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Network Working Group                                       S. Wilkinson
Internet-Draft                                                       YFS
Intended status: Informational                                  B. Kaduk
Expires: November 22, 2015                                           MIT
                                                            May 21, 2015

                       Integrating rxgk with AFS


   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
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 22, 2015.

Copyright Notice

   Copyright (c) 2015 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  The AFS-3 Distributed File System . . . . . . . . . . . .   3
     1.2.  rxgk and AFS  . . . . . . . . . . . . . . . . . . . . . .   3
     1.3.  Providing Keys for the Callback Channel . . . . . . . . .   4
     1.4.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Security Index  . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Authenticator Data  . . . . . . . . . . . . . . . . . . . . .   5
   4.  Application-Specific Constant . . . . . . . . . . . . . . . .   5
   5.  Key Negotiation . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Application-Specific GSSNegotiate Behavior for AFS-3  . .   6
     5.2.  Token Applicability . . . . . . . . . . . . . . . . . . .   6
   6.  Token Format  . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Container . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.2.  Token Encryption  . . . . . . . . . . . . . . . . . . . .   7
     6.3.  Token Contents  . . . . . . . . . . . . . . . . . . . . .   7
   7.  Cache Manager Tokens  . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Keyed Clients . . . . . . . . . . . . . . . . . . . . . .   9
     7.2.  Unkeyed Clients . . . . . . . . . . . . . . . . . . . . .   9
   8.  Combining Tokens  . . . . . . . . . . . . . . . . . . . . . .  10
   9.  The AFSCombineTokens Operation  . . . . . . . . . . . . . . .  10
   10. Server to Server Communication  . . . . . . . . . . . . . . .  12
     10.1.  Token Printing . . . . . . . . . . . . . . . . . . . . .  13
     10.2.  Declaring rxgk Support for a Fileserver  . . . . . . . .  13
       10.2.1.  File Servers With the Cell-Wide Key  . . . . . . . .  14
       10.2.2.  File Servers With Per-Server Keys  . . . . . . . . .  14
     10.3.  Registering Per Server Keys  . . . . . . . . . . . . . .  15
   11. Securing the Callback Channel . . . . . . . . . . . . . . . .  18
     11.1.  Lifetime and scope of the callback channel . . . . . . .  18
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   13. AFS-3 Registry Considerations . . . . . . . . . . . . . . . .  19
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  19
     14.1.  Downgrade attacks  . . . . . . . . . . . . . . . . . . .  19
     14.2.  Per Server Keys  . . . . . . . . . . . . . . . . . . . .  19
     14.3.  Combined Key Materials . . . . . . . . . . . . . . . . .  19
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     15.1.  Informational References . . . . . . . . . . . . . . . .  19
     15.2.  Normative References . . . . . . . . . . . . . . . . . .  20
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  20
   Appendix B.  Changes  . . . . . . . . . . . . . . . . . . . . . .  20
     B.1.  Since 00  . . . . . . . . . . . . . . . . . . . . . . . .  21
     B.2.  Since 01  . . . . . . . . . . . . . . . . . . . . . . . .  21
     B.3.  Since 02  . . . . . . . . . . . . . . . . . . . . . . . .  21
     B.4.  Since 03  . . . . . . . . . . . . . . . . . . . . . . . .  21
     B.5.  Since 04  . . . . . . . . . . . . . . . . . . . . . . . .  22
     B.6.  Since 05  . . . . . . . . . . . . . . . . . . . . . . . .  22
     B.7.  Since 06  . . . . . . . . . . . . . . . . . . . . . . . .  22

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     B.8.  Since 07  . . . . . . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

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, but leaves some aspects of the protocol as application-
   specific.  This document resolves the application-specific portions
   of rxgk for use with the AFS-3 distributed file system, and provides
   additional detail specific to integrating rxgk with AFS-3.

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

   User and group information is stored in the protection database
   (prdb), which is made available by the ptserver(s), colocated with
   the vlservers.  Fileservers check with the prdb before granting
   access to files which are subject to access control.

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, as described in
   Section 5.  Traditional AFS rxkad authentication required that the

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   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 a similar 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
   encrypted in these server-specific keys are returned by an extended
   version of the CombineTokens RPC, AFSCombineTokens.  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.

