Symmetric Encryption System
4.3.1 Symmetric Encryption System
The symmetric encryption system is commonly used to encrypt a large amount of information. Before the 1970s there was only symmetric encryption algorithm in
cryptology. K 1 K 2 K is usually used in symmetric encryption algorithm. As far as symmetric encryption algorithm is concerned, the secrecy of the key is the most important; once the key is divulged or pried out, all the system will be of no secrecy. So the symmetric encryption algorithm is called secret key algorithm. It requires a commonly agreed key before the sender and the receiver begin their communication. The security of symmetric encryption relies on the secrecy of the key, for a divulged key will enable anyone to send or receive the message.
The procedure of symmetric encryption communication: (1) Alice and Bob agree on an encryption system. (2) Alice and Bob agree on a common key K. (3) Alice uses the agreed algorithm and key to encrypt her message and gets
the cipher-text.
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(4) Alice sends the cipher-text to Bob. (5) Bob uses the same key and algorithm to decrypt the cipher-text and gets the
plain text, then reads the message.
The symmetric encryption algorithm can be divided into two categories: stream algorithm, which encrypts or decrypts one bit or byte of the plain text every time; block algorithm, which encrypts or decrypts a fixed length of bytes every time. Now commonly used symmetric encryption algorithms are mostly block algorithms, such as DES and AES. And stream algorithm is usually used for military purpose, so its public literature is less available.
Introduction to DES: DES (Data Encryption Standard) is an improvement of Lucifer algorithm developed by IBM in 1970s. On Nov. 23rd, 1976, this algorithm was formally adopted as the federal encryption standard, and was authorized to
be used in non-confidential government communication. The formal document, numbered FIPS46, was published on Jan. 15, 1977, and came into effect six months later. Afterwards, it was taken as DEA-1 by ISO. It was originally planned that DES was used for only 10 years. But because there was no better symmetric algorithm, the deadline was extended continuously. However, as the computer technology develops, the security of DES becomes weaker and weaker. On Jun.
17, 1997, volunteers from all over the world broke DES successfully. This event changed people’s attitude towards the security of DES. From 1997 the USA began to seek a new generation of symmetric encryption algorithm, which gave
birth to AES. Now the bank system in our country is still using DES. Introduction to AES: AES (Advanced Encryption Standard): AES was the result of four years efforts made by USA to seek a new generation of symmetric
algorithm. Through two rounds of elimination, AES was finally established and published. USA lies in the transitional process between DES and AES. Compared with DES, AES is more clear and reliable in mathematical principle, and it can be
used reliably as a result of analysis. In addition, since the algorithm is not designed by American scientists, it is generally believed that it is more difficult to set trapdoors in AES.
The advantages of symmetric encryption algorithm (compared with public key algorithm) are: encryption is fast and efficient, usually 100 public key algorithm. However, symmetric algorithm also has some obvious demerits:
(1) The key has to be secretly distributed, because knowing the key means knowing the message. So the distribution is quite pivotal.
(2) A problem with symmetric encryption is the tremendous amount of keys, which are difficult to manage. Suppose any pair of sender and receiver have their agreed key, obviously, N users should have N * ( N 1) keep N 1 increase for memorizing all of them rather than keeping them seems impossible.
The existence of such problems gave birth to the generation of public key encryption algorithm.
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