Thursday, October 15, 2015

Backing up your master password and secret sharing

I created a tool implementing a cryptographic secret sharing algorithm, which I use for backing up my master password. The tool is at:

Here's the motivation and how it works:

Let's say you use a password manager to remember your passwords, and the password manager requires a master password. If you forget the master password, you loose access to the other passwords. So how do you backup your master password?

One possibility is giving your master password to trusted people such as family members. A problem with that approach is that one of the people you share it with might accidentally reveal it, post it on the internet, leave the piece of paper on which it is written somewhere a malicious person can see it, etc.

Another possibility is to write your password down, tear it in half and give each half to a trusted person. Then any single person doesn't know the entire password, so if they reveal it, your password still is not entirely revealed. However, if someone malicious sees one half of the password, it makes it much easier to guess the other half. Another problem with this approach is that if either of the two people loose the slip of paper, then you can no longer reconstruct your master password.

Enter Shamir's scheme for secret sharing, based on an old but neat paper. It shows how to divide a secret into a number of "shares" such that one share provides no information about the secret, and any two shares are sufficient to reconstruct the secret. It actually generalizes to any number of minimum shares, but we'll use the version that requires a minimum of two shares to reconstruct the secret.

The idea is that a line is defined by two points. Knowing one point does not help determine the slope or intercept of the line. Knowing two points is enough to determine both. So in Shamir's scheme each point is a share and the line slope or intercept is the secret. The equation for a line in 2D is: y = ax + b.

The tool I created uses this as follows. The user types in a secret. The tool generates a long (128-bit) random number for both of a and b (i.e., the slope and intercept of a line). The slope (parameter a) will be the encryption key used to encrypt the secret. Each share contains the coordinates of one distinct point along the line. Thus knowing one share doesn't tell you anything about the encryption key, and with two shares, it is straight-forward to figure out the slope and then decrypt to recover the secret.

The tool encrypts the secret with AES-128 using the 128-bit random key. Shares are represented in base-64. I took all the easy-to-generate characters on the keyboard, threw away a few confusing ones (such as o, O, l, and .), and that left 64. I could shrink the alphabet down to something like hex, but then the length of the shares would become longer.

The math to manipulate the shares and the secret key is done using a finite field based on the prime 2^128-159. Another note, I also threw in a few bits/bytes of checksum, both into the encrypted secret as well as the shares, to detect mistypes.

Feel free to browse the javascript source code of the tool, it is not minified. It uses the crypto routines in the Forge (client-side javascript library). It does not send any information over the network, except for Google analytics, though feel free to verify that for yourself. I'm happy to provide more details.

Lastly, anything crypto-related is notoriously difficult to get right, including this tool. Please use with care, and it'd be great to have another pair of eyes on the javascript source code if anyone cares to browse!

Saturday, February 21, 2015

Solidworks Macros via Python

I finally figured out how to write Solidworks macros in python (yay!). Almost all the of Solidworks API works with one exception described at the end of this post. The Solidworks API is via the windows COM interface (ugh).

Here's the initial setup:

  1. Download and install Python. I used Active Python 2.7.8
  2. Get Solidworks. Python macros have worked pretty seamlessly across 2012, 2014 and 2015.
  3. Get familiar with the Solidworks online API help. E.g., is for the 2014 API. Note you can change the year in the URL to access the docs for other versions of Solidworks.
Ok, with that, we can dig right in. Before I run a macro, I make sure Solidworks is running and the document I want to modify is active (e.g., visible on the screen). I think you can use macros to start solidworks and open documents, but I'm less familiar with those commands.

Basic startup

This code snippet connects to running instance of Solidworks of the year specified. For example to connect to Solidworks 2015, set swYearLastDigit = 5:
import win32com.client
import pythoncom
swYearLastDigit = 5
sw = win32com.client.Dispatch("SldWorks.Application.%d" % (20+(swYearLastDigit-2)))  # e.g. 20 is SW2012,  23 is SW2015
You can also invoke Dispatch without the year specification, as in ....Dispatch("SldWorks.Application"). If there's only one version on your machine, this connects to that version.

