE = 1030;
const RXGK_SERVER_ENC_TOKEN = 1036;
The application of these key usage numbers is specified in Section 8.
4. Security Levels
rxgk supports the negotiation of a range of different security
levels. These, along with the protocol constants that represent them
during key negotiation, are:
Authentication only (0) Provides only connection authentication,
without either integrity or confidentiality protection. This
mode of operation can provide higher throughput, but is
vulnerable to man in the middle attacks. This corresponds to
the traditional 'clear' security level.
Integrity (1) Provides integrity protection only. Data is protected
from modification by an attacker, but not against
eavesdropping. This corresponds to the traditional 'auth'
security level, authenticating the data payload as well as the
Rx connection.
Encryption (2) Provides both integrity and confidentiality
protection. This corresponds to the traditional 'crypt'
security level.
The authentication only, or clear, security level provides faster
throughput, at the expense of connection security. The 'clear'
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security level is vulnerable to a man in the middle altering the data
passed over the connection, whereas the 'integrity' security level
prevents such attacks by sending a cryptographic checksum of the data
being transmitted.
enum RXGK_Level {
RXGK_LEVEL_CLEAR = 0,
RXGK_LEVEL_AUTH = 1,
RXGK_LEVEL_CRYPT = 2
};
5. Token Format
An rxgk token is an opaque identifier which is specific to a
particular application's implementation of rxgk. The token is
completely opaque to the client, which just transmits it from server
to server. The token MUST permit the receiving server to identify
the corresponding user and session key for the incoming connection --
whether that be by decrypting the information within the token, or
making the token a large random identifier which keys a lookup table
on the server, or some other mechanism. It is assumed that such
mechanisms will conceptually "encrypt" a token by somehow associating
the "encrypted" token with the associated unencrypted data, and will
"decrypt" an encrypted token by using that association to find the
unencrypted data. As such, this document will use "encrypt" and
"decrypt" to refer to these operations on tokens. If the token is an
encrypted blob, it should be encrypted using the key usage
RXGK_SERVER_ENC_TOKEN.
The token MUST NOT expose the session key on the wire. The token
MUST be sufficiently random that an attacker cannot predict suitable
token values by observing other connections. An attacker MUST NOT be
able to forge tokens which convey a particular session key or
identity.
6. Key Negotiation
rxgk uses an independent RX RPC service for key negotiation. The
location of this service is application dependent. Within a given
application protocol, a client MUST be able to locate the key
negotiation service, and that service MUST be able to create tokens
which can be read by the application server. The simplest deployment
has the negotiation service running on every application server, on
the same transport endpoints, but using a separate, dedicated, rx
service ID.
The rxgk key negotiation service uses the service ID 34567.
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GSS security context negotiation requires that the initiator specify
a principal name for the acceptor; in the absence of application-
specific knowledge, when using rxgk over a port number registered
with IANA, the registered service name SHOULD be used to construct
the target principal name as <service name>@<hostname> using the name
type GSS_C_NT_HOSTBASED_SERVICE.
The key negotiation protocol is defined by the following RPC-L:
/* limits for variable-length arrays */
const RXGK_MAXENCTYPES = 10;
const RXGK_MAXLEVELS = 10;
const RXGK_MAXMIC = 16384;
const RXGK_MAXNONCE = 1024;
typedef int RXGK_Enctypes<RXGK_MAXENCTYPES>;
typedef opaque RXGK_Data<>;
struct RXGK_StartParams {
RXGK_Enctypes enctypes;
RXGK_Level levels<RXGK_MAXLEVELS>;
unsigned int lifetime;
unsigned int bytelife;
opaque client_nonce<RXGK_MAXNONCE>;
};
struct RXGK_ClientInfo {
int errorcode;
int enctype;
RXGK_Level level;
unsigned int lifetime;
unsigned int bytelife;
rxgkTime expiration;
opaque mic<RXGK_MAXMIC>;
RXGK_Data token;
opaque server_nonce<RXGK_MAXNONCE>;
};
package RXGK_
GSSNegotiate(IN RXGK_StartParams *client_start,
IN RXGK_Data *input_token_buffer,
IN RXGK_Data *opaque_in,
OUT RXGK_Data *output_token_buffer,
OUT RXGK_Data *opaque_out,
OUT unsigned int *gss_major_status,
OUT unsigned int *gss_minor_status,
OUT RXGK_Data *rxgk_info) = 1;
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The client populates RXGK_StartParams with its preferred options.
