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Network Working Group S. Farrell, Ed.
Request for Comments: 3767 Trinity College Dublin
Category: Standards Track June 2004
Securely Available Credentials Protocol
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document describes a protocol whereby a user can acquire
cryptographic credentials (e.g., private keys, PKCS #15 structures)
from a credential server, using a workstation that has locally
trusted software installed, but with no user-specific configuration.
The protocol's payloads are described in XML. This memo also
specifies a Blocks Extensible Exchange Protocol (BEEP) profile of the
protocol. Security requirements are met by mandating support for
TLS and/or DIGEST-MD5 (through BEEP).
Table Of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. The Protocol. . . . . . . . . . . . . . . . . . . . . . . . . 3
3. BEEP Profile for SACRED. . . . . . . . . . . . . . . . . . . . 9
4. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 12
5. Security Considerations. . . . . . . . . . . . . . . . . . . . 13
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . 16
Appendix A: XML Schema . . . . . . . . . . . . . . . . . . . . . . 17
Appendix B: An Example of Tuning with BEEP . . . . . . . . . . . . 20
Appendix C: Provision SACRED using other Protocols . . . . . . . . 23
Editor's Address . . . . . . . . . . . . . . . . . . . . . . . . . 24
Full Copyright Statement. . . . . . . . . . . . . . . . . . . . . 25
Farrell Standards Track [Page 1]
RFC 3767 Secure Credentials Protocol June 2004
1. Introduction
Digital credentials, such as private keys and corresponding
certificates, are used to support various Internet protocols, e.g.
S/MIME, IPSec, and TLS. In a number of environments, end users wish
to use the same credentials on different end-user devices. In a
"typical" desktop environment, the user already has many tools
available to allow import/export of these credentials. However, this
is not very practical. In addition, with some devices, especially
wireless and other more constrained devices, the tools required
simply do not exist.
This document describes a protocol for the secure exchange of such
credentials and is a realization of the abstract protocol framework
described in [RFC3760].
Many user-chosen passwords are vulnerable to dictionary attacks. So
the SACRED protocol is designed to give no information with which an
attacker can acquire information for launching a dictionary attack,
whether by eavesdropping or by impersonating either the client or
server.
The protocol also allows a user to create or delete an account,
change her account password and/or credentials, and upload the new
values to the server. The protocol ensures that only someone that
knew the old account password is able to modify the credentials as
stored on the credential server. The protocol does not preclude
configuring a server to disallow some operations (e.g. credential
upload) for some users. The account management operations as a whole
are optional implementations for both credential servers and clients.
Note that there are potentially two "passwords" involved when using
this protocol - the first used to authenticate the user to the
credential server, and the second to decrypt (parts of) the
credential following a download operation. Where the context
requires it, we refer to the former as the account password and the
latter as the credential password.
Using a protocol such as this is somewhat less secure than using a
smart card, but can be used until smart cards and smart card readers
on workstations become ubiquitous, and can be useful even after smart
cards are ubiquitous, as a backup strategy when a user's smart card
is lost or malfunctioning.
The protocol sets out to meet the requirements in [REQS].
Cryptographic credentials may take the form of private keys, PKCS #15
[PKCS15], or structures. As stated, a profile based on BEEP [BEEP]
is specified for message transport and security (integrity,
Farrell Standards Track [Page 2]
RFC 3767 Secure Credentials Protocol June 2004
authentication, and confidentiality). In that case, the security
requirements are met by mandating support (via BEEP) for TLS [TLS]
and/or DIGEST-MD5 [DIGEST-MD5].
We assume the only authentication information available to the user
is a username and password.
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 [RFC2119].
2. The Protocol
This section defines the account management and "run-time" operations
for the SACRED protocol.
It also describes the message formats used, which are described in
XML [XMLSCHEMA]. Appendix A provides an XML schema for these
elements.
The approach taken here is to define SACRED elements that are
compatible with the elements used in [XKMS] and [XMLDSIG], so that an
implementation of this protocol can easily also support XKMS, and
vice versa.
It is also intended that other SACRED protocol instances (e.g. using
a different authentication scheme, credential format, or transport
protocol) could re-use many of the definitions here.
2.1. Account Management Operations
These operations MAY be implemented, that is, they are OPTIONAL.
2.1.1. Information Request
This operation does NOT REQUIRE authentication.
The purpose of this operation is to provide the client with the
values required for account creation.
The client sends an InfoRequest message (which has no content).