At this point, the python code looks similar to the VBA code in the API docs. Sometimes you have to play with whether a function wants args or not. Here's the next piece of the boilerplate I have at the beginning of my scripts:

model = sw.ActiveDoc
modelExt = model.Extension
selMgr = model.SelectionManager
featureMgr = model.FeatureManager
sketchMgr = model.SketchManager
eqMgr = model.GetEquationMgr
As an example of difference in arguments, consider the Equation method on the IEquationMGR object (eqMgr in my code above). The 2014 API docs for the Equation member says that you read an equation by reading Equation(idx), and set by putting an equal sign after the expression. In python the binding is a bit different:
print("Equation 1 is: " + eqMgr.Equation(1))
eqMgr.Equation(1, "\"myVar\" = 42")
print("Equation 1 is now: " + eqMgr.Equation(1))
The most common difference I see between the Visual Basic docs and python are whether to put parenthesis after the member name or not. I just try both and see which works.

By the way, I see little rhyme or reason to the return values of method invocations, both at the API level as well as the values returned in practice. I usually go with the API docs, and assert return values, then delete the assertion if/when the method doesn't follow the API docs.

Creating arguments of the correct type (aka, getting SelectById2 to work)

Sometimes the method requires some fancy arguments, like reference arguments, or you otherwise just can't figure out what the thing is expecting. The Visual Basic interface is better at automatically converting types into the appropriate COM objects. The python bindings for the API don't work quite as well all the time. So here's what to do when you need to dig deeper and understand how to invoke a method:
  1. Generate the static python COM bindings for solidworks
    1. First, run python c:\Python27\Lib\site-packages\win32com\client\
    2. Select "SldWorks 2015 Type Library" and hit OK. You'll see output like this:
      Generating to C:\Users\myhappyuser\AppData\Local\Temp\gen_py\2.7\
      Building definitions from type library...
      Importing module
      The exact file name may change depending on your version of Solidworks.
  2. Open up that generated file in a viewer, like Komodo or Notepad
  3. Open up the web page VARIANT Type Constants in a browser
The generated python file has info on what arguments each method is expecting and that web page helps decode the arguments into something a bit more actionable.

Let's work a few common examples.

First let's try the macro command to select an object by name. The recommended version of the method is modelExt.SelectByID2. If you try putting in some actual args, you'll see:

ActivePython (ActiveState Software Inc.) based on
Python 2.7.8 (default, Jul  2 2014, 19:48:49) [MSC v.1500 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import win32com.client
>>> sw = win32com.client.Dispatch("SldWorks.Application")
>>> model = sw.ActiveDoc
>>> modelExt = model.Extension
>>> modelExt.SelectByID2("mysketch", "SKETCH", 0, 0, 0, False, 0, None, 0)
Traceback (most recent call last):
  File "", line 1, in 
  File ">", line 2, in SelectByID2
pywintypes.com_error: (-2147352571, 'Type mismatch.', None, 8)
The last number in the Type mismatch. line indicates the argument that is causing the problem. In this case it is the None, which is the eighth argument. The API man page says it is expecting a "pointer to a callout". We just want to pass None, but the None isn't getting converted to the right COM object, so we have to do the conversion manually.

To do this, open the generated python file, for Solidworks 2015 it should be named, and search for 'SelectById2'. You should find a hit that looks like:

	def SelectByID2(self, Name=defaultNamedNotOptArg, Type=defaultNamedNotOptArg, X=defaultNamedNotOptArg, Y=defaultNamedNotOptArg
			, Z=defaultNamedNotOptArg, Append=defaultNamedNotOptArg, Mark=defaultNamedNotOptArg, Callout=defaultNamedNotOptArg, SelectOption=defaultNamedNotOptArg):
		'Select a specified entity'
		return self._oleobj_.InvokeTypes(68, LCID, 1, (11, 0), ((8, 1), (8, 1), (5, 1), (5, 1), (5, 1), (11, 1), (3, 1), (9, 1), (3, 1)),Name
			, Type, X, Y, Z, Append
			, Mark, Callout, SelectOption)
The list of tuples ( ((8,1), (8, 1), (5, 1), ...) above ) contains info on the expected type of each argument. The eighth tuple corresponds to the problematic eighth argument. That tuple is (9, 1). Now look up '9' in that MSDN web page titled "VARIANT Type Constants" and you'll see it matches with VT_DISPATCH. Here's the magic on how to generate the correct object manually:
    arg1 = win32com.client.VARIANT(pythoncom.VT_DISPATCH, None)
The first arg to VARIANT is the type of the object to create, and the second arg is the initial contents. So now we can use that and:
>>> arg1 = win32com.client.VARIANT(pythoncom.VT_DISPATCH, None)
>>> modelExt.SelectByID2("Sketch1", "SKETCH", 0, 0, 0, False, 0, arg1, 0)