The enctypes and levels parameters are lists of values supported by
the client, and MUST be ordered from best to worst, with the client's
favoured option occurring first within the list. The parameters are:
enctypes: List of encryption types from the Kerberos Encryption Type
Number registry created in RFC3961 and maintained by IANA.
This list indicates the encryption types that the client is
prepared to support.
levels: List of supported rxgk transport encryption levels. See
Section 4 for allowed values.
lifetime: The maximum number of seconds that a connection key should
be used before rekeying. A value of 0 indicates that the
connection should not be rekeyed based on its lifetime. This
lifetime is advisory.
bytelife: The maximum amount of data to be transferred over the
connection before it should be rekeyed, expressed as log base 2
of the number of bytes. A value of 0 indicates that there is
no limit on the number of bytes that may be transmitted. The
byte lifetime is advisory -- a connection that is over its byte
lifetime should be permitted to continue, but endpoints SHOULD
attempt to rekey the connection (as per Section 8.2) at their
earliest convenience.
client_nonce: A client-generated string of random bytes, to be used
as input to the key generation. This nonce SHOULD be at least
20 octets in length.
The client then calls GSS_Init_sec_context() to obtain an output
token to send to the server. The GSS service name is application
dependent.
The client then calls RXGK_GSSNegotiate, as defined above. This
takes the following parameters:
client_start The client params structure detailed above. This
should remain constant across the negotiation.
input_token_buffer The token produced by a call to
GSS_Init_sec_context().
opaque_in An opaque token, which was returned by the server
following a previous call to GSSNegotiate in this negotiation.
If this is the first call, opaque_in should be zero-length.
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output_token_buffer The token output by the server's call to
GSS_Accept_sec_context().
opaque_out An opaque token, which the server may use to preserve
state information between multiple RPCs in the same context
negotiation. The client should use this value as opaque_in in
its next call to GSSNegotiate in this context negotiation.
gss_major_status The major status code output by the server's call
to GSS_Accept_sec_context().
gss_minor_status The minor status code returned by
GSS_Accept_sec_context(). Implementors should note that minor
status codes are not portable between GSSAPI implementations.
rxgk_info If gss_major_status == GSS_S_COMPLETE this contains an
encrypted block containing the server's response to the client.
See below.
The client proceeds through a GSS security context initialization
loop, with alternating calls to GSS_Init_sec_context() and the
GSSNegotiate() RPC, until an error or success condition is reached.
Each call to GSSNegotiate will return an output token from
GSS_Accept_sec_context() and/or an output opaque to be used as an
input for a subsequent call to GSSNegotiate, if such a subsequent
call is necessary.
Different GSS mechanisms will require a different number of full (or
half) round trips. The structure of the loop, with success and error
conditions noted (noting that RX level errors may occur as well but
are not mentioned as part of the loop structure), is as follows:
The client calls GSS_Init_sec_context(), supplying an input
token if one was returned by a previous call to GSSNegotiate().
The client MUST set the mutual_req_flag, conf_req_flag, and
integ_req_flag booleans to true.
If the major status code from GSS_Init_sec_context() includes a
fatal error code, the negotiation loop is in an error condition
and terminates. If the major status code is GSS_S_COMPLETE and
the mutual_state flag is not true, or the major status code is
GSS_S_COMPLETE and the conf_avail flag is not true, or the
major status code is GSS_S_COMPLETE and the integ_avail flag is
not true, the negotiation loop is in an error condition and
terminates. If the major status code is GSS_S_COMPLETE and the
output token is zero length, this is a success condition and
the negotiation loop terminates (this cannot happen on the
first iteration of the loop). Otherwise, if the major status
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code does not include GSS_S_CONTINUE_NEEDED, the negotiation
loop is in an error condition and terminates. If the major
status code includes GSS_S_CONTINUE_NEEDED, the output token is
sent to the server, per the next step.
The client calls GSSNegotiate(), supplying the output token
from GSS_Init_sec_context() and an input opaque if one was
returned by a previous call to GSSNegotiate().
