PROCEEDINGS
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`Symposium on .·.·
`Network· an.d Distributed
`System Security
`
`1996
`
`February 22-23, 1996
`
`San Diego,California
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`MANGROVE 1002
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`}!roceedings of the
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`Sym-posium on ~etwork and Distributed
`System Security
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`2
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`

`
`Proceedings of the
`. '
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`Symposium on Network and Distributed
`System Security (
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`February 22-23, 1996
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`San Diego, California
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`Proceedings of the Symposium on Network and Distributed Systems Security
`
`Table of Contents
`
`General Chair's Message ............................... .. ........................................ ................................ vii
`Program Chairs' Message .................. .................................................................................... viii
`Organizing Committee ............................................................................................................... ix
`Program Committee ..................................................................................................................... x
`Privacy and Security Research Group ................................... ................................................ xi
`
`Session 1: Electronic Mall Security
`Chair: S tephen T . Kent - BBN Corporation
`Mixing E-mail with BABEL ... ...... ... .. ....... .. .... ........ .. ...... .......... ... ...... .. ........ ... .. .... ... ..... ..... .... ..... 2
`C. GUlcU and G. Tsudik
`An Integration of PGP and MIME .......... .... .. .... .... .. .. ... .. .. ................. ... .. ... ........... .. .. .. ........ ....... 17
`K. Yamamoto
`
`Session 2: Distributed Object Systems
`Chair: Danny M. Nessett- Sun Microsystems
`A Security Framework Supporting Domain-Based Access Control in
`Distributed Systems .... ... ... .... .. .. .. .. .............. .. ..... ........ .. ............ ...... ... ...... .. ..... ....... ........ .. .. .. .. .... 26
`N. Yialelis and M. Sloman
`Panel - Scalability of Security in Distributed Object Systems ... ................................ .. ... ........ .40
`Moderator: Danny M. Nessett- Sun Microsystems
`Panelists: Bret Hartman - Odyssey Research Associates
`Danny M. Nessett - Sun Microsystems
`Imperial College, London
`Nicholas Yialelis -
`
`Session 3: Distributed System Security
`Chair: Michael Roe - University of Cambridge
`A Flexible Distributed Authorization Protocol .... .. ... ...... ..................... .. ... .. .. .. ..... ..... ... ... .... ...... . 43
`J. T. Trostle and B. C. Neuman
`Preserving Integrity in Remote File Location and Retrieval ........................... ............................ 53
`T. Jaeger and A.D. Rubin
`C-HTrP - The Development of a Secure, Closed HTfP-Based Network on the Internet ......... 64
`T. Kiuchi and S. Kaihara
`
`v
`
`5
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`

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`6
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`

`
`C-HTTP -- The Development of a Secure, Closed HTTP-based Network
`on the Internet
`
`Takahiro Kiuchi
`Department of Epidemiology and Biostatistics
`Faculty of Medicine, University of Tokyo
`7-3-1 Hongo, Bunkyo~ku, Tokyo 113, Japan·
`
`Shigekoto Kaihara
`Hospital Computer Center
`University of Tokyo Hospital
`7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan
`
`Abstract
`We have designed "C-HITP" which provides secure
`HITP communication mechanisms within a closed group
`of institutions on the Internet, where each member is
`protected by
`its own
`firewall. C-HITP-based
`communications are made possible by the following three
`components: a client-side proxy, a server-side proxy and
`a C-HITP name server. A client-side proxy and server(cid:173)
`side proxy communicate with each other using a secure,
`encrypted protocol while communications between a user
`agent and client-side proxy · or an origin sen,er and
`server-side proxy are performed using current HITP/1. 0.
`In a C-HITP-based network, instead of DM'). a C-HITP(cid:173)
`based secure, encrypted name and certification service is
`used .. The aim of C-HITP is to assure institutional level
`security and is different in scope from other secure HITP
`protocols currently proposed which are oriented toward
`secure end-to-end HITP communications
`in which
`security protection is dependent on each end-user.
