Network Working Group                                       A. Arsenault
Request for Comments: 3157                                    Diversinet
Category: Informational                                       S. Farrell
                                                  Baltimore Technologies
                                                             August 2001

Securely Available Credentials - Requirements

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

Abstract

   This document describes requirements to be placed on Securely
   Available Credentials (SACRED) protocols.

Table Of Contents

   1. Introduction.................................................1
   2. Framework Requirements.......................................4
   3. Protocol Requirements........................................7
   4. Security Considerations.....................................10
   References.....................................................12
   Acknowledgements...............................................12
   Authors' Addresses.............................................13
   Appendix A: A note on SACRED vs. hardware support..............14
   Appendix B: Additional Use Cases...............................14
   Full Copyright Statement.......................................20

1. Introduction

   "Credentials" are information that can be used to establish the
   identity of an entity, or help that entity communicate securely.
   Credentials include such things as private keys, trusted roots,
   tickets, or the private part of a Personal Security Environment (PSE)
   [RFC2510] - that is, information used in secure communication on the
   Internet.  Credentials are used to support various Internet
   protocols, e.g., S/MIME, IPSec and TLS.

   In simple models, users and other entities (e.g., computers like
   routers) are provided with credentials, and these credentials stay in
   one place.  However, the number, and more importantly the number of
   different types, of devices that can be used to access the Internet
   is increasing.  It is now possible to access Internet services and
   accounts using desktop computers, laptop computers, wireless phones,
   pagers, personal digital assistants (PDAs) and other types of
   devices.  Further, many users want to access private information and
   secure services from a number of different devices, and want access
   to the same information from any device.  Similarly credentials may
   have to be moved between routers when they are upgraded.

   This document identifies a set of requirements for credential
   mobility.  The Working Group will also produce companion documents,
   which describe a framework for secure credential mobility, and a set
   of protocols for accomplishing this goal.

   The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", and "MAY"
   in this document are to be interpreted as described in [RFC2119].

1.1 Background and Motivation

   In simple models of Internet use, users and other entities are
   provided with credentials, and these credentials stay in one place.
   For example, Mimi generates a public and private key on her desktop
   computer, provides the public key to a Certification Authority (CA)
   to be included in a certificate, and keeps the private key on her
   computer.  It never has to be moved.

   However, Mimi may want to able to send signed e-mail messages from
   her desktop computer when she is in the office, and from her laptop
   computer when she is on the road, and she does not want message
   recipients to know the difference.  In order to do this, she must
   somehow make her private key available on both devices - that is,
   that credential must be moved.

   Similarly, Will may want to retrieve and read encrypted e-mail from
   either his wireless phone or from his two-way pager.  He wants to use
   whichever device he has with him at the moment, and does not want to
   be denied access to his mail or to be unable to decrypt important
   messages simply because he has the wrong device.  Thus, he must be
   able to have the same private key available on both devices.

   The following scenario relating to routers has also been offered:
   "Once upon a time, a router generated a keypair.  The administrators
   transferred the public key of that router to a lot of other (peer)
   routers and used that router to encrypt traffic to the other routers.
   And this was good for many years.  Then one day, the network

   administrators found that this particular little router couldn't
   handle an OC-192.  So they trashed it and replaced it with a really
   big router.  While they were there, the craft workers inserted a
   smart card into the router and logged into the router.  They gave the
   appropriate commands and entered the correct answers and so the
   credentials (keypair) were transferred to the new, big router.
   Alternatively, the craft people could have logged into the router,
   given it a minimal configuration and transferred the credentials from
   a credential server to the router.  They had to perform the correct
   incantations and authentications for the transfer to be successful.
   In this way, the identity of the router was moved from an old router
   to a new one.  The administrators were glad that they didn't have to
   edit the configurations of all of the peer routers as well."

   It is generally accepted that the private key in these examples must
   be transferred securely.  In the first example, the private key
   should not be exposed to anyone other than Mimi herself (and ideally,
   it would not be directly exposed to her).  Furthermore, it must be
   transferred correctly.  It must be transferred to the proper device,
   and it must not be corrupted - improperly modified - during transfer.