The server calls GSS_Accept_sec_context(), supplying the token
it received from the client as input. If there is an output
token from GSS_Accept_sec_context(), the server returns it to
the client in the output_token_buffer field of the
GSSNegotiate() RPC, along with the major and minor status codes
from the call to GSS_Accept_sec_context(). If the major status
code includes GSS_S_CONTINUE_NEEDED, the server also returns an
opaque identifier in the opaque_out field of the RPC, which
will allow the server to associate a future RPC call with this
partially formed security context. If the major status code is
GSS_S_COMPLETE, the server constructs an RXGK_ClientInfo
structure per below.
The client receives the results of the GSSNegotiate() RPC. If
the major status code is not GSS_S_COMPLETE and does not
include GSS_S_CONTINUE_NEEDED, the negotiation loop is in an
error condition and terminates. If the most recent call to
GSS_Init_sec_context() returned the major status code
GSS_S_COMPLETE and an output token, the negotiation loop is in
a success condition and terminates. If the most recent call to
GSS_Init_sec_context() returned a major status code including
GSS_S_CONTINUE_NEEDED and the output_token_buffer is zero
length, the negotiation loop is in error and terminates.
Otherwise, the client proceeds to begin the next cycle of the
negotiation loop.
In general, an error condition at the server side of the negotiation
loop will be security sensitive (in that the decision made by the
client upon receipt will affect the security class used for future
connections), and as such should be returned to the client in the
errorcode field of ClientInfo, not as an RPC-level return value
(which is transmitted as an RX abort).
Upon successful completion, rxgk_info contains the XDR representation
of a RXGK_ClientInfo structure, encrypted using gss_wrap() with
confidentiality protection. The client should decrypt this structure
using gss_unwrap(). If the returned value of conf_state is zero, the
negotiation has failed.
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RXGK_ClientInfo contains the following server populated fields:
errorcode A policy (rather than connection establishment) error
code. If non-zero, an error has occurred, the resulting key
negotiation has failed, and the rest of the values in this
structure are undefined. These policy error codes are from
com_err tables [COMERR] and may represent such conditions as
insufficient authorization or that the client has too many
active connections to the service. Error codes may be RXGK
errors (see Section 10) or from an application-specific table.
enctype The encryption type selected by the server. This SHALL be
one of the types listed by the client in its StartParams
structure.
level The rxgk security level selected by the server, see Section 4
for allowed values.
lifetime The connection lifetime, in seconds, as determined by the
server. The server MAY honor the client's request, but the
server MUST choose a value at least as restrictive as the value
requested by the client. A value of zero indicates that the
connection should not be rekeyed based on its lifetime.
bytelife The maximum amount of data (as log base 2 of the number of
bytes) that may be transfered using this key. The server MAY
honor the client's request, but the server MUST choose a value
at least as restrictive as the value requested by the client.
A value of 0 indicates that the connection should not be
rekeyed based on the number of bytes transmitted over the
connection.
expiration The time, expressed as an rxgkTime, at which this token
expires. The expiration time MAY be set administratively by
the server, and SHOULD reflect the expiration time of the
underlying GSSAPI credential. The token SHOULD NOT expire
later than the underlying GSSAPI credential.
mic The result of calling gss_get_mic() [RFC2744] over the XDR
encoded representation of the StartParams request received by
the server.
token An rxgk token. This is an opaque blob, as detailed in
Section 5.
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server_nonce The random nonce used by the server to create the K0
contained within the rxgk token. The length of this nonce
SHOULD be the key generation seed length of the selected
enctype.
Upon receiving the server's response, the client MUST verify that the
mic contained within it matches the MIC of the XDR representation of
the StartParams structure it sent to the server (this prevents a man
in the middle from performing a downgrade attack). The client SHOULD
also verify that the server's selected connection properties match
those proposed by the client.
The client may then compute K0, by taking the nonce it sent to the
server (client_nonce) and the one it has just received
(server_nonce), combining them together, and passing them to
GSS_Pseudo_random() with the GSS_C_PRF_KEY_FULL option:
GSS_Pseudo_random(gssapi_context,
GSS_C_PRF_KEY_FULL,
client_nonce || server_nonce,
K,
*K0);
|| is the concatenation operation.
K, the desired output length, is the key generation seed length as
specified in the RFC3961 profile of the negotiated enctype.
The ouput of GSS_Pseudo_random must then be passed through the
random-to-key operation specified in the RFC3961 profile for the
negotiated enctype in order to obtain the actual key K0.
