n2n/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"
#include "n2n_transforms.h"

#ifdef N2N_HAVE_AES

#include <openssl/aes.h>
#include <openssl/sha.h>
#include <openssl/evp.h>
#include <openssl/err.h>

#define N2N_AES_TRANSFORM_VERSION       1  /* version of the transform encoding */
#define N2N_AES_IVEC_SIZE               (AES_BLOCK_SIZE)

#define AES256_KEY_BYTES (256/8)
#define AES192_KEY_BYTES (192/8)
#define AES128_KEY_BYTES (128/8)

/* AES plaintext preamble */
#define TRANSOP_AES_VER_SIZE     1       /* Support minor variants in encoding in one module. */
#define TRANSOP_AES_IV_SEED_SIZE 8	/* size of transmitted random part of IV in bytes; leave it set to 8 for now */
#define TRANSOP_AES_IV_PADDING_SIZE (N2N_AES_IVEC_SIZE - TRANSOP_AES_IV_SEED_SIZE)
#define TRANSOP_AES_IV_KEY_BYTES (AES128_KEY_BYTES) /* use AES128 for IV encryption */
#define TRANSOP_AES_PREAMBLE_SIZE (TRANSOP_AES_VER_SIZE + TRANSOP_AES_IV_SEED_SIZE)

typedef unsigned char n2n_aes_ivec_t[N2N_AES_IVEC_SIZE];

typedef struct transop_aes {
  EVP_CIPHER_CTX	*enc_ctx;	/* openssl's reusable evp_* encryption context */
  EVP_CIPHER_CTX	*dec_ctx;	/* openssl's reusable evp_* decryption context */
  const EVP_CIPHER	*cipher;	/* cipher to use: e.g. EVP_aes_128_cbc */
  uint8_t  		key[32];	/* the pure key data for payload encryption & decryption */
  AES_KEY             iv_enc_key;     /* key used to encrypt the IV */
  uint8_t             iv_pad_val[TRANSOP_AES_IV_PADDING_SIZE]; /* key used to pad the random IV seed to full block size */
} transop_aes_t;

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

  EVP_CIPHER_CTX_free(priv->enc_ctx);
  EVP_CIPHER_CTX_free(priv->dec_ctx);

  if(priv)
    free(priv);

  return 0;
}

/* get any erorr message out of openssl
   taken from https://en.wikibooks.org/wiki/OpenSSL/Error_handling */
char *openssl_err_as_string (void) {
  BIO *bio = BIO_new (BIO_s_mem ());
  ERR_print_errors (bio);
  char *buf = NULL;
  size_t len = BIO_get_mem_data (bio, &buf);
  char *ret = (char *) calloc (1, 1 + len);
  if (ret)
    memcpy (ret, buf, len);
  BIO_free (bio);
  return ret;
}

static void set_aes_cbc_iv(transop_aes_t *priv, n2n_aes_ivec_t ivec, uint64_t iv_seed) {
  uint8_t iv_full[N2N_AES_IVEC_SIZE];

  /* Extend the seed to full block size with padding value */
  memcpy(iv_full, priv->iv_pad_val, TRANSOP_AES_IV_PADDING_SIZE);
  memcpy(iv_full + TRANSOP_AES_IV_PADDING_SIZE, &iv_seed, TRANSOP_AES_IV_SEED_SIZE);

  /* Encrypt the IV with secret key to make it unpredictable.
   * As discussed in https://github.com/ntop/n2n/issues/72, it's important to
   * have an unpredictable IV since the initial part of the packet plaintext
   * can be easily reconstructed from plaintext headers and used by an attacker
   * to perform differential analysis.
   */
  AES_ecb_encrypt(iv_full, ivec, &priv->iv_enc_key, AES_ENCRYPT);
}

