computer science

Ophcrack

Ophcrack is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Ophcrack rather than just read about it. In short: Ophcrack is a free open-source (GPL licensed) program that cracks Windows log-in passwords by using LM hashes through rainbow tables. The program includes the ability to import the hashes from a variety of formats, including dumping directly from the SAM files of Windows, and can be run via the command line or using the program’s GUI (Graphical user interface).

Key takeaways

Ophcrack belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Ophcrack to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Ophcrack from memory before moving on to harder problems.

Reference excerpt

Ophcrack is a free open-source (GPL licensed) program that cracks Windows log-in passwords by using LM hashes through rainbow tables. The program includes the ability to import the hashes from a variety of formats, including dumping directly from the SAM files of Windows, and can be run via the command line or using the program’s GUI (Graphical user interface). On most computers, ophcrack can crack most passwords within a few minutes. Rainbow tables for LM hashes are provided for free by the developers. By default, ophcrack is bundled with tables that allow it to crack passwords no longer than 14 characters using only alphanumeric characters. Available for free download are four Windows XP tables and four Windows Vista tables. Objectif Sécurité has even larger tables that are intended for professional use. Larger rainbow tables are NTLM hash for cracking Windows Vista/Windows 7. In september 2019 these tables have also been made available for free. Ophcrack is also available as Live CD distributions, which automates the retrieval, decryption, and cracking of passwords from a Windows system. One Live CD distribution is available for Windows XP and lower and another for Windows Vista and Windows 7. The Live CD distributions of ophcrack are built with SliTaz GNU/Linux. Starting with version 2.3, Ophcrack also cracks NTLM hashes. This is necessary if the generation of the LM hash is disabled (this is default for Windows Vista) or if the password is longer than 14 characters (in which case the LM hash is not stored). Starting with version 3.7.0, the source code has been moved from SourceForge to GitLab.

See also Aircrack-ng Cain and Abel Crack DaveGrohl Hashcat John the Ripper L0phtCrack NMap RainbowCrack

References

External links Media related to Ophcrack at Wikimedia Commons Cracking Windows XP Passwords at Wikibooks Official website Ophcrack Online Demo - form to submit hashes and instantly crack passwords Ophcrack no table found - how to fix if Ophcrack says "no tables found". Ophcrack LiveCD 2 Tutorial Archived 2016-03-04 at the Wayback Machine Quick fix if Ophcrack doesn't work OPHCRACK (the time-memory-trade-off-cracker) - École Polytechnique Fédérale de Lausanne

Illustrations

Worked examples

Example 1 — a first encounter with Ophcrack

Start with the simplest possible case. Write down what Ophcrack claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Ophcrack before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Ophcrack ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Ophcrack

In research: Ophcrack appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Ophcrack in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Ophcrack is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cryptanalytic software, Free and open-source software stubs, Free security software, so understanding it makes those chapters shorter.
In everyday life: Look for Ophcrack outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Ophcrack in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Ophcrack means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Ophcrack out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Ophcrack in simple terms?

Why does Ophcrack matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Ophcrack?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Ophcrack.