The GSS_Pseudo_random() operation is deterministic, ensuring that the
client and server generate the same K0. The gssapi_context parameter
is the same context used in the client's GSS_Init_sec_context() call
and the server's GSS_Accept_sec_context() call.
7. Combining Tokens
7.1. Overview
A client may elect to combine multiple rxgk tokens in its possession
into a single token. This allows an rx connection to be secured
using a combination of multiple, individually established identities,
which provides additional security for a number of application
protocols.
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Token combination is performed using the CombineTokens RPC call. The
client has two keys -- K0 and K1, and two tokens, T0 and T1. The
client calls the CombineTokens RPC with T0 and T1 and negotiates the
enctype and security level of the new token, received as Tn. Tn
contains the new key Kn, as computed by the server. Using the
negotiated enctype returned by the server, the client then locally
combines the two keys using a defined combination algorithm to
produce Kn.
7.2. Key Combination Algorithm
Assume that the tokens being combined are T0 and T1, with master keys
K0 and K1. The new master key for the combined token, Kn is computed
using the KRB-FX-CF2 operation, described in section 5.1 of
[RFC6113]. The PRF+ operations will correspond to their respective
key enctypes, and the random-to-key operation will correspond to the
negotiated new enctype. The constants pepper1 and pepper2 required
by this operation are defined as the ASCII strings "AFS" and "rxgk"
respectively.
7.3. RPC Definition
The combine keys RPC is defined as:
struct RXGK_CombineOptions {
RXGK_Enctypes enctypes;
RXGK_Level levels<RXGK_MAXLEVELS>;
};
struct RXGK_TokenInfo {
int errorcode;
RXGK_Enctype enctype;
RXGK_Level level;
unsigned int lifetime;
unsigned int bytelife;
rxgkTime expiration;
}
CombineTokens(IN RXGK_Data *token0, IN RXGK_Data *token1,
IN RXGK_CombineOptions *options,
OUT RXGK_Data *new_token,
OUT RXGK_TokenInfo *info) = 2;
7.4. Server Operation
The server receives token0 and token1 from the RPC call, as well as
the options suggested by the client. Upon receipt, the server
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decrypts these tokens using its private key. Providing this
decryption is successful, it now has copies of the master key from
both tokens (K0 and K1). The server then chooses an enctype and
security level from the lists supplied by the client in the options
argument. The server SHOULD select the first entry from each list
which is acceptable in the server's configuration, so as to respect
any preferences indicated by the client. The server then performs
the key combination algorithm detailed above to obtain the new key,
Kn. The server then constructs a new token as follows. The
expiration field is set to the minimum of the expiration values of
the original tokens. The lifetime, bytelife, and any application-
specific data fields are each combined so that the result is the most
restrictive of the two values in each of the original tokens. The
identity information associated with the tokens are combined in an
application-specific manner to yield the identity information in the
combined token (the identity combining operation may be non-
commutative). This new token contains the derived key, Kn. The new
token is encrypted with the server's private key, as normal, and
returned to the client. The enctype and level chosen by the server
are returned in the info parameter, along with the computed lifetime,
bytelife, and expiration.
If the server is unable to perform the CombineTokens operation with
the given arguments, a nonzero value is returned in the errorcode
element of the info parameter; errorcode is zero for a successful
CombineTokens operation. Errors are returned in this in-band field
as opposed to as the RPC return value because the error code is
security sensitive -- the action taken by the client upon receipt
will affect the security of future actions.
If errorcode is nonzero, the values of the other fields in the
RXGK_TokenInfo structure and the value of new_token are undefined.
Nonzero values for errorcode should be com_err codes [COMERR], from
an RX, RXGK, or application-specific table. See Section 10 for RXGK
error codes. For example,
RXGEN_OPCODE used when the server will refuse all CombineTokens
requests.
RXGK_BADETYPE used when none of the enctypes supplied by the client
are acceptable to the server.
RXGK_BADLEVEL used when none of the security levels supplied by the
client are acceptable to the server.
RXGK_EXPIRED used when one or more of the input tokens was already
expired.
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To reduce the potential for denial of service attacks, servers SHOULD
only offer the CombineTokens operation to clients connecting over a
secured rxgk connection. CombineTokens SHOULD NOT be offered over an
RXGK_LEVEL_CLEAR connection.
7.5. Client Operation
As detailed within the overview, the client calls the CombineTokens
RPC using two tokens, T0 and T1, within its possession, as well as an
RXGK_CombineOptions structure containing a list of acceptable
enctypes and a list of acceptable security levels for the new token.