Now let's try a different example. Continuing on, let's get a sketch we selected:

>>> selMgr = model.SelectionManager
>>> aSketch = selMgr.GetSelectedObject(1).GetSpecificFeature2
>>> aSketch.Name
and let's get the plane it came from:
>>> aSketch.GetReferenceEntity(0)
Traceback (most recent call last):
  File "", line 1, in 
  File ">", line 2, in GetReferenceEntity
pywintypes.com_error: (-2147352571, 'Type mismatch.', None, 1)
Oops, that didn't work. Well looking into the generate python file and searching for GetReferenceEntity, we find:
	def GetReferenceEntity(self, LEntityType=defaultNamedNotOptArg):
		'Get entity that this sketch is created on'
		return self._ApplyTypes_(52, 1, (9, 0), ((16387, 3),), u'GetReferenceEntity', None,LEntityType
And then looking at the VARIANT web page, we find that 16387 = pythoncom.VT_BYREF | pythoncom.VT_I4 So GetReferenceEntity uses an output argument to return the entity type. We can construct an output argument similar to what we did for SelectByID2:
    arg1 = win32com.client.VARIANT(pythoncom.VT_BYREF | pythoncom.VT_I4, -1)
    refPlane = aSketch.GetReferenceEntity(arg1)
and now we can see:
>>> arg1 = win32com.client.VARIANT(pythoncom.VT_BYREF | pythoncom.VT_I4, -1)
>>> arg1.value
>>> refPlane = aSketch.GetReferenceEntity(arg1)
>>> arg1.value
To decode the '4', look at the doc for GetReferenceEntity, which points to the doc for an enumeration type swSelectType_e , which says that 4 maps to swSelDATUMPLANES


If you don't want to put '4' and other random constants in your python code, there are two possibilities. The first is to generate the python Solidworks COM constants bindings:
  1. Run python c:\Python27\Lib\site-packages\win32com\client\
  2. Select "SOLIDWORKS 2015 Constant type library"
  3. Add an EnsureModule command early in your python program using the number shown in the output of the makepy command. For example, with Solidworks 2015, that is:
    swconst = win32com.client.gencache.EnsureModule('{4687F359-55D0-4CD3-B6CF-2EB42C11F989}', 0, 23, 0).constants # sw2015
Now, by looking at a man page you can find the appropriate constant name in that module, though there isn't much structure there. For example, you can check the return type of the GetReferenceEntity, above by doing:
    assert arg1.value == swconst.swSelDATUMPLANES
This isn't quite as nice as the Visual Basic interface, which structures the constants.

The downside of the EnsureModule approach is that it requires that anyone using your beautiful python morsel to run makepy. A more crude approach is to copy the constants you need from the man pages. For example, I have:

class swconst:
    swSelDATUMPLANES = 4
    .... more constants here ...

A prayer for GetMathUtility

And here is the one bit of the API that I can't get to work. For the life of me, I can't seem to get GetMathUtility to work, and so can not figure out how to create a MathPoint. What happens is:
>>> mathUtil = sw.GetMathUtility
>>> mathUtil.CreatePoint
Traceback (most recent call last):
  File "", line 1, in 
  File "C:\Python27\lib\site-packages\win32com\client\", line 511, in __getattr__
    ret = self._oleobj_.Invoke(retEntry.dispid,0,invoke_type,1)
pywintypes.com_error: (-2147417851, 'The server threw an exception.', None, None)
The output I expect is an error saying an argument is missing. I can not find any argument that placates this method, nor any other method in the mathUtil object returned. Kudos to anyone who can figure this out! It works fine in Visual Basic, so my guess is something is either screwed up in the python binding, or there's some kind of bug in the interface that the visual basic binding manages to avoid.