One document matched: draft-wilkinson-afs3-rxgk-afs-01.xml
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<rfc category='info' ipr='trust200902'
docName="draft-wilkinson-afs3-rxgk-afs-01"
submissionType="independent">
<front>
<title>Integrating rxgk with AFS</title>
<author surname="Wilkinson" fullname="Simon Wilkinson">
<organization abbrev="YFS">Your File System Inc</organization>
<address>
<email>simon@sxw.org.uk</email>
</address>
</author>
<date month="January" year="2012"/>
<abstract>
<t>
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.
</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>
<xref target="I-D.wilkinson-afs3-rxgk">rxgk</xref> is a new
<xref target="RFC2743">GSSAPI</xref> based security layer for the
<xref target="RX">RX</xref> 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.
</t>
<section title="The AFS-3 distributed file system">
<t>
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.
</t>
<t>
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 syncronised by
an out-of-band mechanism.
</t>
</section>
<section title="rxgk and AFS">
<t>
AFS-3 differs from the standard rxgk implementation in that it
does not require GSSAPI negotiation with each server. Instead,
a client negotiates with the vlserver, and receives a token
which can then be used with any server in the cell. This
requires that all servers have an identical cell wide pre-shared
key for token encryption.
</t>
<t>
For more complex cell topologies, servers which do not share the
cell-wide key are supported by means of an extended CombineTokens
call. This call takes a server identifier, and will return a token
encrypted with a key for a specific server. This extended call,
AFSCombineTokens, also provides support for indicating whether
a specific server is rxgk capable, allowing cells to securely migrate
to rxgk from other security mechanisms.
</t>
<t>
We also define mechanisms for securing the callback channel which
is created between fileserver and client.
</t>
</section>
<section title="Requirements Language">
<t>
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
<xref target="RFC2119">RFC 2119</xref>.
</t>
</section>
</section>
<section title="Security Index">
<t>
When used within the AFS protocol, rxgk has a securityIndex value
of 4
</t>
</section>
<section title="Key negotiation">
<t>
An AFS cell wishing to support rxgk MUST run an rxgk key negotiation
service, as specified in <xref target="I-D.wilkinson-afs3-rxgk"/>,
on each of its vlservers. The service MUST listen on the same port as
the vlserver.
</t>
<t>
The GSS identity afs-rxgk@_afs.<cellname>
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
which wish to support cells using other rx security objects MAY downgrade
if this identity is not available.
</t>
<t>
Tokens returned from the GSSNegotiate call MUST only be used with
database servers. Tokens for fileservers MUST be obtained by
calling AFSCombineTokens before each server is contacted.</t>
<section title="The AFSCombineTokens operation">
<t>
AFS extends the existing CombineTokens operation to provide a more
general token manipulation service. This operation takes a user
token, an optional cache manager token, and a destination identifier,
and returns a token specific to the specified destination.</t>
<artwork>
AFSCombineTokens(IN RXGK_Data *token0<>,
IN RXGK_Data *token1<>,
IN afsUUID destination,
OUT RXGK_Data *new_token<>) = 3;
</artwork>
<list style="hanging" hangIndent="6">
<t hangText="token0:">An rxgk token obtained using the
GSSNegotiate rpc</t>
<t hangText="token1:">Either, an rxgk token obtained using the
GSSNegotiate rpc, or empty (0 length)</t>
<t hangText="destination:">The UUID of the server this token is
intended for. Fileserver UUIDs may be obtained from the VLDB
in the same call as returns their addresses.</t>
<t hangText="new_token:">A new rxgk token, or empty</t>
</list>
<t>The AFSCombineTokens call MUST only be performed over an rxgk
protected channel, with a security level of 1 (integrity) or
more. Servers MUST reject all attempts to perform this operation
over channels that are not protected in this way.</t>
<t>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 below</t>
<t>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.</t>
<t>Clients using a printed token (see below) 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</t>
<t>If the returned token is 0 length, then the destination does not
support rxgk, and the client MAY fall back to using a different
authentication mechanism for that server. This is the only situation
in which an rxgk capable client operating within an rxgk enabled
cell may downgrade its choice of security layer.</t>
<t>Keys and tokens are combined in the same way as the
CombineTokens call, documented in
<xref target="I-D.wilkinson-afs3-rxgk"/>.</t>
</section>
</section>
<section title="Tokens">
<section title="Container">
<t>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 <xref target="RFC3961"/>
encryption type) allows the recipient to identify which
pre-shared key has been used to encrypt the token itself.
