In most cases, computer access is protected by username and password.Usually it is not too difficult to find out some or all user nameson a given computer. Names leak as email addresses and in usenet posts.Utilities like finger or rwho may give some.There are many standard user names, root being the most obvious one.System logs and similar may be visible on the web, and found using Google.
Finding the password is not so simple. Usually one has to brute-force,trying all words in a dictionary, a list of first names, or just allstrings of at most six printable symbols.A good password cracker is John the Ripper.Given the passwd file of some Unix machine, say with two or three dozenuser names and passwords, one normally finds two or three vulnerable oneswithin a day or two.
Various cryptographic hash functions are designed to be fast,and such that constructing collisions or finding preimages isinfeasible. That latter property is precisely what is neededfor password encryption, but a password hash must be slow.Brute force cracking of raw MD5 is very easy.
The PKZIP utility is used to create compressed archives.The format of the outputfileis well-documented. One can protect archives with a password.In the Microsoft world many (usually commercial) brute forceZIP password crackers are available, the most famous beingElcomsoft's AZPR. In the Unix world one has zipcracker(for distributed cracking over a Beowulf network) and fcrackzip(for simple and fast brute force), that come with source code.There is also pkcrack that implements the algorithmdescribed by Eli Biham and Paul Kocher and uses some (at least 13 bytes)known plaintext. Altogether, it is usually feasible to find thepassword of a traditional ZIP archive. Recognizing that the passwordprotection had become too weak, PKZIP 5.0 introduced stronger encryption.
Many sites have lists of cracked MD5 passwords. So if you find one, sayc3875d07f44c422f3b3bc019c23e16ae, then ask Google before tryingto crack. Immediately a dozen sites will tell you that this is denis.
If one is going to do brute force, a lot of time is needed.But if passwords have to be cracked repeatedly, it is possibleto do an (expensive) precomputation with the result that subsequentpassword cracking will be fast.
Now the password cracking works as follows: given f(K) we would liketo find K. Apply f many times to f(K) until we find the end Kn of the thread.Find the pair (K0,Kn) from the table. Apply f many times to K0 until wereach f(K). The step before that we had K.
This approach is best when there are no collisions, no pairs of distinctkeys K, K' with f(K) = f(K'). If there are, then the threads merge andthere wil be several possible K0 for a given Kn, leading to larger tablesand larger cracking time. There is nothing one can do about the function E,it is given, but Oechslin proposes to vary the reduction function R alongthe thread. That speeds up things by a factor of at least 2.
On lasecpc13.epfl.chan "instant NT password cracker" is available, that uses a 0.95 GBprecomputed table to crack alphanumerical WinNT passwords in 5 sec average(and returns "not found" when the password contains non-alphanumerics).The precomputation took about six CPU days.For the theory, see Oech03.pdf.
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This chapter examines password crackers. Because these tools are of such significance in security, I will cover many different types, including those not expressly designed to crack Internet-related passwords.
The term password cracker can be misinterpreted, so I want to define it here. A password cracker is any program that can decrypt passwords or otherwise disable password protection. A password cracker need not decrypt anything. In fact, most of them don't. Real encrypted passwords, as you will shortly learn, cannot be reverse-decrypted.
Instead, simulation tools are used, utilizing the same algorithm as the original password program. Through a comparative analysis, these tools try to match encrypted versions of the password to the original (this is explained a bit later in this chapter). Many so-called password crackers are nothing but brute-force engines--programs that try word after word, often at high speeds. These rely on the theory that eventually, you will encounter the right word or phrase. This theory has been proven to be sound, primarily due to the factor of human laziness. Humans simply do not take care to create strong passwords. However, this is not always the user's fault: 2b1af7f3a8