Question: I'm working on malware analysis at my university and I'm trying to develop ransomware. I'm planning to publish the source code after it's finished. Is there any open source ransomware sample so I can take a look?

Answer: 
I doubt you'll find ransomware source code floating around in the general public. I'd guess there is code out *there somewhere* but I wouldn't risk visiting shady sites on the "Darknet" (I really hate that term).

(Un)fortunately, ransomware is not very complex. For a university project, simply encrypting and replacing files in ~/Documents is probably good enough. The trick is making the decryption key only obtainable through you. 

Encryption:

1. Generate Symmetric Key **K**
2. Encrypt all files in ~/Documents with **K**
3. Send **K** to Server
4. Erase all traces of **K**

Decryption:

1. Obtain **K** from server.
2. Decrypt files with **K**

I know that notorious ransomware use Asymmetric Encryption, but it actually isn't necessary at all. For example, improperly implemented RSA by [CryptoDefense](http://blog.emsisoft.com/2014/04/04/cryptodefense-the-story-of-insecure-ransomware-keys-and-self-serving-bloggers/) actually made it **easier** to write automated decryption tools, because they didn't realize that a Windows Crypto API keeps local copies of generated RSA Private Keys. Putting blind faith in a crypto system won't make it secure. What's important is that the decryption key is not recoverable on the system. Whether or not this is done by zero'ing memory or encrypting **K** with an RSA Public Key really does not matter.
You can visit [this](https://github.com/roothaxor/Ransom) link:

You will be able to understand how the ransomware works in encrypting and decrypting files. It's written in C and Python too. Good luck!