FYI - extracted from: RISKS-LIST: RISKS-FORUM Digest Friday 17 June 1994 Volume 16 : Issue 17 ------------------------------ Date: Thu, 16 Jun 1994 17:29:40 -0400 (EDT) From: ROBACK@ENH.NIST.GOV Subject: NIST Response to Blaze Attack on Clipper Note: The following material was released by NIST in response to recent articles regarding AT&T/Matt Blaze and the key escrow chip. A second more technical response follows. ------------------------- June 2, 1994 Contact: Anne Enright Shepherd (301) 975-4858 The draft paper by Matt Blaze* describes several techniques aimed at circumventing law enforcement access to key escrowed encryption products based on government-developed technologies. As Blaze himself points out, these techniques deal only with the law-enforcement feature, and in no way reduce the key escrow chips' inherent security and data privacy. -- "None of the methods given here permit an attacker to discover the contents of encrypted traffic or compromise the integrity of signed messages. Nothing here affects the strength of the system from the point of view of the communicating parties...." p. 7. Furthermore, Blaze notes that the techniques he is suggesting are of limited use in real-world voice applications. -- "28 minutes obviously adds too much latency to the setup time for real-time applications such as secure telephone calls." p. 7. -- "The techniques used to implement them do carry enough of a performance penalty, however, to limit their usefulness in real-time voice telephony, which is perhaps the government's richest source of wiretap- based intelligence." p. 8. Anyone interested in circumventing law enforcement access would most likely choose simpler alternatives (e.g., use other nonescrowed devices, or super encryption by a second device). More difficult and time-consuming efforts, like those discussed in the Blaze paper, merit continued government review -- but they are very unlikely to be employed in actual communications. All sound cryptographic designs and products consider trade-offs among design complexity, costs, time and risks. Voluntary key escrow technology is no exception. Government researchers recognized and accepted that the law enforcement access feature could be nullified, but only if the user was willing to invest substantial time and trouble, as the Blaze report points out. Clearly, the government's basic design objective for key escrow technology was met: to provide users with very secure communications that will still enable law enforcement agencies to benefit from lawfully authorized wiretaps. It is still the only such technology available today. Today, most Americans using telephones, fax machines, and cellular phones have minimal privacy protection. The key escrow technology -- which is available on a strictly voluntary basis to the private sector -- will provide the security and privacy that Americans want and need. * Statements from "Protocol Failure in the Escrowed Encryption Standard," May 20 draft report by Matt Blaze, AT&T Bell Laboratories ----- Note: The following provides additional technical material in response to questions regarding a recent paper by Matt Blaze on key escrow encryption. -------------------------------------- Technical Fact Sheet on Blaze Report and Key Escrow Encryption Several recent newspaper articles have brought attention to a report prepared by Dr. Matthew Blaze, a researcher at AT&T's Bell Labs. These articles characterize a particular finding in Blaze's report as a ~flaw~ in the U.S. government's key escrow encryption technology. None of the findings in Dr. Blaze's paper in any way undermines the security and privacy provided by the escrow encryption devices. The finding which has received the most publicity could allow a non-compliant or ~rogue~ application to send messages to compliant or ~non-rogue~ users which will not be accessible by law enforcement officials through the escrowed encryption standard field called the Law Enforcement Access Field (LEAF). Dr. Blaze's approach uses the openly disclosed fact that the LEAF contains 16-bit checkword to prevent rogue users from modifying the law enforcement access mechanism. This 16-bit checkword is part of the 128-bit LEAF, which also includes the enciphered traffic key and the unique chip identifier. Dr. Blaze's method is to randomly generate different 128-bit LEAFs until he gets one that passes the checkword. It will take on average 216, or 65,536 tries. This is not a formidable task; it could be done in less than an hour. Dr. Blaze questions the adequacy of a 16-bit checkword and suggests using a larger one, to ensure that the exhaustion attack would be so time consuming as to be impractical. The chip designers recognized the strengths and limitations of a 16-bit checkword. Following are the reasons why they chose to use a checkword of only 16 bits: * There were four fundamental considerations that the designers considered in choosing the LEAF parameters. These were: (1) ease of access by authorized law enforcement agencies, (2) impact on communications, (3) a sufficiently large identifier field which would not constrain manufacturers, and (4) the difficulty required to invalidate the LEAF mechanism by techniques such as those described by Dr. Blaze. * The purpose of the LEAF is to preserve law enforcement's ability to access communications in real-time. The encrypted traffic key, which enables them to do this, is 80 bits long. In addition to this 80-bit field, the LEAF must contain the unique identification number of the key escrow encryption chip doing the encryption. * The size of the identifier field was the subject of considerable deliberation. In the earliest considerations it was only 25 bits long. The chip designers recognized that 25 bits did not offer enough flexibility to provide for multiple manufacturers of key escrow devices. Different chip manufacturers would need manufacturer identifiers as well as their own chip identifiers to ensure that identifiers are unique. Eventually, the designers agreed that 32 bits would adequately meet this requirement. * In many environments, error-free delivery of data is not guaranteed, and there is considerable concern by communication engineers that requiring error-free transmission of a fixed field (the LEAF) could make the encryption device difficult to use. In early discussions with industry, they were opposed to any checkword. In the end, they agreed it would be acceptable if the size of the LEAF was restricted to 128 bits. This left 16 bits for a checkword to inhibit bypassing the LEAF. While recognizing the possibility of exhausting these 16 bits, the designers concluded that 16 bits are adequate for the first intended application. Security enhancements are being made for other applications, such as the TESSERA card. Note that computations are required to search for a matching checkword, which then has to be properly substituted into the communications protocol. The performance and cost penalties of the search operation are significant for telephone, radio, and other such applications, thus providing adequate protection against this technique for bypassing the LEAF. In summary: * Although this technique would allow one to bypass the LEAF, the security provided by the escrow encryption devices would not be altered. Users' information would still be protected by the full strength of the encryption algorithm. * Dr. Blaze was accurate in noting that these attacks are of limited effectiveness in real-time telephony. * When designing the key escrow chip, NSA emphasized sound security and privacy, along with user friendliness. The attacks described by Dr. Blaze were fully understood at the time of initial chip design. The use of 16 bits for the checkword was an appropriate choice in view of the constraints of a 128-bit LEAF. It provides excellent security for real-time telephone applications with high assurance that law enforcement's interests are protected. * Dr. Blaze's research was done using prototype TESSERA cards. As part of the family of planned releases/upgrades, NSA already has incorporated additional security safeguards into the production TESSERA cards to protect against the kinds of attacks described by Dr. Blaze. -------- end of article ---------------------- _______________________________________________________________________________ Paul Ferguson US Sprint Managed Network Engineering tel: 703.904.2437 Herndon, Virginia USA internet: paul@hawk.sprintmrn.com