Attacking WinZip AES Encryption

bden writes "As another tidbit from Bruce Schneier's Crypto-Gram, remember back in January when WinZip was Slashdotted for moving forward with its new AES-based encryption technology? Everything sounded good since we all knew that AES is secure, right? Well, a cryptographer took a look at how WinZip uses AES and found lots of problems. Regardless of how many people actually plan to use WinZip encryption, the lesson, according to Schneier, is that "cryptography is hard, and simply using AES in a product does not magically make it secure." So how can we distinguish between an application that simply uses the right buzzwords, like AES, from an application that is actually secure?"

Comments

Simple

[Anonymous Coward]

So how can we distinguish between an application that simply uses the right buzzwords, like AES, from an application that is actually secure?"

By only using peer reviewed open source software for starters.

FP?

Re:Simple

[Anonymous Coward]

Yes, like sendmail.

Re:Simple

[enditallnow]

Its questionable whether this would help commercial applications. Not every company offering secure programs wants their source code floating about the internet (Insert Microsoft Joke Here).

I agree that the best way to ensure that an application is secure is for it to be reviewed by someone who knows their shit. Quite simply its the only way to be sure at this point in time. Perhaps an authoritive body should be formed comprising of cryptographers that grants their seal of approval on it. Then again, doesn't the US government have to give its authorisation for cryptographic software to be exported? I recall that DES had to go through such motions, and if i'm not mistaken PGP can't be shipped outside of the US because its considered military grade cryptography? If im wrong please correct me, its been a while since I read over this topic and my memory is a bit hazy.

BTW, open source does not necessarily imply increased security. I'd rather have the word that a piece of software is secure from a professional like Bruce Schneier rather than an Open Source zealot who skimmed over a copy "Applied Cryptography" in their local Borders.

-- Enditallnow

Predictable..

[Ckwop]

I think the problem is people approach to the security.
They think you can just take AES and HMAC and glue them together in any way
and arrive at security. I mean both are secure right? The result should be secure?

Wrong! Schneier names one of the chapters in one ofhis book: "Cryptography is hard but that's just the easy part!"

It really is very hard to secure information. It's almost intractable.. We've seen a few articles here in the last week about interesting side-channel attacks. Breaking RSA keys by listening and an earlier one which broke into computers by heating them up.

Cryptography is littered with broken designs fielded designs like WEP and let's not mention software security..

It's going to be twenty years before we have "trustworth computing". It would help if we could modularize cryptography like we can computer programs...

Simon.

How to tell if a product is secure.

[teasea]

Wait for a cryptographer to analyze the product, then read about it on /.

stronger encryption

[Anonymous Coward]

We need 2048-bit buzzwords.

Re:is this a testament to today's computing power?

[TedCheshireAcad]

I took a class in cryptography last semester. The professor offered the best words of advice I ever heard in the subject: "Don't try to create new algorithms. We know how to do that already. What we have is secure. What you need to work on is the implementation. Just because something uses encryption, it is by no means secure."

He then proceeded to explain how easily NTLM can be defeated in a brute force attack.

Quick summary

[pjrc]

Here's the security problems, quickly summarized (and oversimplified). These are in the same order as the paper. The paper is lengthy and not an easy read... if you can't be bothered to RTFA, maybe this will help.

1. Filenames, file sizes, time/date stamps are plaintext. Only the file contents are encrypted. Filenames, dates may be sensitive data (example giving, pinkslips.zip contains file pinkslip-bob.doc).
2. Both compressed and uncompressed sizes are stored without encryption, so an attacker can know the compression ratio and perhaps infer what type of data it is based upon its compressability.
3. File lengths are not authenticated. A man-in-the-middle attack could modify the file sizes recorded inside the zip archive so decrypting produces "garbage" output files (without warning that the zip archive was tampered). The man in the middle intercepts the communication about the problem and impersonates a request from the sender to see the "garbage". The garbage is after decryption, so if the receipient sends it in the clear, the man in the middle can easily turn it into the original plaintext.
4. File names are not authenticated, so an attacker could tamper with the file names and change their extensions. On windows and other systems, the file name extension determines which software will be used to view the file.
5. The CRC is stored without encryption. If an attacker suspects he knows what the message is, he can replace the ciphertext with his guess, and watch if the receipient complains about a CRC error.
6. Zip archives can contain some encrypted files, others plaintext. They worry that a receipient may believe all or most files were protected, when only a few or one was.
7. Key generation isn't random enough, so keys may be reused. I don't fully understand this one... maybe someone who does will reply and explain it??
8. Attacker can create a "self-extracting" archive that mimics the GUI of Winzips, but is actually a trojan horse. They admit this isn't really winzip's problem.

