Integrating rxgk with AFS
draft-wilkinson-afs3-rxgk-afs-04
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| Authors | Simon Wilkinson , Benjamin Kaduk | ||
| Last updated | 2014-02-03 | ||
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draft-wilkinson-afs3-rxgk-afs-04
Network Working Group S. Wilkinson
Internet-Draft YFS
Intended status: Informational B. Kaduk
Expires: August 7, 2014 MIT
February 3, 2014
Integrating rxgk with AFS
draft-wilkinson-afs3-rxgk-afs-04
Abstract
This document describes how the new GSSAPI-based rxgk security class
for RX is integrated with the AFS application protocol. It describes
a number of extensions to the basic rxgk protocol, clarifies a number
of implementation issues, and provides values for the application-
specific elements of rxgk.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on August 7, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. The AFS-3 Distributed File System . . . . . . . . . . . . 3
1.2. rxgk and AFS . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Security Index . . . . . . . . . . . . . . . . . . . . . . . 4
3. Key Negotiation . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. The AFSCombineTokens Operation . . . . . . . . . . . . . 4
4. Tokens . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Container . . . . . . . . . . . . . . . . . . . . . . . . 6
4.2. Token Encryption . . . . . . . . . . . . . . . . . . . . 7
4.3. Token Contents . . . . . . . . . . . . . . . . . . . . . 7
5. Authenticator Data . . . . . . . . . . . . . . . . . . . . . 9
6. Client Tokens . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Keyed Clients . . . . . . . . . . . . . . . . . . . . . . 9
6.2. Unkeyed Clients . . . . . . . . . . . . . . . . . . . . . 10
7. Server to Server Communication . . . . . . . . . . . . . . . 10
7.1. Token Printing . . . . . . . . . . . . . . . . . . . . . 10
8. Declaring rxgk Support for a Fileserver . . . . . . . . . . . 10
9. Per Server Keys . . . . . . . . . . . . . . . . . . . . . . . 11
10. Securing the Callback Channel . . . . . . . . . . . . . . . . 13
10.1. The SetCallBackKey operation . . . . . . . . . . . . . . 14
10.2. Lifetime and scope of the callback channel . . . . . . . 15
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
12. AFS-3 Registry Considerations . . . . . . . . . . . . . . . . 16
13. Security Considerations . . . . . . . . . . . . . . . . . . . 16
13.1. Downgrade attacks . . . . . . . . . . . . . . . . . . . 16
13.2. Per Server Keys . . . . . . . . . . . . . . . . . . . . 16
13.3. Combined Key Materials . . . . . . . . . . . . . . . . . 16
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
14.1. Informational References . . . . . . . . . . . . . . . . 16
14.2. Normative References . . . . . . . . . . . . . . . . . . 17
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 17
Appendix B. Changes . . . . . . . . . . . . . . . . . . . . . . 17
B.1. Since 00 . . . . . . . . . . . . . . . . . . . . . . . . 17
B.2. Since 01 . . . . . . . . . . . . . . . . . . . . . . . . 18
B.3. Since 02 . . . . . . . . . . . . . . . . . . . . . . . . 18
B.4. Since 03 . . . . . . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
rxgk [I-D.wilkinson-afs3-rxgk] is a new GSSAPI-based [RFC2743]
security layer for the RX [RX] remote procedure call system. The
rxgk specification details how it may be used with a generic RX
application. This document provides additional detail specific to
integrating rxgk with the AFS-3 distributed file system.
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1.1. The AFS-3 Distributed File System
AFS-3 is a global distributed network file system. The system is
split into a number of cells, with a cell being the administrative
boundary. Typically an organisation will have one (or more) cells,
but a cell will not span organisations. Each cell contains a number
of fileservers which contain collections of files ("volumes") which
they make available to clients using the AFS-3 protocol. Clients
access these files using a service known as the cache manager.
In order to determine which server a particular file is located upon,
the cache manager looks up the location in the volume location
database (vldb) by contacting the vlserver. Each cell has one or
more vlservers, which are synchronised using an out-of-band
mechanism.
1.2. rxgk and AFS
This document describes the special integration steps needed to use
rxgk with AFS-3 database servers (the PR and VL rx services) and file
servers (the RXAFS, RXAFSCB, and AFSVol rx services), as well as
specifying application-specific portions of the rxgk specification
for use by these services. Other AFS-3 services are not covered by
this document; the generic rxgk document applies to them.
