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The generation of random numbers is an integral part to encryption. So how exactly do you generate random numbers? When a computer does it, we call it the RNG (Random Number Generator). However there is a technique generate quasi-random numbers called PRNG, or Pseudo RNG. It allows you to "seed" the generator with a value. That value determines what numbers get generated. Every time you use the same seed, you get the same results.
Finally I want to go a little further with symmetric encryption algorithms. In particular, I want to mention a few implementations of symmetric encrption. The popular ones are DES, 3DES, and AES. However there is also IDEA and TwoFish. I will not go into the particulars of these right now. Perhaps we will review them next time.