OpenSSL decrypted text length

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 ?

score 2 · Accepted Answer · answered Sep 21 '12 at 13:06

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...

score 1 · Answer 2 · answered Oct 16 '12 at 05:48

If you are using cipher in ECB, CBC or some other chaining modes, you must pad plain text to the length, which is multiple of cipher block length. You can see a PKCS#5 standard for example. High-level functions like in OpenSSL can perform padding transparently for programmer. So, encrypted text can be larger than plain text up to additional cipher block size.

OpenSSL decrypted text length

2 Answers2