Algorithm

NOTE: This post discusses a specific problem which is solved by disabling Nagle's algorithm. Do not try this at home: Nagle's algorithm is implemented in TCP/IP stacks for a reason, and I'm told that 99% of programmers who disable Nagle's algorithm do so errantly due to a lack of understanding of TCP. Disabling Nagle's algorithm is not a silver bullet that magically reduces latency.

While developing an input-only VNC client for Android to act as a remote mouse and keyboard, I noticed that mouse pointer movements were particularly jerky when connecting to a computer running TightVNC. Connections to other VNC servers yielded relatively smooth pointer movements, and using the official TightVNC client to connect to the TightVNC server was also smooth. My VNC client was not alone with this problem—connecting to the TightVNC server using other VNC clients such as Chicken of the VNC also resulted in severe pointer jerkiness. What was special about the VNC connection between the TightVNC client and the TightVNC server that made it work so much more smoothly?

DES, the Data Encryption Standard, was developed by IBM and the US government in the 1970's. Today, DES is considered to be weak and crackable, and a poor choice for anyone in the market for an encryption algorithm. However, many legacy protocols still use DES, so it's important to have implementations handy.

I recently found myself looking for a simple standalone DES implementation to study. Most of the open-source DES implementations are either highly optimized into obfuscation, or sloppily written. Either way, it's hard to find a clearly written and well-commented implementation suitable for educational purposes. I decided it would be a good exercise to write one myself. I wrote the implementation in Java, for the extra challenge of performing bit manipulations on signed primitive types. This implementation is undoubtedly very inefficient, but is well-commented and should be easy to understand for anyone who wants to dive into a sea of Feistel functions, S-boxes, variable rotations, and permutations.