[정보 보안] Classic Encryption Techniques (8)

[1] Classic Encryption Techniques
[1-1] Substitution cipher
[1-2] Transposition cipher

[1-1] Substitution cipher

Monoalphabetic Cipher

Caesar Cipher
- Monoalphabetic Cipher : mapping plaintext character one-to-one with other characters
- Assign numbers from 0 to 25 to english character and combine them with a secret key to perform a modulo operation
- K : secret key (shift interval of plaintext)
- P : plaintext
- C : ciphertext

@CC Encryption
- C = P + K mod 26

@CC Decryption
- P = C - K mod 26

Affine Cryptosystem
- Extension of Caesar Cipher
- Adds a key to plaintext and substitutes a plaintext with ciphertext
- K1 : 배수
- K2 : secret key

@AC Encryption
- C = K1 * P + K2 mod 26

@AC Decryption
- P = (C - K2) / K1 mod 26

Polyalphabetic Cipher

Vigenere Cipher
- The most typical multiple-substitution cryptosystem
- One-to-many
- 위치에 따라 K가 다르다
- A character can be replaced by multiple ciphertext characters
- The character and its position determine the ciphertext
- d : period d

@VC Encryption
- C1 = P1 + K2 mod 26
- Cd = Pd + Kd mod 26

@VC Decryption
- P1 = C1 - K1 mod 26
- Pd = Cd - Kd mod 26


[1-2] Transposition cipher

Keyless Transposition Cipher
- The sequence of character is rearranged according to certain special procedures (permutation cipher)

Rail Fence Cipher
- Plaintext with a length of I is arranged on r rails in a zigzag pattern
- Ciphertexts are generated by reading in a row-sequential pattern

@RFC Encryption
- I : even, C = P0,P2, ... ,P(I-2),P1,P3, ... ,P(I-1)
- I : odd, C = P0,P2, ... ,P(I-1),P1,P3, ... ,P(I-2)
- meetmeatthepark -> memateaketethpr

@RFC Decryption
- I : even, P = C0,C(I/2),C1,C(I/2+1), ... ,C(I/2-1), C(I-1)
- I : odd, P = C0,C(I/2+1),C1,C(I/2+2), ... ,C(I/2-1), C(I-1),C(I/2)

Keyed Transpotition Cipher
- Relocate normal plaintext arrays in order of a specific key

Simple Keyed Transposition Cipher
- Divide the plaintext sentences at refulat intervals
- Arrange the characters devided by the regular interval
- According to the reaarangement order of keys
- d : length of keys

@SKTC Encryption
- 자리를 변경하는 규칙은 없고 key에 따라 변경
- Ciphertext position : 3,5,1,6,4,2
- informations -> frimoninasot

@SKTC Decryption
- 거꾸로