Plaintext: Plaintext is the message that needs to be sent;
Ciphertext: Ciphertext is garbled information obtained by encrypting plaintext;
Key: The key is the input parameter that needs to be selected in the encryption and decryption operation.
Classical code
Spartan code stick device
Caesar Code
Encryption method
Replace password: Disturb the alphabetical order of the input plaintext without changing the value of the plaintext alphabet.
Substitution password: Replace letters in plain text without changing the order.
Steganography: protect messages by hiding their presence
Invisible ink
Character format changes
Image distortion
Tibetan poems
Implicit grammar
Word analysis
Mechanical code
Encryption and decryption machine
Enigma
composition
keyboard
monitor
Rotor (3)
Disadvantages of ENIGMA (one rotor): Type 6 letters in a row, the rotor rotates once, the encryption device returns to the original form, and the encryption is repeated with the original encryption method.
ENIGMA (two rotors): repeat only after encrypting 6 * 6 = 36 letters.
ENIGMA (three rotors): need to encrypt 26 * 26 * 26 = 17576 letters before repeating.
Enigma code buster-"bomb" decoder
New type of decoding machine "Giant" electronic computer
Symmetric cipher
Encryption method
classification
Block cipher
Working method: divide the plain text into fixed length groups
Block cipher algorithm
DES algorithm
Defects and deficiencies
The design of DES may contain traps, the design principle of S box has not been announced so far
The key capacity is too small: 56 bits is unlikely to provide sufficient security
Improve
3DES algorithm: call the DES algorithm three times in a row
low efficiency
AES algorithm
Basic requirements: Supports three different lengths of key input, 128, 192, and 256 bits, and can encrypt 128-bit input plaintext each time.
Operating mode
ECB (easiest)
CBC
CFB
OFB
Stream cipher
Features
Simple to implement
Easy hardware implementation
Fast encryption and decryption
Encryption process
Password distribution for symmetric passwords
Since both parties in communication use the same key, if there are malicious users in both parties in communication, the security of the above password cannot be guaranteed, that is, there is a key security problem.
Every time a pair of users use a symmetric encryption algorithm, they need to use a unique key that no one else knows. This will cause the number of keys owned by both the sending and receiving parties to grow geometrically, causing users to store the management key The burden of the key.
Symmetric password cannot be used for digital signature
insufficient
Key update, management and distribution issues
Cannot be applied to digital signatures
Public key password
Idea: The keys held by the two parties of communication are different, that is, the input parameters required for encryption and decryption are different.
Important characteristics: Knowing the cryptographic algorithm and encryption key, it is not feasible to calculate the decryption key.
Encryption and decryption process
E.g
RSA public key algorithm
It is based on Euler's theorem in elementary number theory and is based on the difficult problem of large integer factorization.
Using the public key idea, everyone can query the public key information of the recipient of the message, which avoids the shortage of storing the session key for each pair of communicating parties.
Users can also use their private keys to encrypt information and decrypt with the corresponding public key, so as to achieve the effect of digital signature.
Application scenario
Network protocols include SSL / TLS, IPsec / IKE, SSH, S / MIME, POP / IMAP, DNSsec, SBGP ...
In real life: online shopping, online banking, smart card, email, digital signature
Classical ciphers, mechanical ciphers, and symmetric ciphers all use two basic design ideas: substitution and replacement.
A cryptographic algorithm using this design idea requires the same encryption and decryption keys