1.3.  Providing Keys for the Callback Channel

   The AFS-3 protocol provides a mechanism by which a client can obtain
   a promise from a fileserver to "call back" when a particular piece of
   data is changed, so that the client does not need to check with the
   fileserver for the current-ness of the data every time it is used.
   At present, this takes the form of a single bit of information about
   whether the callback is still valid, with no authentication of the
   callback break.  It is desired that future work expand the callback
   channel to convey more than a single bit of information, and provide
   an authenticated (and potentially confidential) channel for updating
   callback promises.

   This document provides a mechanism to establish a key and token that
   can be used to provide a secure callback channel.  Though the format
   of that token is flexible and not specified in this document, this
   document does need to specify a mechanism by which a callback key can
   be established between the two parties.  This is done by means of the
   authenticator's appdata field, binding a callback key to an rx
   connection, so that all callbacks generated by that connection will
   use the indicated callback key.

1.4.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Security Index

   When used within the AFS-3 protocol, rxgk has an RX securityIndex
   value of 4.

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3.  Authenticator Data

   The appdata opaque within the RXGK_Authenticator structure used in
   the rx challenge/response authentication exchange contains the
   results of XDR [RFC4506] encoding the RXGK_Authenticator_AFSAppData

       struct RXGK_Authenticator_AFSAppData {
           afsUUID client_uuid;
           RXGK_Data cb_tok;
           RXGK_Data cb_key;
           afs_int32 enctype;
           afsUUID target_uuid;

   client_uuid  the UUID of the client.

   cb_tok  the rxgk token to be used for secure callbacks created by
         RPCs over this connection.  In some implementations this token
         may be empty (zero-length).

   cb_key  the raw key material (k0) to which cb_tok corresponds, to be
         used as the master key for the secure callback connections
         created by RPCs over this connection.

   enctype  the [RFC3961] enctype of the cb_key key material.

   target_uuid  the UUID of the server being authenticated to (if
         applicable).  Database servers do not have UUIDs; when
         authenticating to database servers, this field should be set to
         all zero bits.  File server UUIDs may be obtained from the VLDB
         in the same call that returns their addresses.

4.  Application-Specific Constant

   The constant RXGK_MAXDATA takes the value 1048576 for use with AFS-3.

5.  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

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   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.  Note that not all
   GSS mechanisms can expose to the initiator whether or not a given
   acceptor identity exists.

5.1.  Application-Specific GSSNegotiate Behavior for AFS-3

   The input and output opaques of the GSSNegotiate RPC are left as
   implementation-defined, as needed by the implementation to retain
   information across subsequent calls during a single GSS negotiation

5.2.  Token Applicability

   Tokens returned from the GSSNegotiate and CombineTokens calls MUST
   only be used with database servers.  Tokens for fileservers MUST be
   obtained by calling AFSCombineTokens (Section 9) 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

6.  Token Format

   This section defines the format of rxgk tokens for use with the AFS-3
   protocol.  The same layout is used for database server tokens and
   file server tokens, but file server tokens may be encrypted in a
   different key than database server tokens.

6.1.  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 RFC 3961 encryption type) allows the
   server receiving a token to identify which key has been used to
   encrypt the core token data.

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       struct RXGK_TokenContainer {
           afs_uint32 kvno;
           afs_int32 enctype;
           opaque    encrypted_token<>;

   The RXGK_TokenContainer structure is XDR encoded and transported
   wherever a token is used, such as in the 'token' field of the
   RXGK_ClientInfo structure specified in [I-D.wilkinson-afs3-rxgk].

6.2.  Token Encryption

   rxgk supports encrypting tokens with either a single cell-wide key or
   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 when per-file-server keys are not in
   use.  Cell-wide keys should be for a selected RFC 3961 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 on the vlserver.  The management of per-server keys is
   discussed in more detail in Section 14.2.

   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 existing non-printed (see
   Section 10.1) tokens has elapsed.  Services using printed tokens
   should be prepared to regenerate those tokens in the case of key

   Encryption is performed over the XDR encoded RXGK_Token structure,
   using the RFC 3961 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 encrypted_token field of the
   RXGK_TokenContainer structure described in Section 6.1.

6.3.  Token Contents

   The token itself contains the information expressed by the following

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       struct RXGK_Token {
           afs_int32 enctype;
           opaque K0<>;
           RXGK_Level level;
           afs_uint32 lifetime;
           afs_uint32 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

   level:  The security level, as defined in [I-D.wilkinson-afs3-rxgk],
         that MUST be used for this 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.

