Password Cracking. In most cases, computer access is protected by username and password. Usually it is not too difficult to find out some or all user names on 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 all strings 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 dozen user names and passwords, one normally finds two or three vulnerable ones within a day or two.
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For Windows NT there is the very fast L0phtCrack password cracker. Later versions also work on W2000. How does one obtain the passwd file? On a local machine it is just readable. Sometimes one can obtain it remotely via anonymous ftp, or via a CGI script, using a. /.
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Of course, nowadays people often use shadow password files, and these may be more difficult to obtain. On most Unix systems, passwords are at most 8 characters long. Picking control characters or non-ASCII characters is bound to give problems when logging in remotely via other systems, so it is reasonable to expect characters in the range 32-126. Now 95^6 = 0. 74.
10^12 and 95^8 = 0. 66. 10^16 so if one can check one password in a microsecond then nine days suffice to check all strings of length at most six. (On my computer a DES-type check takes 10 microseconds.
Of course, it is not necessary to try all possible strings. Trying all words in a fat dictionary takes only a few minutes.
Exercise Crack some or all of the following passwords. 4. 1 Common passwords. I find that common passwords include '' (the empty string), 'secret', 'password' (and in Holland the Dutch versions 'geheim', 'wachtwoord'), strings of consecutive digits or letters like '123', '12345', '1234567', 'abc', and proper names like 'eric', 'kevin', 'sandra', 'melissa', 'Nikita'. On 2010-12-12 hackers published about 750000 encrypted passwords of users of Gawker blogs such as Lifehacker, Consumer, Gizmodo, Gawker, Jezebel, io9, Jalopnik, Kotaku, Deadspin, Fleshbot. (This explains the occurrence of these words below. ) I cracked a bit more than 425000 of these passwords in about 12 hours.
The list below gives the 350 most frequent passwords with their frequencies in this cracked set. 4.
2 Unix password algorithms. Having passwords in cleartext in a file is a bad idea - they will be compromised.
Unix introduced the idea of feeding the password to some one-way hash function and storing the result. Now the password file /etc/passwd can (and does) have general read permission. Unix V5 and V6 used a simulation of the M-209 cipher machine, and encrypted (the first 8 characters of) the password to an 8-char string. Anecdote In the Unix V6 days I once gave a Polish colleague a username and password, and told him his username and said that he could guess the password. He sat down and logged in, and was surprised that it worked. `How did you know I was going to try "ladne"?' But I had given him the password "aline". (Thus, we found a collision.
Rechecking: We see another weakness here: this version of passwd required the password on the command line. This means that it would be visible to someone who did ps at the same time. As Morris & Thompson wrote. the encryption algorithm was too fast, and brute force search was too easy. So, in Unix V7 the algorithm was replaced by a modified DES, repeated 25 times.
DES because it was slower and safer, repeated to make it even slower, and modified in order to protect against hardware implementations of the actual DES. (Moreover, there has always been some uncertainty: could it be that there is some backdoor in DES? Maybe a modified DES is more secure than the actual DES. The input to this encryption consists of a 12-bit salt concatenated with the user's password. The 64-bit output is converted to an 11-char string and compared to the entry in /etc/passwd.
which has a 13-char string representing salt and encrypted password. (DES has two inputs: key and data. Here salt plus password is used as 64-bit key, and the initial data is the constant zero. Bits are converted to printing characters in groups of 6, using the alphabet. /0-9A-Za-z (in this order).
The salt makes sure that one cannot precompute encryptions for all dictionary words, say - each word has 4096 different encryptions. It is chosen at random when the user sets his password. These passwords are recognized by their length of 13 characters. Exercise Crack this. This is what the standard Unix routine crypt() does.
Today it is fairly insecure. Exhaustive search is feasible with special purpose hardware, and the speed of 100000 attempts/second is too high. Only 8 characters of the password are used. The salt is too small - it is quite feasible to precompute the encryption for all possible 4096 salts and all words in a large dictionary or word list and store the result on disk. (Also Windows NT uses a form of DES.
It is even weaker and allows 800000 attempts/second. There is no salt. What to do about the weakness of crypt(). The main defense is now the use of shadow password files, that is, the hiding of the password file from the users. But that has all the problems that caused Unix to abandon a plaintext password file.