The server responds with an InfoResponse message which contains the
authentication mechanism parameters for the server and the list of
supported ProcessInfo types. For DIGEST-MD5, this consists of the
list of realms (each as an XML element named "Realm") which the
server supports. There MUST be at least one realm specified.
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RFC 3767 Secure Credentials Protocol June 2004
Clients MUST be able to select one from a list of Realms and MUST be
able to disregard any other information present (allowed for
extensibility).
2.1.2. Create Account
This operation REQUIRES server authentication.
The purpose of this operation is to setup a new account on the
server. The information required for a "new" account will depend on
the SASL [SASL] mechanism used.
The client sends a CreateAccountRequest, which contains the account
name (e.g. username). It also contains the elements required to
create an account for a particular authentication mechanism. The
actual information is defined according to the authentication
mechanism. For DIGEST-MD5, this consists of the password verifier
(the hashed username, password and realm) and the chosen realm.
Although more than one set of such data is allowed by the data
structures defined in the appendix, clients SHOULD only include one
here.
The server responds with an error or acknowledgement message.
2.1.3. Remove Account
This operation REQUIRES mutual authentication.
The purpose of this operation is to delete the entire account.
The client sends a RemoveAccountRequest message (which has no
content) to the server.
The server MUST delete all information relating to the account and
respond with an error or acknowledgement message.
2.1.4. Modify Account
This operation REQUIRES mutual authentication.
The purpose of this operation is to allow the client to change the
information required for authentication. The information required
will depend on the authentication method used.
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RFC 3767 Secure Credentials Protocol June 2004
The client sends a ModifyAccountRequest message, which contains the
elements required to change the authentication information for the
account, for a particular authentication mechanism. The actual
information is defined according to the authentication mechanism. For
[DIGEST-MD5], it will consist of a realm and password verifier value.
Once the account information has been changed, the server will
respond with an error or acknowledgement message.
2.2. "Run-time" Operations
These operations MUST be supported by all conformant implementations.
2.2.1. Credential Upload
This operation REQUIRES mutual authentication.
The purpose of this operation is to allow the client to deposit a
credential with the server.
The client sends an UploadRequest message to the server which MUST
contain one Credential.
If a credential with the same credential selector field as in the
UploadRequest (a "matching" credential) already exists for the
account, then that credential is replaced with the new credential
from the UploadRequest. Otherwise a "new" credential is associated
with that account. If a new credential is being uploaded, then the
client SHOULD include (in LastModified) its local concept of the time
(if it has one), or an indicator that it has no clock. The actual
value of LastModified can be anything, (but the element has to be
present) since this will be overwritten by the server in any case.
If any change is made to the stored credentials associated with the
account, then the server MUST update the corresponding LastModified
value (returned in DownloadResponse messages) to the current time (at
the server).
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RFC 3767 Secure Credentials Protocol June 2004
The LastModified value in the UploadRequest MUST be the value which
was most recently received in a corresponding DownloadResponse for
that credential. This means the clients are strongly RECOMMENDED to
only produce an UploadRequest based on recently downloaded
credentials, since otherwise the LastModified value may be out of
date.
The LastModified value can also be of use in detecting conflicts.
For example, download to platform A, download to platform B, update
from B, update from A. The server could detect a conflict on the
second upload. In this case the server MUST respond with a BEEP
error (which SHOULD be StaleCredential).
The server replaces the provided LastModified value with the current
time at the server before storing the credential. (Note that this
means that it would be unwise for a client to include the
LastModified field in a ClientInfo digital signature which is
calculated over the CredentialType.)
The server responds with an error or acknowledgement message.
2.2.2. Credential Download
This operation REQUIRES mutual authentication.
The purpose of this operation is to allow a client to get one or more
credentials from a server (the purpose of the entire protocol
really!).
The client sends a DownloadRequest message to the server which MAY
contain a credential selector string for the credential. No, or an
empty credential selector means the request is for all credentials
associated with the account.
The server responds with a DownloadResponse or an error message. A
DownloadResponse contains one or more credential payloads, including
the LastModified time which represents the time (at the server) when
the last change was made to each credential associated with the
account (e.g. subsequent to an UploadRequest).
2.2.3. Credential Delete
This operation REQUIRES mutual authentication.
The purpose of this operation is to allow the client to delete one or
all credentials associated with the account.
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RFC 3767 Secure Credentials Protocol June 2004
The client sends a DeleteRequest message to the server which can
contain either a CredentialSelector or an All element.
If the DeleteRequest contains an All element, then all of the
credentials associated with that account are deleted.
