tor/src/common/crypto.c

/* Copyright (c) 2001, Matej Pfajfar.
 * Copyright (c) 2001-2004, Roger Dingledine.
 * Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
 * Copyright (c) 2007-2017, The Tor Project, Inc. */
/* See LICENSE for licensing information */

/**
 * \file crypto.c
 * \brief Wrapper functions to present a consistent interface to
 * public-key and symmetric cryptography operations from OpenSSL and
 * other places.
 **/

#include "orconfig.h"

#ifdef _WIN32
#include <winsock2.h>
#include <windows.h>
#include <wincrypt.h>
/* Windows defines this; so does OpenSSL 0.9.8h and later. We don't actually
 * use either definition. */
#undef OCSP_RESPONSE
#endif /* defined(_WIN32) */

#define CRYPTO_PRIVATE
#include "crypto.h"
#include "compat_openssl.h"
#include "crypto_curve25519.h"
#include "crypto_ed25519.h"
#include "crypto_format.h"
#include "crypto_rsa.h"
#include "crypto_digest.h"

DISABLE_GCC_WARNING(redundant-decls)

#include <openssl/err.h>
#include <openssl/rsa.h>
#include <openssl/pem.h>
#include <openssl/evp.h>
#include <openssl/engine.h>
#include <openssl/rand.h>
#include <openssl/bn.h>
#include <openssl/dh.h>
#include <openssl/conf.h>
#include <openssl/hmac.h>
#include <openssl/ssl.h>

ENABLE_GCC_WARNING(redundant-decls)

#if __GNUC__ && GCC_VERSION >= 402
#if GCC_VERSION >= 406
#pragma GCC diagnostic pop
#else
#pragma GCC diagnostic warning "-Wredundant-decls"
#endif
#endif /* __GNUC__ && GCC_VERSION >= 402 */

#ifdef HAVE_CTYPE_H
#include <ctype.h>
#endif
#ifdef HAVE_UNISTD_H
#include <unistd.h>
#endif
#ifdef HAVE_FCNTL_H
#include <fcntl.h>
#endif
#ifdef HAVE_SYS_FCNTL_H
#include <sys/fcntl.h>
#endif
#ifdef HAVE_SYS_SYSCALL_H
#include <sys/syscall.h>
#endif
#ifdef HAVE_SYS_RANDOM_H
#include <sys/random.h>
#endif

#include "torlog.h"
#include "torint.h"
#include "aes.h"
#include "util.h"
#include "container.h"
#include "compat.h"
#include "sandbox.h"
#include "util_format.h"

#include "keccak-tiny/keccak-tiny.h"

/** Longest recognized */
#define MAX_DNS_LABEL_SIZE 63

/** Largest strong entropy request */
#define MAX_STRONGEST_RAND_SIZE 256

/** 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 */
};

static int tor_check_dh_key(int severity, const BIGNUM *bn);

/** Boolean: has OpenSSL's crypto been initialized? */
static int crypto_early_initialized_ = 0;

/** Boolean: has OpenSSL's crypto been initialized? */
static int crypto_global_initialized_ = 0;

/** Log all pending crypto errors at level <b>severity</b>.  Use
 * <b>doing</b> to describe our current activities.
 */
static void
crypto_log_errors(int severity, const char *doing)
{
  unsigned long err;
  const char *msg, *lib, *func;
  while ((err = ERR_get_error()) != 0) {
    msg = (const char*)ERR_reason_error_string(err);
    lib = (const char*)ERR_lib_error_string(err);
    func = (const char*)ERR_func_error_string(err);
    if (!msg) msg = "(null)";
    if (!lib) lib = "(null)";
    if (!func) func = "(null)";
    if (BUG(!doing)) doing = "(null)";
    tor_log(severity, LD_CRYPTO, "crypto error while %s: %s (in %s:%s)",
              doing, msg, lib, func);
  }
}

