crypto_dh_openssl.c

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/* Copyright (c) 2001, Matej Pfajfar.
 * Copyright (c) 2001-2004, Roger Dingledine.
 * Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
 * Copyright (c) 2007-2021, The Tor Project, Inc. */
/* See LICENSE for licensing information */
 
/**
 * \file crypto_dh_openssl.c
 * \brief Implement Tor's Z_p diffie-hellman stuff for OpenSSL.
 **/
 
#include "lib/crypt_ops/compat_openssl.h"
#include "lib/crypt_ops/crypto_dh.h"
#include "lib/crypt_ops/crypto_digest.h"
#include "lib/crypt_ops/crypto_hkdf.h"
#include "lib/crypt_ops/crypto_util.h"
#include "lib/log/log.h"
#include "lib/log/util_bug.h"
 
DISABLE_GCC_WARNING("-Wredundant-decls")
 
#include <openssl/dh.h>
 
ENABLE_GCC_WARNING("-Wredundant-decls")
 
#include <openssl/bn.h>
#include <string.h>
 
#ifndef ENABLE_NSS
static int tor_check_dh_key(int severity, const BIGNUM *bn,
                            const BIGNUM *dh_p);
 
/** A structure to hold the first half (x, g^x) of a Diffie-Hellman handshake
 * while we're waiting for the second.*/
struct crypto_dh_t {
  DH *dh; /**< The openssl DH object */
};
#endif /* !defined(ENABLE_NSS) */
 
static DH *new_openssl_dh_from_params(BIGNUM *p, BIGNUM *g);
 
/** Shared P parameter for our circuit-crypto DH key exchanges. */
static BIGNUM *dh_param_p = NULL;
/** Shared P parameter for our TLS DH key exchanges. */
static BIGNUM *dh_param_p_tls = NULL;
/** Shared G parameter for our DH key exchanges. */
static BIGNUM *dh_param_g = NULL;
 
/* This function is disabled unless we change the DH parameters. */
#if 0
/** Validate a given set of Diffie-Hellman parameters.  This is moderately
 * computationally expensive (milliseconds), so should only be called when
 * the DH parameters change. Returns 0 on success, * -1 on failure.
 */
static int
crypto_validate_dh_params(const BIGNUM *p, const BIGNUM *g)
{
  DH *dh = NULL;
  int ret = -1;
 
  /* Copy into a temporary DH object, just so that DH_check() can be called. */
  if (!(dh = DH_new()))
      goto out;
 
  BIGNUM *dh_p, *dh_g;
  if (!(dh_p = BN_dup(p)))
    goto out;
  if (!(dh_g = BN_dup(g)))
    goto out;
  if (!DH_set0_pqg(dh, dh_p, NULL, dh_g))
    goto out;
 
  /* Perform the validation. */
  int codes = 0;
  if (!DH_check(dh, &codes))
    goto out;
  if (BN_is_word(g, DH_GENERATOR_2)) {
    /* Per https://wiki.openssl.org/index.php/Diffie-Hellman_parameters
     *
     * OpenSSL checks the prime is congruent to 11 when g = 2; while the
     * IETF's primes are congruent to 23 when g = 2.
     */
    BN_ULONG residue = BN_mod_word(p, 24);
    if (residue == 11 || residue == 23)
      codes &= ~DH_NOT_SUITABLE_GENERATOR;
  }
  if (codes != 0) /* Specifics on why the params suck is irrelevant. */
    goto out;
 
  /* Things are probably not evil. */
  ret = 0;
 
 out:
  if (dh)
    DH_free(dh);
  return ret;
}
#endif /* 0 */
 
/**
 * Helper: convert <b>hex</b> to a bignum, and return it.  Assert that the
 * operation was successful.
 */
static BIGNUM *
bignum_from_hex(const char *hex)
{
  BIGNUM *result = BN_new();
  tor_assert(result);
 
  int r = BN_hex2bn(&result, hex);
  tor_assert(r);
  tor_assert(result);
  return result;
}
 
/** Set the global Diffie-Hellman generator, used for both TLS and internal
 * DH stuff.
 */
static void
crypto_set_dh_generator(void)
{
  BIGNUM *generator;
  int r;
 
  if (dh_param_g)
    return;
 
  generator = BN_new();
  tor_assert(generator);
 
  r = BN_set_word(generator, DH_GENERATOR);
  tor_assert(r);
 
  dh_param_g = generator;
}
 
/** Initialize our DH parameters. Idempotent. */
void
crypto_dh_init_openssl(void)
{
  if (dh_param_p && dh_param_g && dh_param_p_tls)
    return;
 
  tor_assert(dh_param_g == NULL);
  tor_assert(dh_param_p == NULL);
  tor_assert(dh_param_p_tls == NULL);
 
  crypto_set_dh_generator();
  dh_param_p = bignum_from_hex(OAKLEY_PRIME_2);
  dh_param_p_tls = bignum_from_hex(TLS_DH_PRIME);
 