If the DeleteRequest contains a CredentialSelector, then the request
MAY include a LastModified value. If the LastModified value is
present in the DeleteRequest, then it MUST be the value which was
most recently received in a corresponding DownloadResponse for that
credential. If the value does not match, then the server MUST NOT
delete the credentials.
If no "matching" credential exists, the server returns an error.
The server responds to this request with an error or acknowledgement
message.
2.3. Miscellaneous
2.3.1. Session Security
Six SACRED operations are defined above. In this section we specify
the requirements for security for each of the operations (where
supported).
Operation Security REQUIRED
--------- -----------------
Information request NONE
Create account Server authentication,
Confidentiality, Integrity
Remove account Mutual authentication,
Confidentiality, Integrity
Modify account Mutual authentication,
Confidentiality, Integrity
Credential upload Mutual authentication,
Confidentiality, Integrity
Credential download Mutual authentication,
Confidentiality, Integrity
Credential delete Mutual authentication,
Confidentiality, Integrity
The security requirements can be met by several mechanisms. This
document REQUIRES credential servers to support TLS and DIGEST-MD5.
Clients MUST support DIGEST-MD5 and TLS with server authentication.
Farrell Standards Track [Page 7]
RFC 3767 Secure Credentials Protocol June 2004
The mandatory-to-implement TLS cipher suite for SACRED is
TLS_RSA_WITH_3DES-EDE_CBC_SHA. Implementations SHOULD also support
TLS_RSA_WITH_AES_128_CBC_SHA [TLSAES].
When performing mutual authentication using DIGEST-MD5 for the
client, DIGEST-MD5 MUST only be used "within" a TLS server-
authenticated "pipe", and MUST only be used for client
authentication. That is, we do not use the DIGEST-MD5 security
services (confidentiality, integrity etc.).
2.3.2. Handling Multiple Credentials for an Account
When more than one credential is stored under a single account, the
client can select a single credential using the optional credential
selector string.
There is no concept of a "default credential" - all credentials MUST
have an associated selector unique for that account. The selector is
REQUIRED for upload requests and OPTIONAL for download requests. If
the selector is omitted in a download request, it MUST be interpreted
as a request for all the stored credentials.
An empty selector string value (i.e. "") in a credential download
request is to be interpreted as if the selector string were omitted,
i.e. a download request containing this is a request for all
credentials.
It is an error to have more than one credential stored under the same
account where both have the same credential selector string.
2.3.3. Common Fields
All messages sent to the server MAY contain ProcessInfo values. This
field MAY be used by other specifications or for vendor extensions.
For example, a server might require clients to include a phone number
in this field. The information response message contains a list of
the types of ProcessInfo that the server supports. This
extensibility scheme is similar to that used in [XKMS] and [XBULK].
Where no specific response message is defined for an operation (e.g.
for UploadRequest), then the transport will indicate success or
failure.
All of the response messages defined here MAY contain a Status
string, containing a value intended for human consumption.
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RFC 3767 Secure Credentials Protocol June 2004
2.3.4. Credential Format
A number of messages involve the Credential element. It has the
following fields (all optional fields may occur exactly zero or one
times unless otherwise stated):
- CredentialSelector contains a string by which this particular
credential (for this account) can be identified.
- PayLoad contains either a ds:KeyInfo or some other form of
credential. Implementations MUST support the PKCS #15 form of
ds:KeyInfo defined below (the SacredPKCS15 element).
- LastModified is a string containing the time (at the server) at
which this credential was last modified.
- TimeToLive (optional) is a hint clients SHOULD honor, which
specifies the number of seconds the downloaded credential is to be
usable.
- ProcessInfo (optional) MAY contain any (typed) information that
the server is intended to process. If the server doesn't support
any of the ProcessInfo data, it MAY ignore that data.
- ClientInfo (optional) MAY contain any (typed) information that the
client is intended to process, but which the server MUST ignore.
If the client doesn't support any of the ClientInfo data, it MAY
ignore that data (e.g. if the ClientInfo is device specific).
3. BEEP Profile for SACRED
The protocol described in this memo is realized as a [BEEP] profile.
Future memos may define alternative versions of the BEEP profile for
SACRED. When a BEEP peer sends its greeting, it indicates which
profiles it is willing to support. Accordingly, when the BEEP client
asks to start a channel, it indicates the versions it supports, and
if any of these are acceptable to the BEEP server; the latter
specifies which profile it is starting.
