One document matched: draft-ietf-sacred-framework-05.txt
Differences from draft-ietf-sacred-framework-04.txt
D. Gustafson
Future Foundation
M. Just
Entrust
Internet Draft M. Nystrom
Document: draft-ietf-sacred-framework-05.txt RSA Security
Expires: March 2003 September 2002
Securely Available Credentials - Credential Server Framework
Status of this Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026 [RFC2026].
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-
Drafts as reference material or to cite them other than as "work
in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
As the number, and more particularly the number of different
types, of devices connecting to the Internet increases,
credential mobility becomes an issue for IETF standardization.
This document responds to the credential server framework
requirements listed in [RFC3157]. It presents a strawman
framework and outlines protocols for securely available
credentials.
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].
Please send comments on this document to the ietf-sacred@imc.org
mailing list.
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Table of Contents
STATUS OF THIS MEMO...............................................1
ABSTRACT..........................................................1
1 INTRODUCTION....................................................3
2 FUNCTIONAL OVERVIEW.............................................3
2.1 DEFINITIONS.................................................3
2.2 CREDENTIALS.................................................5
2.3 NETWORK ARCHITECTURE........................................6
3 PROTOCOL FRAMEWORK..............................................7
3.1 CREDENTIAL UPLOAD...........................................9
3.2 CREDENTIAL DOWNLOAD........................................10
3.3 CREDENTIAL REMOVAL.........................................12
3.4 CREDENTIAL MANAGEMENT......................................12
4 PROTOCOL CONSIDERATIONS........................................13
4.1 SECURE CREDENTIAL FORMATS..................................13
4.2 AUTHENTICATION METHODS.....................................13
4.3 TRANSPORT PROTOCOL SUITES..................................16
5 SECURITY CONSIDERATIONS........................................17
5.1 COMMUNICATIONS SECURITY....................................17
5.2 SYSTEMS SECURITY...........................................18
6. REFERENCES....................................................19
6.1 NORMATIVE REFERENCES.......................................19
6.2 INFORMATIVE REFERENCES.....................................20
7 AUTHOR'S ADDRESSES.............................................21
FULL COPYRIGHT STATEMENT.........................................22
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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 proposes a general framework for secure exchange of
such credentials and provides a high level outline that will help
guide the development of one or more SACRED credential exchange
protocols.
2 Functional Overview
Requirements for Securely Available Credentials are fully
described in [RFC3157]. These requirements assume that two
distinctly different network architectures will be created to
support credential exchange for roaming users:
a) Client/Server Credential Exchange
b) Peer-to-Peer Credential Exchange
This document describes the framework for one or more
client/server credential exchange protocols.
In all cases, adequate user authentication methods will be used
to ensure credentials are not divulged to unauthorized parties.
As well, adequate server authentication methods will be used to
ensure that each client’s authentication information (see Section
2.1) is not compromised, and to ensure that roaming users
interact with intended/authorized credential servers.
2.1 Definitions
This section provides definitions for several terms or phrases
used throughout this document.
client authentication information: information that is presented
by the client to a server to authenticate the client.
This may include a password token, a registration string
that may have been received out-of-band (and possibly
used for initially registering a roaming user) or data
signed with a signature key belonging to the client (e.g.
as part of TLS [RFC2246] client authentication).
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credentials: cryptographic objects and related data used to
support secure communications over the Internet.
Credentials may consist of public/private key pairs,
symmetric keys, X.509 public key certificates, attribute
certificates, and/or application data. Several
standardized formats for the representation of
credentials exist, e.g. [PKCS12], [PKCS15] (see "secured
credentials" below).
passkey: a symmetric key, derived from a password.
password: a string of characters known only to a client and used
for the purposes of authenticating to a server and/or
securing credentials. A user may be required to remember
more than one password.
password token: a value derived from a password using a one-way
function that may be used by a client to authenticate to
a server. A password token may be derived from a password
using a one-way hash function, for example.
secured credentials: a set of one or more credentials that have
been cryptographically secured, e.g. encrypted/MACed with
a passkey. Secured credentials may be protected using
more than one layer of encryption, e.g. the credential is
secured with a passkey corresponding to a user's password
and also by a key known only to the server (the
credential's stored form). During network transfer, the
passkey-protected credential may be protected with an
additional encryption layer using a symmetric key chosen
by the Credential Server (e.g., the transmitted form).
strong password protocol: a protocol that authenticates clients
to servers securely (see e.g. [SPEKE] for a more detailed
definition of this), where the client need only memorize
a small secret (a password) and carries no other secret
information, and where the server carries a verifier
(password token) which allows it to authenticate the
client. A shared secret is negotiated between client and
server and is used to protect data subsequently
exchanged.