   Making credentials securely available (in an interoperable fashion)
   will provide substantial value to network owners, administrators, and
   end users.  The intent is that this value be provided largely
   independent of the hardware device used to access the secure
   credential and the type of storage medium to which the secure
   credential is written.  Different credential storage devices, (e.g.,
   desktop or laptop PC computer memory, a 3.5 inch flexible diskette, a
   hard disk file, a cell phone, a smart card, etc.) will have very
   different security characteristics and, often very different protocol
   handling capabilities.  Using SACRED protocols, users will be able to
   securely move their credentials between different locations,
   different Internet devices, and different storage media as needed.

   In the remainder of this document we present a set of requirements
   for the secure transfer of software-based credentials.

1.2 Working Group Organization and Documents

   The SACRED Working Group is working on the standardization of a set
   of protocols for securely transferring credentials among devices.  A
   general framework is being developed that will give an abstract
   definition of protocols which can meet the credential-transfer
   requirements.  This framework will allow for the development of a set
   of protocols, which may vary from one another in some respects.
   Specific protocols that conform to the framework can then be
   developed.

   Work is being done on a number of documents.  This document
   identifies the requirements for the general framework, as well as the
   requirements for specific protocols.  Another document will describe
   the protocol framework.  Still others will define the protocols
   themselves.

1.3 Structure of This Document

   Section 1 of this document provides an introduction to the problem
   being solved by this working group.  Section 2 describes requirements
   on the framework.  Section 3 identifies the overall requirements for
   secure credential-transfer protocols, and separate requirements for
   two different classes of solutions.  Section 4 identifies Security
   Considerations.  Appendix A describes the relationship of the SACRED
   solutions and credential-mobility solutions involving hardware
   components such as smart cards.  Appendix B contains some additional
   scenarios which were considered when developing the requirements.

2. Framework Requirements

   This section describes requirements that the SACRED framework has to
   meet, as opposed to requirements that are to be met by a specific
   protocol that uses the framework.

2.1 Credential Server and Direct solutions

   There are at least two different ways to solve the problem of secure
   credential transfer between devices.  One class of solutions uses a
   "credential server" as an intermediate node, and the other class
   provides direct transfer between devices.

   A "credential server" can be likened to a server that sits in front
   of a repository where credentials can be securely stored for later
   retrieval.  The credential server is active in the protocol, that is,
   it implements security enforcing functionality.

   To transfer credentials securely from one end device to another is a
   straightforward two-step process.  Users can have their credentials
   securely "uploaded" from one device, e.g., a wireless phone, to the
   credential server.  They can be stored on the credential server, and
   "downloaded" when needed using another device; e.g., a two-way pager.

   Some advantages of a credential server approach compared to
   credential transfer are:

   1. It provides a conceptually clean and straightforward approach.
      For all end devices, there is one protocol, with a set of actions
      defined to transfer credentials from the device to the server, and
      another set of actions defined to transfer credentials from the
      server to the device.  Furthermore, this protocol involves clients
      (the devices) and a server (the credential server), like many
      other Internet protocols; thus, the design of this protocol is
      likely to be familiar to most people familiar with most other
      Internet protocols.

   2. It provides for a place where credentials can be securely stored
      for arbitrary lengths of time.  Given a reasonable-quality server
      operating under generally accepted practices, it is unlikely the
      credentials will be permanently lost due to a hardware failure.
      This contrasts with systems where credentials are only stored on
      end devices, in which a failure of or the loss of the device could
      mean that the credentials are lost forever.

   3. The credential server may be able to enforce a uniform security
      policy regarding credential handling.  This is particularly the
      case where credentials are issued by an organization for its own
      purposes, and are not "created" by the end user, and so must be
      governed by the policies of the issuer, not the user.

   However, the credential server approach has some potential
   disadvantages, too:

   1. It might be somewhat expensive to maintain and run the credential
      server, particularly if there are stringent requirements on
      availability and reliability of the server.  This is particularly
      true for servers which are used for a large community of users.
      When the credential server is intended for a small community, the
      complexity and cost would be much less.

   2. The credential server may have to be "trusted" in some sense and
      also introduces a point of potential vulnerability.  (See the
      Security Considerations section for some of the issues.)  Good
      protocol and system design will limit the vulnerability that
      exists at the credential server, but at a minimum, someone with
      access to the credential server will be able to delete credentials
      and thus deny the SACRED service to system users.

   Thus, some users may prefer a different class of solution, in which
   credentials are transferred directly from one device to another
   (i.e., having no intermediary element that processes or has any
   understanding of the credentials).