   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.  It is
         RECOMMENDED that an expirationtime of 0 is only used for
         printed tokens.

   identities:  A list of identities represented by this token. struct
         PrAuthName is the identity structure defined in

7.  Cache Manager Tokens

   Some deployment scenarios for AFS-3 involve multi-user machines with
   a single Cache Manager that fetches data on the users' behalf.  When
   multiple users have access to the same content, data that is fetched
   on the behalf of one user may be cached and re-displayed to a second
   user, without re-fetching it from the fileserver hosting the data.
   The initial data aquisition is authenticated by the first user's

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   credentials, and if only that user's credentials are used, it may be
   possible for a malicious user or users to "poison" the cache for
   other users, and introduce bogus data.

   In order to protect users of a multi-user cache manager from each
   other, it is possible to give the cache manager its own token, which
   can be combined (Section 9) with the users' tokens so that the user
   may be authenticated at the fileserver while still preserving the
   integrity of the data obtained by the cache manager.  In order to
   obtain a token, the cache manager must have some means of acquiring/
   using key material.

7.1.  Keyed Clients

   When a host already has key material for a GSSAPI mechanism supported
   by the vlserver, that material MAY be used to key the cache manager.
   The cache manager simply calls the rxgk negotiation service using the
   relevant material, and obtains a token.  This token is a database
   server token; there is no need in the protocol for it to be usable as
   the user_tok input to AFSCombineTokens or for there to be an entry in
   the protection database corresponding to the cache manager's GSS
   identity.  The cache manager should frequently regenerate its token,
   to avoid combined tokens having expiration times which are
   substantially earlier than the expiration time of the corresponding
   user credentials.  The cache manager should not regenerate this token
   so often so as to place excessive load on the vlservers.

   It is recommended that GSS identities created specifically for use by
   a cache manager have the name afs3-callback@<hostname> of name type
   GSS_C_NT_HOSTBASED_SERVICE where <hostname> is the fully qualified
   domain name of the machine upon which the cache manager is running.

7.2.  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.

   In some cases a cache manager may not have any dedicated credentials,
   but have user credentials from multiple different users.  These
   tokens could be combined using the RXGK_CombineTokens operation and
   the combined token used as a proxy cache manager token.  However,
   conspiring malicious users could still be able to manipulate the
   cache, and the differing token expiration times for user tokens would
   make cache management quite complicated with this approach.  As such,
   it is not recommended for general use.

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8.  Combining Tokens

   This section describes the server-side behavior of the
   RXGK_CombineTokens operation for the AFS-3 protocol.

   There are no application-specific fields in RXGK_Token, so only the
   behavior for combination of identity information remains to be

   The identity lists in the 'identities' fields of the two tokens are
   combined via order-preserving concatenation and placed in the
   'identities' field of the output token.

   Printed tokens (Section 10.1) cannot be combined with any other
   token, and servers MUST reject attempts to do so, whether via
   CombineTokens, AFSCombineTokens, or any other token-combining
   procedure.  AFSCombineTokens with a printed user_tok and an empty
   cm_tok is not considered to be token combination for this purpose.

9.  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 fileserver, and a structure containing some
   information describing the returned token.  AFSCombineTokens is the
   only way to obtain a valid file server token (other than printing a
   token, see Section 10.1).

       AFSCombineTokens(IN RXGK_Data *user_tok,
                        IN RXGK_Data *cm_tok,
                        IN RXGK_CombineOptions *options,
                        IN afsUUID *destination,
                        OUT RXGK_Data *new_token,
                        OUT RXGK_TokenInfo *token_info) = TBD;

   user_tok:  An rxgk token for the vlserver.

   cm_tok:  Either an rxgk token for the vlserver, or empty (zero-

   options:  An RXGK_CombineOptions structure containing a list of
         enctypes acceptable to the client and a list of security levels
         acceptable to the client.

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   destination:  The UUID of the server new_token is intended for.  File
         server 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 user_tok) and a token generated from an identity
   that is private to the cache manager (as cm_tok).  This prevents a
   user from poisoning the cache for other users.  Recommendations on
   keying cache managers are contained in Section 7.1.

   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.  To prevent key-reuse attacks, the token master key in the
   output token must be unique per destination file server; the
   destination UUID is incorporated into the key derivation procedure to
   ensure this property.

   Clients using a printed token (see Section 10.1) MUST provide that
   token as user_tok. cm_tok MUST be empty.

