Until the mid-1990s or so, brute force attacks were beyond the capabilities of computers that were within the budget of the attacker community. By that time, however, significant compute power was typically available and accessible. General-purpose computers such as PCs were already being used for brute force attacks. For serious attackers with money to spend, such as some large companies or governments, Field Programmable Gate Array (FPGA) or Application-Specific Integrated Circuits (ASIC) technology offered the ability to build specialized chips that could provide even faster and cheaper solutions than a PC. As an example, the AT&T Optimized Reconfigurable Cell Array (ORCA) FPGA chip cost about $200 and could test 30 million DES keys per second, while a $10 ASIC chip could test 200 million DES keys per second; compare that to a PC which might be able to test 40,000 keys per second. Distributed attacks, harnessing the power of up to tens of thousands of powerful CPUs, are now commonly employed to try to brute-force crypto keys.
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Table 2 — from a 1996 article discussing both why exporting 40-bit keys was, in essence, no crypto at all and why DES' days were numbered — shows what DES key sizes were needed to protect data from attackers with different time and financial resources. This information was not merely academic; one of the basic tenets of any security system is to have an idea of what you are protecting and from whom are you protecting it! The table clearly shows that a 40-bit key was essentially worthless against even the most unsophisticated attacker. On the other hand, 56-bit keys were fairly strong unless you might be subject to some pretty serious corporate or government espionage. But note that even 56-bit keys were clearly on the decline in their value and that the times in the table were worst cases.
Secure use of cryptography requires trust. While secret key cryptography can ensure message confidentiality and hash codes can ensure integrity, none of this works without trust. In SKC, Alice and Bob had to share a secret key. PKC solved the secret distribution problem, but how does Alice really know that Bob is who he says he is? Just because Bob has a public and private key, and purports to be "Bob," how does Alice know that a malicious person (Mallory) is not pretending to be Bob?
It is worth noting that the discussion above describes the Microsoft version of CHAP, or MS-CHAP (MS-CHAPv2 is described in RFC 2759). MS-CHAP assumes that it is working with hashed values of the password as the key to encrypting the challenge. More traditional CHAP (RFC 1994) assumes that it is starting with passwords in plaintext. The relevance of this observation is that a CHAP client, for example, cannot be authenticated by an MS-CHAP server; both client and server must use the same CHAP version.
It is well beyond the scope of this paper to discuss other forms of breaking DES and other codes. Nevertheless, it is worth mentioning a couple of forms of cryptanalysis that have been shown to be effective against DES. Differential cryptanalysis, invented in 1990 by E. Biham and A. Shamir (of RSA fame), is a chosen-plaintext attack. By selecting pairs of plaintext with particular differences, the cryptanalyst examines the differences in the resultant ciphertext pairs. Linear plaintext, invented by M. Matsui, uses a linear approximation to analyze the actions of a block cipher (including DES). Both of these attacks can be more efficient than brute force.
In March 2016, the SSL DROWN (Decrypting RSA with Obsolete and Weakened eNcryption) attack was announced. DROWN works by exploiting the presence of SSLv2 to crack encrypted communications and steal information from Web servers, email servers, or VPN sessions. You might have read above that SSLv2 fell out of use by the early 2000s and was formally deprecated in 2011. This is true. But backward compatibility often causes old software to remain dormant and it seems that up to one-third of all HTTPS sites at the time were vulnerable to DROWN because SSLv2 had not been removed or disabled.
While there did not appear to be any rush to abandon TrueCrypt, it was also the case that you don't want to use old, unsupported software for too long. Another replacement was announced almost immediately upon the demise of TrueCrypt: "TrueCrypt may live on after all as CipherShed." The CipherShed group never produced a product, however, and the CipherShed Web site no longer appeared to be operational sometime after October 2016. The only current, working fork of TrueCrypt appears to be VeraCrypt, which is also open source, multi-platform, operationally identical to TrueCrypt, and compatible with TrueCrypt containers.