#ifndef DISABLE_ENGINES
/** Log any OpenSSL engines we're using at NOTICE. */
static void
log_engine(const char *fn, ENGINE *e)
{
  if (e) {
    const char *name, *id;
    name = ENGINE_get_name(e);
    id = ENGINE_get_id(e);
    log_notice(LD_CRYPTO, "Default OpenSSL engine for %s is %s [%s]",
               fn, name?name:"?", id?id:"?");
  } else {
    log_info(LD_CRYPTO, "Using default implementation for %s", fn);
  }
}
#endif /* !defined(DISABLE_ENGINES) */

#ifndef DISABLE_ENGINES
/** Try to load an engine in a shared library via fully qualified path.
 */
static ENGINE *
try_load_engine(const char *path, const char *engine)
{
  ENGINE *e = ENGINE_by_id("dynamic");
  if (e) {
    if (!ENGINE_ctrl_cmd_string(e, "ID", engine, 0) ||
        !ENGINE_ctrl_cmd_string(e, "DIR_LOAD", "2", 0) ||
        !ENGINE_ctrl_cmd_string(e, "DIR_ADD", path, 0) ||
        !ENGINE_ctrl_cmd_string(e, "LOAD", NULL, 0)) {
      ENGINE_free(e);
      e = NULL;
    }
  }
  return e;
}
#endif /* !defined(DISABLE_ENGINES) */

/** Make sure that openssl is using its default PRNG. Return 1 if we had to
 * adjust it; 0 otherwise. */
STATIC int
crypto_force_rand_ssleay(void)
{
  RAND_METHOD *default_method;
  default_method = RAND_OpenSSL();
  if (RAND_get_rand_method() != default_method) {
    log_notice(LD_CRYPTO, "It appears that one of our engines has provided "
               "a replacement the OpenSSL RNG. Resetting it to the default "
               "implementation.");
    RAND_set_rand_method(default_method);
    return 1;
  }
  return 0;
}

static int have_seeded_siphash = 0;

/** Set up the siphash key if we haven't already done so. */
int
crypto_init_siphash_key(void)
{
  struct sipkey key;
  if (have_seeded_siphash)
    return 0;

  crypto_rand((char*) &key, sizeof(key));
  siphash_set_global_key(&key);
  have_seeded_siphash = 1;
  return 0;
}

/** Initialize the crypto library.  Return 0 on success, -1 on failure.
 */
int
crypto_early_init(void)
{
  if (!crypto_early_initialized_) {

    crypto_early_initialized_ = 1;

#if OPENSSL_VERSION_NUMBER >= OPENSSL_V_SERIES(1,1,0)
    OPENSSL_init_ssl(OPENSSL_INIT_LOAD_SSL_STRINGS |
                     OPENSSL_INIT_LOAD_CRYPTO_STRINGS |
                     OPENSSL_INIT_ADD_ALL_CIPHERS |
                     OPENSSL_INIT_ADD_ALL_DIGESTS, NULL);
#else
    ERR_load_crypto_strings();
    OpenSSL_add_all_algorithms();
#endif

    setup_openssl_threading();

    unsigned long version_num = OpenSSL_version_num();
    const char *version_str = OpenSSL_version(OPENSSL_VERSION);
    if (version_num == OPENSSL_VERSION_NUMBER &&
        !strcmp(version_str, OPENSSL_VERSION_TEXT)) {
      log_info(LD_CRYPTO, "OpenSSL version matches version from headers "
                 "(%lx: %s).", version_num, version_str);
    } else {
      log_warn(LD_CRYPTO, "OpenSSL version from headers does not match the "
               "version we're running with. If you get weird crashes, that "
               "might be why. (Compiled with %lx: %s; running with %lx: %s).",
               (unsigned long)OPENSSL_VERSION_NUMBER, OPENSSL_VERSION_TEXT,
               version_num, version_str);
    }

    crypto_force_rand_ssleay();

    if (crypto_seed_rng() < 0)
      return -1;
    if (crypto_init_siphash_key() < 0)
      return -1;

    curve25519_init();
    ed25519_init();
  }
  return 0;
}

/** Initialize the crypto library.  Return 0 on success, -1 on failure.
 */
int
crypto_global_init(int useAccel, const char *accelName, const char *accelDir)
{
  if (!crypto_global_initialized_) {
    if (crypto_early_init() < 0)
      return -1;

