RFC1805 - Location-Independent Data/Software Integrity Protocol

Network Working Group A. Rubin

Request for Comments: 1805 Bellcore

Category: Informational June 1995

Location-Independent Data/Software Integrity Protocol

Status 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 at http://info.bellcore.com/BETSI/betsi.Html.

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.

High 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 format

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>

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 current

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