The client SHOULD supply these lists sorted by preference, with the
most preferred option appearing first in the list. The client then
receives a new token, Tn, from this call, as well as an
RXGK_TokenInfo structure containing information relating to Tn. The
client needs the level element of the info parameter to determine
what security level to use the new token at, and the enctype
parameter to know which enctype's random-to-key function and key
generation seed length to use in generating Kn. With the negotiated
enctype, the client can then perform the key combination algorithm
described in Section 8.3. The client can only make use of Tn to
establish an rxgk protected connection if it can derive Kn, which it
can only do if it already knows K0 and K1.
Clients MUST use an rxgk secured connection for the CombineTokens
operation.
8. The rxgk Security Class
8.1. Overview
When a new connection using rxgk is created by the client, the client
stores the current timestamp as an rxgkTime (start_time for the rest
of this discussion), and then uses this, along with other connection
information, to derive a transport key from the current master key
(see Section 8.3).
This key is then used to protect the first message the client sends
to the server. The server follows the standard RX security
establishment protocol, and responds to the client with a challenge
[RX]. rxgk challenges simply contain a random nonce selected by the
server.
Upon receiving this challenge, the client uses the transport key to
encrypt an authenticator, which contains the server's nonce, and some
other connection information. The client sends this authenticator,
together with start_time and the current user's rxgk token, back to
the server.
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The server decrypts the rxgk token to determine the master key in
use, uses this to derive the transport key, which it in turn uses to
decrypt the authenticator, and thus validate the connection.
8.2. Rekeying
As part of connection negotiation, the server and client agree upon
advisory lifetimes (both time, and data, based) for connection keys.
Each connection has a key number, which starts at 0. When a
connection exceeds one of its lifetimes, either side may elect to
increment the key number. When the other endpoint sees a key number
increment, it should reset all of its connection counters. Endpoints
should accept packets encrypted with either the current, previous, or
next key number, to allow for resends around the rekeying process.
The key version number is contained within the 16 bit spare field of
the RX header (used by previous security layers as a checksum field),
and expressed as an unsigned value in network byte order. If
rekeying would cause this value to wrap, then the key version number
MAY be stored locally as a 32-bit integer on both endpoints with only
the low 16 bits transmitted on the wire. If an endpoint cannot store
a per-connection 32-bit key version number when the 16-bit key
version number would wrap, that endpoint MUST terminate the
connection.
8.3. Key Derivation
In order to avoid the sharing of keys between multiple connections,
each connection has its own transport key, TK, which is derived from
the master key, K0. Derivation is performed using the PRF+ function
defined in [RFC4402], combined with the random-to-key function of
K0's encryption type, as defined in RFC3961. The PRF input data is
the concatenation of the rx epoch, connection ID, start_time and key
number, all in network byte order. This gives:
TK = random-to-key(PRF+(K0, L,
epoch || cid || start_time || key_number))
L is the key generation seed length as specified in the RFC3961
profile.
epoch, cid and key_number are passed as 32 bit quantities; start_time
is a 64 bit value.
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Note that start_time is selected by the client when it creates the
connection, and shared with the server as part of its response. Thus
both sides of the negotiation are guaranteed to use the same value
for start_time.
8.4. The Challenge
The rxgk challenge is an XDR encoded structure with the following
signature:
struct RXGK_Challenge {
opaque nonce[20];
};
nonce: 20 octets of random data.
8.5. The Response
The rxgk response is an XDR encoded structure, with the following
signature:
const RXGK_MAXAUTHENTICATOR = 1500 /* better fit in a packet! */
struct RXGK_Response {
rxgkTime start_time;
RXGK_Data token;
opaque authenticator<RXGK_MAXAUTHENTICATOR>
};
start_time: The time since the Unix epoch (1970-01-01 00:00:00Z),
expressed as an rxgkTime (see Section 2).
authenticator: The XDR encoded representation of an
RXGK_Authenticator, encrypted with the transport key, and key
usage RXGK_CLIENT_ENC_RESPONSE.
8.5.1. The Authenticator
struct RXGK_Authenticator {
opaque nonce[20];
opaque appdata<>
RXGK_Level level;
unsigned int epoch;
unsigned int cid;
unsigned int call_numbers<>;
};
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nonce: A copy of the nonce from the challenge.
appdata: An application specific opaque blob.
level: The desired security level for this particular connnection.