AFS-3 differs from a standard rxgk deployment in that it does not
require GSSAPI negotiation with each server. Instead, a client
performs GSSAPI negotiation just once, with the vlserver, receiving a
token usable with any database server in the cell. Traditional AFS
rxkad authentication required that the cell-wide key be distributed
to all servers in the cell, both database servers and file servers,
making no distinction between tokens used for database servers and
file servers. rxgk can operate in such a fashion, with a cell-wide
key shared amongst all servers, but is not limited to doing so.
For more complex cell topologies, rxgk also supports configurations
where (some) file servers do not have the cell-wide key. Tokens for
these file servers are obtained by means of an extended version of
the CombineTokens RPC, AFSCombineTokens, which takes a server
identifier and will return a token encrypted in the key for a
specific file server. AFSCombineTokens also provides a mechanism for
indicating whether a specific server is rxgk capable, allowing cells
to securely migrate to rxgk from other security mechanisms.
We also define mechanisms for securing the callback channel that is
created between fileserver and client.
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1.3. 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. Security Index
When used within the AFS protocol, rxgk has an RX securityIndex value
of 4.
3. Key Negotiation
An AFS cell wishing to support rxgk MUST run an rxgk key negotiation
service, as specified in [I-D.wilkinson-afs3-rxgk], on each of its
vlservers. The service MUST listen on the same port as the vlserver.
The GSS identity afs-rxgk@_afs.<cellname> of nametype
GSS_C_NT_HOSTBASED_SERVICE is the acceptor identity for this service.
Where multiple vlservers exist for a single cell, all of these
servers must have access to the key material for this identity, which
MUST be identical across the cell. Clients MAY use the presence of
this identity as an indicator of rxgk support for a particular cell.
Clients that wish to support cells using other rx security objects
MAY downgrade if this identity is not available.
Tokens returned from the GSSNegotiate and CombineTokens calls MUST
only be used with database servers. Tokens for fileservers MUST be
obtained by calling AFSCombineTokens before each server is contacted.
rxgk tokens are in general only usable with the particular rx service
that produced them. For the AFS-3 protocol, the database server
services are grouped together to accept a common type of token, and
the file server services are grouped together to accept a different
common type of token, but it is important to emphasize that a token
for a database server will not in general be useful against a file
server, and vice versa. Tokens for database servers are obtained
from the standard rxgk negotiation services, but tokens for file
servers are obtained through a new procedure, the AFSCombineTokens
RPC described in Section 3.1.
3.1. The AFSCombineTokens Operation
AFS extends the existing CombineTokens operation to provide a more
featured token manipulation and conversion service. This operation
takes a user token, an optional cache manager token, options for
enctype and security level negotiation with the server, and a
destination file server identifier. It returns a token specific to
the specified destination, and a structure containing some
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information describing the returned token. AFSCombineTokens is the
only way to obtain a valid file server token (other than printing a
token, see Section 7.1).
AFSCombineTokens(IN RXGK_Data *token0,
IN RXGK_Data *token1,
IN RXGK_CombineOptions *options,
IN afsUUID destination,
OUT RXGK_Data *new_token,
OUT RXGK_TokenInfo *token_info) = 3;
token0: An rxgk token for the vlserver.
token1: Either an rxgk token for the vlserver, or empty (zero-
length).
options: An RXGK_CombineOptions structure containing a list of
enctypes acceptable to the client and a list of security levels
acceptable to the client.
destination: The UUID of the server new_token is intended for.
Fileserver UUIDs may be obtained from the VLDB in the same call
that returns their addresses.
new_token: The output rxgk token, or empty (zero-length).
token_info: Information describing the returned token.
The AFSCombineTokens call MUST only be performed over a secured rxgk
connection. AFSCombineTokens MUST NOT be offered over an
RXGK_LEVEL_CLEAR connection. Servers MUST reject all attempts to
perform this operation over channels that do not offer integrity
protection. This integrity guarantee protects the returned token
information (token_info) as well as the options and destination
arguments submitted to the server.
Clients which are caching the results of RPCs on behalf of multiple
users (such as a traditional AFS Cache Manager), SHOULD provide both
the user's token (as token0) and a token generated from an identity
that is private to the cache manager (as token1). This prevents a
user from poisoning the cache for other users. Recommendations on
keying cache managers are contained in Section 6.1.