Profile Identification: http://iana.org/beep/sacred
Messages Exchanged during Channel Creation:
InfoRequest,
CreateAccountRequest,
RemoveAccountRequest,
ModifyAccountRequest,
DownloadRequest,
UploadRequest,
DeleteRequest,
InfoResponse,
Farrell Standards Track [Page 9]
RFC 3767 Secure Credentials Protocol June 2004
DownloadResponse,
error,
ok
Messages starting one-to-one exchanges:
InfoRequest,
CreateAccountRequest,
RemoveAccountRequest,
ModifyAccountRequest,
DownloadRequest,
UploadRequest,
DeleteRequest
Messages in positive replies:
ok,
InfoResponse,
DownloadResponse
Messages in negative replies: error
Messages in one-to-many changes: none
Message Syntax: c.f.,Section 3
Message Semantics: c.f., Section 2
Contact Information: c.f., the editor's address section of this memo
3.1. Profile Initialization
Because all but one of the operations of the SACRED profile have
security requirements (cf., Section 2.3.1), before starting the
SACRED profile, the BEEP session will likely be tuned using either
http://iana.org/beep/TLS
or
http://iana.org/beep/TLS followed by
http://iana.org/SASL/DIGEST-MD5
Appendix B gives an example of tuning a BEEP session using DIGEST-
MD5 (i.e. it shows how to turn on BEEP security).
Regardless, upon completion of the negotiation process, a tuning
reset occurs in which both BEEP peers issue a new greeting. Consult
Section 3 of [BEEP] for an example of how a BEEP peer may choose to
issue different greetings based on whether confidentiality is in use.
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RFC 3767 Secure Credentials Protocol June 2004
Any of the messages listed in section 3.2 below may be exchanged
during channel initialization (c.f., Section 2.3.1.2 of [BEEP]),
e.g.,
C: <start number='1'>
C: <profile uri='http://iana.org/beep/sacred'>
C: <![CDATA[<DownloadRequest ...>]]>
C: </profile>
C: </start>
S: <profile uri='http://iana.org/beep/sacred'>
S: <![CDATA[<DownloadResponse ...>]]>
S: </profile>
Note that BEEP imposes both encoding and length limitations on the
messages that are piggybacked during channel initialization.
3.2. Profile Exchange
All messages are exchanged as "application/beep+xml" (c.f., Section
6.4 of [BEEP]):
Role MSG RPY ERR
---- --- --- ---
I InfoRequest InfoResponse error
I CreateAccountRequest ok error
I RemoveAccountRequest ok error
I ModifyAccountRequest ok error
I DownloadRequest DownloadResponse error
I UploadRequest ok error
I DeleteRequest Ok error
3.3. Error Handling
The "error" message from Section 2.3.1.5 of [BEEP] is used to convey
error information. Typically, after flagging an error, a peer will
initiate a graceful release of the BEEP session.
The following BEEP error reply codes from [BEEP] are to be used:
code Meaning
==== =======
421 service not available
450 requested action not taken (e.g., lock already in
use)
451 requested action aborted (e.g., local error in
processing)
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RFC 3767 Secure Credentials Protocol June 2004
454 temporary authentication failure
500 general syntax error (e.g., poorly-formed XML)
501 syntax error in parameters (e.g., non-valid XML)
504 parameter not implemented
530 authentication required
534 authentication mechanism insufficient (e.g., too
weak, sequence exhausted, etc.)
535 authentication failure
537 action not authorized for user
538 authentication mechanism requires encryption
550 requested action not taken (e.g., no requested
profiles are acceptable)
553 parameter invalid
554 transaction failed (e.g., policy violation)
The following SACRED-specific error reply codes can also be used:
code Meaning
==== =======
555 Extension (ProcessInfo) used not supported
556 Required extension (ProcessInfo) not present
557 StaleCredential (A bad LastModified value was
contained in an UploadRequest.)
3.4. SASL Authorization Identity
The use of the SASL authorization identity in this protocol is
implementation-specific. If used, the authorization identity is not
a substitute for the credential selector field, but may be used to
affect authorization for access to credentials.
4. IANA Considerations
The IANA has registered the BEEP profile specified in Section 4.
http://iana.org/beep/sacred
The sacred protocol SHOULD be run over port 1118.
The GSSAPI service name (required when using SASL) for this protocol
SHALL be "sacred".
Farrell Standards Track [Page 12]
RFC 3767 Secure Credentials Protocol June 2004
5. Security Considerations
[REQS] calls for specifications to state how they address the
vulnerabilities listed below.
V1. A passive attacker can watch all packets on the network and
later carry out a dictionary attack.
- The use of DIGEST-MD5 and/or TLS counters this
vulnerability.
