本文利用仿射密碼，對一個只含可打印字符的txt文件進行加密和解密。

//加解密時，文件內容為所有可打印字符，即ASCII碼從32-126的95個字符 #include<iostream> #include<fstream> using namespace std;//加密類 class Encrypt { public:void set_AB(int a, int b) {A = a; B = b;} //設置加密密鑰int gcd(int a, int b) {while(b) {int r = a % b;a = b;b = r;}return a;}bool Judge(int a, int b) { //判斷密鑰是否合法if(gcd(a, 95) == 1) {set_AB(a, b);return true;}else {cout << "密鑰不合法，請重新輸入！" << endl;return false;}}int get_A() {return A;} //獲取加密密鑰int get_B() {return B;}int encrypt(char ch) { //對單個字符進行加密int plaintext_id = ch - ' ';int A = get_A(), B = get_B();int ciphertext_id = (A * plaintext_id + B) % 95;return ciphertext_id + 32;}private:int A, B; //加密密鑰 };//解密類 class Decrypt { public:int x, y; //a*x + b*y = gcd(a, b)int extend_eulid(int a, int b) {if(b == 0) {x = 1;y = 0;}else {extend_eulid(b, a%b);int temp = x;x = y;y = temp - a / b * y;}}int get_inv(int a) { //求A的乘法逆元extend_eulid(a, 95);int inv = x;if(inv < 0) inv += 95;return inv;}void set_A(int a) {A = get_inv(a);}void set_B(int b) {B = b;}int get_A() {return A;} //獲取解密密鑰int get_B() {return B;}int decrypt(char ch) { //對單個字符進行解密int ciphertext_id = ch - ' ';int A = get_A(), B = get_B();int plaintext_id = A * (ciphertext_id - B + 95) % 95;return plaintext_id + 32;}private:int A, B; //解密密鑰 };int main() {Encrypt en;Decrypt de;int A, B;char ch;while(1) { //輸入加密密鑰cin >> A >> B;if(en.Judge(A, B) == true) break;}de.set_A(A), de.set_B(B); //設置解密密鑰ifstream fin("F:text.txt", ios::in); //以輸入方式打開明文文件"text.txt"ofstream fout("F:ciphertext.txt", ios::out); //以輸出方式打開保存密文的文件"ciphertext.txt"，并將其原有的內容清除if(!fin) {cout << "Can not open the plaintext file!" << endl;}while(fin.get(ch) != NULL) {int ciphertext_id = en.encrypt(ch);char ciphertext_ch = char(ciphertext_id);fout << ciphertext_ch;} //從明文文件"F:test.txt"中一次讀取一個字符進行加密，將加密后的文件保存為 "ciphertext.txt"fout.close();ifstream fin1("F:ciphertext.txt", ios::in); //以輸入方式打開密文文件"ciphertext.txt"ofstream fout1("F:plaintext.txt", ios::out); //以輸出方式打開保存解密后的文件"plaintext.txt"，并將其原有的內容清除if(!fin1) {cout << "Can not open the ciphertext file!" << endl;}while(fin1.get(ch) != NULL) {int plaintext_id = de.decrypt(ch);char plaintext_ch = char(plaintext_id);fout1 << plaintext_ch;} //從密文文件"ciphertext.txt"中一次讀取一個字符進行解密，將解密后的文件保存為"plaintext.txt"fout1.close();return 0; }

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