    crypto_global_initialized_ = 1;

    if (useAccel > 0) {
#ifdef DISABLE_ENGINES
      (void)accelName;
      (void)accelDir;
      log_warn(LD_CRYPTO, "No OpenSSL hardware acceleration support enabled.");
#else
      ENGINE *e = NULL;

      log_info(LD_CRYPTO, "Initializing OpenSSL engine support.");
      ENGINE_load_builtin_engines();
      ENGINE_register_all_complete();

      if (accelName) {
        if (accelDir) {
          log_info(LD_CRYPTO, "Trying to load dynamic OpenSSL engine \"%s\""
                   " via path \"%s\".", accelName, accelDir);
          e = try_load_engine(accelName, accelDir);
        } else {
          log_info(LD_CRYPTO, "Initializing dynamic OpenSSL engine \"%s\""
                   " acceleration support.", accelName);
          e = ENGINE_by_id(accelName);
        }
        if (!e) {
          log_warn(LD_CRYPTO, "Unable to load dynamic OpenSSL engine \"%s\".",
                   accelName);
        } else {
          log_info(LD_CRYPTO, "Loaded dynamic OpenSSL engine \"%s\".",
                   accelName);
        }
      }
      if (e) {
        log_info(LD_CRYPTO, "Loaded OpenSSL hardware acceleration engine,"
                 " setting default ciphers.");
        ENGINE_set_default(e, ENGINE_METHOD_ALL);
      }
      /* Log, if available, the intersection of the set of algorithms
         used by Tor and the set of algorithms available in the engine */
      log_engine("RSA", ENGINE_get_default_RSA());
      log_engine("DH", ENGINE_get_default_DH());
#ifdef OPENSSL_1_1_API
      log_engine("EC", ENGINE_get_default_EC());
#else
      log_engine("ECDH", ENGINE_get_default_ECDH());
      log_engine("ECDSA", ENGINE_get_default_ECDSA());
#endif /* defined(OPENSSL_1_1_API) */
      log_engine("RAND", ENGINE_get_default_RAND());
      log_engine("RAND (which we will not use)", ENGINE_get_default_RAND());
      log_engine("SHA1", ENGINE_get_digest_engine(NID_sha1));
      log_engine("3DES-CBC", ENGINE_get_cipher_engine(NID_des_ede3_cbc));
      log_engine("AES-128-ECB", ENGINE_get_cipher_engine(NID_aes_128_ecb));
      log_engine("AES-128-CBC", ENGINE_get_cipher_engine(NID_aes_128_cbc));
#ifdef NID_aes_128_ctr
      log_engine("AES-128-CTR", ENGINE_get_cipher_engine(NID_aes_128_ctr));
#endif
#ifdef NID_aes_128_gcm
      log_engine("AES-128-GCM", ENGINE_get_cipher_engine(NID_aes_128_gcm));
#endif
      log_engine("AES-256-CBC", ENGINE_get_cipher_engine(NID_aes_256_cbc));
#ifdef NID_aes_256_gcm
      log_engine("AES-256-GCM", ENGINE_get_cipher_engine(NID_aes_256_gcm));
#endif

#endif /* defined(DISABLE_ENGINES) */
    } else {
      log_info(LD_CRYPTO, "NOT using OpenSSL engine support.");
    }

    if (crypto_force_rand_ssleay()) {
      if (crypto_seed_rng() < 0)
        return -1;
    }

    evaluate_evp_for_aes(-1);
    evaluate_ctr_for_aes();
  }
  return 0;
}

/** Free crypto resources held by this thread. */
void
crypto_thread_cleanup(void)
{
#ifndef NEW_THREAD_API
  ERR_remove_thread_state(NULL);
#endif
}

/** Used by tortls.c: Get the DH* from a crypto_dh_t.
 */
DH *
crypto_dh_get_dh_(crypto_dh_t *dh)
{
  return dh->dh;
}