</t>
<artwork>
struct RXGK_TokenContainer {
afs_int32 kvno;
afs_int32 enctype;
opaque encrypted_token<>;
}
</artwork>
</section>
<section title="Token Encryption">
<t>Token contents are encrypted using a pre-shared key. rxgk supports
the use of both a single cell-wide key and the use of per-server
keys. The cell-wide key must be installed on all servers which are
capable of accepting cell-wide tokens. Cell-wide keys should be for
a selected RFC3961 encryption mechanism which is supported by all
servers within the cell. Per-server keys should be for an
encryption mechanism which is supported by both the destination
server, and the negotiation service. The management of per-server
keys is discussed in more detail below.</t>
<t>Key rollover is permitted by means of a key version number. When the
key is changed, a different key version number MUST be selected. Servers
SHOULD accept tokens using the old key until the lifetime of all
existing tokens has elapsed.</t>
<t>Encryption is performed over the XDR encoded RXGK_Token structure,
using the RFC3961 encrypt operation, with a key usage value of
1036 (RXGK_SERVER_ENC_TICKET)</t>
</section>
<section title="Token Contents">
<t>The token itself contains the information expressed by the following
XDR:</t>
<artwork>
struct RXGK_Token {
afs_int32 enctype;
opaque K0<>;
afs_int32 level;
afs_int64 starttime;
afs_int32 lifetime;
afs_int32 bytelife;
rxgkTime expirationtime;
struct PrAuthName identities<>;
}
</artwork>
<list style="hanging" hangIndent="6">
<t hangText="enctype:">The RFC3961 encryption type of the session key
contained within this ticket</t>
<t hangText="K0:">The session key (see the rxgk specification for
details of how this key is negotiated between client and negotiation
service).</t>
<t hangText="level:">The security level that MUST be used for this
connection</t>
<t hangText="starttime:">The time, expressed as a 100ns value, since
the Unix epoch. Servers MUST reject attempts to
start connections with tokens that are not yet valid. </t>
<t hangText="lifetime:">The maximum number of seconds that a
key derived from K0 may be used for. This is an advisory limit. If 0,
keys have no time based limit</t>
<t hangText="bytelife:">The maximum amount of data (expressed as
log 2 byes) that may be transferred using a key derived from K0.
This is an advisory limit. If 0, there is no data based limit on
key usage</t>
<t hangText="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 has passed. A time of 0 indicates that this token never
expires.</t>
<t hangText="identities:">A list of identities represented by this
token. struct PrAuthName is the identity structure defined in
<xref target="I-D.brashear-afs3-pts-extended-names"/></t>
</list>
</section>
</section>
<section title="Authenticator data">
<t>The appdata opaque within the RXGK_Authenticator contains the XDR
encoded UUID of the client. The UUID is encoded using the afsUUID
type.</t>
</section>
<section title="Client tokens">
<t>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.</t>
<section title="Keyed clients">
<t>Where 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. The client should frequently
renew this token, to avoid combined tokens having unnecessarily
close expiration times.</t>
<t>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.</t>
</section>
<section title="Unkeyed clients">
<t>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 <xref target="RFC2743">anon_req_flag</xref> and the
default credentials set.</t>
</section>
</section>
<section title="Server to server communication">
<t>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. </t>
<section title="Ticket printing">
<t>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 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.</t>
<t>The session key within a printed token MUST use the same encryption
type as the pre-shared key. When connecting to a fileserver, a client
SHOULD use the combine tokens service as discussed above to ensure that
they are using the correct key for the fileserver.</t>
</section>
</section>
<section title="Declaring rxgk support for a fileserver">
<t>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.
</t>
<t>Fileservers currently register with the vlserver using the
VL_RegisterAddrs RPC. Fileservers which support rxgk MUST call this
RPC over a rxgk protected connection. The vlserver should then note
the rx security layer used in registration, and infer rxgk support
from that. To prevent downgrade attacks, once a fileserver has
registered as being rxgk capable, the vlserver MUST NOT remove that
registration without administrator intervention.
</t>
<t>Once a fileserver has been marked as supporting rxgk, VL_RegisterAddrs
calls for that fileserver MUST only be accepted over an rxgk
protected link.