V2. An attacker can attempt to masquerade as a credential server
in an attempt to get a client to reveal information online
that allows for a later dictionary attack.
- The use of server or mutual authentication counters this
vulnerability.
V3. An attacker can attempt to get a client to decrypt a chosen
"ciphertext" and get the client to make use of the resulting
plaintext - the attacker may then be able to carry out a
dictionary attack (e.g. if the plaintext resulting from
"decryption" of a random string is used as a DSA private
key).
- The use of server or mutual authentication counters this
vulnerability.
V4. An attacker could overwrite a repository entry so that when
a user subsequently uses what they think is a good
credential, they expose information about their password
(and hence the "real" credential).
- Server implementations SHOULD take measures to protect the
database. Clients MAY use the ClientInfo field to store
e.g. a signature over the Credential, which they then verify
before using the private component.
V5. An attacker can copy a credential server's repository and
carry out a dictionary attack.
- Server implementations SHOULD take measures to protect the
database.
V6. An attacker can attempt to masquerade as a client in an
attempt to get a server to reveal information that allows
for a later dictionary attack.
- The mutual authentication requirements of this protocol
counter this to a great extent. Additionally, credential
servers MAY choose to provide mechanisms that protect
against online dictionary attacks against user account
passwords, either by repeated access attempts to a single
user account (varying the password) or by attempting to
access many user accounts using the same password.
V7. An attacker can persuade a server that a successful login
has occurred, even if it hasn't.
- Client authentication prevents this.
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RFC 3767 Secure Credentials Protocol June 2004
V8. (Upload) An attacker can overwrite someone else's
credentials on the server.
- Only if they know the account password already (thanks to
mutual authentication).
V9. (When using password-based authentication) An attacker can
force a password change to a known (or "weak") password.
- Client authentication counters this.
V10. An attacker can attempt a man-in-the-middle attack for lots
of reasons...
- Mutual authentication and the encryption of subsequent
messages prevents this.
V11. User enters password instead of name.
- Since the DIGEST-MD5 mechanism is only used after TLS
tuning, the user's name is also protected.
V12. An attacker could attempt various denial-of-service attacks.
- No specific countermeasures against DoS are proposed.
If the CreateAccountRequest message were sent over a cleartext
channel (or otherwise exposed), then an attacker could mount a
dictionary attack and recover the account password. This is why the
server authenticated TLS transport is REQUIRED for this operation.
If someone steals the server database they can launch a dictionary
attack. If the dictionary attack is successful, the attacker can
decrypt the user's credentials. An attacker that has learned the
user's account password can also upload new credentials, assuming the
user is authorized to modify the credentials, because someone who
knows the user's account password is assumed to be the user.
However, if someone steals the server database and is unsuccessful at
obtaining the user's account password through a dictionary attack,
they will be unable to upload new credentials.
Credential servers SHOULD incorporate measures that act to counter
denial of service attacks. In particular, they SHOULD drop inactive
connections and minimize the use of resources by un-authenticated
connections. A number of recommendations are listed at [DDOS].
Various operations in the SACRED protocol depend upon server
authentication being provided by server authenticated TLS. SACRED
clients SHOULD take care that the correct server is at the far end of
the TLS "pipe" by performing the checks which are listed in section
3.1 of RFC 2818 [RFC2818]. Clients SHOULD also include the optional
BEEP serverName field in their "start" message and SHOULD then ensure
that the BEEP serverName is consistent with the checks on the TLS
server described in RFC 2818. Failure to carry out these checks
could allow a spoof server access to a user's credential.
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RFC 3767 Secure Credentials Protocol June 2004
If the SACRED account password were to be used in some other, less
secure protocol, using DIGEST-MD5, then it might appear to be the
case that a man-in-the-middle (MITM) attack could be mounted.
However, this is not the case since the DIGEST-MD5 client hash
includes a client-selected "digest-uri-value", which in SACRED's case
will be "sacred/<serverName>". In a MITM attack, those values will
be something else. A MITM attack as described is therefore thwarted,
because digest-uri-value wouldn't match what the SACRED server is
expecting.
6. References
6.1. Normative References
[BEEP] Rose, M., "The Blocks Extensible Exchange Protocol
Core", RFC 3080, March 2001.
[DIGEST-MD5] Leach, P. and C. Newman, "Using Digest Authentication as
a SASL Mechanism", RFC 2831, May 2000.
[PKCS15] "PKCS #15 v1.1: Cryptographic Token Information Syntax
Standard," RSA Laboratories, June 2000.
