Week 11

Group key decryption was added to the extension this week. The group key is encrypted with some symmetric ciphers, for example, aes-256-cbc, and the key of the symmetric cipher is then encrypted under a user's public key using RSA. Therefore, only the user knowing the corresponding private key can retrieve the key for the symmetric cipher, and then decrypt the group key, which, in our case, is indeed the information of the current group state, and can be used to derived the actual group key.

At present, the encrypted group key obtained from a key server is in the following format:

1. The initialization vector of the aes-cbc cipher in hexadecimal.
2. The aec-cbc key encrypted with the user's public key in base64
3. The encrypted group key in base64

A sample encrypted group key looks as follows:

For the RSA decryption compatible with openssl, I have imported another library called jsencrypt. To be honest, I'm feeling uncomfortable with importing so many different libraries in javascript, so I have started implementing my own crypto library in javascript. For now, the library only provide some basic interface for big number arithmetic including addition, subtraction, multiplication, and division. I am going to work on the library further so that encryption and decryption with various ciphers will be possible. The library is currently held on github. Here is the link: https://github.com/qiyuh/CLJS.

During the following weeks, I will have the extension check the signature of the message to ensure the authenticity. It seems that I have to find some other libraries to do so because openssl signs a message with a pretty complicated process. The message is first hashed, and then ASN1 encoded. So what's signed in the end is the ASN1 encoded hash of the message.