/** Allocate and return a new symmetric cipher using the provided key and iv.
 * The key is <b>bits</b> bits long; the IV is CIPHER_IV_LEN bytes.  Both
 * must be provided. Key length must be 128, 192, or 256 */
crypto_cipher_t *
crypto_cipher_new_with_iv_and_bits(const uint8_t *key,
                                   const uint8_t *iv,
                                   int bits)
{
  tor_assert(key);
  tor_assert(iv);

  return aes_new_cipher((const uint8_t*)key, (const uint8_t*)iv, bits);
}

/** Allocate and return a new symmetric cipher using the provided key and iv.
 * The key is CIPHER_KEY_LEN bytes; the IV is CIPHER_IV_LEN bytes.  Both
 * must be provided.
 */
crypto_cipher_t *
crypto_cipher_new_with_iv(const char *key, const char *iv)
{
  return crypto_cipher_new_with_iv_and_bits((uint8_t*)key, (uint8_t*)iv,
                                            128);
}

/** Return a new crypto_cipher_t with the provided <b>key</b> and an IV of all
 * zero bytes and key length <b>bits</b>.  Key length must be 128, 192, or
 * 256. */
crypto_cipher_t *
crypto_cipher_new_with_bits(const char *key, int bits)
{
  char zeroiv[CIPHER_IV_LEN];
  memset(zeroiv, 0, sizeof(zeroiv));
  return crypto_cipher_new_with_iv_and_bits((uint8_t*)key, (uint8_t*)zeroiv,
                                            bits);
}

/** Return a new crypto_cipher_t with the provided <b>key</b> (of
 * CIPHER_KEY_LEN bytes) and an IV of all zero bytes.  */
crypto_cipher_t *
crypto_cipher_new(const char *key)
{
  return crypto_cipher_new_with_bits(key, 128);
}

/** Free a symmetric cipher.
 */
void
crypto_cipher_free_(crypto_cipher_t *env)
{
  if (!env)
    return;

  aes_cipher_free(env);
}

/** Copy <b>in</b> to the <b>outlen</b>-byte buffer <b>out</b>, adding spaces
 * every four characters. */
void
crypto_add_spaces_to_fp(char *out, size_t outlen, const char *in)
{
  int n = 0;
  char *end = out+outlen;
  tor_assert(outlen < SIZE_T_CEILING);

  while (*in && out<end) {
    *out++ = *in++;
    if (++n == 4 && *in && out<end) {
      n = 0;
      *out++ = ' ';
    }
  }
  tor_assert(out<end);
  *out = '\0';
}

/* symmetric crypto */

/** Encrypt <b>fromlen</b> bytes from <b>from</b> using the cipher
 * <b>env</b>; on success, store the result to <b>to</b> and return 0.
 * Does not check for failure.
 */
int
crypto_cipher_encrypt(crypto_cipher_t *env, char *to,
                      const char *from, size_t fromlen)
{
  tor_assert(env);
  tor_assert(env);
  tor_assert(from);
  tor_assert(fromlen);
  tor_assert(to);
  tor_assert(fromlen < SIZE_T_CEILING);

  memcpy(to, from, fromlen);
  aes_crypt_inplace(env, to, fromlen);
  return 0;
}

/** Decrypt <b>fromlen</b> bytes from <b>from</b> using the cipher
 * <b>env</b>; on success, store the result to <b>to</b> and return 0.
 * Does not check for failure.
 */
int
crypto_cipher_decrypt(crypto_cipher_t *env, char *to,
                      const char *from, size_t fromlen)
{
  tor_assert(env);
  tor_assert(from);
  tor_assert(to);
  tor_assert(fromlen < SIZE_T_CEILING);

  memcpy(to, from, fromlen);
  aes_crypt_inplace(env, to, fromlen);
  return 0;
}

/** Encrypt <b>len</b> bytes on <b>from</b> using the cipher in <b>env</b>;
 * on success. Does not check for failure.
 */
void
crypto_cipher_crypt_inplace(crypto_cipher_t *env, char *buf, size_t len)
{
  tor_assert(len < SIZE_T_CEILING);
  aes_crypt_inplace(env, buf, len);
}