It is better to replace crypt(). Various cryptographic hash functions are designed to be fast, and such that constructing collisions or finding preimages is infeasible. That latter property is precisely what is needed for password encryption, but a password hash must be slow. Brute force cracking of raw MD5 is very easy. FreeBSD-MD5 and bcrypt. According to US law, exporting cryptographic software was a form of munitions export. This caused a lot of stupid annoyances.
Of course everybody in the whole world had DES source code, but nevertheless distribution was restricted. In order to overcome this difficulty, FreeBSD 4. 2 switched to a complicated algorithm based on MD5.
That had several advantages: it is a bit stronger, with 128-bit output instead of 64-bit, it uses the entire password instead of only the first 8 characters, and it is slower (the digest is rehashed a thousand times), so brute force takes longer. (On my machine 2000 attempts/sec, against 100000 for modified DES. ) Also RedHat 6.
0 and up uses MD5 (but SuSE does not by default - ach). These FreeBSD-type MD5 passwords can be recognized as 34-char encrypted passwords starting with $1$. The first 8 characters following are the salt. Poul-Henning Kamp described his design criteria.
Niels Provos and David MaziГЁres developed bcrypt(). the best choice for a password hash today. It is based on Blowfish, and contains facilities for making the algorithm arbitrarily expensive. It is used by OpenBSD, and has passwords starting with $2$.
or $2y$. Brute force is even slower here, at 100 attempts/sec. Various implementations of crypt() have suffered from problems in an 8-bit environment since the programmers expected ASCII input. What to do with non-ASCII bytes? In some implementations they were replaced by '?', so that a strong password turned into the constant string ". ". In some implementations the high order bit was masked off, so that 0x80 became end-of-string. In 2011 a sign-extension bug was discovered in the Openwall implementation of Blowfish.
The $2y$ -prefix in bcrypt() -generated passwords indicates that they were generated by a post-fix algorithm. 4. 3 MySQL passwords. Before version 4. 1 MySQL had a very weak password algorithm.
Here it is. The input is an arbitrary string. The output is a 16-hexdigit hash, 62 bits. This is weak for many reasons.
It is fast, so brute force cracking is easy. There is no salt, so precomputation is possible. The value of nr2 is not used in the computation of nr. so a cracker can forget about the second half of the hash and work with the first 31 bits only. On the other hand, nr2 provides valuable information.
Since the final nr and nr2 are known (except for their high order bit) one can find the value of nr2 before the last step, so that incorrect candidate passwords can be discarded without looking at their last byte, greatly speeding up the search. And with a bit of work one can also derive information about the stages after all but two or all but three bytes of the password, making the search even faster. This means that cracking 8-symbol passwords is quick. See also Philippe Vigier's poc. c.
Recent versions of MySQL use a double application of SHA1, and do not have obvious weaknesses. However, many sites still use the old scheme, e. for compatibility reasons. Exercise Crack some or all of the following passwords. 4. 4 ZIP passwords.
The PKZIP utility is used to create compressed archives. The format of the outputfile is well-documented.
One can protect archives with a password. In the Microsoft world many (usually commercial) brute force ZIP password crackers are available, the most famous being Elcomsoft'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 algorithm described by Eli Biham and Paul Kocher and uses some (at least 13 bytes) known plaintext. Altogether, it is usually feasible to find the password of a traditional ZIP archive. Recognizing that the password protection had become too weak, PKZIP 5. 0 introduced stronger encryption. Concerning the cracking speed one can expect: a moment ago I needed to crack a ZIP password and found that zipcracker did approximately 1000000 tries/sec on a 1400MHz machine. Exercise In my mailbox I found the password protected zip file Message.
zip. What is the password? What does this file contain. 4.
5 PDF passwords. Adobe's Portable Document Format is one of the more popular formats in which to distribute files representing printed material. Such files are commonly viewed with Acrobat Reader or with xpdf. The format allows the creator of the file to set certain protections. The protection comes in two flavours: protection bits and password protection.
There may be two passwords: the owner's password and the user password. The permission bits are not enforced in any way. But Adobe asks implementers of PDF readers to respect their settings. The Linux xpdf indeed respects the bits. Of course it is trivial to modify the source removing the tests on okToPrint().
etc. Revision 2 knows about four permission bits (in a 16-bit short):.