Clients which are working on behalf of a single user can provide an
empty token1, but MUST use AFSCombineTokens to obtain a destination-
specific token for each fileserver they contact.
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The output token from AFSCombineTokens is a token specific to the
fileserver indicated by the destination argument. As such, it is not
a valid input token for a successor AFSCombineTokens operation, as
the input tokens for AFSCombineTokens must be tokens for the
vlserver.
Clients using a printed token (see Section 7.1) MUST provide that
token as token0. token1 MUST be empty. Printed tokens cannot be
combined with any other token, and servers MUST reject attempts to do
so, whether via the CombineTokens RPC, the AFSCombineTokens RPC, or
any other token-combining procedure.
If the server is unable to perform the AFSCombineTokens operation
with the given arguments, a nonzero value is returned. Clients MUST
NOT use such an error as an indication to fall back to to a different
security class.
If the returned token is zero-length, then the destination does not
support rxgk, and the client MAY fall back to using a different
authentication mechanism for that server. An rxgk capable client
operating within an rxgk enabled cell MUST NOT downgrade its choice
of security layer in any other situation.
Other aspects of the operation of AFSCombineTokens, including the
combination of keys and tokens, are largely the same as the
CombineTokens RPC, documented in CombineTokens call, documented in
[I-D.wilkinson-afs3-rxgk]. The only differences pertain to the case
where the supplied token1 is empty. In this case, there is only one
input token master key, so the KRB-FX-CF2() algorithm is not
applicable; in these cases the master key K0 from token0 is reused
unchanged for the output token. The case where token1 is empty also
requires special handling for the identities list in new_token, which
is discussed in Section 4.3.
The combination of identity information into the new_token is not
specified in [I-D.wilkinson-afs3-rxgk], being left as application-
specific function. For the AFS-3 protocols, identity information in
tokens is stored as a list of PrAuthNames, and the identity
combination algorithm is order-preserving concatenation.
4. Tokens
4.1. Container
rxgk tokens for AFS take the form of some key management data,
followed by an encrypted data blob. The key management data (a
version number, followed by an [RFC3961] encryption type) allows the
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server receiving a token to identify which key has been used to
encrypt the core token data.
struct RXGK_TokenContainer {
afs_int32 kvno;
afs_int32 enctype;
opaque encrypted_token<>;
};
The RXGK_TokenContainer structure is XDR encoded and transported
within the 'token' field of the RXGK_ClientInfo structure specified
in [I-D.wilkinson-afs3-rxgk].
4.2. Token Encryption
rxgk supports encrypting tokens both with a single cell-wide keys and
with per-file-server keys. The cell-wide key must be installed on
all database servers in the cell, and may additionally be installed
on non-database file servers if the use of per-file-server keys is
not desired. Cell-wide keys should be for a selected RFC3961
encryption mechanism that is supported by all servers within the cell
that will use that key. Per-server keys should be for an encryption
mechanism that is supported by both the destination server and the
negotiation service. The management of per-server keys is discussed
in more detail in Section 13.2.
Key rollover is permitted by means of a key version number. When a
key is changed, whether cell-wide or per-server, a different (larger)
key version number MUST be selected. Servers SHOULD accept tokens
using old keys until the lifetime of all non-printed (see Section 7.1
existing tokens has elapsed. Services using printed tokens should be
prepared to regenerate those tokens in the case of key rollover.
Encryption is performed over the XDR encoded RXGK_Token structure,
using the RFC3961 encrypt operation, with a key usage value of
RXGK_SERVER_ENC_TOKEN (defined in [I-D.wilkinson-afs3-rxgk]). The
enrypted data is stored in the encryted_token field of the
RXGK_TokenContainer structure described in Section 4.1.
4.3. Token Contents
The token itself contains the information expressed by the following
RPC-L:
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struct RXGK_Token {
afs_int32 enctype;
opaque K0<>;
RXGK_Level level;
rxgkTime start_time;
afs_int32 lifetime;
afs_int32 bytelife;
rxgkTime expirationtime;
struct PrAuthName identities<>;
};
enctype: The RFC3961 encryption type of the session key contained
within this ticket.
