AES256加密 CBC模式 pkcs7_padding C语言 函数，相关代码

我理解您的要求。以下是示例代码：

#include <openssl/aes.h>
#include <openssl/rand.h>
#include <string.h>

int aes256_encrypt(const unsigned char* key,
                   const unsigned char* iv,
                   const unsigned char* plaintext,
                   size_t plaintext_len,
                   unsigned char* ciphertext) {
  if (plaintext_len % AES_BLOCK_SIZE != 0) {
    return -1;  // plaintext长度必须是AES块大小的倍数
  }
  AES_KEY aes_key;
  if (AES_set_encrypt_key(key, 256, &aes_key) < 0) {
    return -1;  // AES密钥初始化失败
  }
  AES_cbc_encrypt(plaintext, ciphertext, plaintext_len, &aes_key, iv, AES_ENCRYPT);
  return 0;
}

int pkcs7_padding(const unsigned char* in,
                  size_t in_len,
                  unsigned char* out,
                  size_t* out_len) {
  size_t pad_len = AES_BLOCK_SIZE - in_len % AES_BLOCK_SIZE;
  *out_len = in_len + pad_len;
  if (pad_len > 255) {
    return -1;  // 数据太长
  }
  memcpy(out, in, in_len);
  memset(out + in_len, pad_len, pad_len);
  return 0;
}

此处只是示例代码，需根据具体情况做相应调整。