`
`1. Introduction
`In the medical community, there is a strong need for
`closed networks among hospitals and related institutions,
`such as coordinating centers for clinical trials or clinical
`laboratories. Secure transfer of patient information for
`clinical use is obviously essential. In addition, some
`medical information has to be shared among some
`hospitals, but it should not be made available to other
`sites. This includes, for example, information concerning
`multi-institutional clinical trials and documents for case
`conferences although patients' names are usually not
`specified in such information. In this paper, we discuss
`the design and implementation of a closed HTTP
`(Hypertext Transfer Protocol)-based network (C-HTTP)
`which can be built on the Internet.
`
`2. Design and specification of C-HTTP
`2.1 Overview
`C-HTTP is assumed to be used .in a closed group of
`institutions on the Internet, in which each member is
`protected
`by
`its
`own
`firewall. C-HTTP-based
`communication is made possible with the following three
`components: 1) a client-side proxy on the firewall of one
`institution, 2) a server-side proxy on the firewall of
`another institution and 3) a C-HTI'P name server, which
`manages a given C-H'ITP-based network and
`the
`information for its all proxies. A client-side proxy and
`server-side proxy communicate with each other using a
`secure, encrypted protocol (C-HTTP). Communications
`between two kinds of proxies and HTTP/1.0 compatible
`servers/user agents within the firewalls are performed
`based on HTTP/1.0 with current C-HfTP implementation
`under way[l]. The DNS name service is not used for
`hostname resolution as the original secure name service,
`including certification, is used for the C-HTTP-based
`network. A summary of the protocol specification is
`described in the Appendices.
`
`2.2 Security technology and key information
`InC-HTTP, five kinds of~curity technologies are used.
`They are: 1 j asymmetric key encryption for the secure
`exchange of data encryption keys between two types of
`proxies and host information between a proxy and C(cid:173)
`HTTP name server, 2) symmetric key encryption for the
`encryption of C-HTTP encrypted headers and HTTP/1.0
`requests, 3) electronic signature for the request/response
`authentication, 4) a one-way hash function for checking
`data tampering and 5) random key generation technology.
`In the C-HTTP name service, symmetric encryption is not
`used because the amount of information transferred is
`small.
`Each client-side or server-side proxy has its own
`private and public asymmetric keys and the C-HTTP
`name server's public key. Proxies do not exchange their
`
`0-8186-7222-6/96 $5.00 © 1996 IEEE
`Proceedings of SNDSS '96
`
`64
`
`7
`
`

`
`public keys with each other directly. Instead, the C-HTTP
`name server provides both client-side and server-side
`proxies with each peer's public key. In addition, Nonce
`values for both request and response are also generated
`and provided by the C-HTTP name server, which will be
`specified as
`initial values
`in Request-Nonce and
`Response-Nonce headers contained in the first C-HTTP
`request dispatched by a client -side proxy and in the first
`C-HTTP response dispatched by a server-side proxy,
`respectively. The C-HTTP name server manages its own
`private and public asymmetric keys and the public keys of
`all proxies which participate in the closed network. Two
`data encryption keys (symmetric keys) for requests and
`responses respectively are generated randomly during
`each C-HTTP session.
`An origin server which is compatible with HTTP/1.0 is
`responsible for user authentication if necessary. It uses
`the built-in HTTP/1.0
`authentication mechanism.
`Information concerning a user's
`ID, password and
`security realm (HTTP/1.0) are encrypted by proxies and
`are transferred only in encrypted form through tl1e
`Internet. Replay attacks are blocked by checking values of
`the Request-Nonce header field of each request.
