Implementing AES-CFB in C requires two parts:
-
AES implementation: This handles the core AES encryption/decryption logic. There are libraries like OpenSSL you can use, or for a lightweight option, consider libraries like tiny-AES-c.
-
CFB mode implementation: This uses the AES functionality to perform encryption/decryption in Cipher Feedback mode. Here’s a basic structure:
#include <stdint.h>
// Assuming you have functions for AES encryption/decryption (e.g., AES_encrypt, AES_decrypt)
void aes_cfb_encrypt(const uint8_t* key, size_t key_len, const uint8_t* iv,
const uint8_t* input, size_t input_len, uint8_t* output) {
uint8_t shift_register[AES_BLOCK_SIZE]; // AES block size (usually 16 bytes)
memcpy(shift_register, iv, AES_BLOCK_SIZE);
for (size_t i = 0; i < input_len; ++i) {
AES_encrypt(key, key_len, shift_register, shift_register);
output[i] = shift_register[0] ^ input[i];
// Shift the register for next round
memmove(shift_register, shift_register + 1, AES_BLOCK_SIZE - 1);
shift_register[AES_BLOCK_SIZE - 1] = output[i];
}
}
void aes_cfb_decrypt(const uint8_t* key, size_t key_len, const uint8_t* iv,
const uint8_t* input, size_t input_len, uint8_t* output) {
uint8_t shift_register[AES_BLOCK_SIZE];
memcpy(shift_register, iv, AES_BLOCK_SIZE);
for (size_t i = 0; i < input_len; ++i) {
AES_encrypt(key, key_len, shift_register, shift_register);
output[i] = shift_register[0] ^ input[i];
// Shift the register for next round
memmove(shift_register, shift_register + 1, AES_BLOCK_SIZE - 1);
shift_register[AES_BLOCK_SIZE - 1] = input[i];
}
}
Note:
This is a basic example and doesn’t handle padding (important for CBC mode). You’ll need to implement padding for real-world use cases. Remember to replace AES_encrypt and AES_decrypt with your actual implementation functions. This code assumes the same key and IV are used for encryption and decryption.
Further considerations:
Error handling is not included in this example. Make sure to handle potential errors during encryption/decryption. This is a basic implementation for educational purposes. For production use, consider using well-established cryptographic libraries like OpenSSL.