The origin and basic concept of cryptography

One, some ancient encryption methods:

1. Steganography----Hide Secret messages in other messages

The most common is invisible ink: the sender uses onions or milk to write a secret message on the paper, and the receiver heats it slightly on the back of the paper to see the ciphertext.

2. The squares of the Greek poppy

	1	2	3	4	5
1	A	B	C	D	E
2	F	G	H	Ij	K
3	L	M	N	O	P
4	Q	R	S	T	U
5	V	W	X	Y	Z

Plaintext	A	P	P	L	E
Ciphertext	11	35	35	31	15

Thought: Replace

Just remember the password table and be able to encrypt and decrypt.

3. Caesar Password

A	B	C	D	E	F	G	...	X	Y	Z
D	E	F	G	H	I	J	...	A	B	C

Plaintext	A	P	P	L	E
Ciphertext	D	S	S	O	H

Thought: Replace

The Caesar Code table is easier to remember than the Greek squares above, because it only moves the clear text right 3 bits.

Second, the basic concept of cryptography:

Research on the Science of Information system security technology, including two branches: cryptography, encoding information to realize the knowledge of hidden information; cryptography: Research and analysis of the knowledge of breaking passwords or forgery. The two oppose each other, and promote each other to develop forward.

Sender: The sender of the message

Recipient: Destination of message sent

Eavesdroppers: Non-authorized person in the information transmission process and processing system

PlainText: Messages that will be hidden

Ciphertext: The form of clear text after transformation

Cryptography: The process of transforming plaintext into ciphertext

Decryption: The process of restoring ciphertext to plaintext

Encryption algorithm: A set of rules used to encrypt plaintext

Decryption algorithm: A set of rules used to decrypt ciphertext

Key: The data that controls the operation of the encryption and decryption algorithms, respectively, with the encryption key and decryption key

Third, the security system model:

Security System (M,C,K1,K2,EK1,DK2):

Clear text message space m

Ciphertext message Space C

Key space K1 and K2, under a single-key system K1=k2=k, when the key K belongs to K

Iv. Classification of cryptographic algorithms:

The basis for confidentiality dependence is divided into:

1. Restricted algorithm: The secrecy of the algorithm is based on the secrecy of the retention algorithm (secrecy algorithm is not disclosed).

2. Key-based algorithm: the confidentiality of the algorithm is based on the confidentiality of the key.

According to the key features are divided into:

Symmetric cipher algorithm: The encryption key and decryption key are the same, or substantially equivalent, from one easy to eject another. Also known as the Tammy key algorithm.

Asymmetric key algorithm: Encryption key and decryption key is not the same, from one difficult to launch another, also known as Public key algorithm.

Security of cryptographic algorithms:

1. Unconditional security: Even if the translator has unlimited resources, it will not be able to solve the corresponding plaintext

2. Computational safety: The cost of deciphering exceeds the value of the information itself, deciphering the time beyond the validity of the information

The origin and basic concept of cryptography