K0: The session key (see [I-D.wilkinson-afs3-rxgk] for details of
how this key is negotiated between client and negotiation
service).
level: The security level, as defined in [I-D.wilkinson-afs3-rxgk],
that MUST be used for this connection.
start_time: The time at which the token's validity begins. Servers
MUST reject attempts to use tokens with a start_time value
later than the current time.
lifetime: The maximum number of seconds that a key derived from K0
may be used for, before the connection is rekeyed. If 0, keys
have no time-based limit.
bytelife: The maximum amount of data (expressed as the log base 2 of
the number of bytes) that may be transferred using a key
derived from K0 before the connection is rekeyed. If 0, there
is no data-based limit on key usage.
expirationtime: The time (expressed as an rxgkTime) beyond which
this token may no longer be used. Servers MUST reject attempts
to use connections secured with this token after this time. A
value of 0 indicates that this token never expires.
identities: A list of identities represented by this token. struct
PrAuthName is the identity structure defined in
[I-D.brashear-afs3-pts-extended-names].
The above token structure is used for both database server tokens and
file server tokens, however, there is a key distinction between the
two: in file server tokens, the last identity in the list of
identities is special, and corresponds to the "cache manager"
identity that was presented to AFSCombineTokens in token1. File
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server tokens must always have a cache manager identity present; if
the token1 presented to AFSCombineTokens is empty, a placeholder
identity is introduced into the identity list of the output token
from AFSCombineTokens to indicate that fact. This placeholder
identity is of a new kind, PRAUTHTYPE_EMPTY:
#define PRAUTHTYPE_EMPTY XXX
PrAuthNames of kind PRAUTHTYPE_EMPTY MUST have empty (zero-length)
data and display components.
5. Authenticator Data
The appdata opaque within the RXGK_Authenticator structure contains
the results of XDR encoding the RXGK_Authenticator_AFSAppData
structure. The uuid field contains the UUID of the client.
struct RXGK_Authenticator_AFSAppData {
afsUUID uuid;
};
6. Client Tokens
In order to protect users of a multi-user cache manager from each
other, it must be impossible for an individual user to determine the
key used to protect operations which affect the cache. This requires
that the cache manager have key material of its own which can be
combined with that of the user. This functionality is provided by
the AFSCombineTokens call specified earlier in this document.
However, this call requires that a cache manager have access to a
token for this purpose.
6.1. Keyed Clients
When a host already has key material for a GSSAPI mechanism supported
by rxgk, that material MAY be used to key the client. The client
simply calls the rxgk negotiation service using the relevant
material, and obtains a token. Combined tokens may not be used as
cache manager tokens. The client SHOULD frequently regenerate this
token, to avoid combined tokens having unnecessarily close expiration
times. The client SHOULD NOT regenerate this token so often so as to
place excessive load on the vlservers.
It is recommended that identities created specifically for use by a
cache manager have the name afs3-callback@<hostname> where <hostname>
is the fully qualified domain name of the cache manager.
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6.2. Unkeyed Clients
When a client has no key material, it is possible that an anonymous
GSSAPI connection may succeed. Clients MAY attempt to negotiate such
a connection by calling GSS_Init_sec_context() with the anon_req_flag
[RFC2743] and the default credentials set.
7. Server to Server Communication
A number of portions of the AFS protocol require that servers
communicate amongst themselves. To secure this with rxgk we require
both a mechanism of generating tokens for these servers to use, and a
definition of which identities are permitted for authorisation
purposes. We refer to the process of forging tokens for local use,
given access to the relevant key, as "token printing".
7.1. Token Printing
A server with access to the cell-wide pre-shared key may print its
own tokens for server-to-server access. To do so, it should
construct a database server token with suitable values. The list of
identities in such a token MUST be empty. It can then encrypt this
token using the pre-shared key, and use it in the same way as a
normal rxgk token. The receiving server can identify it is a printed
token by the empty identity list.
The session key within a printed database server token MUST use the
same encryption type as the pre-shared key. When connecting to a
fileserver, a client SHOULD use the AFSCombineTokens service as
discussed above to ensure that they are using the correct key for the
fileserver.
File servers with per-server keys may also print tokens, though these
tokens are in general of limited utility. (Being file server tokens,
they are not valid inputs to AFSCombineTokens, etc..) In this case,
the printed token should have a single identity in the list, of kind
PRAUTHTYPE_EMPTY.
8. Declaring rxgk Support for a Fileserver
The AFSCombineTokens call has specific behaviour when a destination
endpoint does not support rxgk. Implementing this behaviour requires
that the vlserver be aware of whether a fileserver supports rxgk.