/** Encrypt <b>fromlen</b> bytes (at least 1) from <b>from</b> with the key in
 * <b>key</b> to the buffer in <b>to</b> of length
 * <b>tolen</b>. <b>tolen</b> must be at least <b>fromlen</b> plus
 * CIPHER_IV_LEN bytes for the initialization vector. On success, return the
 * number of bytes written, on failure, return -1.
 */
int
crypto_cipher_encrypt_with_iv(const char *key,
                              char *to, size_t tolen,
                              const char *from, size_t fromlen)
{
  crypto_cipher_t *cipher;
  tor_assert(from);
  tor_assert(to);
  tor_assert(fromlen < INT_MAX);

  if (fromlen < 1)
    return -1;
  if (tolen < fromlen + CIPHER_IV_LEN)
    return -1;

  char iv[CIPHER_IV_LEN];
  crypto_rand(iv, sizeof(iv));
  cipher = crypto_cipher_new_with_iv(key, iv);

  memcpy(to, iv, CIPHER_IV_LEN);
  crypto_cipher_encrypt(cipher, to+CIPHER_IV_LEN, from, fromlen);
  crypto_cipher_free(cipher);
  memwipe(iv, 0, sizeof(iv));
  return (int)(fromlen + CIPHER_IV_LEN);
}

/** Decrypt <b>fromlen</b> bytes (at least 1+CIPHER_IV_LEN) from <b>from</b>
 * with the key in <b>key</b> to the buffer in <b>to</b> of length
 * <b>tolen</b>. <b>tolen</b> must be at least <b>fromlen</b> minus
 * CIPHER_IV_LEN bytes for the initialization vector. On success, return the
 * number of bytes written, on failure, return -1.
 */
int
crypto_cipher_decrypt_with_iv(const char *key,
                              char *to, size_t tolen,
                              const char *from, size_t fromlen)
{
  crypto_cipher_t *cipher;
  tor_assert(key);
  tor_assert(from);
  tor_assert(to);
  tor_assert(fromlen < INT_MAX);

  if (fromlen <= CIPHER_IV_LEN)
    return -1;
  if (tolen < fromlen - CIPHER_IV_LEN)
    return -1;

  cipher = crypto_cipher_new_with_iv(key, from);

  crypto_cipher_encrypt(cipher, to, from+CIPHER_IV_LEN, fromlen-CIPHER_IV_LEN);
  crypto_cipher_free(cipher);
  return (int)(fromlen - CIPHER_IV_LEN);
}

/* DH */

/** Our DH 'g' parameter */
#define DH_GENERATOR 2

/** 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;

/** 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;
#ifdef OPENSSL_1_1_API
  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;
#else /* !(defined(OPENSSL_1_1_API)) */
  if (!(dh->p = BN_dup(p)))
    goto out;
  if (!(dh->g = BN_dup(g)))
    goto out;
#endif /* defined(OPENSSL_1_1_API) */

  /* 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;
}

/** 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;
}

/** Set the global TLS Diffie-Hellman modulus.  Use the Apache mod_ssl DH
 * modulus. */
void
crypto_set_tls_dh_prime(void)
{
  BIGNUM *tls_prime = NULL;
  int r;

  /* If the space is occupied, free the previous TLS DH prime */
  if (BUG(dh_param_p_tls)) {
    /* LCOV_EXCL_START
     *
     * We shouldn't be calling this twice.
     */
    BN_clear_free(dh_param_p_tls);
    dh_param_p_tls = NULL;
    /* LCOV_EXCL_STOP */
  }

  tls_prime = BN_new();
  tor_assert(tls_prime);

  /* This is the 1024-bit safe prime that Apache uses for its DH stuff; see
   * modules/ssl/ssl_engine_dh.c; Apache also uses a generator of 2 with this
   * prime.
   */
  r = BN_hex2bn(&tls_prime,
               "D67DE440CBBBDC1936D693D34AFD0AD50C84D239A45F520BB88174CB98"
               "BCE951849F912E639C72FB13B4B4D7177E16D55AC179BA420B2A29FE324A"
               "467A635E81FF5901377BEDDCFD33168A461AAD3B72DAE8860078045B07A7"
               "DBCA7874087D1510EA9FCC9DDD330507DD62DB88AEAA747DE0F4D6E2BD68"
               "B0E7393E0F24218EB3");
  tor_assert(r);

  tor_assert(tls_prime);

  dh_param_p_tls = tls_prime;
  crypto_set_dh_generator();
  tor_assert(0 == crypto_validate_dh_params(dh_param_p_tls, dh_param_g));
}

