Selections from RISKS DIGEST 18.54

[Alan Olsen <alan@ctrl-alt-del.com>]

The latest issue of RISKS has some info of Crypto interest.  The second one
may have been posted to the list, but the first is well worth reading.
(Shamir Adi on new methods of breaking sealed module cryptosystems.  Maybe
this is why they hauled him away...)

>Date: Fri, 18 Oct 1996 16:58:50 +0200
>From: Shamir Adi <shamir@wisdom.weizmann.ac.il>
>Subject: A new attack on DES 
>
>You have recently referred in RISKS [18.50, 18.52] to the ingenious new
>attack against public key cryptosystems developed at Bellcore. All the
>published information on the subject (including Bellcore's press release)
>stress that the attack is not applicable to secret key cryptosystems.  Well,
>Eli Biham and I have just released a research announcement in which we show
>that an extension of the attack can, under the same realistic fault model,
>break almost any secret-key algorithm, including DES, multiple DES, IDEA,
>etc. The attack on DES was actually implemented on a PC, and it found the
>key by analysing fewer than 200 ciphertexts generated from unknown
>cleartexts.
>
>Adi Shamir
>
>= = = = = =
>
>Research announcement: A new cryptanalytic attack on DES
>
>Eli Biham                                 Adi Shamir
>Computer Science Dept.                    Applied Math Dept.
>The Technion                              The Weizmann Institute
>Israel                                    Israel
>
>                 18 October 1996
>                     (DRAFT)
>
>In September 96, Boneh Demillo and Lipton from Bellcore announced an
>ingenious new type of cryptanalytic attack which received widespread
>attention (see, e.g., John Markoff's 9/26/96 article in the New York Times).
>Their full paper had not been published so far, but Bellcore's press release
>and the authors' FAQ (available at
>http://www.bellcore.com/PRESS/ADVSRY96/medadv.html) specifically state that
>the attack is applicable only to public key cryptosystems such as RSA, and
>not to secret key algorithms such as the Data Encryption Standard (DES).
>According to Boneh, "The algorithm that we apply to the device's faulty
>computations works against the algebraic structure used in public key
>cryptography, and another algorithm will have to be devised to work against
>the nonalgebraic operations that are used in secret key techniques." In
>particular, the original Bellcore attack is based on specific algebraic
>properties of modular arithmetic, and cannot handle the complex bit
>manipulations which underly most secret key algorithms.
>
>In this research announcement, we describe a related attack (which we call
>Differential Fault Analysis, or DFA), and show that it is applicable to
>almost any secret key cryptosystem proposed so far in the open literature.
>In particular, we have actually implemented DFA in the case of DES, and
>demonstrated that under the same hardware fault model used by the Bellcore
>researchers, we can extract the full DES key from a sealed tamperproof DES
>encryptor by analysing fewer than 200 ciphertexts generated from unknown
>cleartexts.  The power of Differential Fault Analysis is demonstrated by the
>fact that even if DES is replaced by triple DES (whose 168 bits of key were
>assumed to make it practically invulnerable), essentially the same attack
>can break it with essentially the same number of given ciphertexts.
>
>We would like to greatfully acknowledge the pioneering contribution of Boneh
>Demillo and Lipton, whose ideas were the starting point of our new attack.
>
>In the rest of this research announcement, we provide a short technical
>summary of our practical implementation of Differential Fault Analysis of 
>DES. Similar attacks against a large number of other secret key cryptosystems
>will be described in the full version of our paper.
>
>TECHNICAL DETAILS OF THE ATTACK
>
>The attack follows the Bellcore fundamental assumption that by exposing a
>sealed tamperproof device such as a smart card to certain physical effects
>(e.g., ionizing or microwave radiation), one can induce with reasonable
>probability a fault at a random bit location in one of the registers at some
>random intermediate stage in the cryptographic computation. Both the bit
>location and the round number are unknown to the attacker.
>
>We further assume that the attacker is in physical possession of the
>tamperproof device, so that he can repeat the experiment with the same
>cleartext and key but without applying the external physical effects. As a
>result, he obtains two ciphertexts derived from the same (unknown) cleartext
>and key, where one of the ciphertexts is correct and the other is the result
>of a computation corrupted by a single bit error during the computation. For
>the sake of simplicity, we assume that one bit of the right half of the data
>in one of the 16 rounds of DES is flipped from 0 to 1 or vice versa, and
