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Network Working Group                                           A. RubinRequest for Comments: 1805                                      BellcoreCategory: Informational                                        June 1995Location-Independent Data/Software Integrity ProtocolStatus of this Memo   This memo provides information for the Internet community.  This memo   does not specify an Internet standard of any kind.  Distribution of   this memo is unlimited.Abstract   This memo describes a protocol for adding integrity assurance to   files that are distributed across the Internet.  This protocol is   intended for the distribution of software, data, documents, and any   other file that is subject to malicious modification.  The protocol   described here is intended to provide assurances of integrity and   time.  A trusted third party is required.Introduction   One problem with any system for verifying the integrity of a file is   that the verifying program itself may be attacked. Thus, although   users may be reassured by their software that a file has not changed,   in reality, the file, and the verifier might have both changed.   Because of this danger, a protocol that does not rely on the   distribution of some special software, but rather, is based entirely   on widely used standards, is very useful. It allows users to build   their own software, or obtain trusted copies of software to do   integrity checking independently. Therefore, the protocol described   in this memo is composed of ASCII messages that may be sent using e-   mail or any other means. There is an existing implementation, Betsi   [1], that is designed this way. Betsi has been in existence since   August, 1994, and is operational on the Internet. It can be accessed   by sending e-mail to certify@bellcore.com with subject 'help', or via   the world wide web athttp://info.bellcore.com/BETSI/betsi.html.Rubin                        Informational                      [Page 1]

RFC 1805 Location-Independent Data/Software Integrity Protocol June 1995   The purpose of the proposed protocol is for authors to be able to   distribute their files to users on the internet with guarantees of   time and integrity, by use of a trusted third party. The protocol is   divided into several phases:           I.   Author registration           II.  Author verification           III. File Certification           IV.  File Distribution           V.   File Integrity Verification   Phases I, III, IV, and V are defined in the protocol. Phase II is   intentionally not defined. Author verification can be different for   different applications, and the particular method chosen for phase II   is identified in phases III and V.  It is the hope that further   Internet Drafts will describe the various possibilities for phase II.   This memo describes the method for author verification in the Betsi   system, and makes several recommendations.Requirements   It is important that the integrity and time information be   independent from the location of the file. Lowry [2] defines a syntax   and protocols for location-independent objects.  His system requires   that end-users possess special software, and is still in the   prototype stage.  The protocol described in this memo has been   implemented, and is already in wide-spread use across the Internet.   It is simple, compact and easy to understand.  The disadvantage of a   very complex system is that users may not be inclined to trust the   designers' claims if they cannot understand how it works.Assumptions   The three entities in the protocol are Authors (A), Users (U), and a   Trusted third party (T).  The protocol described here is algorithm   independent, and all of the messages are in ASCII.  It is assumed   that for each signature scheme used, there is a well-known   verification key associated with T.   Any signature scheme may be used, as long as there is a standard   ASCII representation of a digital signature. PGP [3] meets all of the   above requirements, but it also requires encryption, and thus, export   restrictions may deter some users. The DSS [4] is recommended, but   some suspect that it contains a trapdoor [5] based on some results by   Simmons [6]. It is also not clear that there is a standard for   generating an ASCII signature using the DSS.Rubin                        Informational                      [Page 2]

RFC 1805 Location-Independent Data/Software Integrity Protocol June 1995High level view   The protocol works as follows. In the first phase, authors request to   register with the trusted third party, T.  Any registered author can   distribute files with integrity and time assurance. Time assurance   means that there is a guarantee that a file existed at a given time.   In the second phase, T somehow verifies the identity of an author who   requests to register.  Registration is not complete until this   verification takes place.   To distribute a file, a registered author computes a cryptographic   hash of the file, and sends it over an integrity protected channel to   T. T then creates an object containing the hash, the current time,   the name of the author, the name of the file, and some other   information, seals the object, and returns it to the author. The   author can then use the sealed object as a location-independent proof   of the integrity and timeliness of the file.   Any user who obtains the file and the sealed object, can compute the   cryptographic hash of the file, check the seal on the object, and   verify that the object has not changed.   The trusted third party must maintain a widely available, dated, and   signed, certificate revocation list (CRL). Users who access a file   with a certificate must check that the CRL is current and complete,   and that the certificate is not listed.Author registration   In the first phase, authors request to register with the trusted   third party, T. The author sends an ASCII message to T containing   keywords followed by values. Some of the fields are optional, and are   marked with a *. The values are represented with angle brackets < >.     AUTHOR-NAME= <first m. last>   * AUTHOR-ORGANIZATION= <Company, school, etc.>   * AUTHOR-EMAIL= <e-mail address>     AUTHOR-LOCATION= <city, state>   * AUTHOR-PHONE-1= <Home phone>   * AUTHOR-PHONE-2= <Work phone>     SIGNATURE-SYSTEM= <name of signature system>   * MISC-FIELD-n= <Any number of additional fields can be defined here>   * AUTHOR-PUBLIC-KEY=   * <public key of author>   Each of the fields contains the keyword and the value on the same   line, except for the public key. An ASCII version of the key is   pasted on the line after the AUTHOR-PUBLIC-KEY keyword.  The formatRubin                        Informational                      [Page 3]