Note the distinction between an "account password" and a
"credential password." An account password (and corresponding
password token) is used to authenticate to a Credential Server
and to negotiate a key that provides session level encryption
between client and server.
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A credential password is used to derive a passkey that’s used to
provide persistent encryption and authentication for a stored
credential. Applicable secured credential standards documents
(e.g. [PKCS#15]) describe the technical details of specific
password-based-encryption (pbe) techniques that are used to
protect credentials from unauthorized use.
Although the same password value may be used to provide both
services, it is likely that different, algorithm specific
passkeys would be generated from this password (i.e. because of
different salt values, etc.).
In addition, although it may be more convenient for a user to
remember only a single password, differing security policies
(e.g. password rules) between the credential server and the
credential issuers may result in a user having to remember
multiple passwords.
2.2 Credentials
This document is concerned with the secure exchange and online
management of credentials in a roaming or mobile environment.
Credentials MAY be usable with any end user device that can
connect to the Internet, such as:
- desktop or laptop PC
- mobile phone
- personal digital assistant (PDA)
- etc.
The end user system may, optionally, store its credential
information on special hardware devices that provide enhanced
portability and protection for user credentials.
Since the credential usually contains sensitive information that
is known only to the credential holder, credentials MUST NOT be
sent in the clear during network transmission and SHOULD NOT be
in the clear when stored on an end user device such as a diskette
or hard drive. For this reason, a secured credential is defined.
Throughout this document we assume that, at least from the point
of view of the protocol, a secured credential is an opaque (and
at least partially privacy and integrity protected) data object
that can be used by a network connected device. Once downloaded,
clients must be able to recover their credentials from this
opaque format.
At a minimum, all supported credential formats SHOULD provide
privacy and integrity protection for private keys, secret keys,
and any other data objects that must be protected from disclosure
or modification. Typically, these security capabilities are part
of the basic credential format such that the credential (e.g., a
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data file) is protected when stored on hard drives, flexible
diskettes, etc.
During network transmission, the secured credential is protected
with a second (outer) encryption layer. The outer encryption
layer is created using a session-level encryption key that was
derived during the mutual authentication process. Effectively,
secured credentials traverse an "encrypted tunnel" that provides
an additional layer of privacy protection for credentials (and
any other) information exchanged.
2.3 Network Architecture
The network diagram below shows the components involved in the
SACRED client/server framework.
+--------+ +------------+
| Client +-----------| Credential |
+--------+ 1 | Server |
\ +-----+------+
\ |
\ | 2
\ |
\ 3 +-----+------+
-----------| Credential |
| Store(s) |
+------------+
Client - The entity that wants to retrieve their credentials from
a credential server.
Credential Server - The server that downloads secure credentials
to and uploads them from the client. The server is
responsible for authenticating the client to ensure
that the secured credentials are exchanged only with an
appropriate end user. The credential server is
authenticated to the client to ensure that the client's
authentication information is not compromised and so
that the user can trust the credentials retrieved.
Credential Store - The repository for secured credentials. There
might be access control features but those generally
aren't sufficient in themselves for securing
credentials. The credential server may be capable of
splitting credentials across multiple credential stores
for redundancy or to provide additional levels of
protection for user credentials.
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Protocol 1 - The protocol used to authenticate the client and
credential server, and download and upload user
credentials from a credential server.
Protocol 2 - The protocol used by the Credential Server to store
and retrieve user credentials (LDAP, LDAP/SSL, or
other).
Protocol 3 - The protocol used by the client to store and
retrieve user credentials from the credential store
(LDAP, LDAP/SSL, or other).
This framework describes the high level design for protocol 1.
Protocols 2 and 3 are closely related (but out of scope for this
document) and could be implemented using standard protocols, such
as LDAP or secure LDAP, or other standard or proprietary
protocols. Note also that any administrator-credential server
protocols are assumed to be server vendor specific and are not
the subject of SACRED standardization efforts at this time.