/** Initialize dh_param_p and dh_param_g if they are not already
 * set. */
static void
init_dh_param(void)
{
  BIGNUM *circuit_dh_prime;
  int r;
  if (BUG(dh_param_p && dh_param_g))
    return; // LCOV_EXCL_LINE This function isn't supposed to be called twice.

  circuit_dh_prime = BN_new();
  tor_assert(circuit_dh_prime);

  /* This is from rfc2409, section 6.2.  It's a safe prime, and
     supposedly it equals:
        2^1024 - 2^960 - 1 + 2^64 * { [2^894 pi] + 129093 }.
  */
  r = BN_hex2bn(&circuit_dh_prime,
                "FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E08"
                "8A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B"
                "302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9"
                "A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE6"
                "49286651ECE65381FFFFFFFFFFFFFFFF");
  tor_assert(r);

  /* Set the new values as the global DH parameters. */
  dh_param_p = circuit_dh_prime;
  crypto_set_dh_generator();
  tor_assert(0 == crypto_validate_dh_params(dh_param_p, dh_param_g));

  if (!dh_param_p_tls) {
    crypto_set_tls_dh_prime();
  }
}

/** 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 handhake go faster.
 */
#define DH_PRIVATE_KEY_BITS 320

/** 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)
    init_dh_param();

  if (!(res->dh = DH_new()))
    goto err;

#ifdef OPENSSL_1_1_API
  BIGNUM *dh_p = NULL, *dh_g = NULL;

  if (dh_type == DH_TYPE_TLS) {
    dh_p = BN_dup(dh_param_p_tls);
  } else {
    dh_p = BN_dup(dh_param_p);
  }
  if (!dh_p)
    goto err;

  dh_g = BN_dup(dh_param_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;
#else /* !(defined(OPENSSL_1_1_API)) */
  if (dh_type == DH_TYPE_TLS) {
    if (!(res->dh->p = BN_dup(dh_param_p_tls)))
      goto err;
  } else {
    if (!(res->dh->p = BN_dup(dh_param_p)))
      goto err;
  }

  if (!(res->dh->g = BN_dup(dh_param_g)))
    goto err;

  res->dh->length = DH_PRIVATE_KEY_BITS;
#endif /* defined(OPENSSL_1_1_API) */

  return res;

  /* LCOV_EXCL_START
   * This error condition is only reached when an allocation fails */
 err:
  crypto_log_errors(LOG_WARN, "creating DH object");
  if (res->dh) DH_free(res->dh); /* frees p and g too */
  tor_free(res);
  return NULL;
  /* LCOV_EXCL_STOP */
}

/** 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)
{
#ifndef OPENSSL_1_1_API
 again:
#endif
  if (!DH_generate_key(dh->dh)) {
    /* LCOV_EXCL_START
     * To test this we would need some way to tell openssl to break DH. */
    crypto_log_errors(LOG_WARN, "generating DH key");
    return -1;
    /* LCOV_EXCL_STOP */
  }
#ifdef OPENSSL_1_1_API
  /* 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)<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;
  }
#else /* !(defined(OPENSSL_1_1_API)) */
  if (tor_check_dh_key(LOG_WARN, dh->dh->pub_key)<0) {
    /* LCOV_EXCL_START
     * If this happens, then openssl's DH implementation is busted. */
    log_warn(LD_CRYPTO, "Weird! Our own DH key was invalid.  I guess once-in-"
             "the-universe chances really do happen.  Trying again.");
    /* Free and clear the keys, so OpenSSL will actually try again. */
    BN_clear_free(dh->dh->pub_key);
    BN_clear_free(dh->dh->priv_key);
    dh->dh->pub_key = dh->dh->priv_key = NULL;
    goto again;
    /* LCOV_EXCL_STOP */
  }
#endif /* defined(OPENSSL_1_1_API) */
  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 \>= DH_BYTES.
 */
int
crypto_dh_get_public(crypto_dh_t *dh, char *pubkey, size_t pubkey_len)
{
  int bytes;
  tor_assert(dh);

  const BIGNUM *dh_pub;