   For example, consider the case where Mimi sends a message from her
   wireless phone containing the credentials in question, and retrieves
   it using her two-way pager.  In getting from one place to another,
   the bits of the message cross the wireless phone network to a base
   station.  These bits are likely transferred over the wired phone
   network to a message server run by the wireless phone operator, and
   are transferred from there over the Internet to a message server run
   by the paging operator.  From the paging operator they are
   transferred to a base station and then finally to Mimi's pager.
   Certainly, there are devices other than the original wireless phone
   and ultimate pager that are involved in the credential transfer, in
   the sense that they transmit bits from one place to another.
   However, to all devices except the pager and the wireless phone, what
   is being transferred is an un-interpreted and unprocessed set of
   bits.  No security-related decisions are made, and no actions are
   taken based on the fact that this message contains credentials, at
   any of the intermediate nodes.  They exist simply to forward bits.
   Thus, we consider this to be a "direct" transfer of credentials.

   Solutions involving the direct transfer of credentials from one
   device to another are potentially somewhat more complex than the
   credential-server approach, owing to the large number of different
   devices and formats that may have to be supported.  Complexity is
   also added due to the fact that each device may in turn have to
   exhibit the behavior of both a client and a server.

   We believe that both classes of solutions are useful in certain
   environments, and thus that the SACRED framework will have to define
   solutions for both.  The extent to which elements of the above
   solutions overlap remains to be determined.

   This all leads to our first set of requirements:

   F1.   The framework MUST support both "credential server" and
         "direct" solutions.
   F2.   The "credential server" and "direct" solutions SHOULD use the
         same technology as far as possible.

2.2 User authentication

   There is a wide range of deployment options for credential mobility
   solutions.  In many of these cases, it is useful to be able to re-use
   an existing user authentication scheme, for example where passwords
   have previously been established, it may be more secure to re-use
   these than try to manage a whole new set of passwords.  Different
   devices may also limit the types of user authentication scheme that
   are possible, e.g., not all mobile devices are practically capable of
   carrying out asymmetric cryptography.

F3. The framework MUST allow for protocols which support different
user authentication schemes

2.3 Credential Formats

         Today there is no single standard format for credentials and
         this is not likely to change in the near future.  There are a
         number of fairly widely deployed formats, e.g., [PGP],
         [PKCS#12] that have to be supported.  This means that the
         framework has to allow for protocols supporting any credential
         format.

   F4.   The details of the actual credential type or format MUST be
         opaque to the protocol, though not to processing within the
         protocol's peers.  The protocol MUST NOT depend on the internal
         structure of any credential type or format.

2.4 Transport Issues

   Different devices allow for different transport layer possibilities,
   e.g., current WAP 1.x devices do not support TCP.  For this reason
   the framework has to be transport "agnostic".

   F5.   The framework MUST allow use of different transports.

3. Protocol Requirements

   In this section, we identify the requirements for secure credential-
   transfer solutions.  We will begin by listing a set of relevant
   vulnerabilities and the requirements that must be met by all
   solutions.  Then we identify additional requirements that must be met
   by solutions involving a credential server, followed by additional
   requirements that must be met by solutions involving direct transfer
   of credentials.

3.1 Vulnerabilities

   This section lists the vulnerabilities against which a SACRED
   protocol SHOULD offer protection.  Any protocol claiming to meet the
   requirements listed in this document MUST explicitly indicate how (or
   whether) it offers protection for each of these vulnerabilities.

   V1.      A passive attacker can watch all packets on the network and
            later carry out a dictionary attack.
   V2.      An attacker can attempt to masquerade as a credential server
            in an attempt to get a client to reveal information on line
            that allows for a later dictionary attack.

   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).
   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).
   V5.      An attacker can copy a credential server's repository and
            carry out a dictionary attack.
   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.
   V7.      An attacker can persuade a server that a successful login
            has occurred, even if it hasn't.
   V8.      (Upload) An attacker can overwrite someone else's
            credentials on the server.
   V9.      (When using password-based authentication) An attacker can
            force a password change to a known (or "weak") password.
   V10.     An attacker can attempt a man-in-the-middle attack for lots
   V11.     User enters password instead of name.
   V12.     An attacker could attempt various denial-of-service attacks.