/** The aes packet format consists of:
 *
 *  - a 8-bit aes encoding version in clear text
 *  - a 64-bit random IV seed
 *  - encrypted payload.
 *
 *  [V|II|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)
{
  int len2=-1;
  transop_aes_t * priv = (transop_aes_t *)arg->priv;
  uint8_t assembly[N2N_PKT_BUF_SIZE] = {0};

  if ( in_len <= N2N_PKT_BUF_SIZE) {
    if ( (in_len + TRANSOP_AES_PREAMBLE_SIZE) <= out_len) {
      int len=-1;
      size_t idx=0;
      uint64_t iv_seed = 0;
      uint8_t padding = 0;
      n2n_aes_ivec_t enc_ivec = {0};

      traceEvent(TRACE_DEBUG, "encode_aes %lu", in_len);

      /* Encode the aes format version. */
      encode_uint8( outbuf, &idx, N2N_AES_TRANSFORM_VERSION);

      /* Generate and encode the IV seed.
       * Using two calls to rand() because RAND_MAX is usually < 64bit
       * (e.g. linux) and sometimes < 32bit (e.g. Windows).
       */
      iv_seed = ((((uint64_t)rand() & 0xFFFFFFFF)) << 32) | rand();
      encode_buf(outbuf, &idx, &iv_seed, TRANSOP_AES_IV_SEED_SIZE);

      /* Encrypt the assembly contents and write the ciphertext after the iv seed. */
      /* len is set to the length of the cipher plain text to be encrpyted 
	 which is (in this case) identical to original packet lentgh */
      len = in_len;

      /* The assembly buffer is a source for encrypting data.
       * The whole contents of assembly are encrypted. */
      memcpy( assembly, inbuf, in_len);

      /* Need at least one encrypted byte at the end for the padding. */
      len2 = ( (len / AES_BLOCK_SIZE) + 1) * AES_BLOCK_SIZE; /* Round up to next whole AES adding at least one byte. */
      padding = (len2-len);
      assembly[len2 - 1] = padding;
      traceEvent(TRACE_DEBUG, "padding = %u, seed = %016llx", padding, iv_seed);

      set_aes_cbc_iv(priv, enc_ivec, iv_seed);

      EVP_CIPHER_CTX *ctx = priv->enc_ctx;
      int evp_len;
      int evp_ciphertext_len;
      if (1 == EVP_EncryptInit_ex(ctx, priv->cipher, NULL, priv->key, enc_ivec)) {
	if (1 == EVP_CIPHER_CTX_set_padding(ctx, 0)) {
	  if (1 == EVP_EncryptUpdate(ctx, outbuf + TRANSOP_AES_PREAMBLE_SIZE, &evp_len, assembly, len2)) {
	    evp_ciphertext_len = evp_len;
	    if (1 == EVP_EncryptFinal_ex(ctx, outbuf + TRANSOP_AES_PREAMBLE_SIZE + evp_len, &evp_len)) {
	      evp_ciphertext_len += evp_len;
	      if ( evp_ciphertext_len != len2)
		traceEvent (TRACE_ERROR, "encode_aes openssl encryption: encrypted %u bytes where %u were expected.\n",
			    evp_ciphertext_len, len2);
	    } else
	      traceEvent(TRACE_ERROR, "encode_aes openssl final encryption: %s\n", openssl_err_as_string());
	  } else
	    traceEvent(TRACE_ERROR, "encode_aes openssl encrpytion: %s\n", openssl_err_as_string());
	} else
	  traceEvent(TRACE_ERROR, "encode_aes openssl padding setup: %s\n", openssl_err_as_string());
      } else
	traceEvent(TRACE_ERROR, "encode_aes openssl init: %s\n", openssl_err_as_string());
      EVP_CIPHER_CTX_reset(ctx);

      len2 += TRANSOP_AES_PREAMBLE_SIZE; /* size of data carried in UDP. */
    } else
      traceEvent(TRACE_ERROR, "encode_aes outbuf too small.");
  } else
    traceEvent(TRACE_ERROR, "encode_aes inbuf too big to encrypt.");

  return len2;
}

/* 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) {
  int len=0;
  transop_aes_t * priv = (transop_aes_t *)arg->priv;
  uint8_t assembly[N2N_PKT_BUF_SIZE];

  if ( ( (in_len - TRANSOP_AES_PREAMBLE_SIZE) <= N2N_PKT_BUF_SIZE) /* Cipher text fits in assembly */
       && (in_len >= TRANSOP_AES_PREAMBLE_SIZE) /* Has at least version, iv seed */
       )
    {
      size_t rem=in_len;
      size_t idx=0;
      uint8_t aes_enc_ver=0;
      uint64_t iv_seed=0;

