Uncompress password protected WinRar file without password

Question

I would like to know how to extract contents of the password protected WinRar file without the password.

I have downloaded that WinRar file from a file hosting website and found out that it was password protected after I downloaded it. I would like to know is there any way to “unprotect” a password protected file so I can decompress the contained files.

Brute-force attacks take ages. One thing is that I found out that brute force attacking tries every combination of words from the word-list. Therefore, I would like to know if it is possible to brute force attack the file using individual passwords one-by-one, instead of going through all possible permutations of all the individual passwords contained in my word-list.

Answer 1

Why can't you simply ask the person who made that file to tell you the password?

No one knows how to decode an AES-encrypted file, such as password-protected WinRar files, without knowing the password.

As far as we know, the only way to decode AES-encrypted files is to somehow obtain the right password, and then use that password to decrypt the file.

In the movies, sometimes you see people "cracking" a password. They try all possibilities for the first letter, and then when they get the first letter right, they try all the possibilities for the second letter, etc. Eventually all the letters fill in, and the then the actor has the whole password.

It doesn't work like the movies with AES. AES passwords are all-or-nothing. If you get all-but-one of the letters right when typing in your password, you get exactly the same "failed" message as if you typed in a completely wrong password. (It's not possible to distinguish the random-looking gibberish from decoding with an almost-correct key from other random-looking gibberish from decoding with a completely wrong key). It's not possible to tell if one guess is "closer" to the right password than another guess.

(There are various "timing attacks" that, with some systems, can tell a researcher if one guess is "closer" to the right password than another, but they don't work on AES-encrypted files).

There are various "password unlocker" programs that will try one guess at a time until they stumble on the correct WinRar password. If the person who made the password-protected WinRar file picked a "strong" password -- for example, a passphrase of 8 words randomly chosen using dice from a short dictionary -- we expect that it will take more than a thousand years to stumble across the correct password. If that person picked a "very weak" password -- such as a single word in the dictionary, or a single dictionary word followed by a single digit, or a single word repeated 8 times, or a sequence of 5 completely random characters -- we expect that it will take less than a day for that program to run through all those very weak passwords and find the correct one.

EDIT:

According to Wikipedia: WinRAR, the "method of encryption" used by WinRAR is the AES (Advanced Encryption Standard) with a 128-bit key.

I agree with Thomas. It's probably hopeless trying to crack it without knowing anything about the password. If you do find a way to crack AES, Bruce Schnier, the readers of crypto stackexchange, and every other cryptographer on the planet would be fascinated to find out, especially those cryptographers who work at the SVR, the NSA, and other intelligence agencies.

Answer 2

I've got two possible solutions for you:

1. On file hosting platforms uploaders tend to use one password for all files they upload. So on huge file-sharing forums there are not as many possible passwods as files but as many possible passwords as users. That reduces the set of passwords that are very likely to be correct to a few hundred. And I'd bet someone collects all these passwords and makes them available.

2. As long as you recently downloaded the file all the pages that finally led you to the download-link are still in your browsers history. Uploading something password-protected and meant for the public without providing the password at some central place is nonsense. Therefore, the password should be on one of these webpages in your browser-history.

In general, as the others already said, password protected ZIP and RAR files are encrypted. "Undoing" encryption without the correct password is impossible. Guessing a password may work but file-sharing passwords (e.g. x.X.RiDDiCK.X.x) will definitel resist a general dictionary attack.

Answer 3

According to the RarLab website as well as the RAR changelogs, the encryption algorithm of (Win)Rar was changed to AES (Advanced Encryption Standard) with a 128 bit key length, starting with version 3.0. WinRar 3.0 was released around and about Jun 22, 2002, so chances are high you’re facing AES here!

Trying to be faster than brute-forcing the unknown password would mean you would need to find a practical way to attack on AES which quicker than brute-forcing.

At the time of writing, no such practical attack is known.
^{(In the unlikely case you find such an attack, publish a paper and become famous!)}

Long story short: you will not be able to take a short-cut here. Your only two options are: “brute-force the file”, or “forget-about-it”. That is, unless you learn the password by asking the owner who protected the file, or by finding the place where the owner who protected the file may have published the password.

As for your question if brute-forcing using word-lists instead of symbol/password permutations could work: yes, it “could” work… if the password is contained in your word-list. To try that option, you can create a simple script or program that reads the word-list and tries to apply the contained passwords one-by-one. But explaining how to practically do that is too broad, and greatly depends on the scripting or coding language you prefer and choose to use. Personally, I’ld probably go for C/C++ using the available RarLib API as that’s most probably one of the fastest options to handle things in this case.