Similar to IANAL, I'm not a crypto expert. I probably botched some of these a bit, especially the key collisions one. If I've misunderstood any of these, please reply.

not everything in the paper a Winzip vulnerability

[Anonymous Coward]

While most of the points raised in the paper seem valid, some done make sense. Case in point: "someone may use a keystroke logger to find out what your passphrase is". How the fuck is this a Winzip vulnerability?

Yoshi's broken other things too....

[meese]

Might I note that this is the same Yoshi Kohno who broke the Diebold voting system and SSH.

Much ado over nothing?

[BillX]

This may be non-news to those who read the paper, but it seems like the "vulnerabilities" here are overstated. Plenty of "rah, rah, should've used open-source, all your data are belong..." comments, but successful use of any of the exploits in the paper seems highly unlikely at best.

The vulnerabilities listed basically boil down to:

* Filenames and sizes aren't encrypted. If you store sensitive data in the filename, it can be read. (The paper uses the example of Bob intercepting a zip file containing a file named PinkSlipForBob.doc)

* The type of encryption method used is not authenticated. If a malicious user is able to perform a man-in-the-middle attack and edit the file so that it specifies a different (incorrect) encryption method, the final recipient will decrypt it and get a file of nothing but garbage. Now, if the attacker can also social-engineer the victim to send him that garbage file, the original file can be reconstructed.

* File names stored in the .zip are not authenticated. Like above, if the attacker can change the file extension, (s)he can cause the file to open in the wrong application when the victim unzips that file. This will likely be a nuisance at best; while the paper states that this method could be used to mount an attack similar to the above (getting garbage decrypted by a different method), it's unclear how this would actually work (since the file decrypted successfully, and there isn't any garbage). The attacker would have to coerce the user to send the unencrypted file itself.

* The next attack involves the attacker actually knowing the entire contents of the file (s)he wants to intercept, which to me at least, seems to defeat the purpose of intercepting it. Actually, that's a slight oversimplification: for this attack, the attacker needs to know 1 of n possibilities of what the exact file contents could be, and with this information, has a 1 in n chance of finding out if (s)he was right, by replacing the file in the archive with the "guess" (again, requiring the ability to modify the file in transit), and use the fact of whether (s)he intercepts a "Hey Bob, that zip file you sent was corrupted" message to find out whether the guess was right. (If it was a 1-byte file named "yesorno.txt", and the attacker wanted to know whether it contained "Y" or "N", this could be a useful attack. For less trivial files, however, this doesn't seem very feasible.)

* WinZip allows both encrypted and un-encrypted files in the same archive, so the end-user doesn't know if any given file was encrypted or not. An attacker can (man-in-middle, yadayada) add files to the archive before it reaches its recipient, and the recipient won't know they're not part of the original archive. A definite flaw, however, not directly a data leak of any kind. (Although, if one of the 'unofficial files' is a keylogger, and you can get the luser to run it....)

* A weakness in key randomization will cause a repeat key to be generated once every 2^32 files rather than the theoretical maximum of 2^64 files. So, "all" the attacker needs to do is find a victim who will use WinZip to encrypt, oh, 4.2 billion files or so, and they will have a good chance that one of the encryption keys is a repeat. Supposing there was a repeat, now they just have to know the entire contents of the larger of the two files, and they can determine the contents of the smaller one.

The paper also briefly mentions attacks like "plant a keylogger" or "replace Winzip with a program that looks like Winzip", but I wouldn't exactly call these flaws in the AES implementation. (The paper also comes to pretty much this conclusion, and so doesn't dwell on these possibilities.)