Question

I am using this simple function for decrypting a AES Encrypted string

unsigned char *aes_decrypt(EVP_CIPHER_CTX *e, unsigned char *ciphertext, int *len)
{
  int p_len = *len, f_len = 0;
  unsigned char *plaintext = (unsigned char*)malloc(p_len + 128);
  memset(plaintext,0,p_len);
  EVP_DecryptInit_ex(e, NULL, NULL, NULL, NULL);
  EVP_DecryptUpdate(e, plaintext, &p_len, ciphertext, *len);
  EVP_DecryptFinal_ex(e, plaintext+p_len, &f_len);

  *len = p_len + f_len;
  return plaintext;
}

The problem is that len is returning a value that does not match the entire decoded string. What could be the problem ?

Answer 1

When you say “string”, I assume you mean a zero-terminated textual string. The encryption process is dependent on a cipher block size, and oftentimes padding. What’s actually being encoded and decoded is up to the application… it’s all binary data to the cipher. If you’re textual string is smaller than what’s returned from the decrypt process, your application needs to determine the useful part. So for example if you KNOW your string inside the results is zero-terminated, you can get the length doing a simple strlen. That’s risky of course if you can’t guarantee the input… probably better off searching the results for a null up to the decoded length…