n2n/src/transform_aes.c

/**
 * (C) 2007-20 - ntop.org and contributors
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not see see <http://www.gnu.org/licenses/>
 *
 */


#include "n2n.h"

#ifdef N2N_HAVE_AES


#define AES_BLOCK_SIZE           16

// size of random value prepended to plaintext defaults to AES BLOCK_SIZE;
// gradually abandoning security, lower values could be chosen;
// however, minimum transmission size with cipher text stealing scheme is one
// block; as network packets should be longer anyway, only low level programmer
// might encounter an issue with lower values here
#define AES_PREAMBLE_SIZE       (AES_BLOCK_SIZE)

#define AES_IV_SIZE             (AES_BLOCK_SIZE)

// cbc mode is being used with random value prepended to plaintext
// instead of iv so, actual iv is null_iv
const uint8_t null_iv[AES_IV_SIZE] = {0};

typedef struct transop_aes {
  aes_context_t       *ctx;
} transop_aes_t;

/* ****************************************************** */

static int transop_deinit_aes(n2n_trans_op_t *arg) {
  transop_aes_t *priv = (transop_aes_t *)arg->priv;

  if(priv->ctx) free(priv->ctx);

  if(priv) free(priv);

  return 0;
}

/* ****************************************************** */

// the aes packet format consists of
//
//  - a random AES_PREAMBLE_SIZE-sized value prepended to plaintext
//    encrypted together with the...
//  - ... payload data
//
//  [VV|DDDDDDDDDDDDDDDDDDDDD]
//  | <---- encrypted ---->  |
//
static int transop_encode_aes(n2n_trans_op_t * arg,
			      uint8_t * outbuf,
			      size_t out_len,
			      const uint8_t * inbuf,
			      size_t in_len,
			      const uint8_t * peer_mac) {

  transop_aes_t * priv = (transop_aes_t *)arg->priv;

  // the assembly buffer is a source for encrypting data
  // the whole contents of assembly are encrypted
  uint8_t assembly[N2N_PKT_BUF_SIZE];
  size_t idx = 0;
  int padded_len;
  uint8_t padding;
  uint8_t buf[AES_BLOCK_SIZE];

  if(in_len <= N2N_PKT_BUF_SIZE) {
    if((in_len + AES_PREAMBLE_SIZE + AES_BLOCK_SIZE) <= out_len) {

        traceEvent(TRACE_DEBUG, "transop_encode_aes %lu bytes plaintext", in_len);

        // full block sized random value (128 bit)
        encode_uint64(assembly, &idx, n2n_rand());
        encode_uint64(assembly, &idx, n2n_rand());
        // adjust for maybe differently chosen AES_PREAMBLE_SIZE
        idx = AES_PREAMBLE_SIZE;

        // the plaintext data
        encode_buf(assembly, &idx, inbuf, in_len);

        // round up to next whole AES block size
        padded_len = (((idx - 1) / AES_BLOCK_SIZE) + 1) * AES_BLOCK_SIZE;
        padding = (padded_len-idx);
        // pad the following bytes with zero, fixed length (AES_BLOCK_SIZE) seems to compile
        // to slightly faster code than run-time dependant 'padding'
        memset (assembly + idx, 0, AES_BLOCK_SIZE);

        aes_cbc_encrypt(outbuf, assembly, padded_len, null_iv, priv->ctx);

        if(padding) {
          // exchange last two cipher blocks
          memcpy (buf, outbuf+padded_len - AES_BLOCK_SIZE, AES_BLOCK_SIZE);
          memcpy (outbuf + padded_len - AES_BLOCK_SIZE, outbuf + padded_len - 2 * AES_BLOCK_SIZE, AES_BLOCK_SIZE);
          memcpy (outbuf + padded_len - 2 * AES_BLOCK_SIZE, buf, AES_BLOCK_SIZE);
        }
    } else
      traceEvent(TRACE_ERROR, "transop_encode_aes outbuf too small");
  } else
    traceEvent(TRACE_ERROR, "transop_encode_aes inbuf too big to encrypt");

  return idx;
}

/* ****************************************************** */

// see transop_encode_aes for packet format
static int transop_decode_aes(n2n_trans_op_t * arg,
			      uint8_t * outbuf,
			      size_t out_len,
			      const uint8_t * inbuf,
			      size_t in_len,
			      const uint8_t * peer_mac) {

  transop_aes_t * priv = (transop_aes_t *)arg->priv;
  uint8_t assembly[N2N_PKT_BUF_SIZE];

  uint8_t rest;
  size_t penultimate_block;
  uint8_t buf[AES_BLOCK_SIZE];
  int len=-1;