`When a given institution wants to participate in a
`closed network, it must 1) install a client-side and/or
`server-side prox-y on its firewall, 2) register an IP address
`( for a server-side proxy, a port number should also be
`registered) and hostname (which does not have to be the
`same as its DNS name) for a firewall gateway, 3) give the
`proxy's public key to the C-HTTP name server, and 4)
`obtain the C-HTTP name server's public key. In the
`present C-HTTP specification, there is only one name
`server in a given C-HTTP network, although one can
`define any possible combination of closed subnetworks
`within the network.
`
`2.3 C-HTTP-based communication
`C-HTTP-based communication
`is
`follows:
`
`summarized as
`
`1) Connection of a client to a client -side proxy
`A client-side proxy behaves as an HTTP/1.0 compatible
`proxy, and it should be specified as a proxy server for
`external (outside the firewall) access in each user agent
`within the firewall. In C-HTTP, as different from
`ordinary HTTP, a session (virtual C-HTTP connection) is
`established between a client-side proxy and server-side
`proxy and, thus, it is not stateless. The session is finished
`when the client accesses another C-HTTP server or an
`ordinary WWW server or when the client-side or server(cid:173)
`side proxy times out. The following ad-hoc mechanism is
`employed to define a session in stateless HTTP/1.0-based
`communication between a client-side proxy and user
`
`agent. Suppose that the HTI\.1L specified in Figure (a) is
`retrieved and sent to a client-side proxy after a C-HTTP
`session is established. In the client-side-proxy, the HTI\.1L
`document is rewritten as specified in Figure (b) and
`forwarded to a user agent. When one of these resource
`names with a connection ID, for example,
`"http://server.in.current.connection/sample.htm1=@=6zd
`DfldfcZLj8V!i" in Figure (b), is selected and requested by
`an end-user, tl1e client-side proxy takes off the connection
`ID and forwards the stripped, the original resource name
`to the server in its request as described in Figure (c).
`When the connection ID is not found
`in the current
`connection table in the client-side-proxy, the current
`connection is disconnected. Thus a new connection is
`established if the host is in the closed network and an
`ordinary HTTP I 1. 0 request is dispatched otherwise.
`
`2) Lookup of server-side proxy information (Appendix 3.
`a,b)
`A client-side proxy asks the C-HTTP name server
`whether it can communicate with the host specified in a
`given URL. If the name server confirms that the query is.
`legitimate, it examines whether the requested server-side
`proxy is registered in the closed network and is permitted
`to accept the connection from the client-side proxy. If the
`connection is permitted, the C-HTTP name server sends
`the IP address and public key of the server-side proxy and
`both request and response Nonce values. If it is not
`permitted, it sends a status code which indicates an error.
`If a client-side proxy receives an error status, then it
`performs DNS
`lookup, behaving
`like an ordinary
`HTTP/1.0 proxy.
`Both the request to and response from the C-HTTP
`name
`server are encrypted and certified, using
`asymmetric key encryption and digital
`signature
`technology.
`
`3) Request for connection to the server-side proxy
`(Appendix 3. c)
`When the C-HTTP name server confirms that the
`specified server-side prox-y is an appropriate closed
`network member, a client-side prox-y sends a request for
`connection to the server-side proxy, which is encrypted
`using the server-side proxy's public key and contains the
`client-side proxy's IP address, hostname, request Nonce
`value and symmetric data exchange key for request
`encryption.
`
`4) Lookup of client-side proxy information (Appendix 3.
