Assignment 1

Symmetric keys are exchanged by private communications, such as through a private meeting or something similar. Once the key is distributed, it could be used multiple times,
but no one can change to another key anymore - Not until they meet another time.

Symmetric keys could only be cracked by trying all the possibilities of the keyspace, which often turns out to be very big. And even if the cracked tries out all the possible keys, it still may or may not generate multiple understandable meanings, which could confuse the cracker.

Since it is the same key used both on encryption and decryption, so the only one that can send a code which could be decrypted by the key in the receiver's hand, is the one that owns the same key(What a sentence!). In other words, only the ones that share keys can communicate.

The advantage of the symmetric key technique is that it is fast and safe. Increasing key space doesn't make many performance changes for the sender and receiver who owns the key. But it would be like impossible to crack a 256-bit keyspace. However, the key distributing may cost a lot, especially in the cases where the two objects don't need to communicate a lot of times. And communicating between strangers is also hard to fix in this method.

In the Asymmetric method, the public key can be know by everyone, it can be posted at public places, or mainpages, or everywhere possible. The private key is kept by oneself ,which doen't need to be distributed.

The security of cryptographic in this case is based on the well admitted assumption that it is hard to factor large numbers.

The only message that a public key can decrypt is the one that is encrypted by the owner of the private key. And it is same the other way, which provides the way to send messages and signature documents.

The advantage of asymmetric keys is that there is neither risk or cost by sharing public keys. And if we want, we can change our public key and private key at any time. Which is convenient for totally strangers. However, the encryption and decryption turned out to be much slower and the keys are also larger. And it is not safe enough in the "Man sit in the middle case".