Encrypting and decrypting a small file using openssl


I want to write a small program in C/C++ which reads a small text file, and encrypts it, using a "internal" key. Then I also want to write another small program which can decrypt the encrypted file using internally the same key.

I looked at openSSL site and googled but found not simple example, has somebody ever tried to do this thing?

Best Solution

Ideally, you could use an existing tool like ccrypt, but here goes:

#include <openssl/aes.h>

/* ... */

  int bytes_read, bytes_written;
  unsigned char indata[AES_BLOCK_SIZE];
  unsigned char outdata[AES_BLOCK_SIZE];

  /* ckey and ivec are the two 128-bits keys necesary to
     en- and recrypt your data.  Note that ckey can be
     192 or 256 bits as well */
  unsigned char ckey[] =  "thiskeyisverybad";
  unsigned char ivec[] = "dontusethisinput";

  /* data structure that contains the key itself */
  AES_KEY key;

  /* set the encryption key */
  AES_set_encrypt_key(ckey, 128, &key);

  /* set where on the 128 bit encrypted block to begin encryption*/
  int num = 0;

  while (1) {
    bytes_read = fread(indata, 1, AES_BLOCK_SIZE, ifp);

    AES_cfb128_encrypt(indata, outdata, bytes_read, &key, ivec, &num,

    bytes_written = fwrite(outdata, 1, bytes_read, ofp);
    if (bytes_read < AES_BLOCK_SIZE)

Decryption is done by calling AES_cfb128_encrypt with AES_DECRYPT as the last parameter. Note that this code hasn't been given anything more than the most elementary of testing, and that you really should use proper 8-bits random data for ckey and ivec.

EDIT: It seems AES_cfb128_encrypt accepts data of arbitrary length, so you're not required to encrypt in blocks of AES_BLOCK_SIZE (16) bytes.

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