Clients are not precluded from exchanging credentials directly
with a credential store (or any other server of it’s choosing).
However, mutual authentication with roaming users and a
consistent level of protection for credential data while stored
on network servers and while in transit is provided by SACRED
protocols exchanged with the credential server. Depending on
credential server design, user credentials may flow through the
credential server to the credential store or directly between the
client and the credential store.
Also, users may upload their credentials to several credential
servers to obtain enhanced levels of availability. Coordination
(automatic replication) of user information or credential data
among several credential servers is currently beyond the scope of
this document.
3 Protocol Framework
This section provides a high level description of client/server
protocols that can be used to exchange and manage SACRED
credentials.
The client/server credential exchange protocol is based on three
basic and abstract operations; "GET", "PUT", and "DELETE". The
secured credential exchange protocol is accomplished as follows:
connect - the client initiates a connection to a credential
server for the purpose of secure credential
exchange.
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mutual authentication/key negotiation - using a strong
password protocol (or equivalent) the client
authenticates to the server, the server
authenticates to the client, and a session level
encryption key is negotiated. The details of the
mutual authentication protocol exchange are
dependent upon the particular authentication method
used. In all cases, the end result is to
authenticate the client to the server and server to
the client, and establish a strong, shared secret
between the two parties.
client request(s) - the SACRED client issues one or more
high level credential exchange requests (e.g., GET,
PUT, or DELETE).
server response(s) - the SACRED credential server responds
to each request, either performing the operation
successfully or indicating an appropriate error.
close - the client indicates it has no more requests for the
server at this time. The security context between
client and server is no longer needed. Close is a
logical, session management operation.
disconnect - the parties disconnect the transport level
connection between client and server. Note that
"connect" and "disconnect" are logical, transport-
layer dependent operations that enclose the protocol
exchange between the two communicating processes.
Each high-level credential exchange operation is made up of
a series of request-response pairs. The client initiates
each request, which the server processes before returning an
appropriate response. Each request must complete (server
reports success or failure) before the client issues the
next request. The server SHOULD be willing to service at
least one upload or download request following successful
mutual authentication but either party can terminate the
logical connection at any time.
In the following sections, secured credentials and related values
are represented using the following notation:
SC-x is the secured credential file, which includes a format
identifier field and credential data. The
credential data is an opaque, encrypted data object
(e.g. PKCS#15 or PKCS#12 file). The format
identifier is needed to correctly parse the
credential data.
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Name-x is an account-defined selector or locator (a user
friendly name) that is used to indicate a specific
secured credential. The name of each credential
stored under a given user account MUST be unique
e.g. there may be one credential called "financial"
and another called "healthcare", etc. At a minimum,
credential names MUST be unique across a given
account/user name. When no name is supplied for a
GET operation, all credentials stored for the given
username will be returned.
ID-x is a distinct credential version indicator that MAY be
used to request a conditional GET/PUT/DELETE
operation. This credential-ID value SHOULD contain
the server’s "last-modified" date and time (e.g. the
time that this particular credential version was
stored on the server) and MAY contain additional
information such as a sequence number or a (complete
or partial) credential fingerprint that is used to
ensure the credential-ID is unique from other
credential versions stored under the same user
account and credential name.
All named credentials may be accessed by authenticating under a
single username. If a user needs or prefers to use more than one
distinct authentication password (and/or authentication method)
to protect access to several secured credentials, he/she SHOULD
register those credentials under distinct user/account names, one
for each different authentication method used.
3.1 Credential Upload
The purpose of a credential upload operation is to allow a client
to register new credentials, or replace currently stored
credentials (e.g. credentials that may have been updated by the
client using appropriate key management software).
The framework for the credential upload, as implemented using the
PUT operation, is:
. The client and server establish a mutually authenticated
session and negotiate a shared secret.
. The client will then issue a PUT message that contains the
upload credential and related data fields.
. The server will respond to the PUT, indicating the credential
was successfully stored on the server or that an error
occurred.