[REQS] Arsenault, A. and S. Farrell, "Securely Available
Credentials - Requirements", RFC 3157, August 2001.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[SASL] Myers, J., "Simple Authentication and Security Layer
(SASL)", RFC 2222, October 1997.
[TLS] Dierks, T. and C. Allen, "The TLS Protocol - Version
1.0", RFC 2246, January 1999.
[TLSAES] Chown, P., "Advanced Encryption Standard (AES)
Ciphersuites for Transport Layer Security (TLS)", RFC
3268, June 2002.
[XMLDSIG] Eastlake, 3rd, D., Reagle, J. and D. Solo, "(Extensible
Mark-Up Language) XML-Signature Syntax and Processing",
RFC 3275, March 2002.
[XMLSCHEMA] "XML Schema Part 1: Structures", D. Beech, M. Maloney,
N. Mendelsohn, and H. Thompson. W3C Recommendation, May
2001. Available at http://www.w3.org/TR/2001/REC-
xmlschema-2-20010502/
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RFC 3767 Secure Credentials Protocol June 2004
6.2. Informative References
[DDOS] "Recommendations for the Protection against Distributed
Denial-of-Service Attacks in the Internet",
http://www.iwar.org.uk/comsec/resources/dos/ddos_en.htm
[RFC2818] Rescorla, E., "HTTP over TLS", RFC 2818, May 2000.
[RFC3760] Gustafson, D., Just, M. and M. Nystrom, "Securely
Available Credentials - Credential Server Framework,"
RFC 3760, April 2004.
[XKMS] Hallam-Baker, P. (ed), "XML Key Management
Specification", http://www.w3.org/TR/xkms2/
[XBULK] Hughes, M (ed), "XML Key Management Specification - Bulk
Operation", http://www.w3.org/TR/xkms2-xbulk/
Acknowledgements
Radia Perlman (radia.perlman@sun.com) and Charlie Kaufman
(charliek@microsoft.com) co-authored earlier versions of this
document. Michael Zolotarev (mzolotar@tpg.com.au) did much of the
initial work, adapting an earlier version to the use of SRP (though
SRP was subsequently dropped, much of the framework survives).
Marshall Rose (mrose@dbc.mtview.ca.us) helped out a lot, in
particular, with the BEEP profile. And the following people were
actively involved in the mailing list discussions leading to this
document:
David Chizmadia,
Dave Crocker (dcrocker@brandenburg.com),
Lawrence Greenfield (leg+@andrew.cmu.edu),
Dale Gustafson (degustafson@comcast.net),
Mike Just (just.mike@tbs-sct.gc.ca),
John Linn (jlinn@rsasecurity.com),
Neal McBurnett (neal@bcn.boulder.co.us),
Keith Moore (moore@cs.utk.edu),
RL "Bob" Morgan (rlmorgan@washington.edu),
Magnus Nystrom (magnus@rsasecurity.com),
Eamon O'Tuathail (eamon.otuathail@clipcode.com),
Gareth Richards (grichards@rsasecurity.com)
Of course, any and all errors remain the editor's responsibility.
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RFC 3767 Secure Credentials Protocol June 2004
Appendix A: XML Schema
<?xml version="1.0" encoding="UTF-8"?>
<schema
targetNamespace="urn:sacred-2002-12-19"
xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
xmlns:sacred="urn:sacred-2002-12-19"
xmlns="http://www.w3.org/2001/XMLSchema">
<import namespace="http://www.w3.org/2000/09/xmldsig#"
schemaLocation=
"http://www.w3.org/TR/xmldsig-core/xmldsig-core-schema.xsd"/>
<!-- extensibility holes -->
<complexType name="ProcessInfoType">
<sequence maxOccurs="unbounded">
<any namespace="##other"/>
</sequence>
</complexType>
<element name="ProcessInfo" type="sacred:ProcessInfoType"/>
<complexType name="ClientInfoType">
<sequence maxOccurs="unbounded">
<any namespace="##other"/>
</sequence>
</complexType>
<element name="ClientInfo" type="sacred:ClientInfoType"/>
<!-- Where to put authenentication information -->
<complexType name="AuthInfoType">
<choice maxOccurs="unbounded">
<element name="DigestMD5AuthInfo">
<complexType>
<sequence>
<element name="PasswordVerifier" type="base64Binary"/>
<element name="Realm" type="string" />
</sequence>
</complexType>
</element>
<any namespace="##other"/>
</choice>
</complexType>
<element name="AuthInfo" type="sacred:AuthInfoType"/>
<!-- authentication mechanism parameters -->
<complexType name="AuthParamsType">
<choice maxOccurs="unbounded">
<element name=" DigestMD5AuthParams">
<complexType>
<sequence>