   The server uses a zero-length new_token to indicate that the
   generation of rxgk tokens for the specified fileserver cannot work at
   the present time.  Upon receipt of such a zero-length new_token, 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.  (Such a client may still not attempt to use rxgk at
   all for an AFS cell if it has determined that there is no suitable
   GSS acceptor identity to be used for that cell.)

   In other cases where the server is unable to perform the
   AFSCombineTokens operation with the given arguments, a nonzero value

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   is returned.  Clients MUST NOT use such an error as an indication to
   fall back to to a different security class.

   The 'identities' list from user_tok is copied to the 'identities'
   field of the new_token.  The 'identities' list from cm_tok is
   discarded unused.

   Other aspects of the operation of AFSCombineTokens, including the
   combination of keys and tokens, are largely the same as the
   CombineTokens RPC, documented in [I-D.wilkinson-afs3-rxgk] and
   Section 8.  However, the AFSCombineTokens operation needs to include
   the destination file server's UUID in the key combination process to
   ensure that the resulting key is unique for each file server (and
   different from the key in the input tokens); AFSCombineTokens must
   also handle the case where the supplied cm_tok is absent (empty).  In
   the two-token case, the KRB-FX-CF2 operation is still used, but the
   pepper1 and pepper 2 inputs will both include the destination UUID:

       pepper1 :=  "AFS" || 00 || destination || enctype
       pepper2 := "rxgk" || 00 || destination || enctype

   where the strings "AFS" and "rxgk" exclude the NUL terminator; 00 is
   a NUL octet; destination is the XDR-encoding of the destination
   afsUUID; enctype is the enctype selected by the server and returned
   in the enctype field of token_info, encoded as a 32-bit integer in
   network byte order; and || is the concatenation operator.  In the
   one-token case,

       Kn := random-to-key(PRF+(K0, pepper0))
       pepper0 := "rxgkAFS" || 00 || destination || enctype

   where the string "rxgkAFS" excludes the NUL terminator.  Note that
   the PRF+ function here is the one used in the KRB-FX-CF2 operation
   specified in [RFC6113], which differs from the PRF+ function
   specified in [RFC4402] and used elsewhere in this document.  random-
   to-key is the function specified by the RFC3961 profile of the
   selected enctype.

10.  Server to Server Communication

   A number of portions of the AFS-3 protocol require that servers
   communicate amongst themselves.  To name a limited subset of
   examples, file servers must register their location (IP addresses)
   with the vldb, and must query the prdb when serving data; moving
   volumes from one file server to another requires that the file
   servers communicate with each other directly.

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   A server with the cell-wide shared key can forge a token for its use
   in server-to-server communication, which we refer to as "token
   printing".  Printed tokens take on a special form (Section 10.1) and
   are limited in that they cannot be combined with any other token.

   However, file servers with a server-specific key (that is, without
   the cell-wide shared key), can only print a token to themselves.
   Such tokens are not usable to communicate with database servers or
   other file servers.  As such, file servers with a per-server key will
   need GSS credentials (but, as with keyed clients, not necessarily
   entries in the protection database) in order to function.  These
   credentials can be used to acquire an rxgk token, allowing queries to
   the database servers.  They can also be used to register the file
   server in the vldb, and to create and update the file server's
   server-specific key in the vldb.

10.1.  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, place it in an RXGK_TokenContainer
   describing the key used to perform the encryption, and use it in the
   same way as a normal rxgk token.  The receiving server can identify
   it as 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 starting from a printed token, a client MUST 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..)

10.2.  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 vldb have a record 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 10.3), and either one may be used to
   indicate that a fileserver supports rxgk.  Fileservers which support
   rxgk MUST call these RPCs over an rxgk protected connection.  The

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   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.

   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 and VL_RegisterAddrsAndKey from a printed token, an
   administrator, or the identity registered for the fileserver using a
   prior call to VL_RegisterAddrsandKey.

   There are two tracks for registering a file server as being rxgk-
   enabled; one for file servers with the cell-wide key, and another for
   file servers with per-server keys.

10.2.1.  File Servers With the Cell-Wide Key

   When a file server that will use the cell-wide key is registered as
   rxgk-capable, there is no need to register a new key for that server
   (and in fact it would be actively harmful!), so there is no need to
   use VL_RegisterAddrsAndKey.  In this case, VL_RegisterAddrs is
   sufficient, and using a printed token for the rxgk connection for
   VL_RegisterAddrs indicates that the file server possesses the cell-
   wide key.  Since the file server has the cell-wide shared key, it
   will get its key updated when the cell-wide key is updated, and does
   not need to update its own key separately.  As such, it will never
   need to call VL_RegisterAddrsAndKey.