</t>
<!-- XXX - we need to define a VL_ RPC which can be used to remove
the rxgk capable marking -->
</section>
<section title="Per server keys">
<t>The provision of 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 down to the implementor, who MAY decided to use a
manual, or out of band key management system
</t>
<t>Implementations supporting automatic key management through the AFS3
protocol MUST provide the following RPC
<artwork>
struct RXGK_ServerKeyDataRequest {
afs_int32 enctypes<>
opaque nonce1<>
};
struct RXGK_ServerKeyDataResponse {
afs_int32 enctype;
afs_int32 kvno;
opaque nonce2<>
};
VL_RegisterAddrsAndKey(
IN afsUUID *uuidp,
IN afs_int32 spare1,
IN bulkaddrs *ipaddr,
IN afs_int32 secIndex,
IN opaque *keyDataRequest<>,
OUT opaque *keyDataResponse<>) = XXX;
</artwork>
</t>
<list style="hanging" hangIdent="6">
<t hangText="uuidp:">As the existing VL_RegisterAddrs RPC</t>
<t hangText="spare1:">As the existing VL_RegisterAddrs RPC</t>
<t hangText="ipaddr:">As the existing VL_RegisterAddrs RPC</t>
<t hangText="secIndex:">The index of the security mechanism for
which a key is being set. For rxgk, this value should be '4'</t>
<t hangText="keyDataRequest:">An opaque blob of data, specific to
the security mechanism defined by secIndex. For rxgk it is,
the xdr encoded representation of RXGK_ServerKeyDataRequest</t>
<t hangText="keyDataResponse:">An opaque blob of data, specific to
the security mechanism defined by secIndex. For rxgk it is
the xdr encoded representation of RXGK_ServerDataResponse</t>
</list>
<t>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.</t>
<t>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.</t>
<t>The server key can then be derived by both client and server using
<artwork>
random-to-key(PRF+(K0, K, nonce1 || nonce2))
</artwork>
</t>
<t>random-to-key is the function specified by the RFC3961 profile of the
encryption type chosen by the server, and returned in enctype.</t>
<t>PRF+ is the function of that name specified by
<xref target="RFC4402"/></t>
<t>K0 is the master key of the current rxgk session, as originally
determined by the GSSNegotiate call.</t>
<t>K is the key generation seed length as specified in enctype's RFC3961
profile</t>
</section>
<section title="Securing the callback channel">
<t>AFS has traditionally had an unprotected callback channel. However,
extended callbacks requires 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 earlier, which is required in order for a client to
accept secure callbacks
</t>
<t>A cache manager may set a key for secure callbacks by issuing
the following RPC (part of the RXAFS_ family)</t>
<artwork>
RXAFS_SetCallbackKey(afs_int32 securityIndex,
opaque mech_data<>) = XXX;
</artwork>
<list style="hanging" hangIndent="6">
<t hangText="securityIndex:">The securityIndex of the mechanism for
which this key is being set. In the rxgk case, this will be
rxgk's security index, as defined earlier.</t>
<t hangText="mech_data:">This contains the security object specific
data. In rxgk's case this is an XDR encoded RXGK_Token structure.</t>
</list>
<t>When used with rxgk, this RPC MUST be performed over an rxgk protected
link established using solely the cache manager's token. This connection
MUST have a security level of 2 (encrypted).</t>
<t>If a fileserver receives a AFS_SetCallbackKey protected with a different
cache manager identity than the previous call from that client, it MUST
break all secure callbacks held by that client using the old key before
this RPC completes.</t>
<t>Only RPCs issued over an rxgk protected connection should receive rxgk
protected callbacks</t>
<t>The fileserver MUST only send rxgk protected callbacks when one of the
identities performing the RPC establishing that callback matches the
identity associated with that clients callback channel.</t>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>This memo includes no request to IANA.</t>
</section>
<section title="Security Considerations">
<section title="Downgrade attacks">
<t>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.
</t>
</section>
<section title="Per server keys">
<t>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 life one
(the server key)
</t>
</section>
<section title="Combined key materials">
<t>As described earlier, 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.
</t>
</section>
</section>
</middle>
<back>
<references title="Informational References">
<reference anchor='RX'>
<front>
<title>RX protocol specification</title>
<author initials='N.' surname='Zeldovich'
fullname='Nickolai Zeldovich'>
<organization />
</author>
</front>
<format type='HTML' target='http://web.mit.edu/kolya/afs/rx/rx-spec' />
</reference>
</references>
<references title="Normative References">
&I-D.draft-brashear-afs3-pts-extended-names;
&I-D.draft-wilkinson-afs3-rxgk;
&RFC2119;
&RFC2743;
&RFC3961;
&RFC4402;
&RFC4506;
</references>
<section title="Acknowledgements">
<t>
RXGK has been the work of many contributors over the years. A partial
list is contained in the previous document. All errors and omissions
are, however, mine.
</t>
</section>
<section title="Changes">
<section title="Since 00">
<t>Add references to RX and XDR specifications</t>
<t>Add introductory material on AFS</t>
<t>Change expirationTime to be expressed using the rxgkTime type</t>
<t>Document how encryption types are chosen for printed tokens,
and how they are used against fileservers</t>
<t>Expand security considerations section to cover combined tokens</t>
<t>Rename AFS_SetCallbackKey as RXAFS_SetCallbackKey</t>
</section>
</section>
</back>
</rfc>
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