`d,e)
`When a server-side proxy accepts a request for
`connection from a client-side proxy, it asks the C-HTTP
`
`65
`
`8
`
`

`
`Figure. Conversion of stateless HTTP
`stateful C-HTTP
`
`to
`
`a. The HTML document sent from a origin server to a
`client-side proxy
`
`<TITLE>SAMPLE</TITLE>
`<BODY>
`<A HREF =
`"http:/ /server.in.current.connectionjsample.html">
`Please click here.</ A>
`~
`1
`<A HREF =
`"http: I I another .server .in .closed .network/">
`Another server.</ A>
`</BODY>
`
`b. The HTML document rewritten and forwarded to a
`use agent by the client-side proxy. The string,
`"6zdDfldfcZLj8V!i", attached to the end of the URLs
`is a connection ID
`
`<TITLE> SAMPLE< /TITLE>
`<BODY>
`<A HREF =
`"http:; /server.in.current.connection/sq.mple.html=@
`=6zdDfldfcZLj8V!i">
`Please click here.</ A>
`<A HREF =
`"http:/ ;another.server.in.closed.network/=@=6zdDfl
`dfcZLj8V!i">
`Another server.</ A>
`</BODY>
`
`~,,
`
`c. HTTP/1.0 request from the user agent (1) and
`HTTP/1.0 request encrypted and wrapped in C-HTTP
`request dispatched by the client-side proxy (2)
`
`(1)
`GET "http:/ jserver.in.current.connection/
`sample.html=@=6zdDfldfcZLj8V!i"
`HTTP/l.O<CR><LF>
`
`(2)
`GET "http: I I server.in.cu rrent.connection/
`sample.html"
`HTTP/l.O<CR><LF>
`
`name server whether
`the client-side proxy
`is an
`appropriate member of the closed network. If the name
`server confirms that the query is legitimate, it then
`examines whether the client-side proxy is permitted to
`access to the server-side proxy. If access is permitted, the
`C-HTTP name server sends the IP address and public key
`of the client-side proxy and both request and response
`Nonce values, which are the same as those sent to the
`client-side proxy. The C-HTTP name server keeps both of
`the Nonce values for thirty seconds. If not, it sends a
`status code which indicates an error and the server-side
`proxy refuses the connection from the client-side proxy.
`
`5) Connection establishment (Fig. 2f)
`When the sever-side proxy obtains the client-side
`proxy's
`IP address, hostname and public key,
`it
`authentiCates the client-side proxy, checks the integrity of
`the request and the request Nonce value and generates
`both a connection ID derived from the server-side proxy's
`name, date and random numbers (32 bits) using MD5,
`and also a second symmetric data exchange key for
`response encryption, which are sent to the client-side
`proxy. When the client-side proxy accepts and checks
`them, the connection is established.
`
`6) Sending C-HTTP requests to the server-side proxy (Fig.
`2g)
`Once the connection is established, a client-side pro~
`forwards HTTP/1.0 requests from the user agent in
`encrypted form using C-HTTP format.
`
`7) Forwarding requests to an origin server
`Using HTTP/1.0, a server-side proxy communicates
`with an origin server inside the firewall. From the view of
`the user agent or client-side proxy, all resources appear to
`be located in a server-side proxy on the firewall. In reality,
`however, the server-side proxy forwards requests to the
`origin server. It is possible to map any of the virtual
`directories on the server-side proxy to any of the
`directories in one or more origin servers inside the
`firewall.
`
`8) Origin server responses to the user agent through the
`server-side and client-side proxies (Fig. 2h)
`An HTTP/1.0 response sent from the origin server to
`the server-side proxy is encrypted in C-HTTP format by
`the server-side proxy, and is forwarded to the client-side
`proxy. Then, in the client-side proxy, the C-HTTP
`response
`is decrypted and
`the HTTP/1.0 response
`extracted. If the transferred object is in HTML format, the
`connection ID is attached to the anchor URLs contained
`in the document. The resulting HTTP/1.0 response is sent
`to the user agent.
`
`66
`
`9
`
`

`
`---
`
`9) Request for closing the connection (Appendix 3. i,j)
`A client-side proxy can send a request for closing the
`connection. The server-side proxy returns a status which
`indicates the connection is closed. On the other hand, if
`the server-side proxy detects that a given connection
`times out,
`it deletes
`the connection ID from
`the
`connection list, informing the client-side prox-y that the
`connection is closed when an error status is returned in
`response to the request.
