Difference Between a Block Cipher and A Stream Cipher Questions

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