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Cryptographic hash functions

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Original author: 
Megan Geuss


List your passwords alphabetically, so it's easy for you and others to find them!

Give three password crackers a list of 16,000 cryptographically hashed passwords and ask them to come up with the plaintext phrases they correspond to. That's what Ars did this week in Dan Goodin's Anatomy of a hack: How crackers ransack passwords like “qeadzcwrsfxv1331.” Turns out, with just a little skill and some good hardware, three prominent password crackers were able to decode up to 90 percent of the list using common techniques.

The hashes the security experts used were converted using the MD5 cryptographic hash function, something that puzzled our readers a bit. MD5 is seen as a relatively weak hash function compared to hashing functions like bcrypt. flunk wrote, "These articles are interesting but this particular test isn't very relevant. MD5 wasn't considered a secure way to hash passwords 10 years ago, let alone now. Why wasn't this done with bcrypt and salting? That's much more realistic. Giving them a list of passwords that is encrypted in a way that would be considered massively incompetent in today's IT world isn't really a useful test."

To this, Goodin replied that plenty of Web services employ weak security practices: "This exercise was entirely relevant given the huge number of websites that use MD5, SHA1, and other fast functions to hash passwords. Only when MD5 is no longer used will exercises like this be irrelevant." Goodin later went on to cite the recent compromises of "LinkedIn, eHarmony, and LivingSocial," which were all using "fast hashing" techniques similar to MD5.

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Original author: 
Dan Goodin


Thanks to the XKCD comic, every password cracking word list in the world probably has correcthorsebatterystaple in it already.

Aurich Lawson

In March, readers followed along as Nate Anderson, Ars deputy editor and a self-admitted newbie to password cracking, downloaded a list of more than 16,000 cryptographically hashed passcodes. Within a few hours, he deciphered almost half of them. The moral of the story: if a reporter with zero training in the ancient art of password cracking can achieve such results, imagine what more seasoned attackers can do.

Imagine no more. We asked three cracking experts to attack the same list Anderson targeted and recount the results in all their color and technical detail Iron Chef style. The results, to say the least, were eye opening because they show how quickly even long passwords with letters, numbers, and symbols can be discovered.

The list contained 16,449 passwords converted into hashes using the MD5 cryptographic hash function. Security-conscious websites never store passwords in plaintext. Instead, they work only with these so-called one-way hashes, which are incapable of being mathematically converted back into the letters, numbers, and symbols originally chosen by the user. In the event of a security breach that exposes the password data, an attacker still must painstakingly guess the plaintext for each hash—for instance, they must guess that "5f4dcc3b5aa765d61d8327deb882cf99" and "7c6a180b36896a0a8c02787eeafb0e4c" are the MD5 hashes for "password" and "password1" respectively. (For more details on password hashing, see the earlier Ars feature "Why passwords have never been weaker—and crackers have never been stronger.")

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hypnosec writes "BLAKE2 has been recently announced as a new alternative to the existing cryptographic hash algorithms MD5 and SHA-2/3. With applicability in cloud storage, software distribution, host-based intrusion detection, digital forensics and revision control tools, BLAKE2 performs a lot faster than the MD5 algorithm on Intel 32- and 64-bit systems. The developers of BLAKE2 insist that even though the algorithm is faster, there are no loose ends when it comes to security. BLAKE2 is an optimized version of the then SHA-3 finalist BLAKE."

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Enlarge / A slide from Steube's presentation outlining a more efficient way to crack passwords protected by the SHA1 cryptographic algorithm.

hashcat.net

A researcher has devised a method that reduces the time and resources required to crack passwords that are protected by the SHA1 cryptographic algorithm.

The optimization, presented on Tuesday at the Passwords^12 conference in Oslo, Norway, can speed up password cracking by 21 percent. The optimization works by reducing the number of steps required to calculate SHA1 hashes, which are used to cryptographically represent strings of text so passwords aren't stored as plain text. Such one-way hashes—for example 5baa61e4c9b93f3f0682250b6cf8331b7ee68fd8 to represent "password" (minus the quotes) and e38ad214943daad1d64c102faec29de4afe9da3d for "password1"—can't be mathematically unscrambled, so the only way to reverse one is to run plaintext guesses through the same cryptographic function until an identical hash is generated.

Jens Steube—who is better known as Atom, as the pseudonymous developer of the popular Hashcat password-recovery program—figured out a way to remove identical computations that are performed multiple times from the process of generating of SHA1 hashes. By precalculating several steps ahead of time, he's able to skip the redundant steps, shaving 21 percent of the time required to crack large numbers of passwords. Slides from Tuesday's presentation are here.

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An overview of a chosen-prefix collision

Marc Stevens

Flame

The Flame espionage malware that infected computers in Iran achieved mathematic breakthroughs that could only have been accomplished by world-class cryptographers, two of the world's foremost cryptography experts said.

"We have confirmed that Flame uses a yet unknown MD5 chosen-prefix collision attack," Marc Stevens and B.M.M. de Weger wrote in an e-mail posted to a cryptography discussion group earlier this week. "The collision attack itself is very interesting from a scientific viewpoint, and there are already some practical implications."

"Collision" attacks, in which two different sources of plaintext generate identical cryptographic hashes, have long been theorized. But it wasn't until late 2008 that a team of researchers made one truly practical. By using a bank of 200 PlayStation 3 consoles to find collisions in the MD5 algorithm—and exploiting weaknesses in the way secure sockets layer certificates were issued—they constructed a rogue certificate authority that was trusted by all major browsers and operating systems. Stevens, from the Centrum Wiskunde & Informatica in Amsterdam, and de Weger, of the Technische Universiteit Eindhoven were two of the driving forces behind the research that made it possible.

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