3.2 General Protocol Requirements

   Looking again at the examples described in Section 1.1, we can
   readily see that there are a number of requirements that must apply
   to the transfer of credentials if the ultimate goal of supporting the
   Internet security protocols (e.g., TLS, IPSec, S/MIME) is to be met.
   For example, the credentials must remain confidential at all times;
   it is unacceptable for nodes other than the end-user's device(s) to
   see the credentials in any readable, cleartext form.

   These, then, are the requirements that apply to all secure
   credential-transfer solutions:

   G1.      Credential transfer both to and from a device MUST be
            supported.
   G2.      Credentials MUST NOT be forced by the protocol to be present
            in cleartext at any device other than the end user's.
   G3.      The protocol SHOULD ensure that all transferred credentials
            be authenticated in some way (e.g., digitally signed or
            MAC-ed).
   G4.      The protocol MUST support a range of cryptographic
            algorithms, including symmetric and asymmetric algorithms,
            hash algorithms, and MAC algorithms.

   G5.      The protocol MUST allow the use of various credential types
            and formats (e.g., X.509, PGP, PKCS12, ...).
   G6.      One mandatory to support credential format MUST be defined.
   G7.      One mandatory to support user authentication scheme MUST be
            defined.
   G8.      The protocol MAY allow credentials to be labeled with a text
            handle, (outside the credential), to allow the end user to
            select amongst a set of credentials or to name a particular
            credential.
   G9.      Full I18N support is REQUIRED (via UTF8 support) [RFC2277].
   G10.     It is desirable that the protocol be able to support
            privacy, that is, anonymity for the client.
   G11.     Transferred credentials MAY incorporate timing information,
            for example a "time to live" value determining the maximum
            time for which the credential is to be usable following
            transfer/download.

3.3 Requirements for Credential Server-based solutions

   The following requirements assume that there is a credential server
   from which credentials are downloaded to the end user device, and to
   which credentials are uploaded from an end user device.

   S1.      Credential downloads (to an end user) and upload (to the
            credential server) MUST be supported.
   S2.      Credentials MUST only be downloadable following user
            authentication or else only downloaded in a format that
            requires completion of user authentication for deciphering.
   S3.      It MUST be possible to ensure the authenticity of a
            credential during upload.
   S4.      Different end user devices MAY be used to
            download/upload/manage the same set of credentials.
   S5.      Credential servers SHOULD be authenticated to the user for
            all operations except download.  Note: This requirement can
            be ignored if the credential format itself is strongly
            protected, as there is no risk (other than user confusion)
            from an unauthenticated credential server.
   S6.      It SHOULD be possible to authenticate the credential server
            to the user as part of a download operation.
   S7.      The user SHOULD only have to enter a single secret value in
            order to download and use a credential.
   S8.      Sharing of secrets across multiple servers MAY be possible,
            so that penetration of some servers does not expose the
            private parts of a credential ("m-from-n" operation).
   S9.      The protocol MAY support "away-from-home" operation, where
            the user enters both a name and a domain (e.g.
            RoamingStephen@baltimore.ie) and the domain can be used in
            order to locate the user's credential server.

   S10.     The protocol MUST provide operations allowing users to
            manage their credentials stored on the credential server,
            e.g., to retrieve a list of their credentials stored on a
            server; add credentials to the server; delete credentials
            from the server.
   S11.     Client-initiated authentication information (e.g., password)
            change MUST be supported.
   S12.     The user SHOULD be able to retrieve a list of
            accesses/changes to their credentials.
   S13.     The protocol MUST support user self-enrollment.  One
            scenario calling for this is where a previously unknown user
            uploads his credential without requiring manual operator
            intervention.
   S14.     The protocol MUST NOT prevent bulk initializing of a
            credential server's repository.
   S15.     The protocol SHOULD require minimal client configuration.

3.4 Requirements for Direct-Transfer Solutions

   The full set of requirements for this case has not been elucidated,
   and this list is therefore provisional.  An additional requirements
   document (or a modification of this one) will be required prior to
   progression of a direct-transfer protocol.

   The following requirements apply to solutions supporting the "direct"
   transfer of credentials from one device to another.  (See Section 2
   for the note on the meaning of "direct" in this case.)

   D1.   It SHOULD be possible for the receiving device to authenticate
         that the credential package indeed came from the purported
         sending device.
   D2.   In order for a sender to know that a credential has been
         received by a recipient, it SHOULD be possible for the
         receiving device to send an acknowledgment of credential
         receipt back to the sending device, and for the sending device
         to authenticate this acknowledgment.

