Question

I have the following function that decrypts ciphertexts.

However i have a problem that i would like to decrypt the data without having the plaintext length! How do i do that? As if i send a encrypted data over, it would not be appropriate to send the ciphertext with the plain-text length.

int main()
{
/*some code*/
char input[] = "123456789abcdef";
int olen, len;
len = strlen(input)+1;

plaintext = (char *)aes_decrypt(&de, ciphertext, &len);
/*some code*/
}

Decryption method

unsigned char *aes_decrypt(EVP_CIPHER_CTX *e, unsigned char *ciphertext, int *len)
    {
      /* plaintext will always be equal to or lesser than length of ciphertext*/
      int p_len = *len, f_len = 0;
      unsigned char *plaintext = (unsigned char *)malloc(p_len);

      if(!EVP_DecryptInit_ex(e, NULL, NULL, NULL, NULL)){
        printf("ERROR in EVP_DecryptInit_ex \n");
        return NULL;
      }

      if(!EVP_DecryptUpdate(e, plaintext, &p_len, ciphertext, *len)){
        printf("ERROR in EVP_DecryptUpdate\n");
        return NULL;
      }

      if(!EVP_DecryptFinal_ex(e, plaintext+p_len, &f_len)){
        printf("ERROR in EVP_DecryptFinal_ex\n");
        return NULL;
      }

      *len = p_len + f_len;
      return plaintext;
    }

Thanks in advance!! 🙂

Answer 1

Typically you would prefix the cleartext with a length indicator before encryption. This can be as small as a single byte “valid bytes in last block”.