      /* Get the encoding version to make sure it is supported */
      decode_uint8( &aes_enc_ver, inbuf, &rem, &idx );

      if ( N2N_AES_TRANSFORM_VERSION == aes_enc_ver) {
	/* Get the IV seed */
	decode_buf((uint8_t *)&iv_seed, TRANSOP_AES_IV_SEED_SIZE, inbuf, &rem, &idx);

	traceEvent(TRACE_DEBUG, "decode_aes %lu with seed %016llx", in_len, iv_seed);

	len = (in_len - TRANSOP_AES_PREAMBLE_SIZE);

	if ( 0 == (len % AES_BLOCK_SIZE)) {
	  uint8_t padding;
	  n2n_aes_ivec_t dec_ivec = {0};

	  set_aes_cbc_iv(priv, dec_ivec, iv_seed);

	  EVP_CIPHER_CTX *ctx = priv->dec_ctx;
	  int evp_len;
	  int evp_plaintext_len;
	  if (1 == EVP_DecryptInit_ex(ctx, priv->cipher, NULL, priv->key, dec_ivec)) {
	    if (1 == EVP_CIPHER_CTX_set_padding(ctx, 0)) {
	      if (1 == EVP_DecryptUpdate(ctx, assembly, &evp_len, inbuf + TRANSOP_AES_PREAMBLE_SIZE, len)) {
		evp_plaintext_len = evp_len;
		if (1 == EVP_DecryptFinal_ex(ctx, assembly + evp_len, &evp_len)) {
		  evp_plaintext_len += evp_len;
		  if ( evp_plaintext_len != len)
		    traceEvent (TRACE_ERROR, "decode_aes openssl decryption: decrypted %u bytes where %u were expected.\n",
				evp_plaintext_len, len);
		} else
		  traceEvent(TRACE_ERROR, "decode_aes openssl final decryption: %s\n", openssl_err_as_string());
	      } else
		traceEvent(TRACE_ERROR, "decode_aes openssl decrpytion: %s\n", openssl_err_as_string());
	    } else
	      traceEvent(TRACE_ERROR, "decode_aes openssl padding setup: %s\n", openssl_err_as_string());
	  } else
	    traceEvent(TRACE_ERROR, "decode_aes openssl init: %s\n", openssl_err_as_string());
	  EVP_CIPHER_CTX_reset(ctx);

	  /* last byte is how much was padding: max value should be
	   * AES_BLOCKSIZE-1 */
	  padding = assembly[ len-1 ] & 0xff; 

	  if ( len >= padding)
	    {
	      /* strictly speaking for this to be an ethernet packet
	       * it is going to need to be even bigger; but this is
	       * enough to prevent segfaults. */
	      traceEvent(TRACE_DEBUG, "padding = %u", padding);
	      len -= padding;

	      memcpy( outbuf, 
		      assembly, 
		      len);
	    } else
	    traceEvent(TRACE_WARNING, "UDP payload decryption failed.");
	} else {
	  traceEvent(TRACE_WARNING, "Encrypted length %d is not a multiple of AES_BLOCK_SIZE (%d)", len, AES_BLOCK_SIZE);
	  len = 0;
	}
      } else
	traceEvent(TRACE_ERROR, "decode_aes unsupported aes version %u.", aes_enc_ver);
    } else
    traceEvent(TRACE_ERROR, "decode_aes inbuf wrong size (%ul) to decrypt.", in_len);

  return len;
}

static int setup_aes_key(transop_aes_t *priv, const uint8_t *key, ssize_t key_size) {
  size_t aes_key_size_bytes;
  size_t aes_key_size_bits;

  uint8_t key_mat_buf[SHA512_DIGEST_LENGTH + SHA256_DIGEST_LENGTH];
  size_t key_mat_buf_length;