Fileservers currently register with the vlserver using the
VL_RegisterAddrs RPC. This document introduces an extended version,
VL_RegisterAddrsAndKey (Section 9), and either one may be used to
indicate that a fileserver supports rxgk. Fileservers which support
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rxgk MUST call these RPCs over an rxgk protected connection. The
vlserver then infers rxgk support from the rx security layer used in
registration. To prevent downgrade attacks, once a fileserver has
registered as being rxgk capable, the vlserver MUST NOT remove that
registration without administrator intervention. When the first
rxgk-enabling call from a fileserver is VL_RegisterAddrs, that call
MUST be made using a printed token, and the implication is that the
token-encrypting key for that file server is the cell-wide shared
key.
Once a fileserver has been marked as supporting rxgk,
VL_RegisterAddrs calls for that fileserver MUST only be accepted over
an rxgk protected connection. vlservers MUST only accept calls to
VL_RegisterAddrs from a printed token, an administrator, or the
identity registered for the fileserver using a prior call to
VL_RegisterAddrsandKey.
9. Per Server Keys
The provisioning of file servers with their own keys, rather than the
cell-wide master key, requires the ability to maintain a directory of
these keys on the vlserver, so that the AFSCombineTokens RPC can
encrypt the outgoing token with the correct key. The manner in which
this directory is maintained is left to the implementor, who MAY
decide to use a manual, or out of band, key management system.
Otherwise, the automated keying mechanism described as follows will
be used.
Implementations supporting automatic key management through the AFS-3
protocol MUST provide the VL_RegisterAddrsAndKey RPC (similar to the
VL_RegisterAddrs RPC). This RPC is called by a fileserver to
register itself with the VLDB; it MUST be called over a secure
connection. In particular, it MUST NOT be called over an rxkad
connection.
For the purpose of this RPC, the fileserver acts as the client and
the vlserver as the server. Once the RPC completes, both peers of
the RPC call can generate a key to be used as the fileserver's long-
term server key.
vlservers MUST NOT permit calls to VL_RegisterAddrsAndKey for UUIDs
which already exist within the vldb, unless that UUID already has a
server-specific key registered. The identity information stored in
the token used to authenticate and secure the VL_RegisterAddrsAndKey
connection is used to provide continuity between multiple calls to
VL_RegisterAddrsAndKey, so a printed token is not appropriate here,
as it would only be useful for the first call and there would be no
identity available for cross-checking that a second call was from the
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same source. To support this cross-check, when a new fileserver
first registers with the vldb using VL_RegisterAddrsAndKey, the
vlserver MUST store the identity list from the token used to make
this connection. The vlserver MUST only permit subsequent calls to
VL_RegisterAddrsAndKey for this UUID when they come over a connection
authenticated with that same identity list, an administrator's token,
or a printed token. New fileserver UUIDs register themselves with
the vldb in a "leap of faith", binding a GSSAPI identity to the
fileserver UUID for future authenticated operations. Fileservers
SHOULD use VL_RegisterAddrsAndKey to rekey themselves periodically,
in accordance with key lifetime best practices.
The VL_RegisterAddrsAndKey RPC is described by the following RPC-L:
struct RXGK_ServerKeyDataRequest {
afs_int32 enctypes<>;
opaque nonce1[20];
};
struct RXGK_ServerKeyDataResponse {
afs_int32 enctype;
afs_int32 kvno;
opaque nonce2[20];
};
const RXGK_MAXKEYDATAREQUEST = 16384;
const RXGK_MAXKEYDATARESPONSE = 16384;
typedef opaque keyDataRequest<RXGK_MAXKEYDATAREQUEST>;
typedef opaque keyDataResponse<RXGK_MAXKEYDATARESPONSE>;
VL_RegisterAddrsAndKey(
IN afsUUID *uuidp,
IN afs_int32 spare1,
IN bulkaddrs *ipaddr,
IN afs_int32 secIndex,
IN keyDataRequest *request,
OUT keyDataResponse *response) = XXX;
uuidp: The fileserver's UUID.
spare1: Unused. (Clients SHOULD pass zero.)
ipaddr: The list of addresses to register as belonging to this
fileserver.
secIndex: The index of the security mechanism for which a key is
being set. For rxgk, this value MUST be 4.
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keyDataRequest: An opaque blob of data, specific to the security
mechanism defined by secIndex. For rxgk, it is the XDR-encoded
representation of an RXGK_ServerKeyDataRequest structure.
keyDataResponse: An opaque blob of data, specific to the security
mechanism defined by secIndex. For rxgk, it is the XDR-encoded
representation of an RXGK_ServerDataResponse structure.
