1. Explain the difference between symmetric-key and asymmetric-key encryption.

How are keys distributed with each technique?

symmetric-key:
It only needs one key to encrypt the message. And both user only need the same key to decode the message. And the in order to create the key is by moving the bit.

asymmetric-key:
It needs two different keys- public key and private key. Everyone can see the public key and only the person who has private key can decode the message.

Why is each technique cryptographically secure? In other words, what needs to be done to crack a ciphertext encrypted with each algorithm?

symmetric-key:
because the symmetric-key system only need one key, in order to crack the message, you only need the key. Or you can find the pattern how they encrypt the message. For example: the encrypted message is “Lbj-Xfo,” and you can probably guess that the real message is “Kai-Wen” by moving each letter to the prior letter. (but you probably need to try out every single possible.)

asymmetric-key:
asymmetric-key is like double encryption. First, the user use his private key to encrypt the message, them he/she public the message with public key; although everyone can see the message, only the person with his own private key can decode the message. So, in order to crack the system, you need the person’s private key, or need to know how they created the private key. It is almost impossible to crack the system because the key is too big.

How can each technique be used to confirm a sender's identity?

symmetric-key:
Symmetric-key confirms sender’s identity by knowing who can encrypt the message or decode the message; in other words, by knowing who has the key.

asymmetric-key:
Asymmetric-key confirms the sender’s identity by double the encryption. One person encrypt the message with his private key, and send that with public key. And only the person with another private key can decode the message. The person can also use his private to encrypt his sign in order to let the other person know who he is. (Digital Signature)

What are the strengths and weaknesses of each approach?

symmetric-key:

• advantages
  Safer (lots of probability), and faster.

• Disadvantages
  One time transactions, how to give the key to the other person. And once other people know the key, you have to change the key at both sides.

asymmetric-key:

• advantages:
  Allow letting other people read the encrypted message without any risk.
  No problem for distributing the key.

• Disadvantages:
  Big and slow.