<element name="Realm" type="string"
minOccurs="1" maxOccurs="unbounded"/>
</sequence>
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</complexType>
</element>
<any namespace="##other"/>
</choice>
</complexType>
<element name="AuthParams" type="sacred:AuthParamsType"/>
<!-- Protocol messsages -->
<!-- "account handling" operations -->
<!-- Information request -->
<element name="InfoRequest"/>
<element name="InfoResponse">
<complexType>
<sequence>
<element name="Status" type="string" minOccurs="0"/>
<element name="ServerId" type="string"/>
<element ref="sacred:AuthParams"/>
<element ref="sacred:ProcessInfo" minOccurs="0"/>
</sequence>
</complexType>
</element>
<!-- Create Account Request -->
<element name="CreateAccountRequest">
<complexType>
<sequence>
<element name="UserId" type="string"/>
<element ref="sacred:AuthInfo"/>
<element ref="sacred:ProcessInfo" minOccurs="0"/>
</sequence>
</complexType>
</element>
<!-- remove account request -->
<element name="RemoveAccountRequest">
<complexType>
<sequence>
<element ref="sacred:ProcessInfo" minOccurs="0"/>
</sequence>
</complexType>
</element>
<!-- password change request -->
<element name="ModifyAccountRequest">
<complexType>
<sequence>
<element ref="sacred:AuthInfo"/>
<element ref="sacred:ProcessInfo" minOccurs="0"/>
</sequence>
</complexType>
</element>
<!-- "run-time" operations -->
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<!-- DownLoad Request -->
<element name="DownloadRequest">
<complexType>
<sequence>
<element name="CredentialSelector" type="string"
minOccurs="0"/>
<element ref="sacred:ProcessInfo" minOccurs="0"/>
</sequence>
</complexType>
</element>
<!-- Download Response -->
<element name="DownloadResponse">
<complexType>
<sequence>
<element name="Status" type="string" minOccurs="0"/>
<element name="Credential" type="sacred:CredentialType"
maxOccurs="unbounded"/>
</sequence>
</complexType>
</element>
<!-- Upload request -->
<element name="UploadRequest">
<complexType>
<sequence>
<element name="Credential" type="sacred:CredentialType"/>
</sequence>
</complexType>
</element>
<element name="DeleteRequest">
<complexType>
<sequence>
<choice>
<sequence>
<element name="CredentialSelector" type="string"/>
<element name="LastModified" type="dateTime"
minOccurs="0"/>
</sequence>
<element name="All"/>
</choice>
<element ref="sacred:ProcessInfo" minOccurs="0"/>
</sequence>
</complexType>
</element>
<!-- Credential related structures -->
<!-- A new ds:KeyInfo thing -->
<element name="SacredPKCS15" type="base64Binary"/>
<!-- credential -->
<complexType name="CredentialType">
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<sequence>
<element name="CredentialSelector" type="string"/>
<element name="LastModified" type="dateTime"/>
<element name="Payload" type="ds:KeyInfoType" minOccurs="0"/>
<element name="TimeToLive" type="string" minOccurs="0"/>
<element ref="sacred:ProcessInfo" minOccurs="0"/>
<element ref="sacred:ClientInfo" minOccurs="0"/>
</sequence>
</complexType>
</schema>
Appendix B: An Example of Tuning with BEEP
Here is what tuning BEEP for authentication and confidentiality
looks like using TLS and SASL's DIGEST-MD5:
L: <wait for incoming connection>
I: <open connection>
... each peer sends a greeting indicating the services that
it offers ...
L: RPY 0 0 . 0 233
L: Content-Type: application/beep+xml
L:
L: <greeting>
L: <profile uri='http://iana.org/beep/SASL/DIGEST-MD5' />
L: <profile uri='http://iana.org/beep/TLS' />
L: <profile uri='http://iana.org/beep/sacred' />
L: </greeting>
L: END
I: RPY 0 0 . 0 52
I: Content-Type: application/beep+xml
I:
I: <greeting />
I: END
... the initiator starts a channel for TLS and piggybacks a request
to start the TLS negotiation ...
I: MSG 0 1 . 52 149
I: Content-Type: application/beep+xml
I:
I: <start number='1' serverName="sacred.example.org">
I: <profile uri='http://iana.org/beep/TLS'>
I: <ready />
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I: </profile>
I: </start>
I: END
... the listener creates the channel and piggybacks its readiness to
start TLS ...