>that both the bit position and the round number are uniformly distributed.
>
>In the first step of the attack we identify the round in which the fault
>occurred.  This identification is very simple and effective: If the fault
>occurred in the right half of round 16, then only one bit in the right half
>of the ciphertext (before the final permutation) differs between the two
>ciphertexts. The left half of the ciphertext can differ only in output bits
>of the S box (or two S boxes) to which this single bit enters, and the
>difference must be related to non-zero entries in the difference
>distribution tables of these S boxes.  In such a case, we can guess the six
>key bit of each such S box in the last round, and discard any value which
>disagree with the expected differences of these S boxes (e.g., differential
>cryptanalysis). On average, about four possible 6-bit values of the key
>remain for each active S box.
>
>If the faults occur in round 15, we can gain information on the key bits
>entering more than two S boxes in the last round: the difference of the
>right half of the ciphertext equals the output difference of the F function
>of round 15.  We guess the single bit fault in round 15, and verify whether
>it can cause the expected output difference, and also verify whether the
>difference of the right half of the ciphertext can cause the expected
>difference in the output of the F function in the last round (e.g., the
>difference of the left half of the ciphertext XOR the fault).  If
>successful, we can discard possible key values in the last round, according
>to the expected differences.  We can also analyse the faults in the 14'th
>round in a similar way.  We use counting methods in order to find the key.
>In this case, we count for each S box separately, and increase the counter
>by one for any pair which suggest the six-bit key value by at least one of
>its possible faults in either the 14'th, 15'th, or 16'th round.
>
>We have implemented this attack on a personal computer.  Our analysis
>program found the whole last subkey given less than 200 ciphertexts,
>with random single-faults in all the rounds.
>
>This attack finds the last subkey.  Once this subkey is known, we can
>proceed in two ways: We can use the fact that this subkey contains 48 out of
>the 56 key bits in order to guess the missing 8 bits in all the possible
>2^8=256 ways. Alternatively, we can use our knowledge of the last subkey to
>peel up the last round (and remove faults that we already identified), and
>analyse the preceding rounds with the same data using the same attack. This
>latter approach makes it possible to attack triple DES (with 168 bit keys),
>or DES with independent subkeys (with 768 bit keys).
>
>This attack still works even with more general assumptions on the fault
>locations, such as faults inside the function F, or even faults in the key
>scheduling algorithm.  We also expect that faults in round 13 (or even prior
>to round 13) might be useful for the analysis, thus reducing the number of
>required ciphertext for the full analysis.
>
>OTHER VULNERABLE CIPHERS
>
>Differential Fault Analysis can break many additional secret key
>cryptosystems, including IDEA, RC5 and Feal.  Some ciphers, such as Khufu,
>Khafre and Blowfish compute their S boxes from the key material.  In such
>ciphers, it may be even possible to extract the S boxes themselves, and the
>keys, using the techniques of Differential Fault Analysis.  Differential
>Fault Analysis can also be applied against stream ciphers, but the
>implementation might differ by some technical details from the
>implementation described above.
>
>------------------------------
>
>Date: Fri, 18 Oct 1996 09:21:58 -0400 (EDT)
>From: Edupage Editors <educom@elanor.oit.unc.edu>
>Subject: "Key Recovery" Replaces "Key Escrow" in Encryption Plan (Edupage)
>
>The latest government proposal for encryption software controls touts a new
>approach called "key recovery."  This provision would allow law enforcement
>officials to rebuild, or "recover" the mathematical key to encoded messages
>with the help of third-party code-breakers.  The new policy reflects
>suggestions made in a National Research Council report released earlier this
>year.  Under the Clinton plan, encryption keys would be expanded from 40
>bits to 56 bits in products to be exported, provided the company agrees to
>the key recovery process.  In addition, authority to issue licenses for
>overseas sales of such products would move from the State Department, where
>they're handled as "munitions," over to the Commerce Department.  The
>Business Software Alliance, however, is still not completely happy with the
>compromise.  "We expect to go back to Congress," says a BSA spokeswoman.
>"Although the announcement was clearly a step in the right direction, it's
>not at all what the industry was looking for in its entirety."  (*Investor's
>Business Daily* 17 Oct 1996 A4; Edupage 17 October 1996)