10.2.2.  File Servers With Per-Server Keys

   This section describes the case when the automated keying mechanism
   described in Section 10.3 is used.  If the record of per-server keys
   in the vldb is being manually maintained, cell administrators should
   manually register the file servers in the vldb using VL_RegisterAddrs

   Since the goal is to establish a per-server key,
   VL_RegisterAddrsAndKey is necessary for the first call.  However,
   best practices require that the file server change its long-term key
   periodically, so it must retain the ability to perform subsequent
   VL_RegisterAddrsAndKey calls in the future, to register those new
   keys in the vldb.  For this reason, a printed token is not a useful
   choice for performing the initial call to VL_RegisterAddrsAndKey,
   since only a printed token would be able to perform a subsequent
   call.  The printed token would require the cell-wide shared key,
   eliminating any benefit from having a server-specific key.  As such,
   a regular (non-printed) token is required for the initial call to

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   VL_RegisterAddrsAndKey.  A cell administrator's token could be used,
   but it is advantageous to allow file servers with per-server keys to
   operate without intervention by the central cell administrators (so
   that these file servers could be run solely by a local administrator
   without need for central administrator intervention).

   Thus, it is expected that a file server with a per-server key will
   have a dedicated GSS identity and credentials that it will use for
   registering with the vldb (VL_RegisterAddrsAndKey) and that will also
   be used for securing the file server's regular connections to the
   database servers during normal operation.  The vlserver will store in
   the vldb what GSS identity is used to perform VL_RegisterAddrsAndKey
   for a given file server UUID, and allow that identity to perform
   successor calls to VL_RegisterAddrsAndKey and VL_RegisterAddrs for
   that UUID.

   Is is RECOMMENDED that GSS identities created solely for use on file
   servers with per-server keys be of the form
   afs3-fileserver@<hostname> of name type GSS_C_NT_HOSTBASED_SERVICE.

10.3.  Registering 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 in the vldb, 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, 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 that provides confidentiality protection.

   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 SHOULD NOT permit calls to VL_RegisterAddrsAndKey for
   fileserver UUIDs which already exist within the vldb, unless that
   UUID already has a server-specific key registered.  Requiring the
   separation facilitates a workflow wherein existing servers retain the
   cell-wide key, and new file servers are created with per-server keys.
   Data volumes can then be gradually migrated to the new file servers,
   and old file servers decommissioned.  Permitting file servers to

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   convert from cell-wide key to per-server keys would involve
   complicated access checking and update logic for which it is
   difficult to ensure correctness of implementation.

   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_uint32 kvno;
           opaque nonce2[20];

       const RXGK_MAXKEYDATAREQUEST = 16384;
       const RXGK_MAXKEYDATARESPONSE = 16384;
       typedef opaque keyDataRequest<RXGK_MAXKEYDATAREQUEST>;
       typedef opaque keyDataResponse<RXGK_MAXKEYDATARESPONSE>;
           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

   secIndex:  The index of the security mechanism for which a key is
         being set.

   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.

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   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 kvno
   field of the RXGK_ServerKeyDataResponse is used to indicate to the
   client what key version number it should use for the key it will
   compute using these nonces.  The kvno will be used in the
   RXGK_TokenContainer bearing file server tokens for this file server,
   to indicate which key was used to encrypt the RXGK_Token.

   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.  Such subsequent calls using an administrator's
   token or a printed token do not update the identity list associated
   with this UUID's key.  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.

   For rxgk, the file server key can then be derived by both client and
   server using

       random-to-key(PRF+(K0, K,
                          pepper || 00 || nonce1 || nonce2 || enctype));

   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].

   [[The PRF+ function defined in RFC 4402 specifies that the values of
   the counter 'n' should begin at 1, for T1, T2, ... Tn.  However,
   implementations of that PRF+ function for the gss_pseudo_random()
   implementation for the krb5 mechanism have disregarded that
   specification and started the counter 'n' from 0.  Since there is no
   interoperability concern between krb5 gss_pseudo_random() and rxgk
   key derivation, implementations of the RFC 4402 PRF+ function for
   rxgk key derivation should use the RFC 4402 version as specified,
   that is, with the counter 'n' beginning at 1.]]