L: RPY 0 1 . 233 112
L: Content-Type: application/beep+xml
L:
L: <profile uri='http://iana.org/beep/TLS'>
L: <proceed />
L: </profile>
L: END
... upon receiving the reply, the initiator starts up TLS ...
... successful transport security negotiation ...
... a new greeting is sent (cf., Section 9 of RFC 3080), note that
the listener no longer advertises TLS (we're already running
it)
L: RPY 0 0 . 0 186
L: Content-Type: application/beep+xml
L:
L: <greeting>
L: <profile uri='http://iana.org/beep/SASL/DIGEST-MD5' />
L: <profile uri='http://iana.org/beep/sacred' />
L: </greeting>
L: END
I: RPY 0 0 . 0 52
I: Content-Type: application/beep+xml
I:
I: <greeting />
I: END
... the initiator starts a channel for DIGEST-MD5 and piggybacks
initialization information for the mechanism ...
I: MSG 0 1 . 52 178
I: Content-Type: application/beep+xml
I:
I: <start number='1'>
I: <profile uri='http://iana.org/beep/SASL/DIGEST-MD5'>
I: <blob> ... </blob>
I: </profile>
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I: </start>
I: END
... the listener creates the channel and piggybacks a challenge ...
L: RPY 0 1 . 186 137
L: Content-Type: application/beep+xml
L:
L: <profile uri='http://iana.org/beep/SASL/DIGEST-MD5'>
L: <blob> ... </blob>
L: </profile>
L: END
... the initiator sends a response to the challenge ...
I: MSG 1 0 . 0 58
I: Content-Type: application/beep+xml
I:
I: <blob> ... </blob>
I: END
... the listener accepts the challenge and tells the initiator
that it is now authenticated ...
L: RPY 1 0 . 0 66
L: Content-Type: application/beep+xml
L:
L: <blob status='complete' />
L: END
... the initiator starts a channel for SACRED and piggybacks its
initial SACRED request ...
I: MSG 0 2 . 230 520
I: Content-Type: application/beep+xml
I:
I: <start number='3'>
I: <profile uri='http://iana.org/beep/sacred' />
I: <?xml version="1.0" encoding="UTF-8"?>
I: <sacred:DownloadRequest
I: xmlns:sacred="urn:sacred-2002-12-19"
I: xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
I: xsi:schemaLocation="urn:sacred-2002-12-19 sacred.xsd">
I: <CredentialSelector>
I: magnus-credentials</CredentialSelector>
I: </sacred:DownloadRequest>
I: </start>
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I: END
... the listener creates the channel and piggybacks the response to
the initial SACRED request
L: RPY 0 2 . 323 805
L: Content-Type: application/beep+xml
L:
L: <profile uri='http://iana.org/beep/sacred' />
L: <?xml version="1.0" encoding="UTF-8"?>
L: <sacred:DownloadResponse
L: xmlns:sacred="urn:sacred-2002-12-19"
L: xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
L: xsi:schemaLocation="urn:sacred-2002-12-19 sacred.xsd">
L: <Status>Success</Status>
L: <Credential>
L: <CredentialSelector>
L: magnus-credential</CredentialSelector>
L: <LastModified>2002-11-22T00:00:08Z</LastModified>
L: <Payload>
L: <sacred:SacredPKCS15
L: xmlns:sacred="urn:sacred-2002-12-19">GpM7
L: </sacred:SacredPKCS15>
L: </Payload>
L: </Credential>
L: </sacred:DownloadResponse>
L: </profile>
L: END
Appendix C: Provision SACRED using other Protocols
SACRED may be implemented in a non-BEEP environment, provided that
before any SACRED PDUs are sent, the application protocol must be
protected according to the security mandates provided in Section 2.3.
For example, if SACRED is provisioned as the payload of an
application protocol that supports SASL and TLS, then the appropriate
SASL and/or TLS negotiation must successfully occur before exchanging
Sacred PDUs.
Alternatively, if the application protocol doesn't support SASL, then
one or more PDUs are defined to facilitate a SASL negotiation, and
the appropriate negotiation must occur before exchanging Sacred PDUs.
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RFC 3767 Secure Credentials Protocol June 2004
Editor's Address
Stephen Farrell,
Distributed Systems Group,
Computer Science Department,
Trinity College Dublin,
IRELAND
Phone: +353-1-608-3070
EMail: stephen.farrell@cs.tcd.ie
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RFC 3767 Secure Credentials Protocol June 2004
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