Feistel C语言程序如何实现核心逻辑？

Feistel Cipher C Implementation

Here's a complete implementation of a Feistel cipher in C. This example includes the Feistel structure, a simple round function, and example usage for encryption and decryption.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
// Define block size and number of rounds
#define BLOCK_SIZE 8  // 64-bit block (8 bytes)
#define NUM_ROUNDS 16
// Example round function (simple XOR with round key)
// In practice, this would be much more complex
uint32_t round_function(uint32_t right, uint32_t round_key) {
    return right ^ round_key;
}
// Feistel encryption function
void feistel_encrypt(uint8_t *block, uint32_t *keys) {
    uint32_t left, right;
    // Split block into left and right halves
    memcpy(&left, block, sizeof(uint32_t));
    memcpy(&right, block + sizeof(uint32_t), sizeof(uint32_t));
    // Apply Feistel rounds
    for (int i = 0; i < NUM_ROUNDS; i++) {
        uint32_t temp = right;
        right = left ^ round_function(right, keys[i]);
        left = temp;
    }
    // Combine halves (note: final swap is already done in the loop)
    memcpy(block, &right, sizeof(uint32_t));
    memcpy(block + sizeof(uint32_t), &left, sizeof(uint32_t));
}
// Feistel decryption function (same as encryption but with reversed keys)
void feistel_decrypt(uint8_t *block, uint32_t *keys) {
    uint32_t left, right;
    // Split block into left and right halves
    memcpy(&left, block, sizeof(uint32_t));
    memcpy(&right, block + sizeof(uint32_t), sizeof(uint32_t));
    // Apply Feistel rounds in reverse order
    for (int i = NUM_ROUNDS - 1; i >= 0; i--) {
        uint32_t temp = right;
        right = left ^ round_function(right, keys[i]);
        left = temp;
    }
    // Combine halves (note: final swap is already done in the loop)
    memcpy(block, &right, sizeof(uint32_t));
    memcpy(block + sizeof(uint32_t), &left, sizeof(uint32_t));
}
// Simple key scheduling function (in practice, this would be more complex)
void generate_keys(uint32_t master_key, uint32_t *keys) {
    for (int i = 0; i < NUM_ROUNDS; i++) {
        keys[i] = master_key + i;  // Simple key derivation
    }
}
// Helper function to print a block as hex
void print_block(const char *label, const uint8_t *block) {
    printf("%s: ", label);
    for (int i = 0; i < BLOCK_SIZE; i++) {
        printf("%02x", block[i]);
    }
    printf("\n");
}
int main() {
    // Example plaintext (64-bit block)
    uint8_t plaintext[BLOCK_SIZE] = {0x12, 0x34, 0x56, 0x78, 0x9A, 0xBC, 0xDE, 0xF0};
    // Master key (32-bit in this example)
    uint32_t master_key = 0x12345678;
    // Generate round keys
    uint32_t keys[NUM_ROUNDS];
    generate_keys(master_key, keys);
    // Create a copy for decryption
    uint8_t ciphertext[BLOCK_SIZE];
    uint8_t decrypted[BLOCK_SIZE];
    memcpy(ciphertext, plaintext, BLOCK_SIZE);
    memcpy(decrypted, ciphertext, BLOCK_SIZE);
    // Print original plaintext
    print_block("Plaintext", plaintext);
    // Encrypt
    feistel_encrypt(ciphertext, keys);
    print_block("Ciphertext", ciphertext);
    // Decrypt
    feistel_decrypt(decrypted, keys);
    print_block("Decrypted", decrypted);
    // Verify
    if (memcmp(plaintext, decrypted, BLOCK_SIZE) == 0) {
        printf("Success: Decrypted text matches original plaintext.\n");
    } else {
        printf("Error: Decrypted text does not match original plaintext.\n");
    }
    return 0;
}

Explanation:

1. Feistel Structure:

   • The block is split into two 32-bit halves (left and right)
   • For each round, the right half is transformed using a round function and the round key
   • The result is XORed with the left half
   • The halves are then swapped

2. Key Components:

   • round_function(): A simple example (XOR with round key)
   • generate_keys(): Creates round keys from a master key
   • feistel_encrypt() and feistel_decrypt(): The main encryption/decryption functions

3. Example Usage:

   • Encrypts a 64-bit block of plaintext
   • Decrypts it back
   • Verifies the decrypted text matches the original

Notes:

• This is a simplified example for educational purposes. Real Feistel ciphers (like DES) use much more complex round functions and key schedules.
• For production use, you would want to:
  • Add proper padding for messages longer than one block
  • Implement a more secure round function (like in DES or Blowfish)
  • Use a proper key derivation function
  • Add input validation and error handling
• The block size is fixed at 64 bits (8 bytes) in this example.

Would you like me to explain any specific part in more detail or modify the implementation for a particular use case?