transform_aes.c

/**
 * (C) 2007-18 - 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>

#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 {
    AES_KEY             enc_key;        /* tx key */
    AES_KEY             dec_key;        /* tx key */
    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;

    if(priv)
        free(priv);

    return 0;
}

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);

            AES_cbc_encrypt( assembly, /* source */
                             outbuf + TRANSOP_AES_PREAMBLE_SIZE, /* dest */
                             len2, /* enc size */
                             &(priv->enc_key), enc_ivec, AES_ENCRYPT);

            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);

                AES_cbc_encrypt( (inbuf + TRANSOP_AES_PREAMBLE_SIZE),
                                 assembly, /* destination */
                                 len, 
                                 &(priv->dec_key),
                                 dec_ivec, AES_DECRYPT);

                /* 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->enc_key), 0, sizeof(priv->enc_key) );
    memset( &(priv->dec_key), 0, sizeof(priv->dec_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)
    {
        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)
    {
        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
    {
        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 enc_key/dec_key, used for the CBC encryption */
    aes_key_size_bits = 8 * aes_key_size_bytes;
    AES_set_encrypt_key(key_mat_buf, aes_key_size_bits, &(priv->enc_key));
    AES_set_decrypt_key(key_mat_buf, aes_key_size_bits, &(priv->dec_key));

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

#endif /* N2N_HAVE_AES */