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The client’s PUT request MAY contain an optional identifier
(credential-ID) field. If present, the new credential will only
be stored if a credential with the same name and credential-ID is
currently stored on the server (e.g. a logical REPLACE operation
is performed). The server MUST return an error if a client
attempts to replace a credential that does not exist on the
server.
The credential server’s response to a PUT request MUST contain a
credential version identifier (credential-ID) for the newly
stored credential that MAY be used by clients to optimize
subsequent download operations and avoid credential version
mismatches.
3.1.1 Credential Upload Protocol Sequence
The following gives an example of a "credential upload" protocol
sequence:
client server
------- -------
< connect > -->
<--- mutual authentication --->
< PUT SC-1, Name-1, [ID-1] > -->
<-- < Name-1, new-ID-1 >
< PUT SC-2, Name-2, [ID-2] > -->
<-- < Name-2, new-ID-2 >
...
< close > -->
<-- OK (+ disconnect)
new-ID-x is the credential-ID of the newly stored credential.
3.2 Credential Download
Roaming clients can download their credentials at any time after
they have been uploaded to the server.
The framework for a credential download, as implemented using the
GET operation, is:
. The client SHOULD authenticate the server.
. The user MUST be authenticated (by the server).
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. A GET request for the credential download is issued.
. The response contains the credential and format identifier.
The specific user credential being requested may be identified by
name in the message sent to the credential server. If
successful, the response MUST contain the requested credential
data element (format ID and data) as defined above.
If the user issues a GET request with a NULL credential name
field, the server SHOULD return all credentials stored under the
current user account.
Optionally, the client MAY include a credential-ID to indicate a
conditional download request. In this case, the server will
return the requested credential if and only if the ID of the
credential currently stored on the server does NOT match the ID
specified.
The server should return either the requested credential or a
distinct response indicating that the conditional download was
not performed (e.g., the client already has a copy of this exact
credential). In addition, to the credential, the server returns the
credential-ID for the client to use in later PUT requests.
3.2.1 Credential Download Protocol Sequence
The following gives an example of a "credential download"
protocol sequence:
client server
------- --------
< connect > -->
<--- mutual authentication -->
< GET Name-1, [ID-1] > -->
<-- < SC-1, ID-1'>
< GET Name-2, [ID-2] > -->
<-- < GET response >
...
< close > -->
<-- OK (+ disconnect)
Notice that for the second request, no credential has been
returned since ID-2, as included in the client’s request, matched
the identifier for the Name-2 credential.
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3.3 Credential Removal
The framework for the credential removal, as implemented with the
DELETE operation, is:
. The credential server MUST be authenticated (by the client)
using a method-dependent protocol sequence.
. The user MUST be authenticated (by the server) using a method-
dependent protocol sequence.
. The user then sends a DELETE request message that contains the
credential name indicating which credential to remove.
. Optionally, the client may include a credential-ID in the
DELETE request. In this case, the credential will be deleted if
the request ID matches the ID of the credential currently
stored on the server. This may be done to ensure that a client
intending to delete their stored credential does not mistakenly
delete a different version of the credential.
3.3.1 Credential Removal Protocol Sequence
The following gives an example of a "credential removal" protocol
sequence:
client server
------- --------
< connect > -->
<-------- mutual authentication -------->
< DEL Name-1, [ID1] > -->
<-- < Name-1 deleted >
< DEL Name-2, [ID2] > -->
<-- < Name-2 deleted >
...
< close > -->
<-- OK (+ disconnect)
3.4 Credential Management
Note that the three operations defined above (GET, PUT, DELETE)
can be used to perform the basic credential management
operations:
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- add a new credential on the server,
- update (replace) an existing credential, and
- delete an existing credential.
The information provided for these basic operations might be used
to help guide the design of more complex operations such as user
registration (add account), user deregistration (remove account),
change account password, or list all credentials.
Note that, in the case where a credential with the same name
exists on the server, uploading a NULL credential is logically
equivalent to removing a previously stored credential.
4 Protocol Considerations
4.1 Secure Credential Formats
To ensure that credentials created on, and uploaded from, one
device can be downloaded and used on any other device, there is a
need to define a single "mandatory to implement" credential
format that must be supported by all conforming client
implementations.
At least two well-defined credential formats are available today:
[PKCS12] and [PKCS15].