This MUST NOT be less secure than the security level negotiated
for the associated token.
epoch: The rx connection epoch.
cid: The rx connection ID.
call_numbers: The set of current rx call numbers for all available
channels; unused channels should report a call number of zero.
The length of this vector indicates the maximum number of calls
per connection supported by the client.
8.6. Checking the Response
To check the validity of an rxgk response, the authenticator should
be decrypted, the nonce from the decrypted authenticator compared
with the nonce sent in the RXGK_Challenge, and the connection ID and
epoch compared with that of the current connection. The call number
vector (call_numbers) should be supplied to the rx implementation.
The security level should be confirmed to be at least as secure as
the security level of the token. Failure of any of these steps MUST
result in the failure of the security context.
8.7. Packet Handling
The way in which the rxgk security class handles packets depends upon
the requested security level. As noted in Section 4, 3 levels are
currently defined -- authentication only, integrity protection and
encryption.
Connection parameters used when preparing a packet for transmission
MUST be verified when processing a received packet. Packet handling
when receiving packets is the inverse of the packet preparation
procedures, with explicit data length fields used to remove padding
added for encryption.
8.7.1. Authentication Only
When running at the clear security level, RXGK_LEVEL_CLEAR, no
manipulation of the payload is performed by the security class.
8.7.2. Integrity Protection
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Packet payloads transmitted at the auth security level,
RXGK_LEVEL_AUTH, consist of an opaque blob of MIC data followed by
the unencrypted original payload data.
The MIC data is generated by calling the RFC3961 get_mic operation
using a key and a data input. The RXGK_CLIENT_MIC_PACKET key usage
number MUST be used for packets transmitted from the client to the
server. The RXGK_SERVER_MIC_PACKET key usage number MUST be used for
packets transmitted from the server to the client. The following
data structure is the get_mic operation data input:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| epoch |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cid |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| call number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| security index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ original packet payload ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
All fields MUST be in network byte order. The data length field
specifies the length of the original packet payload in octets,
excluding padding required for encryption routines.
The packet is transmitted with the following payload:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ MIC ~
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+ |
| |
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~ original packet payload ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Note: The length of the MIC depends on which RFC3961 encryption type
is used. In particular, the original packet payload may not be word-
aligned.
Note: The data prepended to the original packet payload during the
MIC generation is not transmitted.
8.7.3. Encryption
Using the encryption security level, RXGK_LEVEL_CRYPT, provides both
integrity and confidentiality protection.
The existing payload is prefixed with a psuedo header, to produce the
following plaintext data for encryption before transmission. All
fields MUST be represented in network byte order for encryption.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| epoch |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cid |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| call number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| security index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ original packet payload ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The data length is the length of the following data in octets, and is
necessary so the receiving end can remove any padding added by the
encryption routines.
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This plaintext is encrypted using an RFC3961 style encrypt()
function, with the connection's transport key, using key usage
RXGK_CLIENT_ENC_PACKET for messages from client to server, and
RXGK_SERVER_ENC_PACKET for messages from server to client. The
encrypted block is transmitted to the peer as the payload of the
packet.
9. RXGK protocol error codes
This document specifies several error codes for use by RXGK
implementations (see Section 10 for the com_err table). In general,
when an endpoint receives any such error code, it should abort the
current operation. The various codes allow some information about
why the operation failed to be conveyed to the peer so that future
requests will be more likely to succeed. The circumstances in which
each error code should be used are as follows:
RXGK_INCONSISTENCY Used for errors internal to the security class,
such as when invariant assertions are violated. For example,
when an incoming packet to a server contains flags that do not
match the server's idea of the connection state, or attempting
to allocate a new connection where a connection already exists.
RXGK_PACKETSHORT The size of the packet is too small. Used when a
server is constructing a challenge packet but the required data
would be larger than the server's allowed packet size. Used
when a reply packet received by the server is smaller than the
expected size of a response packet. Also used for the
analogous situations on the other side of the challenge/
response exchange.
RXGK_BADCHALLENGE A challenge or response packet (of the expected
size) failed to decode properly or contained nonsense or
useless data.
RXGK_BADETYPE Used when the supplied encryption type(s) are invalid
or impermissible, such as for the GSSNegotiate and
CombineTokens RPCs or when the client-supplied enctype list
does not contain any entries that are acceptable to the server.