The client provides, in the RXGK_ServerKeyDataRequest structure, a
list of the RFC3961 encryption types that it will accept as a server
key. It also provides a nonce containing 20 random data bytes.
The server selects an encryption type shared by it and the client,
and returns that, along with 20 bytes of random data that it has
generated, in RXGK_ServerKeyDataResponse. If there is no common
encryption type, then the server MUST fail the request.
The file server key can then be derived by both client and server
using
random-to-key(PRF+(K0, K, nonce1 || nonce2));
random-to-key is the function specified by the RFC3961 profile of the
encryption type chosen by the server and returned in enctype.
PRF+ is the function of that name specified by [RFC4402].
K0 is the master key of the current rxgk session, e.g., as originally
determined by the GSSNegotiate call.
K is the key generation seed length as specified in enctype's RFC3961
profile.
|| is the concatenation operation.
10. Securing the Callback Channel
AFS has traditionally had an unprotected callback channel. However,
extended callbacks [I-D.benjamin-extendedcallbackinfo] require a
mechanism for ensuring that callback breaks and, critically, data
updates, are protected. This requires that there is a strong
connection between the key material used initially to perform the
RPC, and that which is used to protect any resulting callback. We
achieve this using the cache manager token discussed in Section 6.1,
which is required in order for a client to accept secure callbacks.
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10.1. The SetCallBackKey operation
A cache manager may set a key for secure callbacks by issuing the
following RPC (in the RXAFS service):
RXAFS_SetCallBackKey(afs_int32 securityIndex,
opaque mech_data<>) = XXX;
securityIndex: The security index of the mechanism for which this
key is being set. The security index for rxgk is 4.
mech_data: This contains the security object specific data. In
rxgk's case this is an XDR encoded RXGK_CallBackKeyData
structure.
struct RXGK_CallBackKeyData {
afs_int32 enctype;
opaque K0<16384>;
RXGK_Level level;
/* no rxgkTime start_time */
afs_int32 lifetime;
afs_int32 bytelife;
/* no rxgkTime expirationtime */
RXGK_Data token;
/* no identities needed */
}
enctype The encryption type of K0.
K0 The raw key data for the callback connection's connection
master key.
level The security level to be used for the callback connection.
lifetime The maximum number of seconds that a key derived from K0
may be used for, before the connection is rekeyed. If 0, keys
have no time-based limit.
bytelife: The maximum amount of data (expressed as the log base 2 of
the number of bytes) that may be transferred using a key
derived from K0 before the connection is rekeyed. If 0, there
is no data-based limit on key usage.
token An opaque token that permits the client to identify the key and
connection parameters used for the callback connection. This
token behaves as a generic rxgk token, as described in section
5 of [I-D.wilkinson-afs3-rxgk], and its contents and encoding
are implementation-defined. In particular, a client
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implementation might be able to leave this token empty and
store the key and connection parameters in an internal per-
fileserver storage location. A client implementation might
also reuse wholesale the token format defined in this document,
filling the token field with an XDR-encoded RXGK_TokenContainer
containing an encrypted encoded RXGK_Token, with a per-client
secret key.
No expiration or start time need be transferred in this RPC (as are
included in an RXGK_Token and RXGK_ClientInfo), because the callback
connection is implicitly authorized to continue for as long as the
client is interested in data from the fileserver.
10.2. Lifetime and scope of the callback channel
The RXAFS_SetCallBackKey RPC must be performed over a secure channel.
When used to set callback keys for rxgk, this means that the RPC MUST
be performed over an rxgk protected connection of security level
RXGK_LEVEL_CRYPT. Additionally, the connection MUST have been
established using solely the cache manager's token.
The callback channel key is inherently tied to the identity of the
cache manager token used to establish it. A fileserver which is
providing secure callbacks MUST store the cache manager identity used
to establish each callback connection key and associate that identity
with the cache manager UUID. In the abstract, the fileserver is
storing triples of (UUID, identity, key). A cache manager may make
multiple calls to RXAFS_SetCallBackKey, and the fileserver MAY store
multiple cache manager identity/callback connection key pairs for a
given cache manager UUID. If a fileserver receives an
RXAFS_SetCallBackKey call which will cause it to stop storing an
identity/key pair (whether because the fileserver only stores one
such pair for a given cache manager, or some larger fixed limit is
reached), it MUST break all secure callbacks held by that client that
are using the old key before the RPC terminates.