Other optional credential formats may also be supported if
necessary. For example, additional credential formats might be
defined for use with specific (compatible) client devices. Each
credential format MUST provide adequate privacy protection for
user credentials when they are stored on flexible diskettes, hard
disks, etc.
Throughout this document, the credential is treated as an opaque
(encrypted) data object and, as such, the credential format does
not affect the basic credential exchange protocol.
4.2 Authentication Methods
Authentication is vitally important to ensure that credentials
are accepted from and delivered to the authorized end user only.
If an unsecured credential is delivered to some other party, the
credential may be more easily compromised. If a credential is
accepted from an unauthorized party, the user might be tricked
into using a credential that has been substituted by an attacker
(e.g. an attacker might replace a newer credential with an older
credential belonging to the same user).
Ideally, the list of authentication methods should be open ended,
allowing new methods to be added as needs are identified and as
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they become available. For all credentials, the user
authentication method and data is defined when a user is first
registered with the credential server and may be updated from
time to time thereafter by the authorized user.
To adequately protect user credentials from unauthorized
disclosure or modification in a roaming environment, all SACRED
authentication methods MUST provide protection for user
credentials in network environments where attackers might attempt
to exploit potential security vulnerabilities. See SACRED
Requirements [RFC3157], Section 3.1, Vulnerabilities.
At a minimum, each SACRED authentication method SHOULD ensure
that:
. The server authenticates the client
. The client authenticates the server
. The client and server securely negotiate (or derive) a
cryptographically strong, secret key (e.g., a session
key).
. The exchange of one or more user credentials is protected
using this session key.
It is expected that all SACRED client/server protocols will
provide each of these basic security functions. Some existing
authentication protocols that might be used for this purpose
include:
- Strong password protocols
- TLS
Sections 4.2.1 and 4.2.2 provide some guidance about when to use
these authentication methods based on the generic security
capabilities they provide and the security elements (passwords,
key pairs, user certificates, CA certificates) that must be
available to the SACRED client.
4.2.1 Strong Password Protocols
Strong password protocols such as those described in [RFC2945],
[BM92], [BM94], and [SPEKE] MAY be used to provide mutual
authentication and privacy for SACRED protocols.
All strong password protocols require that user-specific values
(i.e. a passtoken and related values) be configured within the
server. The verifier value can only be calculated by a party who
knows the password. It must be securely delivered to the server
at a time when the client establishes a relationship with the
server. At connect time, messages are exchanged between the two
parties and complementary algorithms are used to compute a shared
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common value known only to the legitimate user and the server.
Both parties derive a strong (symmetric) key that may be used to
secure communications between the two parties.
4.2.2 TLS Authentication
TLS authentication may either be mutual between the client and
server or unilateral where only the server is authenticated to
the client. These options are described in the next two
subsections.
In both cases, TLS can be used to authenticate the server
whenever the TLS client has been pre-configured with the
necessary certificates needed to validate the server’s
certificate chain (including revocation status checking).
TLS Server Authentication (sTLS)
TLS provides a basic secure session capability (sometimes called
server-side TLS) whereby the client authenticates the server and
a pair of session level encryption keys is securely exchanged
between client and server. Following server authentication and
security context setup, all client requests and server responses
exchanged are integrity and privacy protected.
When necessary, and after a TLS session has been established
between the two parties, the credential server can request that
the client provide her user id and password information to
authenticate the remote user. Preferably, client and server can
cooperate to perform an authentication operation that allows the
server to authenticate the client (and perhaps vice-versa) in a
"zero knowledge manner". In such cases, the client need not have
a security credential.
TLS with Client Authentication (cTLS)
TLS provides an optional, secure session capability (sometimes
called client-side TLS) whereby the TLS server can request client
authentication by verifying the client’s digital signature.
In order to use cTLS to provide mutual authentication, the client
must also be configured with at least one security credential
that is acceptable to the TLS server for remote client
authentication purposes.
4.2.3 Other Authentication Methods
Other authentication methods that provide necessary security
capabilities MAY also be suitable for use with SACRED credential
exchange protocols.
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4.3 Transport Protocol Suites
It is intended that one or more underlying protocol stacks may
carry the SACRED credential exchange protocols. It is recognized
at the outset that the use of several underlying protocol suites,
although not ideal from an interoperability standpoint, may well
be required to support the wide variety of needs anticipated.
