Basic functions used in encryption algorithms

2 What are the two basic functions used in encryption algorithms?
3 How many keys are required for two people to communicate via a symmetric cipher?
4 What is the difference between a block cipher and a stream cipher?
5 What are the two general approaches to attacking a cipher?
6 Why do some block cipher modes of operation only use encryption while others use both encryption and decryption?
7 What is triple encryption?
8 Why is the middle portion of 3DES a decryption rather than an encryption?”

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