  /* Checks below are disabled unless we change the hardcoded DH parameters. */
#if 0
  tor_assert(0 == crypto_validate_dh_params(dh_param_p, dh_param_g));
  tor_assert(0 == crypto_validate_dh_params(dh_param_p_tls, dh_param_g));
#endif
}
 
/** Number of bits to use when choosing the x or y value in a Diffie-Hellman
 * handshake.  Since we exponentiate by this value, choosing a smaller one
 * lets our handshake go faster.
 */
#define DH_PRIVATE_KEY_BITS 320
 
/** Used by tortls.c: Get the DH* for use with TLS.
 */
DH *
crypto_dh_new_openssl_tls(void)
{
  return new_openssl_dh_from_params(dh_param_p_tls, dh_param_g);
}
 
#ifndef ENABLE_NSS
/** Allocate and return a new DH object for a key exchange. Returns NULL on
 * failure.
 */
crypto_dh_t *
crypto_dh_new(int dh_type)
{
  crypto_dh_t *res = tor_malloc_zero(sizeof(crypto_dh_t));
 
  tor_assert(dh_type == DH_TYPE_CIRCUIT || dh_type == DH_TYPE_TLS ||
             dh_type == DH_TYPE_REND);
 
  if (!dh_param_p)
    crypto_dh_init();
 
  BIGNUM *dh_p = NULL;
  if (dh_type == DH_TYPE_TLS) {
    dh_p = dh_param_p_tls;
  } else {
    dh_p = dh_param_p;
  }
 
  res->dh = new_openssl_dh_from_params(dh_p, dh_param_g);
  if (res->dh == NULL)
    tor_free(res); // sets res to NULL.
  return res;
}
#endif /* !defined(ENABLE_NSS) */
 
/** Create and return a new openssl DH from a given prime and generator. */
static DH *
new_openssl_dh_from_params(BIGNUM *p, BIGNUM *g)
{
  DH *res_dh;
  if (!(res_dh = DH_new()))
    goto err;
 
  BIGNUM *dh_p = NULL, *dh_g = NULL;
  dh_p = BN_dup(p);
  if (!dh_p)
    goto err;
 
  dh_g = BN_dup(g);
  if (!dh_g) {
    BN_free(dh_p);
    goto err;
  }
 
  if (!DH_set0_pqg(res_dh, dh_p, NULL, dh_g)) {
    goto err;
  }
 
  if (!DH_set_length(res_dh, DH_PRIVATE_KEY_BITS))
    goto err;
 
  return res_dh;
 
  /* LCOV_EXCL_START
   * This error condition is only reached when an allocation fails */
 err:
  crypto_openssl_log_errors(LOG_WARN, "creating DH object");
  if (res_dh) DH_free(res_dh); /* frees p and g too */
  return NULL;
  /* LCOV_EXCL_STOP */
}
 
#ifndef ENABLE_NSS
/** Return a copy of <b>dh</b>, sharing its internal state. */
crypto_dh_t *
crypto_dh_dup(const crypto_dh_t *dh)
{
  crypto_dh_t *dh_new = tor_malloc_zero(sizeof(crypto_dh_t));
  tor_assert(dh);
  tor_assert(dh->dh);
  dh_new->dh = dh->dh;
  DH_up_ref(dh->dh);
  return dh_new;
}
 
/** Return the length of the DH key in <b>dh</b>, in bytes.
 */
int
crypto_dh_get_bytes(crypto_dh_t *dh)
{
  tor_assert(dh);
  return DH_size(dh->dh);
}
 
/** Generate <x,g^x> for our part of the key exchange.  Return 0 on
 * success, -1 on failure.
 */
int
crypto_dh_generate_public(crypto_dh_t *dh)
{
  if (!DH_generate_key(dh->dh)) {
    /* LCOV_EXCL_START
     * To test this we would need some way to tell openssl to break DH. */
    crypto_openssl_log_errors(LOG_WARN, "generating DH key");
    return -1;
    /* LCOV_EXCL_STOP */
  }
 
  /* OpenSSL 1.1.x doesn't appear to let you regenerate a DH key, without
   * recreating the DH object.  I have no idea what sort of aliasing madness
   * can occur here, so do the check, and just bail on failure.
   */
  const BIGNUM *pub_key, *priv_key;
  DH_get0_key(dh->dh, &pub_key, &priv_key);
  if (tor_check_dh_key(LOG_WARN, pub_key, DH_get0_p(dh->dh))<0) {
    log_warn(LD_CRYPTO, "Weird! Our own DH key was invalid.  I guess once-in-"
             "the-universe chances really do happen.  Treating as a failure.");
    return -1;
  }
 