`
`3. Trial implementation
`Trial implementation is under way using I) RSA as the
`asymmetric key encryption method (OSISEC RSA
`Iibrary)[2], 2) DES as the symmetric key encryption
`method (GNU DES Iibrary)[3], 3) RSA as the electronic
`signature method (OSISEC RSA library) and 4) a one(cid:173)
`way hash function based on lviD5[4]). As for random key
`generation, programs included in the OSISEC RSA
`library and GNU DES
`library are used for RSA
`asymmetric keys and DES symmetric keys, respectively.
`In the implementation, we employed the following
`methods to enhance security.
`
`1) Key protection
`In C-HTTP, keys are stored only on the firewall of a
`given institution. C-HTTP proxy software is provided as
`source code, and the keys are designed not to be stored in
`a separate "key file." A key generation program generates
`a C program file, which contains key information for
`proxies. It is more difficult to steal keys using this
`method than if they were stored in a separate file.
`
`2) No simultaneous data transfer to both sides
`Only after receiving all the data transferred from one
`side, does a proxy server begin to forward it to the other
`side, except for image and sound data. In this method, the
`performance of data transfer is not good, however, the
`data transfer is separated between the internal and
`external sides. For the secure implementation of this
`feature, the size of HTML documents and object bodies
`should be limited and checked by each proxy. We plan to
`implement routines which check the contents of object
`bodies (especially concerning form data used in POST
`method) in the future.
`
`3) Closure ofTCP connection after each transaction
`C-HTTP itself is stateful, but the TCP connection is
`closed after each transaction (request and response pair)
`in order to reduce the possibility of it being intercepted by
`attackers.
`
`4. Discussion
`4.1 Why HTTP?
`It is possible to develop a secure application level
`protocol available only to a closed group in the Internet,
`making use of cipher technology. The reasons we chose
`HTTP as the communication protocol for a closed
`network are as follows:
`
`1) Flexibility of HTTP
`Different application
`level protocols have been
`developed for individual network services, such as FTP,
`SMTP, NNTP or GOPHER[5],[6],[7],[8]. HTTP has the
`flexibility to be able to provide services similar to those
`which have been provided by these protocols . For
`example, file transfer by FTP is accomplished by the
`object
`transfer mechanism of HTTP and, from a
`functional viewpoint,
`the Gopher protocol can be
`considered a subset of HTTP.
`Internet news and
`electronic mail services are available with an HTTP(cid:173)
`based graphical user interface via gateways for protocol
`conversions[9]. Electronic mail services within. a given
`group of institutions can be also developed using HTTP
`and CGI (Common Gateway Interface)[IO].
`
`2) Hypertext-based user-friendly graphical interface
`Using HTTP and the Hypertext Markup Language
`(HTML), distributed multimedia information systems
`with user-friendly graphical interfaces based on hypertext
`can be easily developed[ 11].
`
`3) User agents and servers available on almost all
`platforms
`HTTP has now gained widespread popularity and
`various kinds of user agents and servers are available on
`almost all platforms. Even if new protocols for closed
`networks are developed which are superior in function or
`flexibility, new clients and servers have to be developed
`for compatibility, which is costly and an obstacle to their
`universal acceptance.
`
`4.2 Proxy-proxy vs. end-to-end secure HTTP(cid:173)
`based information exchange
`As for hospitals, from which the Internet is available,
`in-hospital networks are usually protected using a dual
`home gateway and packet filter (firewall) and the Internet
`can only be accessed through proxies on the firewalls.
`The role of proxies in HTTP communication has been
`considered as important in communicating over firewalls
`and transferring information efficiently by caching. Other
`secure HTTP protocols are designed to be implemented in
`origin servers and user agents in order to assure "end-to(cid:173)
`end" security protection[l2-15]. Our approach is aimed at
`
`67
`
`10
`
`

`
`assuring proxy-pro~"Y security and is fundamentally
`>
`different from theirs.
