1. Reactive Android Development
CS T & CS T
Summer 2016
Encryption Continued

2. Encoding lots of data
Last time, we saw that there was a limit to the size of message that could be encrypted by the RSA algorithm
This could be solved by breaking the data into blocks
But the cipher text was 50% larger than the clear text

3. Stream Cipher
There are symmetric encryption algorithms that don't increase the messages size nearly as much.
But both sides of the conversation must have the same key

4. Key Exchange
There are ways of exchanging keys without fear of eavesdroppers
But our clients have published public keys, so we can simply use them.

5. Log-in
Last time, I had proposed that we could use a form of digital signature as a way of logging in.
That is, encode a message with your private key, so that the server would know that it really came from the owner of the public key.
But I was concerned that doing so would potentially expose your private key

6. Log-in But there's a simpler way
The server can encrypt a random message with the user's public key
The user can then decrypt the message and re-encrypt with the server's public key
The encrypted message can be used as a session identifier for future communication with the server.

7. Code Example
GitHub
UTSA-Reactive-Android-2016
kbaldor
CryptoTest