The SACRED list members have discussed several protocol suites
that have been considered on their technical merits, each with
distinct benefits and protocol design/implementation costs. Among
these protocols are:
. TCP
. BEEP
. HTTP
All protocol suites listed here depend on TCP to provide a
reliable, end-to-end transport layer protocol. Each of these
building block approaches provides a different way of handling
the remaining application layer issues (basic session management,
session level security, presentation/formatting, application
functionality).
4.3.1 TCP
This approach (layering a SACRED credential exchange protocol
directly on top of a TCP connection) requires the development of
a custom credential exchange messaging protocol that interfaces
to a TCP connection/socket. The primary benefit of this approach
is the ability to provide exactly the protocol functionality
needed and no more. Most server and client development
environments already provide the socket level API needed.
4.3.2 BEEP
This approach builds on the Blocks Extensible Exchange Protocol
(BEEP) described in [RFC3080]. BEEP provides general purpose,
peer-to-peer message exchange over any of several transport
mechanisms where the necessary transport layer mappings have been
defined for operation over TCP, TLS, etc. See also [RFC3081].
BEEP provides the necessary user authentication/session security
and session management capabilities needed to support SACRED
credential exchange operations.
4.3.3 HTTP
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This approach builds on the Hypertext Transport Protocol (HTTP)
described in [RFC1945] and [RFC2616]. HTTP provides general
purpose typing and negotiation of data representation, allowing
systems to be built independently of the data objects being
transferred. HTTP support is available in a wide variety of
server and client platforms, including portable devices that
apply to roaming environments (laptop PCs, PDAs, mobile phones,
etc.).
HTTP is layered over TCP and can be used, optionally, with TLS to
provide authenticated, session level security. Either or both
TLS authentication options, sTLS or cTLS, may be used whenever
TLS is supported.
5 Security Considerations
The following security considerations identify general
observations and precautions to be considered for a framework
supporting credential mobility. When designing or implementing a
protocol to support this framework, one should recognize these
security considerations, and furthermore consult the SACRED
Requirements document [RFC3157] Security Considerations.
5.1 Communications Security
A SACRED PDU will contain information pertaining to client or
server authentication, or communication of credentials.
This information is subject to the traditional security concerns
identified below.
5.1.1 Confidentiality
The password or password verifier should be protected when
communicated from the client to credential server. The
communicated value should be resistant to a dictionary attack.
Similarly, the entity credentials must be confidentiality
protected, when communicated from the client to the server and
vice-versa. The communicated value should also resist a
dictionary attack.
5.1.2 Integrity
Communication integrity between the client and the credential
server is required. In this way, intended client operations may
not be altered (e.g. from an update to a deletion of
credentials), nor may clients be maliciously given "old"
credentials (e.g. possibly by an attacker replaying a previous
credential download).
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5.1.3 Entity Authentication
Proper authentication of the client and server is required to
achieve communication confidentiality and integrity.
The server must properly authenticate the client, so that
credentials are not mistakenly revealed to an attacker.
The client must ensure the proper identification of the
credential server so as to prevent revealing their password to an
attacker. These goals may be achieved implicitly with a strong
password-based protocol or explicitly. If the server is
identified explicitly, the user or client must ensure that the
user password is conveyed to a trusted server. This might be
achieved by installing appropriate trusted key(s) in the client.
5.1.4 Non-repudiation
Although credential confidentiality is one of many factors
required to support non-repudiation, there are no requirements
upon the SACRED protocol itself to support non-repudiation.
5.2 Systems Security
Systems security is concerned with protection of the protocol
endpoints (i.e. the client and server) and information
stored at the server in support of the SACRED protocol.
5.2.1 Client Security
As with most security protocols, secure use of the client often
relies, in part, upon secure behavior by the user. In the
case of a password-based SACRED protocol, users should be
educated, or enforced through policy, to choose passwords with a
reasonable amount of entropy. Additionally, users should be made
aware of the importance of protecting the confidentiality of
their account password.
In addition, the client interface should be designed to thwart
"shoulder surfing" where an attacker can observe the password as
entered by a user. This is often achieved by not echoing the
exact characters of the password when entered.