`All proposals for secure HITP communications are
`designed to be secure against the following attacks: 1)
`network tampering, 2) replay attacks and 3) middle of the
`man attack[l2- 15]. C-HITP is also designed to be secure
`against these attacks and, in addition, it has the following
`enhancements for security protection.
`
`1) No end-user has any chance to obtain keys for
`encryption or decryption.-
`Much cost and time are necessary to decode ciphers·
`which have been used for a long time and are considered
`confidential, such as DES or RSA, so an easier and more
`practical way to obtain original information is not to
`decode them, but to "steal a key" instead. It is not realistic
`for hospital information managers to expect that all
`individual end-users, including those who connect their
`PCs to in-hospital LANs, manage their keys in a secure
`manner.
`As currently proposed secure HTTP protocols aim at
`providing end-to-end security mechanisms, responsibility
`for security is attributed to each individual user. Secure
`transfer of data exchange keys
`is performed by
`exchanging public keys (in most cases with certificates)
`between both parties. In this situation, once a private key
`is stolen, it is possible to obtain information from WWW
`servers outside the hospital.
`Undoubtedly, the purpose of security protection is
`secure commercial
`information services or on-line
`shopping services which are provided by profit-making
`companies for the masses. For commercial services, it is
`reasonable that individual users (payers) are responsible
`for "their own risks," but, as for patient information, it is
`each hospital that should be responsible for "their
`patients' risks." Each hospital should take measures to
`assure security at the institutional leveL
`
`2) Name service
`As C-HITP includes its own secure name service,
`which contains a certification mechanism, it is impossible
`to know the IP address of a server-side prO:.\"Y even if its
`C-HTTP hostname (not necessarily the same as its DNS
`name ) is known and vice versa. The C-HITP name
`service is efficient because it can do name resolution and
`host certification simultaneously.
`
`3) Difficulty in accessing from outside the closed network
`It is difficult to access any servers in a closed network
`from outside. A cracker has to take the following steps:
`
`a) To fmd the IP address and port number of a server-side
`proxy
`
`b) To get the public key of the server-side pro~"Y in order
`to send a valid C-HTTP request for C-HTfP connection.
`c) To make a TCP connection to a target server-side
`proxy using a certain client-side proxy's IP address
`d) To make the server-side proxy believe that request
`comes from a legitimate client-side proxy within the
`closed network. For this, it is necessary to know the
`private key and C-HTTP hostname of the client-side
`proxy.
`
`There are other merits in favor of C-HITP over other
`secure HTTP protocols, although they are not the original
`purposes of the development.
`
`1) Easy installation
`A C-HTTP based network is made available simply by
`installing proxies on the firewall and registering their
`information with the C-HTTP name server. Current
`HITP/ 1.0 compatible servers and clients can be used as
`they are.
`
`2) Simplicity
`There are no negotiations concerning security options
`or type and representation of objects in C-HTTP because
`C-HTTP-based communication
`is performed only
`between two types of C-HTTP proxies and between a C(cid:173)
`HTTP proxy and C-HTTP name server. They do not
`communicate directly with various types of user agents
`and servers using C-HITP. Negotiations concerning type
`and representation of objects are done between an origin
`server and user agent, using HTTP/1.0. As for these
`negotiations, C-HTTP is transparent to both of them.
`This makes the design and implementation of C-HTTP
`simple.
`
`3) Easy manipulation by end-users
`End-users do not have to employ security protection
`procedures. They do not even have to be conscious of
`using C-HTTP based communications.
`
`4.3 Disadvantages and limitations
`Our proposal has some disadvantages and limitations,
`and it should be used where its use is appropriate and
`suitable, taking them into account.
`The key technology used in the Internet is dedicated to
`assure connectivity between
`the huge number of
`computers, which may be added or removed at any time.
`Such connectivity is attractive to commercial companies
`and, in this conte:.\1,