How secure are password-protected zip files?

Question

In modern zip utilities like 7zip, you can specify a password when creating a .ZIP file. But, how secure is this? What encryption algorithm(s) are used to secure password-protected zip files?

Solution 1

7-zip uses AES-256 encrpytion for 7z/zip archives.

http://7-zip.org/7z.html says:

7-Zip also supports encryption with AES-256 algorithm. This algorithm uses cipher key with length of 256 bits. To create that key 7-Zip uses derivation function based on SHA-256 hash algorithm. A key derivation function produces a derived key from text password defined by user. For increasing the cost of exhaustive search for passwords 7-Zip uses big number of iterations to produce cipher key from text password.

The help files of 7-zip says this about the security of a passworded 7z file:

Solution 2

Brute force attacks are a waste of time. I won't go into details why, I will instead direct you to Jeff Atwood's blog, specifically Brute Force Key Attacks Are for Dummies.

Solution 3

As secure as the length of the password:

From my personal experience trying to crack 7zip, password protected files by the combo of dictionary and brute-force attack the way to go is:

1. Dictionary attack 200k^1
   1.1 dictionary attack with the first letter capitalized 200k^1
   1a. Dictionary and brute-force 3 digits at the back (idiot123) 10^3*200k
   1a.1 Dictionary with the first letter capitalized and brute-force 3 digits at the back (Boston777) 10^3*200k
   Less than an hour.

If above didn't fly you have less than 1 in 10 to crack with following (using cRARk).

2. All numbers (0-9) 9 digits long (10^9) - will take about a day, will unlock all pins (4-5 digits) and all dates(20191111, 10102019)

3. Latin lower and digits (a-z0-9) 6 symbols (25^6) will take about 2 days, have cracked some this way (asdfaf)

4. Latin lower and upper and digits and . and - (a-zA-Z0-9.-) 5 symbols (60^5) will take another day, have scored some (A.1983)

After this point, it's a longshot (Nowadays I stop here)

4. Dictionary combo 2 words 200k^2
   4.1 Dictionary combo with 2 words first letter cap 200k^2
   4.2 Dictionary combo 2 words space in between 200k^2
   4.3 Dictionary combo with 2 words first letter cap space in between 200k^2
   another two to three days

After this point just give up, lol

Comments

[Mike1980]

Brute force attacks aren't always a waste of time, most users don't put an awful lot of thought into passwords, and a dictionary attack against the password they use can be a very effective way to decrypt a file. A brute force attack is only a waste of time (at present it becomes easier by the year) against a secure key.

[Mircea Chirea]

A brute force attack is a waste of time. Dictionary attacks are not brute force, and are a LOT faster than trying every possible combination of a set of characters up to a specific length.

[Oorang]

+1 For posting a link to a blog with a link to a blog with that discusses an email that pertains to the original question.

[Mircea Chirea]

@Oorang, I did that on purpose :P

[Oorang]

@iconiK, And it was awesome.

[zylstra]

Jeff Atwood is a smart guy, but his article was written in 2006, and he didn't quite have a grasp of the fundamentals like we do now with dictionary attacks, password frequency, rainbow tables, etc.