Having nothing to do with TrueCrypt, but having something to do with plausible deniability and devious crypto schemes, is a new approach to holding password cracking at bay dubbed Honey Encryption. With most of today's crypto systems, decrypting with a wrong key produces digital gibberish while a correct key produces something recognizable, making it easy to know when a correct key has been found. Honey Encryption produces fake data that resembles real data for every key that is attempted, making it significantly harder for an attacker to determine whether they have the correct key or not; thus, if an attacker has a credit card file and tries thousands of keys to crack it, they will obtain thousands of possibly legitimate credit card numbers. See "'Honey Encryption' Will Bamboozle Attackers with Fake Secrets" (Simonite) for some general information or "Honey Encryption: Security Beyond the Brute-Force Bound" (Juels & Ristenpart) for a detailed paper.
CRCs were developed in the early 1960s to provide message integrity, bit-error detection, and, in some cases, bit-error correction in data communication systems. There are many CRC codes in use today, almost all in some sort of networking application. CRCs are expressed as an n-order polynomial yielding an n-bit result; i.e., a CRC-n polynomial is one with n+1 terms and is used to compute an n-bit checksum. An n-bit CRC code can be used with an arbitrary length input, can detect 100% of burst errors up to n bits in length, and can detect bursts of n bits or more with probability (1-2-n).
In theory, a quantum computer can solve problems that are too computationally complex for a today's conventional computers. The implications for using quantum methods to attack classic cryptography algorithms should be readily apparent. Since we can theoretically build a computer where an n-qubit device can take on 2n states at the same time, it renders an n-bit keyspace susceptible to a nearly immediate brute force attack. Before any panic sets in, recognize that quantum computers today are relatively small, so a large key (say, 256 bits or larger) is as safe today from a quantum computer brute force attack as a smaller key (e.g., 128 bits or smaller) is against a brute-force attack from a classic computer. Clearly, there will be this battle for some time keeping the keys large enough to withstand quantum attacks. (This is similar to the increasing size of RSA keys to keep them computationally infeasible to factor.) Meanwhile, a functional quantum computer that can pose a serious attack of cryptography schemes as we know them today does not exist, and won't for many years — if ever. Meanwhile, it would be folly to ignore the potential threat and be blindsided.
As a slight aside, another way that people try to prove that their new crypto scheme is a good one without revealing the mathematics behind it is to provide a public challenge where the author encrypts a message and promises to pay a sum of money to the first person — if any — who cracks the message. Ostensibly, if the message is not decoded, then the algorithm must be unbreakable. As an example, back in 2011, a $10,000 challenge page for a new crypto scheme called DioCipher was posted and scheduled to expire on 1 January 2013 — which it did. That was the last that I heard of DioCipher. I leave it to the reader to consider the validity and usefulness of the public challenge process.
Gary C. Kessler, Ph.D., CCE, CISSP, is the president and janitor of Gary Kessler Associates, a consulting, research, and training firm specializing in computer and network security (with a focus on maritime cybersecurity), computer forensics, and TCP/IP networking. He has written over 75 papers, articles, and book chapters for industry publications, is co-author of ISDN, 4th. edition (McGraw-Hill, 1998), a past editor-in-chief of the Journal of Digital Forensics, Security and Law, and co-author of Maritime Cybersecurity: A Guide for Leaders and Managers, 2/e (2022). Gary retired as Professor of Cybersecurity at Embry-Riddle Aeronautical University in Daytona Beach, Florida, and is an Adjunct Professor at Edith Cowan University in Perth, Western Australia. Gary was formerly an Associate Professor and Program Director of the M.S. in Information Assurance program at Norwich University in Northfield, Vermont, and a member of the Vermont Internet Crimes Against Children (ICAC) Task Force; he started the M.S. in Digital Investigation Management and undergraduate Computer & Digital Forensics programs at Champlain College in Burlington, Vermont. Gary's e-mail address is gck@garykessler.net and his PGP public key can be found at . Gary is also a SCUBA instructor and U.S. Coast Guard licensed captain. 2ff7e9595c
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