计算机问题求解一论题4-4 密码算法 2017年04月05日
计算机问题求解 – 论题4-4 - 密码算法 2017年04月05日
问题1： Cryptosystem是什么意 思？它要解决的基本问题 是什么？
最古典：Caesar's Cipher Plain b c d e h n Cipher D E F G H K L M P Plain n 0 p t U 0 2 Cipher Q R S T U X Z A B C a b d e h i k m 0 1 2 3 4 5 6 7 8 9 10 11 12 n 0 P 9 u U w E y 13 14 15 1617 18 19 20 21 22 23 2425 Cipher Plain +3mod 26 你试试：NKH GLH LV FDVW The die is cast!
最古典: Caesar’s Cipher Cipher = Plain + 3mod 26 你试试: WKH GLH LV FDVW The die is cast!
问题2： 你能不能用数学概念来描述 加密和解密？
Transformations An enciphering transformation (also called an enciphering function)is a bijective function Ee:M→e, where the key eE K uniquely determines Ee acting upon plaintert message units m E M to get ciphertert message units Ee(m)=c∈C. A deciphering transformation (or deciphering function)is a bijective function Da:CM, which is uniquely determined by a given key d E K,acting upon ciphertert message units c Ee to get plaintert message units Da(c）=m. The application of Ee to m,namely the operation Ee(m),is called enciphering, encoding,or encrypting m EM,whereas the application of Da to c is called deciphering,decoding,or decrypting ceC
Transformations