RXGK_BADLEVEL Used when the supplied security level(s) are invalid
or impermissible, such as for the GSSNegotiate and
CombineTokens RPCs or when the client-supplied list of security
levels does not contain any entries that are acceptable to the
server.
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RXGK_BADKEYNO The client or client's token indicates the use of a
key version number that is not present on the server. May also
be used when a key is presented that is not a valid key.
RXGK_EXPIRED The client presented an expired credential or token.
RXGK_NOTAUTH The caller is not authorized for the requested
operation or the presented credentials are invalid. In
particular, may also be used when credentials are presented
that have a start time in the future. Note that many
application error tables already include codes for "permission
denied", which take precedence over this general error code.
RXGK_BAD_TOKEN The client failed to present a token or the presented
token is invalid. For cases including but not limited to:
wrong size, fails to decode, zero or negative lifetime, starts
too far in the future, and too long a lifetime.
RXGK_SEALED_INCON Encrypted or checksummed data does not verify or
correctly decode. The checksum is invalid, the sealed copy of
the sequence and/or call number does not match the current
state, or similar situations.
RXGK_DATA_LEN The packet is too large, contains a zero-length iovec
entry, or otherwise presents an unacceptable or invalid data
length.
10. AFS-3 Registry Considerations
This document requests that the AFS-3 registrar include a com_err
error table for the RXGK module, as follows:
error_table RXGK
ec RXGK_INCONSISTENCY, "Security module structure inconsistent"
ec RXGK_PACKETSHORT, "Packet too short for security challenge"
ec RXGK_BADCHALLENGE, "Invalid security challenge"
ec RXGK_BADETYPE, "Invalid or impermissible encryption type"
ec RXGK_BADLEVEL, "Invalid or impermissible security level"
ec RXGK_BADKEYNO, "Key version number not found"
ec RXGK_EXPIRED, "Token has expired"
ec RXGK_NOTAUTH, "Caller not authorized"
ec RXGK_BAD_TOKEN, "Security object was passed a bad token"
ec RXGK_SEALED_INCON, "Sealed data inconsistent"
ec RXGK_DATA_LEN, "User data too long"
end
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The error table base should be 1233242880, with codes within the
table assigned relative numbers starting from 0 in the order
appearing above.
11. IANA Considerations
This memo includes no request to IANA.
12. Security Considerations
12.1. Abort Packets
RX Abort packets are not protected by the RX security layer.
Therefore, caution should be exercised when relying on their results.
In particular, clients MUST NOT use an error from GSSNegotiate or
CombineTokens to determine whether to downgrade to another security
class.
12.2. Token Expiry
This document permits tokens to be issued with expiration times after
the expiration time of the underlying GSSAPI credential, though
implementations SHOULD NOT do so. Allowing the expiration time of a
credential to be artificially increased can break the invariants
assumed by a security system, with potentially disastrous
consequences. For example, with the krb5 GSSAPI mechanism, access
revocation may be implemented by refusing to issue new tickets (or
renew existing tickets) for a principal; all access is assumed to be
revoked once the maximum ticket lifetime has passed. If an rxgk
token is created with a longer lifetime than the kerberos ticket,
this assumption is invalid, and the user whose access has supposedly
been revoked may gain access to sensitive materials. An application
should only allow token expiration times to be extended after a
security review of the assumptions made about credential expiration
for the GSSAPI mechanism(s) in use with that application. Such a
review is needed to confirm that allowing token expiration times to
be extended will not introduce vulnerabilities into the security
eocsystem in which the application operates.
12.3. Nonce Lengths
The key negotiation protocol includes both client-and server-
generated nonces as input. Both nonces are important, but serve
slightly different purposes. A random nonce is also used in the
challenge-response authentication protocol, which serves yet a
different purpose.
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The client_nonce is important for using the MIC of the StartParams
structure as a defense against man-in-the-middle attacks, and should
be long enough that the client will not generate collisions within
the lifetime of a given set of GSS credentials. The client_nonce
also contributes to the uniqueness of the generated key when GSS
initiator credentials are used to establish multiple GSS security
contexts.
The server_nonce serves primarily to add entropy to the generated
key. The maximum amount of entropy possible in the generated key is
the key generation seed length, so using a longer nonce gives no
benefit (unless the nonce is nonrandom).