RFC 1805 Location-Independent Data/Software Integrity Protocol June 1995   of this ASCII key will depend on the signature system used.  The   public key field is optional. The user may include his own, or one   can be supplied by T during phase II.  T responds with a message that   the request was received, and that the user should wait for off-line   verification.  If a user receives this confirmation message, and he   did not request to register, he knows that somebody may be attempting   to register on his behalf.Author verification   The trusted third party, T, must verify the identity of the author   who sent the request message in phase I.  The rest of the information   in the request is also confirmed.  This process takes place off-line.   The method used is intentionally left open, but whatever technique is   used must be identified in phases III and V.   In the Betsi implementation, T uses the phone company infrastructure.   T calls directory assistance (1-xxx-555-1212) in the city of the   author and asks for the author's number. Then, that number is called,   and T asks the author to verify the information sent in the request.   In particular, T insures that the author has registered his correct   public key. Or, in some cases, T assigns a public key to the author.   As Betsi is only operational in the United States, other mechanisms   need to be in place for verifying identities of people   internationally. Hopefully, standards for doing this will arise. The   rest of the protocol is independent of whatever mechanism is used for   off-line identity and public key verification.File certification   Registered authors can obtain location-independent objects from the   trusted third party, T, that vouch for the integrity and time of any   file.   An author generates the following ASCII message and signs it with the   signature key that corresponds to the public key that was registered.     AUTHOR-NAME= <first m. last>     HASH-FUNCTION= <md5,sha, etc.>   * FILE-LOCATION= <ftp site/directory>     <list of hashes>   Each entry in the <list of hashes> consists of two mandatory fields   and one optional one, as follows:     <fixed-length hash of file> <name of file> <version number>Rubin                        Informational                      [Page 4]

RFC 1805 Location-Independent Data/Software Integrity Protocol June 1995   The <fixed-length hash of file> is a fixed-length hexadecimal value   corresponding to the hash of the contents of the file.  For MD5, the   output is 32 hexadecimal digits. There is one space between the   fields, and the name of the file contains no spaces.  The <version   number> is optional.  The <list of hashes> contains at least one   entry, and may contain as many as the author wants.  The message is   signed and sent to the trusted third party, T.   When T receives the request for file certification, he verifies the   signature on the request and creates a location-independent   certificate for the request. The certificate is signed by T, and   contains the following information:     TRUSTED-PARTY= <identity of T>     AUTHOR-VERIFICATION-METHOD= <how authors are verified off-line>     AUTHOR-NAME= <first m. last>     AUTHOR-ORGANIZATION= <company, school, etc.>     HASH-FUNCTION= <md5,sha, etc.>     DATE= <date>     <list of hashes>   The <list of hashes> is the same as the one in the author's request.   T signs the message and sends it to the author, who verifies the   signature and the contents of the certificate.  Note that the method   for off-line author verification is included in the certificate.File distribution   In the file distribution phase, the author distributes his file,   along with the certificate from T. The file and certificate are   location-independent. That is,  the integrity and timeliness of the   file can be verified independently from the location of the file and   the certificate. This means that files can be distributed from   insecure sites, and over insecure networks.File integrity verification   The final phase is file integrity verification. A user obtains the   public key of the trusted third party, T, from several independent   sources, until he is convinced of its authenticity.  The user then   verifies the certificate for a file, and decides whether or not he   trusts the method of off-line verification that was used by T. If so,   then he extracts the name of the hash function in the certificate,   and performs the hash function on the actual file. Finally, the user   compares the hash of the file to the hash in the certificate. The   user also checks the date in the certificate if he is concerned with   this information.  As a last step, the user checks the highly   available certificate revocation list of T, to see if the currentRubin                        Informational                      [Page 5]

RFC 1805 Location-Independent Data/Software Integrity Protocol June 1995   certificate is listed.  When all of this has concluded, if none of   the assumptions of the system has been violated, then the user is   assured of the integrity and timeliness of the file.References   [1] Rubin, A., "Trusted Distribution of Software over the Internet",       Internet Society Symposium on Network and Distributed System       Security," pp. 47-53, 1995.   [2] Lowrey, J., "Location-Independent Information Object Security",       Internet Society Symposium on Network and Distributed System       Security," pp. 54-62, 1995.   [3] Zimmerman, P., "PGP User's Guide", 1992.   [4] National Institute for Standards and Technology, Digital       Signature Standard (DSS), Federal Register 56(169), 1991.   [5] Schneier, B., "Applied Cryptography", ISBN 0-471-59756-2.   [6] Simmons, G., "The Subliminal Channels of the U.S.  Digital       Signature Algorithm (DSA)", Proceedings of the 3rd Symposium on:       State and Progress of research in Cryptography, pp. 35-54, 1993.Security Considerations   Security issues are discussed throughout this memo.Author's Address   Aviel D. Rubin   Bellcore   Morristown, NJ 07960   USA   Phone: +1 201 829 5922   Fax: +1 201 829 2645   EMail: rubin@bellcore.comRubin                        Informational                      [Page 6]