  /* Clear out any old possibly longer key matter. */
  memset( &(priv->key), 0, sizeof(priv->key) );
  memset( &(priv->iv_enc_key), 0, sizeof(priv->iv_enc_key) );
  memset( &(priv->iv_pad_val), 0, sizeof(priv->iv_pad_val) );

  /* Let the user choose the degree of encryption:
   * Long input keys will pick AES192 or AES256 with more robust but expensive encryption.
   *
   * The input key always gets hashed to make a more unpredictable use of the key space and
   * also to derive some additional material (key for IV encrpytion, IV padding).
   *
   * The following scheme for key setup was discussed on github:
   * https://github.com/ntop/n2n/issues/101
   */

  /* create a working buffer of maximal occuring hashes size and generate
   * the hashes for the aes key material, key_mat_buf_lengh indicates the
   * actual "filling level" of the buffer
   */

  if (key_size >= 65)
    {
      priv->cipher = EVP_aes_256_cbc();
      aes_key_size_bytes = AES256_KEY_BYTES;
      SHA512(key, key_size, key_mat_buf);
      key_mat_buf_length = SHA512_DIGEST_LENGTH;
    }
  else if (key_size >= 44)
    {
      priv->cipher = EVP_aes_192_cbc();
      aes_key_size_bytes = AES192_KEY_BYTES;
      SHA384(key, key_size, key_mat_buf);
      /* append a hash of the first hash to create enough material for IV padding */
      SHA256(key_mat_buf, SHA384_DIGEST_LENGTH, key_mat_buf + SHA384_DIGEST_LENGTH);
      key_mat_buf_length = SHA384_DIGEST_LENGTH + SHA256_DIGEST_LENGTH;
    }
  else
    {
      priv->cipher = EVP_aes_128_cbc();
      aes_key_size_bytes = AES128_KEY_BYTES;
      SHA256(key, key_size, key_mat_buf);
      /* append a hash of the first hash to create enough material for IV padding */
      SHA256(key_mat_buf, SHA256_DIGEST_LENGTH, key_mat_buf + SHA256_DIGEST_LENGTH);
      key_mat_buf_length = 2 * SHA256_DIGEST_LENGTH;
    }

  /* is there enough material available? */
  if (key_mat_buf_length < (aes_key_size_bytes + TRANSOP_AES_IV_KEY_BYTES + TRANSOP_AES_IV_PADDING_SIZE))
    {
      /* this should never happen */
      traceEvent( TRACE_ERROR, "AES missing %u bits hashed key material\n",
		  (aes_key_size_bytes + TRANSOP_AES_IV_KEY_BYTES + TRANSOP_AES_IV_PADDING_SIZE - key_mat_buf_length) * 8);
      return(1);
    }

  /* setup of key, used for the CBC encryption */
  aes_key_size_bits = 8 * aes_key_size_bytes;
  memcpy (priv->key, key_mat_buf, aes_key_size_bytes);

  /* setup of iv_enc_key (AES128 key) and iv_pad_val, used for generating the CBC IV */
  AES_set_encrypt_key(key_mat_buf + aes_key_size_bytes, TRANSOP_AES_IV_KEY_BYTES * 8, &(priv->iv_enc_key));
  memcpy(priv->iv_pad_val, key_mat_buf + aes_key_size_bytes + TRANSOP_AES_IV_KEY_BYTES, TRANSOP_AES_IV_PADDING_SIZE);

  traceEvent(TRACE_DEBUG, "AES %u bits setup completed\n",
	     aes_key_size_bits);

  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, "cannot allocate transop_aes_t memory");
    return(-1);
  }
  ttt->priv = priv;

  /* Setup openssl's reusable evp_* contexts for encryption and decryption*/
  if (!(priv->enc_ctx = EVP_CIPHER_CTX_new())) {
    traceEvent(TRACE_ERROR, "openssl's evp_* encryption context creation: %s\n", openssl_err_as_string());
    return(-1);
  }
  if (!(priv->dec_ctx = EVP_CIPHER_CTX_new())) {
    traceEvent(TRACE_ERROR, "openssl's evp_* decryption context creation: %s\n", openssl_err_as_string());
    return(-1);
  }

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

#endif /* N2N_HAVE_AES */