4. Security Considerations

4.1 Hardware vs. Software

   Mobile credentials will never be as secure as a "pure" hardware-based
   solution, because of potential attacks through the operating system
   of the end-user device.  However, an acceptable level of security may
   be accomplished through some simple means.  In fact the level of
   security may be improved (compared to password encrypted files)
   through the use of SACRED protocols.

   The platforms to which credentials are downloaded usually cannot be
   regarded as tamper-resistant, and it therefore is not too hard to
   analyze contents of their memories.  Further, storage of private
   keys, even if they are encrypted, on a credential server, will be
   unacceptable in some environments.  Lastly, replacement of installed
   or downloaded SACRED client software with a Trojan horse program will
   always be possible, such a program could email the username and
   password to the program's author.

4.2 Auditing

   Although out of scope of the SACRED protocol development work,
   implementations should carefully audit events that may be security
   relevant.  In particular credential server implementations should
   audit all operations and should include information about the time
   and source (e.g., IP address) of the operation, the claimed identity
   of the client (possibly masked - see below), the type and result of
   the operation and possibly other operation specific information.
   Implementations should also take care not to include security
   sensitive information in the audit trail, especially not sensitive
   authentication information.

   It may be sensible to mask the claimed identity in some way in order
   to ensure that even if a user enters her password in a "username"
   field, that that information is not in clear in the audit trail,
   regardless of whether or not it was received in clear.

   Similar mechanisms which should be supported, but which are out of
   scope of protocol development include alerts and account locking, in
   particular following repeated authentication failures.

4.3 Defense against attacks

   Credential servers are major targets.  Someone who can successfully
   attack a credential server might be able to gain access to the
   credentials of a number of users, unless those credentials are
   sufficiently protected (e.g., encrypted sufficiently that they cannot
   be read or used by someone who gains access to them).  Attackers
   might also be able to substitute credentials of users, to carry out
   other system attacks (e.g., an attacker could provide a user with a
   "trusted root" credential that the attacker controls, which would
   later allow the attacker to have some other certificate accepted by
   the user counter to policy).

   In addition, a credential server is a major target for denial of
   service attacks.  Ensuring that a credential server is unavailable to
   legitimate users can be of great assistance to attackers.  Users who
   were not able to retrieve needed credentials might be forced to

   operate insecurely, or not operate at all.  Credential servers are
   especially vulnerable to denial of service attacks if they do lots of
   expensive cryptographic operations - it might not take very many
   operations for the attacker to bring service to an unacceptable
   level.

   Thus, great care should be taken in designing systems that use
   credential servers to protect against these attacks.

References

   [PGP]       Callas, J., Donnerhacke, L., Finney, H. and R. Thayer,
               "OpenPGP Message Format", RFC 2440, November 1998.

   [PKCS12]    "PKCS #12 v1.0: Personal Information Exchange Syntax
               Standard", RSA Laboratories, June 24, 1999.

   [CMS]       Housley, R., "Cryptographic Message Syntax", RFC 2630,
               June 1999.

   [PKCS15]    "PKCS #15 v1.1: Cryptographic Token Information Syntax
               Standard," RSA Laboratories, June 2000.

   [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", BCP 14, RFC 2119, March 1997.

   [RFC2277]   Alvestrand, H., " IETF Policy on Character Sets and
               Languages", BCP 18, RFC 2277, January 1998.

   [RFC2510]   Adams, C. and S. Farrell, "Internet X.509 Public Key
               Infrastructure Certificate Management Protocols", RFC
               2510, March 1999.

   [RFC2616]   Fielding, R., Gettys, J., Mogul, J., Frysyk, H.,
               Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext
               Transfer Protocol - HTTP/1.1", RFC 2616, June 1999.

Acknowledgements

   The authors gratefully acknowledge the text containing additional use
   cases in Appendix B that was supplied by Neal Mc Burnett
   (nealmcb@avaya.com).

Authors' Addresses

   Alfred Arsenault
   Diversinet Corp.
   P.O. Box 6530
   Ellicott City, MD 21042
   USA

   Phone: +1 410-480-2052
   EMail: aarsenault@dvnet.com

   Stephen Farrell,
   Baltimore Technologies,
   39 Parkgate Street,
   Dublin 8,
   IRELAND

   Phone: +353-1-881-6000
   EMail: stephen.farrell@baltimore.ie

Appendix A: A note on SACRED vs. hardware support.

