playfair算法 c语言版

Playfair算法是一种密码加密算法，通过使用一个5x5的矩阵来加密明文。首先要在矩阵中填充关键字，并按照一定规则处理重复字母。接下来，将明文按照规则进行分块，每一对字母通过矩阵中的规则进行加密，最终得到密文。

以下是一个简单的Playfair算法的C语言版实现：

#include <stdio.h>
#include <string.h>

void generateKeyTable(char key[], int key_len, char keytable[5][5]) {
  char a[26];
  memset(a, 0, sizeof(a));
  
  int i, j, k;
  j = 0;

  for (i = 0; i < key_len; i++) {
    if (key[i] == 'j') key[i] = 'i';
    if (a[key[i] - 97] == 0) {
      a[key[i] - 97] = 1;
      keytable[j / 5][j % 5] = key[i];
      j++;
    }
  }

  for (i = 0, k = 0; i < 26; i++) {
    if (a[i] == 0) {
      keytable[j / 5][j % 5] = (char)(i + 97);
      j++;
    }
  }
}

void encrypt(char* str, char keytable[5][5]) {
  int i = 0;
  while (str[i + 1] != '\0') {
    if (str[i] == str[i + 1]) {
      if (i % 2 == 0) {
        str[i + 1] = 'x';
      } else {
        str[i] = 'x';
      }
    }
    i++;
  }

  if (i % 2 != 0) {
    str[i + 1] = 'x';
  }

  i = 0;
  while (str[i] != '\0') {
    if (str[i] == 'j') str[i] = 'i';
    i++;
  }

  i = 0;
  while (str[i] != '\0') {
    if (str[i] != ' ') {
      int row1, col1, row2, col2;
      for (int j = 0; j < 5; j++) {
        for (int k = 0; k < 5; k++) {
          if (str[i] == keytable[j][k]) {
            row1 = j;
            col1 = k;
          }
          if (str[i + 1] == keytable[j][k]) {
            row2 = j;
            col2 = k;
          } 
        }
      }

      if (row1 == row2) {
        if (col1 == 4) {
          str[i] = keytable[row1][0];
        } else {
          str[i] = keytable[row1][col1 + 1];
        }
        if (col2 == 4) {
          str[i + 1] = keytable[row2][0];
        } else {
          str[i + 1] = keytable[row2][col2 + 1];
        }
      } else if (col1 == col2) {
        if (row1 == 4) {
          str[i] = keytable[0][col1];
        } else {
          str[i] = keytable[row1 + 1][col1];
        }
        if (row2 == 4) {
          str[i + 1] = keytable[0][col2];
        } else {
          str[i + 1] = keytable[row2 + 1][col2];
        }
      } else {
        str[i] = keytable[row1][col2];
        str[i + 1] = keytable[row2][col1];
      }
    }
    i += 2;
  }
}

int main() {
  char key[100], str[100];
  char keytable[5][5];

  printf("Enter key: ");
  scanf("%s", key);

  generateKeyTable(key, strlen(key), keytable);

  printf("Enter the string to encrypt: ");
  scanf("%s", str);

  encrypt(str, keytable);

  printf("Encrypted string: %s\n", str);
  
  return 0;
}

这是一个简单的C语言版的Playfair算法实现，可以根据用户输入的关键字和明文进行加密并输出密文。