  return 0;
}
 
/** Generate g^x as necessary, and write the g^x for the key exchange
 * as a <b>pubkey_len</b>-byte value into <b>pubkey</b>. Return 0 on
 * success, -1 on failure.  <b>pubkey_len</b> must be >= DH1024_KEY_LEN.
 */
int
crypto_dh_get_public(crypto_dh_t *dh, char *pubkey, size_t pubkey_len)
{
  int bytes;
  tor_assert(dh);
 
  const BIGNUM *dh_pub;
 
  const BIGNUM *dh_priv;
  DH_get0_key(dh->dh, &dh_pub, &dh_priv);
 
  if (!dh_pub) {
    if (crypto_dh_generate_public(dh)<0)
      return -1;
    else {
      DH_get0_key(dh->dh, &dh_pub, &dh_priv);
    }
  }
 
  tor_assert(dh_pub);
  bytes = BN_num_bytes(dh_pub);
  tor_assert(bytes >= 0);
  if (pubkey_len < (size_t)bytes) {
    log_warn(LD_CRYPTO,
             "Weird! pubkey_len (%d) was smaller than key length (%d)",
             (int) pubkey_len, bytes);
    return -1;
  }
 
  memset(pubkey, 0, pubkey_len);
  BN_bn2bin(dh_pub, (unsigned char*)(pubkey+(pubkey_len-bytes)));
 
  return 0;
}
 
/** Check for bad Diffie-Hellman public keys (g^x).  Return 0 if the key is
 * okay (in the subgroup [2,p-2]), or -1 if it's bad.
 * See http://www.cl.cam.ac.uk/ftp/users/rja14/psandqs.ps.gz for some tips.
 */
static int
tor_check_dh_key(int severity, const BIGNUM *bn, const BIGNUM *dh_p)
{
  BIGNUM *x;
  char *s;
  tor_assert(bn);
  x = BN_new();
  tor_assert(x);
  BN_set_word(x, 1);
  if (BN_cmp(bn,x)<=0) {
    log_fn(severity, LD_CRYPTO, "DH key must be at least 2.");
    goto err;
  }
  BN_copy(x,dh_p);
  BN_sub_word(x, 1);
  if (BN_cmp(bn,x)>=0) {
    log_fn(severity, LD_CRYPTO, "DH key must be at most p-2.");
    goto err;
  }
  BN_clear_free(x);
  return 0;
 err:
  BN_clear_free(x);
  s = BN_bn2hex(bn);
  log_fn(severity, LD_CRYPTO, "Rejecting insecure DH key [%s]", s);
  OPENSSL_free(s);
  return -1;
}
 
/** Given a DH key exchange object, and our peer's value of g^y (as a
 * <b>pubkey_len</b>-byte value in <b>pubkey</b>) generate
 * g^xy as a big-endian integer in <b>secret_out</b>.
 * Return the number of bytes generated on success,
 * or -1 on failure.
 *
 * This function MUST validate that g^y is actually in the group.
 */
ssize_t
crypto_dh_handshake(int severity, crypto_dh_t *dh,
                    const char *pubkey, size_t pubkey_len,
                    unsigned char *secret_out, size_t secret_bytes_out)
{
  BIGNUM *pubkey_bn = NULL;
  size_t secret_len=0;
  int result=0;
 
  tor_assert(dh);
  tor_assert(secret_bytes_out/DIGEST_LEN <= 255);
  tor_assert(pubkey_len < INT_MAX);
 
  if (BUG(crypto_dh_get_bytes(dh) > (int)secret_bytes_out)) {
    goto error;
  }
 
  if (!(pubkey_bn = BN_bin2bn((const unsigned char*)pubkey,
                              (int)pubkey_len, NULL)))
    goto error;
  if (tor_check_dh_key(severity, pubkey_bn, DH_get0_p(dh->dh))<0) {
    /* Check for invalid public keys. */
    log_fn(severity, LD_CRYPTO,"Rejected invalid g^x");
    goto error;
  }
  result = DH_compute_key(secret_out, pubkey_bn, dh->dh);
  if (result < 0) {
    log_warn(LD_CRYPTO,"DH_compute_key() failed.");
    goto error;
  }
  secret_len = result;
 
  goto done;
 error:
  result = -1;
 done:
  crypto_openssl_log_errors(LOG_WARN, "completing DH handshake");
  if (pubkey_bn)
    BN_clear_free(pubkey_bn);
  if (result < 0)
    return result;
  else
    return secret_len;
}
 
/** Free a DH key exchange object.
 */
void
crypto_dh_free_(crypto_dh_t *dh)
{
  if (!dh)
    return;
  tor_assert(dh->dh);
  DH_free(dh->dh);
  tor_free(dh);
}
#endif /* !defined(ENABLE_NSS) */
 
void
crypto_dh_free_all_openssl(void)
{
  if (dh_param_p)
    BN_clear_free(dh_param_p);
  if (dh_param_p_tls)
    BN_clear_free(dh_param_p_tls);
  if (dh_param_g)
    BN_clear_free(dh_param_g);
 
  dh_param_p = dh_param_p_tls = dh_param_g = NULL;
}