   One way of accomplishing many of the goals of the SACRED WG is to put
   the credentials on hardware tokens - e.g., smart cards, PCMCIA cards,
   or other devices.  There are a number of types of hardware tokens
   today that provide secure storage for sensitive information, some
   degree of authentication, and interfaces to a number of types of
   wireless and other devices.  Thus, in the second example from section
   1.1, Will could simply put his private key on a smart card, always
   take the smart card with him, and be assured that whichever device he
   uses to retrieve his e-mail, he will have all of the information
   necessary to decrypt and read messages.

   However, hardware tokens are not appropriate for every environment.
   They cost more than software-only solutions, and the additional
   security they provide may or may not be worth the incremental cost.
   Not all devices have interfaces for the same hardware tokens.  And
   hardware tokens are subject to different failure modes than typical
   computers - it is not at all unusual for a smart card to be lost or
   stolen; or for a PCMCIA card to physically break.

   Thus, it is appropriate to develop complementary software-based
   solution that allows credentials to be moved from one device to
   another, and provides a level of security sufficient for its
   environments.  While we recognize that the level of security provided
   by a software solution may not be as high as that provided by the
   hardware solutions discussed above, and some organizations may not
   consider it sufficient at all, we believe that a worthwhile solution
   can be developed.

   Finally, SACRED protocols can also complement hardware credential
   solutions by providing standard mechanisms for the update of
   credentials which are stored on the hardware device.  Today, this
   often requires returning (with) the device to an administrative
   centre, which is often inconvenient and may be costly.  SACRED
   protocols provide a way to update and manage credentials stored on
   hardware devices without requiring such physical presence.

Appendix B: Additional Use Cases

   This appendix describes some additional use cases for SACRED
   protocols.  SACRED protocols are NOT REQUIRED to support all these
   use cases, that is, this text is purely informative.

B.1 Home/Work Desktop Computer

   Scenario Overview

   A university utilizing a PKI for various applications and services
   on-campus is likely to find that many of its users would like to make
   use of the same PKI-enabled services and applications on computers
   located in their residence.  These home computers may be owned either
   by the university or by the individual but are permanently located at
   the residence as opposed to laptop systems that may be taken home.
   The usage depicted in this scenario may be motivated by formal
   telecommuting arrangements or simply by the need to catch up on work
   from home in the evenings.  The basic scenario should apply equally
   well to the commercial, health care, and higher education
   environments.

   Assumptions

   This scenario assumes that the institution has not implemented a
   hardware token-based PKI mobility solution

   The home computer has a dial-up as opposed to a permanent network
   connection.

   The PKI applications, whenever practical, should be functional in
   both on-line and off-line modes.  For example, the home user signing
   an email message to be queued for later bulk sending and the reading
   of a received encrypted message may be supported off-line while
   composing and queuing of an encrypted message might not be supported
   in off-line mode.

   Applications using digital signatures may require "non-repudiation".

   The institution prefers that the user be identified via a single
   certificate / key-pair from all computers used by the individual.

   The home computer system can not be directly supported by the
   institution's IT staff.  Hardware, operating system versions, and
   operating system configurations will vary widely.  Significant
   software installation or specialized configurations will be difficult
   to implement.

   Uniqueness of Scenario

   vThe PKI mobility support needed for this scenario is, in general,
   similar to the other mobility scenarios.  However, it does have
   several unique aspects:

   1. The home-user scenario differs from the general public workstation
      case in that it provides the opportunity to permanently store the
      user's certificate and key-pair on the workstation.

   2. Likewise the appropriate CA certificates and even certificates for
      other users can be permanently stored or cached on the home
      workstation.

   3. Another key difference is the need to support off-line use of the
      PKI credentials given the assumed dial-up network connection.

   4. The level of hardware and software platform consistency (operating
      system versions and configurations) will vary widely.

   5. Finally, the level of available technical support is significantly
      less for home systems than for equivalent systems managed by the
      IT staff at the office location.