The authentication nonce is used to prevent replays of the
authenticator. It is specified as a fixed length to allow the length
of the challenge packet to be used to indicate a new version of the
challenge/response protocol, but is chosen to be long enough that the
server will not accidentally reuse a nonce in a reasonable timeframe.
13. References
13.1. Informational References
[RX] Zeldovich, N., "RX protocol specification", October 2002.
[COMERR] Raeburn, K., "A Common Error Description Library for
UNIX", January 1989.
This paper is available as com_err.texinfo within
com_err.tar.Z.
13.2. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC2744] Wray, J., "Generic Security Service API Version 2 :
C-bindings", RFC 2744, January 2000.
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, February 2005.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
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[RFC4401] Williams, N., "A Pseudo-Random Function (PRF) API
Extension for the Generic Security Service Application
Program Interface (GSS-API)", RFC 4401, February 2006.
[RFC4402] Williams, N., "A Pseudo-Random Function (PRF) for the
Kerberos V Generic Security Service Application Program
Interface (GSS-API) Mechanism", RFC 4402, February 2006.
[RFC4506] Eisler, M., "XDR: External Data Representation Standard",
STD 67, RFC 4506, May 2006.
[RFC6113] Hartman, S. and L. Zhu, "A Generalized Framework for
Kerberos Pre-Authentication", RFC 6113, April 2011.
Appendix A. Acknowledgements
rxgk was originally developed over a number of AFS Hackathons. The
editor of this document has assembled the protocol description from a
number of notes taken at these meetings, and from a partial
implementation in the Arla AFS client.
Thanks to Derrick Brashear, Jeffrey Hutzelman, Love Hornquist Astrand
and Chaskiel Grundman for their original design work, and comments on
this document, and apologies for any omissions or misconceptions in
my archaeological work.
Marcus Watts and Jeffrey Altman provided invaluable feedback on an
earlier version of this document at the 2009 Edinburgh AFS Hackathon.
The text describing the rxgkTime type is based on language from
Andrew Deason.
Appendix B. Changes
B.1. Since 00
Add a reference to RFC4402, which describes the PRF+ algorithm we are
using.
Change references to RXGK_Token to RXGK_Data for clarity, and add a
definition of that type.
Rename the 'ticket' member of RXGK_ClientInfo to 'token', for
consistency, and make it a simple opaque.
Add a length field to the packet header, so that we can remove
padding.
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Remove versioning in the challenge and the response.
Clarify that both bytelife and lifetime are advisory.
Remove the RXGK_CLIENT_COMBINE_ORIG and RXGK_SERVER_COMBINE_NEW key
derivations, as these are no longer used.
Update the reference to draft-ietf-krb-wg-preauth-framework.
Require that CombineTokens be offered over an rxgk authenticated
connection.
Pull our time definition out into its own section and define a type
for it.
Define an enum for the security level, and use that throughout.
B.2. Since 01
Spell check.
Remove a couple of stray references to afs_ types.
Update start_time text to clarify that it uses rxgkTime.
Make expiration also be an rxgkTime.
Add a definition for RXGK_LEVEL_BIND.
Add reference to RX.
Add reference to XDR.
Rename the gss_status output parameter from the GSSNegotiate RPC to
gss_major_status, and update the supporting text.
Add a new gss_minor_status output paramter to the GSSNegotiate RPC,
but make clear that it is there for informational use only.
B.3. Since 02
Edit for grammar and punctuation.
Remove RXGK_LEVEL_BIND.
Make CombineTokens negotiate level and enctype.
Allow key version rollover at 16 bits when rekeying.
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Add Security Considerations for increasing token expiry.
Clarify behavior at RXGK_LEVEL_AUTH.
Add RXGK com_err table and descriptions.
Clean up call number vector and maxcalls support.
Improve the description of the GSS negotiation loop.
Give suggestions for acceptor principal names.
B.4. Since 03
Give guidance on the length of key negotiation nonces.
Supply bounds for (most) variable-length arrays.
Note that in-band errorcodes are for security sensitive errors.
Use abstract GSSAPI routine names, not the C binding names.
Discuss packet handling for received packets.
Authors' Addresses
Simon Wilkinson
Your File System Inc
Email: simon@sxw.org.uk
Benjamin Kaduk
MIT Kerberos Consortium
Email: kaduk@mit.edu
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