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   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

   pepper is the ASCII string "RXGKRegisterAddrsAndKey" (without
   trailing NUL).

   00 is a NUL octet.

   enctype is the selected enctype, encoded as a 32-bit integer in
   network byte order.

   || is the concatenation operation.

11.  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 by binding the key used to secure the callback
   connection into the authenticator used to create the original rxgk
   connection.  Callbacks created as a result of RPCs performed on that
   rxgk connection will use the callback key given in the authenticator.

11.1.  Lifetime and scope of the callback channel

   The RXGK_Authenticator_AFSAppData structure contains a key and
   enctype, but no key version number field.  This restricts the
   connection to only ever having one key to secure callbacks created as
   a result of calls on that connection, even if there are multiple Rx
   challenge/response exchanges where a new authenticator could be
   constructed.  This is acceptable, because if the client needs to
   rotate the key used for secure callbacks to it, the client can
   initiate a new connection to the server, with a new callback key.

   It may be reasonable for a cache manager to only ever use one key for
   secure callbacks (until the cache manager is restarted), such as in a
   cell where all fileservers have the cell-wide shared key or where all
   fileservers are equally trusted.  Alternately, a cache manager may
   use just one callback key per fileserver.  In either case, which key
   to use for incoming callback connections is known just from the
   context of the connection, so there is no need to provide a callback
   token in the authenticator.

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   In all cases, both cache manager and file server must retain the
   callback key until all callbacks using that key are expired.

   Only RPCs issued over an rxgk protected connection should receive
   rxgk protected callbacks.

12.  IANA Considerations

   This memo includes no request to IANA.

13.  AFS-3 Registry Considerations

   This document requrests that the AFS-3 registry allocate code points
   for the new RPCs AFSCombineTokens (for the RXGK service) and
   RegisterAddrsAndKey (for the VL service).

14.  Security Considerations

14.1.  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.

14.2.  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.

14.3.  Combined Key Materials

   As described in Section 7, 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.

15.  References

15.1.  Informational References

   [RX]       Zeldovich, N., "RX protocol specification", October 2002.

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              Benjamin, M., "AFS Callback Extensions (Draft 14)", draft-
              benjamin-extendedcallbackinfo-02 (work in progress),
              December 2011.

15.2.  Normative References

              Brashear, D., "Authentication Name Mapping extension for
              AFS-3 Protection Service", draft-brashear-afs3-pts-
              extended-names-09 (work in progress), March 2011.

              Wilkinson, S., "rxgk: GSSAPI based security class for RX",
              draft-wilkinson-afs3-rxgk-00 (work in progress), January

   [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.

   [RFC6113]  Hartman, S. and L. Zhu, "A Generalized Framework for
              Kerberos Pre-Authentication", RFC 6113, April 2011.

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

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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.

   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

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   Place bounds on some XDR opaque arrays.

   Expound more about printed tokens, for dbservers and fileservers.

B.5.  Since 04

   Rearrange content within the document in attempt to give a more
   coherent structure and improve readability.

   Add specifications for the remaining pieces of rxgk behavior which
   the core document left as application-specific.

   Change the token format.  Instead of having the last entry in the
   identities list be the CM identity, use an explicit separate field
   for the identity to be used for callbacks.

   As a result, PRAUTHTYPE_EMPTY is no longer necessary.

   General edits for grammar and readability.

   Add security considerations for the DoS attach that is possible by
   setting fake callback keys.

   Add a clarifying note for the RFC 4402 PRF+ implementation.

B.6.  Since 05

   Remove start_time from the token format.

   Remove the SetCallBackKey RPC, in favor of putting a callback key in
   the authenticator appdata.  This provides a simpler solution to the
   problem of establishing a secure callback channel.

   While here, add the server UUID into the appdata as well as the
   client UUID, to prevent some possible routes to data corruption.

B.7.  Since 06

   General edits for clarity.

   Use afs_uint32 for token lifetimes, to match the core spec.

B.8.  Since 07

   Incorporate the destination UUID and target enctype into
   AFSCombineTokens key generation.

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   Add (fixed) pepper strings for AFSCombineTokens and
   RegisterAddrsAndKey key generation.

   General editing for clarity.

   Use unsigned types for kvnos.

   Use a pointer type for afsUUID RPC arguments.

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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