Python: Convert string to instance

Question

I am new to Python and I have been stuck for hours with this problem... I don't know how to convert a variable (type string) to another variable (type instance).

>>from Crypto.PublicKey import RSA
>>from Crypto import Random
>>randomValue = Random.new().read
>>priv = RSA.generate(512, randomValue)

After these lines of code, "priv" is created, and this has type "instance". And I had to convert this "priv" to type string using str(priv).

>>convertedToStr = str(priv)
>>type(convertedToStr)
<type 'str'>

Now, I need to convert it back to 'instance' and want to get the same thing in value and type as the original "priv". Assume that I cannot use "priv" anymore, and I need to convert "convertedToStr" (type string) into "convertedToStr" (type instance). Is this ever possible?

Note: The reason I am doing this complex thing is because I have client and server sides and when one side sends a message to the other using sendall(var), it does not allow me to send variable of type 'instance'. So I had to convert it to string before sending it. Now, I want to use that on the receiver side as an variable of type 'instance' but I do not know how to convert it back.

Answer 1

The instance type is used for instances of old-style classes in Python 2. You may want to look at priv.__class__ instead of type(priv) to find out what class it actually has. I expect you'll find that it's class is Crypto.PublicKey.RSA._RSAObject , since that's what the generate function is documented to return.

I don't have the Crypto package installed, so I don't actually know what string you get when you call str on a private key instance. You might be able to parse the string and then call the function Crypto.PublicKey.RSA.construct with appropriate values to reconstruct the key object.

But I think that is doing more work than necessary. Instead of calling str on the key, you should instead call its exportKey method . Then, after you send the string you get back to the other system, you can pass it to Crypto.PublicKey.RSA.importKey .

Note that sending a private key over a network may expose it to eavesdropping, making it useless! You probably shouldn't do it unless the connection between your two systems is encrypted with some other system. Your system is only as secure as its weakest link.

Answer 2

Type instance is nothing specific, you can make a custom class and instantiate it, and it will have type instance :

>>> class x:
...     y=1
...     
>>> type(x())
<type 'instance'>

You can't arbitrarily convert things to a string by calling str() and guarantee get useful results - it merely asks the object to return a string that could say anything at all. In this case you asked for an RSA private key 512 bytes long and the str() output is ~45 bytes long, there's not 10% of the information needed to get the full object state back from that.

The general problem you're trying to solve is serialization/deserialization, and it's the topic of many modules, libraries and protocols - but luckily RSA keys are easy to convert to useful text and back again (not all objects are).

>>> out = priv.exportKey()
>>> new = RSA.importKey(out)
>>> new == priv
True

NB. when I tried your code, it clearly complained at me that 512 byte keys are weak and refused to generate them, insisting on 1024 bytes or more. You possibly are on an older version, but should specify a longer keylength.