  if( ((in_len - AES_PREAMBLE_SIZE) <= N2N_PKT_BUF_SIZE) // cipher text fits in assembly
    && (in_len >= AES_PREAMBLE_SIZE) // has at least random number
    && (in_len >= AES_BLOCK_SIZE)    // minimum size requirement for cipher text stealing
    ) {

    traceEvent(TRACE_DEBUG, "transop_decode_aes %lu bytes ciphertext", in_len);

    rest = in_len % AES_BLOCK_SIZE;
    if(rest) {
      // cipher text stealing
      penultimate_block = ((in_len / AES_BLOCK_SIZE) - 1) * AES_BLOCK_SIZE;
      // everything normal up to penultimate block
      memcpy(assembly, inbuf, penultimate_block);
      // prepare new penultimate block in buf
      aes_ecb_decrypt(buf, inbuf + penultimate_block, priv->ctx);
      memcpy(buf, inbuf + in_len - rest, rest);
      // former penultimate block becomes new ultimate block
      memcpy(assembly + penultimate_block + AES_BLOCK_SIZE, inbuf + penultimate_block, AES_BLOCK_SIZE);
      //  write new penultimate block from buf
      memcpy(assembly + penultimate_block, buf, AES_BLOCK_SIZE);
      // regular cbc decryption on the re-arranged ciphertext
      aes_cbc_decrypt(assembly, assembly, in_len + AES_BLOCK_SIZE - rest, null_iv, priv->ctx);
      // check for expected zero padding and give a warning otherwise
      if (memcmp(assembly + in_len, null_iv, AES_BLOCK_SIZE - rest)) {
        traceEvent(TRACE_WARNING, "transop_decode_aes payload decryption failed with unexpected cipher text stealing padding");
        return -1;
      }
    } else {
      // regular cbc decryption on multiple block-sized payload
      aes_cbc_decrypt(assembly, inbuf, in_len, null_iv, priv->ctx);
    }
     len = in_len    - AES_PREAMBLE_SIZE;
     memcpy(outbuf,
            assembly + AES_PREAMBLE_SIZE,
            len);
  } else
    traceEvent(TRACE_ERROR, "transop_decode_aes inbuf wrong size (%ul) to decrypt", in_len);

  return len;
}

/* ****************************************************** */

static int setup_aes_key(transop_aes_t *priv, const uint8_t *password, ssize_t password_len) {

  unsigned char   key_mat[32];     // maximum aes key length, equals hash length
  unsigned char   *key;
  size_t          key_size;

  // let the user choose the degree of encryption:
  // long input passwords will pick AES192 or AES256 with more robust but expensive encryption

  // the input password always gets hashed to make a more unpredictable use of the key space
  // just think of usually reset MSB of ASCII coded password bytes
  pearson_hash_256(key_mat, password, password_len);

  // the length-dependant scheme for key setup was discussed on github:
  // https://github.com/ntop/n2n/issues/101
  if(password_len >= 65) {
    key_size = AES256_KEY_BYTES;       // 256 bit
  } else if(password_len >= 44) {
    key_size = AES192_KEY_BYTES;       // 192 bit
  } else {
    key_size = AES128_KEY_BYTES;       // 128 bit
  }
  // and use the last key-sized part of the hash as aes key
  key = key_mat + sizeof(key_mat) - key_size;

  // setup the key and have corresponding context created
  if (aes_init (key, key_size, &(priv->ctx))) {
    traceEvent(TRACE_ERROR, "setup_aes_key %u-bit key setup unsuccessful",
	       key_size * 8);
    return -1;
  }

  traceEvent(TRACE_DEBUG, "setup_aes_key %u-bit key setup completed",
	     key_size * 8);
  return 0;
}

/* ****************************************************** */

static void transop_tick_aes(n2n_trans_op_t * arg, time_t now) { ; }

/* ****************************************************** */

// AES initialization function
int n2n_transop_aes_cbc_init(const n2n_edge_conf_t *conf, n2n_trans_op_t *ttt) {
  transop_aes_t *priv;
  const u_char *encrypt_key = (const u_char *)conf->encrypt_key;
  size_t encrypt_key_len = strlen(conf->encrypt_key);

  memset(ttt, 0, sizeof(*ttt));
  ttt->transform_id = N2N_TRANSFORM_ID_AESCBC;

  ttt->tick = transop_tick_aes;
  ttt->deinit = transop_deinit_aes;
  ttt->fwd = transop_encode_aes;
  ttt->rev = transop_decode_aes;

  priv = (transop_aes_t*) calloc(1, sizeof(transop_aes_t));
  if(!priv) {
    traceEvent(TRACE_ERROR, "n2n_transop_aes_cbc_init cannot allocate transop_aes_t memory");
    return(-1);
  }
  ttt->priv = priv;

  // setup the cipher and key
  return(setup_aes_key(priv, encrypt_key, encrypt_key_len));
}

#endif // N2N_HAVE_AES