List the following briefly each question in 25 words

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2.1 What are the essential ingredients of a symmetric cipher?
2.2 What are the two basic functions used in encryption algorithms?
2.3 How many keys are required for two people to communicate via a symmetric cipher?
2.4 What is the difference between a block cipher and a stream cipher?
2.5 What are the two general approaches to attacking a cipher?
2.6 Why do some block cipher modes of operation only use encryption while others use both encryption and decryption?
2.7 What is triple encryption?
2.8 Why is the middle portion of 3DES a decryption rather than an encryption?