As well, the interface should encourage the entering of the
password in the appropriate interface field so that protections
can be properly enforced. For example, a user should be guided to
not mistakenly enter their password in the "username" field
(since their password would likely be echoed to the screen in
this case, and might not be encrypted when communicated to the
server). This might be accomplished via the automatic insertion
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of the user name or several user name choices in the appropriate
on-screen dialog field, for example.
5.2.2 Server Security
Password verifiers and user credentials must be afforded a high
level of protection at the credential server. In addition to
salting and super-encrypting each (to ensure resistance to
offline dictionary attacks), a system should ensure that
credential server keys are protected using sufficient procedural
and physical access controls.
The login to the credential server should be resistant to replay
attacks.
Online attempts to access a particular user account should be
controlled, or at least monitored. Control might be enforced by
incorporating a time delay after a number of unsuccessful logins
to a particular account, or possibly the locking of the account
all together. Alternatively, one might simply log unsuccessful
attempts where an administrative notice is produced once a
threshold of unsuccessful credential access attempts is reached.
5.2.3 DoS
As with most protocols, Denial of Service (DoS) issues must also
be considered. In the case of SACRED, most DoS issues are a
concern for the underlying transport protocol. However, some
concerns may still be mitigated.
Service to a user might be denied in case their account is locked
after numerous unsuccessful login attempts. Consideration of
protection against online attacks must therefore be considered
(as described above). Proper user authentication should ensure
that an attacker does not maliciously overwrite a user's
credentials. Credential servers should be wary of repeated logins
to a particular account (which also identifies a possible
security breach, as described above) or abnormal volumes of
requests to a number of accounts (possibly identifying a DoS
attack).
6. References
6.1 Normative references
[RFC2026] Bradner, S., "The Internet Standards Process - Revision
3", BCP 9, RFC 2026, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
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Credential Server Framework
[RFC3157] Arsenault, A., Farrell, S., "Securely Available
Credentials - Requirements", RFC 3157, August 2001.
6.2 Informative references
[BM92] S. Bellovin and M. Merritt, "Encrypted Key Exchange:
Password-based protocols secure against dictionary
attacks", Proceedings of the IEEE Symposium on Research
in Security and Privacy, May 1992.
[BM94] S. Bellovin and M. Merritt, "Augmented Encrypted Key
Exchange: a Password-Based Protocol Secure Against
Dictionary Attacks and Password File Compromise, ATT
Labs Technical Report, 1994.
[PKCS12] "PKCS 12 v1.0: Personal Information Exchange Syntax",
RSA Laboratories, June 24, 1999
[PKCS15] "PKCS #15 v1.1: Cryptographic Token Information Syntax
Standard", RSA Laboratories, June 2000.
[RFC1945] Berners-Lee, T., Fielding, R. and H. Frystyk,
"Hypertext Transfer Protocol-- HTTP/1.0", RFC 1945, May
1996.
[RFC2246] Dierks, T., Allen, C., "The TLS Protocol Version 1.0,"
RFC 2246, January 1999.
[RFC2616] R. Fielding, J. Gettys, J. Mogul, H. Frystyk, L.
Masinter, M. Leach, T. Berners-Lee, "Hypertext Transfer
Protocol - HTTP/1.1", RFC 2616.
[RFC2945] Wu, T., "The SRP Authentication and Key Exchange
System", RFC 2945, September 2000.
[RFC3080] Rose, M., "The Blocks Extensible Exchange Protocol
Core", RFC 3080, March 2001.
[RFC3081] Rose, M., "Mapping the BEEP Core onto TCP", RFC 3081,
March 2001.
[SPEKE] Jablon, D., "Strong Password-Only Authenticated Key
Exchange", September 1996.
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7 Author's Addresses
Dale Gustafson
Future Foundation Inc.
3001 Broadway St NE
Suite 100
Minneapolis, MN 55413 Phone: +1 651-452-9033
USA Email: dale.gustafson@bpsi.net
Mike Just
Entrust, Inc.
1000 Innovation Drive
Ottawa, ON K2K 3E7 Phone: +1 613-270-3685
Canada Email: mike.just@entrust.com
Magnus Nystrom
RSA Security
Box 10704
121 29 Stockholm Phone: +46 8 725 0900
Sweden Email: magnus@rsasecurity.com
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Full Copyright Statement
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