Only RPCs issued over an rxgk protected connection should receive
rxgk protected callbacks.
Since the callback connection key is tied to the cache manager
identity, it should only be used to protect callbacks relating to
data accessed using that identity. Since extended callbacks are only
available when a cache manager token is in use, and the cache manager
token's identity is always the last identity in the list, this means
that the fileserver MUST only send rxgk protected extended callbacks
when the cache manager identity in the token authenticating the RPC
establishing the callback matches an identity associated with that
cache manager's UUID. Additionally, the fileserver MUST use the
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callback key (and token) registered to that identity for the extended
callbacks established as a result of the RPC.
11. IANA Considerations
This memo includes no request to IANA.
12. AFS-3 Registry Considerations
This document requrests that the AFS-3 registry allocate code points
for the new RPCs AFSCombineTokens (for the RXGK service),
RegisterAddrsAndKey (for the VL service), and SetCallBackKey (for the
RXAFS service). It also requests the allocation of a value for the
PRAUTHTYPE_EMPTY kind of PrAuthName.
13. Security Considerations
13.1. Downgrade attacks
Using the presence of a GSSAPI key to determine a cell's ability to
perform rxgk is vulnerable to a downgrade attack, as an attacker may
forge error responses. Cells which no longer support rxkad should
remove their afs@REALM and afs/cell@REALM Kerberos keys.
13.2. Per Server Keys
The mechanism for automatically registering per-server keys is
potentially vulnerable, as it trades a short-lived key (the rxgk
session key, which protects the key exchange) for a long-lived one
(the server key). There is precedent for this sort of key exchange,
such as when using kadmin to extract a new kerberos keytab.
13.3. Combined Key Materials
As described in Section 6, combined tokens are used to prevent cache
poisoning attacks on multi-user systems. In order for this
protection to be effective, cache managers MUST NOT provide user
access to keys produced through the combine tokens operation, unless
those keys will not be used by the cache manger itself.
14. References
14.1. Informational References
[RX] Zeldovich, N., "RX protocol specification", October 2002.
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[I-D.benjamin-extendedcallbackinfo]
Benjamin, M., "AFS Callback Extensions (Draft 14)", draft-
benjamin-extendedcallbackinfo-02 (work in progress),
December 2011.
14.2. Normative References
[I-D.brashear-afs3-pts-extended-names]
Brashear, D., "Authentication Name Mapping extension for
AFS-3 Protection Service", draft-brashear-afs3-pts-
extended-names-09 (work in progress), March 2011.
[I-D.wilkinson-afs3-rxgk]
Wilkinson, S., "rxgk: GSSAPI based security class for RX",
draft-wilkinson-afs3-rxgk-00 (work in progress), January
2010.
[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.
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, February 2005.
[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.
Appendix A. Acknowledgements
rxgk has been the work of many contributors over the years. A
partial list is contained in the [I-D.wilkinson-afs3-rxgk]. All
errors and omissions are, however, mine.
Appendix B. Changes
B.1. Since 00
Add references to RX and XDR specifications.
Add introductory material on AFS.
Change expirationTime to be expressed using the rxgkTime type.
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Document how encryption types are chosen for printed tokens, and how
they are used against fileservers.
Expand security considerations section to cover combined tokens.
Rename AFS_SetCallbackKey as RXAFS_SetCallbackKey.
B.2. Since 01
Rename RXAFS_SetCallbackKey to RXAFS_SetCallBackKey.
Add an AFS-3 Registry Considerations section.
Clarify the vlserver/dbserver/fileserver relationship.
AFSCombineTokens prototype changes.
Clarify the scope of the document.
Use a leap of faith for RegisterAddrsAndKey.
Specify the nametype of the acceptor identity.
B.3. Since 02
Deal with fallout of errorcode's removal from RXGK_TokenInfo.
Rework "securing the callback channel".
B.4. Since 03
Clarify the distinction between dbserver and fileserver tokens.
AFSCombineTokens is the only way to get file server tokens.
Add new kind of PrAuthName, PRAUTHTYPE_EMPTY.
Specify how cache manager token identities are stored in file server
tokens.
Place bounds on some XDR opaque arrays.
Expound more about printed tokens, for dbservers and fileservers.
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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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