Overview

Digital certificates, signed by a certificate authority (CA), bind a public key to an identity string, e.g. the a name of an individual or organization. Digital certificates also contain expiry information that a user checks prior to communication. However, due to various reasons, both malicious and benign, certificates may need to be revoked or terminated prematurely. It is not sufficient just to check if a certificate is expired.

Prior to communication a user must be able to inquire about the status of a certificate to be assured that communication is still secure. This is done by issuing a revocation query to the CA service containing revocation information. However, each time a user issues a revocation query a loss of privacy occurs. Namely, two things can discovered:

• Source of revocation query
• Target of query

We propose a technique, called range queries, that allows users to inquire about the status of a certificate while maintaining their privacy. A distribution center can not determine, with a certain statistical probability, exactly which certificate a user is interested in. Full details are located in our paper which can be downloaded here.

People

• Gene Tsudik - Research Professor
• John Solis - Graduate Student

Publications

• Simple and Flexible Revocation Checking with Privacy
  John Solis and Gene Tsudik
  Privacy Enhancing Technologies (PET 2006), June 2006 [PDF] , [PS]

Presentations

• Simple and Flexible Revocation Checking with Privacy [PPT]
  Conference presentation at PET 2006

Implementation & Source Code

Client:

Begin by downloading the binary or source code. Compile the source code and execute the client. The following interface will be displayed to the user:

     After this information has been entered the user clicks on the "Query" button and the client attempts to establish a connection with the server.  Once a connection has been established to the server, the server returns the public parameters of the system, as well as the results of the client query.  An example of these results can be seen in the following figure:

Server

The server itself has no interactive interface. All information is provided to the server through a config file that is passed during execution time. This configuration file is a simple text file that has all the information needed by the server. The header information from the a sample (default.conf) file is shown here:

        ####################################################################################
	#  Server Configuration File Format
	#
	#   1st line: Permutation Algorithm to use:  des,rc4,blowfish
	#   2nd line: Key to use for permutation algorithm
	#   3rd line: Serial number of smallest possible node value
	#   4th line: Serial number of largest possible node value
	#
	#   Remaing lines:
	#	  
	#
	#   ** ReasonID is just a number to be used by administrators and the actual mapping
	#		between ID and a reason is determined by each individual administrator
	#
	####################################################################################
        des
	pkomnij7
	5
	1000000000
	36753 22 ./sslcerts/100001.pem
	409661 22 ./sslcerts/100002.pem
	1257298 22 ./sslcerts/100003.pem
	2218829 22 ./sslcerts/100004.pem

	

Two implementations of the Ranged Query CRT implementation are available for download. The first download is the original Unix implementation which was written in C++ in Slackware Linux 10.0. The second download is a port of the Unix imlementation to the Windows enviornment with both source code and binaries available for download. Both implementations use version 'openssl-0.9.8a' of the OpenSSL library.

     Note: All packages require that the OpenSSL libraries be installed and can be downloaded from the OpenSSL website.