B.2 Public Lab / On-campus Shared Workstation

   Scenario Overview

   Many colleges and universities operate labs full of computer systems
   that are available for use by the general student population.  These
   computers are typically configured with identical hardware and an
   operating system build that is replicated to all of the systems in
   the lab.  Many typical configurations provide no permanent storage of
   any type while others may offer individual disk space for personal
   files on a central server.  Some scheme is generally used to ensure
   that the configuration of the operating system is preserved across
   users and that temporary files created by one user are removed before
   the next user logs in.  Students generally sit down at the next
   available workstation without any clear pattern of usage.

   The same basic technical solutions used to operate public labs are
   often also used in general environments where several people share a
   single workstation.  This is often found in locations with shift work
   such as medical facilities and service bureaus that provide services
   to multiple time zones.

   Assumptions

   1. This scenario assumes that the institution has not implemented a
      hardware token-based PKI mobility solution.

   2. The computer systems are permanently networked with LAN
      connections.

   3. The configuration of the computer system is centrally maintained
      and customizations are relatively easy to implement.  For example
      it would be easy to load enterprise root certificates, LDAP server
      configurations, specialized software, and any other needed
      components of the PKI on to the workstations.

   4. Applications using digital signatures may require "non-
      repudiation" in some of the anticipated environments.  Examples of
      this might include homework submission in a public lab environment
      or medical records in a health care environment.

5. The institution prefers that the user be identified via a single
certificate / key-pair from all computers used by the individual.

   6. Many anticipated implementations of this scenario will not
      implement any user authentication at the desktop operating system
      level.  Instead, user authentication will occur at during the
      startup of networked applications such as email, web-based
      services, etc.  Login at the desktop level may be with generic
      user names that are more targeted at matching printouts to
      machines than identifying users.

   7. Users, with almost ridiculous frequency, will walk away from a
      system forgetting to first logout from running authenticated
      applications.

   Uniqueness of Scenario

   The PKI mobility support needed for this scenario is, in general,
   similar to the other mobility scenarios.  However, it does have
   several unique aspects:

   1. Unlike situations with personal workstations, there is no
      permanent storage available to hold user key pairs and
      certificates.

   2. Appropriate CA certificates and custom software are easily added
      and maintained for these types of shared systems.

   3. The workstations are installed in public locations and users will
      frequently forget to close applications before permanently walking
      away from the workstation.

B.3 Public Kiosk Mobility

   Overview

   This scenario describes the needs of the traveler or the shopper.
   This person is traveling light (no computer) or is burdened with
   everything but a computer.  It recognizes the increasing availability
   of Internet access points in public spaces, such as libraries,
   airports, shopping malls, and "cyber cafes".

   The Need

   In our increasingly mobile society, the chances of needing
   information when away from the normal computing place are great.  One
   may need to look up a telephone number.  Have you tried to find a
   phone book at a public phone lately?  It may become necessary to use
   a data device to find the next place to rush to.  With the
   proliferation of wireless devices (electronic leashes), others have
   the ability to create a need for quick access to electronic
   information.  A pager can generate a need to check the email inbox or
   address book.  A cell phone can drive you to your database to answer
   a pressing question.

   The ability to quickly access sensitive or protected information or
   services from publicly available devices will only become more
   necessary as we become more and more "connected".

   The Device

   The access device is more a function of the best discount or
   marketing effort than of design.  Any number of hardware platforms
   will be encountered.

   Since these devices are open to the public I/O ports are not likely
   to be.  In order to protect the device and its immediate network
   environment, most devices will be in some sort of protective
   container.  Access to serial, parallel, USB, firewire, SCSI, or
   PCMCIA connections will not be possible.  Likewise floppy, zip, or CD
   drives.  Therefore, any software "token" must be obtained from the
   network itself.

   The Concerns

   1. Getting the "token".  Since it will be necessary to obtain the
      token (key, certificate, credential) from across the network.  How
      can it be protected during transit?

   2. Where did you get it?  One of the primary controls in PKI is
      protection of the private key.  Placing the key on a host that is
      accessible from a public network means that there is an inherent
      exposure from that network.  The access controls and other
      security measures on the host machine are an area of concern.

   3. How did you get it?  When you obtained the token from the server,
      how did it know that you are you?  Authentication becomes
      critical.

   4. What happens to the token when you leave?  You've checked your
      mail, downloaded a recipe from that super-secure recipe server,
      found out how to get to the adult beverage store for the... uh...
      accessories... for the meal, and you're off!  Is your token?  Or
      is it still sitting there on the public kiosk waiting for those
      youngsters coming out of the music store to notice and cruise the
      information highway on your ticket?

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