#ifdef OPENSSL_1_1_API
  const BIGNUM *dh_priv;
  DH_get0_key(dh->dh, &dh_pub, &dh_priv);
#else
  dh_pub = dh->dh->pub_key;
#endif /* defined(OPENSSL_1_1_API) */

  if (!dh_pub) {
    if (crypto_dh_generate_public(dh)<0)
      return -1;
    else {
#ifdef OPENSSL_1_1_API
      DH_get0_key(dh->dh, &dh_pub, &dh_priv);
#else
      dh_pub = dh->dh->pub_key;
#endif
    }
  }

  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 DH_BYTES (%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)
{
  BIGNUM *x;
  char *s;
  tor_assert(bn);
  x = BN_new();
  tor_assert(x);
  if (BUG(!dh_param_p))
    init_dh_param(); //LCOV_EXCL_LINE we already checked whether we did this.
  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_param_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
 * <b>secret_bytes_out</b> bytes of shared key material and write them
 * to <b>secret_out</b>.  Return the number of bytes generated on success,
 * or -1 on failure.
 *
 * (We generate key material by computing
 *         SHA1( g^xy || "\x00" ) || SHA1( g^xy || "\x01" ) || ...
 * where || is concatenation.)
 */
ssize_t
crypto_dh_compute_secret(int severity, crypto_dh_t *dh,
                         const char *pubkey, size_t pubkey_len,
                         char *secret_out, size_t secret_bytes_out)
{
  char *secret_tmp = NULL;
  BIGNUM *pubkey_bn = NULL;
  size_t secret_len=0, secret_tmp_len=0;
  int result=0;
  tor_assert(dh);
  tor_assert(secret_bytes_out/DIGEST_LEN <= 255);
  tor_assert(pubkey_len < INT_MAX);

  if (!(pubkey_bn = BN_bin2bn((const unsigned char*)pubkey,
                              (int)pubkey_len, NULL)))
    goto error;
  if (tor_check_dh_key(severity, pubkey_bn)<0) {
    /* Check for invalid public keys. */
    log_fn(severity, LD_CRYPTO,"Rejected invalid g^x");
    goto error;
  }
  secret_tmp_len = crypto_dh_get_bytes(dh);
  secret_tmp = tor_malloc(secret_tmp_len);
  result = DH_compute_key((unsigned char*)secret_tmp, pubkey_bn, dh->dh);
  if (result < 0) {
    log_warn(LD_CRYPTO,"DH_compute_key() failed.");
    goto error;
  }
  secret_len = result;
  if (crypto_expand_key_material_TAP((uint8_t*)secret_tmp, secret_len,
                                     (uint8_t*)secret_out, secret_bytes_out)<0)
    goto error;
  secret_len = secret_bytes_out;

  goto done;
 error:
  result = -1;
 done:
  crypto_log_errors(LOG_WARN, "completing DH handshake");
  if (pubkey_bn)
    BN_clear_free(pubkey_bn);
  if (secret_tmp) {
    memwipe(secret_tmp, 0, secret_tmp_len);
    tor_free(secret_tmp);
  }
  if (result < 0)
    return result;
  else
    return secret_len;
}

/** Given <b>key_in_len</b> bytes of negotiated randomness in <b>key_in</b>
 * ("K"), expand it into <b>key_out_len</b> bytes of negotiated key material in
 * <b>key_out</b> by taking the first <b>key_out_len</b> bytes of
 *    H(K | [00]) | H(K | [01]) | ....
 *
 * This is the key expansion algorithm used in the "TAP" circuit extension
 * mechanism; it shouldn't be used for new protocols.
 *
 * Return 0 on success, -1 on failure.
 */
int
crypto_expand_key_material_TAP(const uint8_t *key_in, size_t key_in_len,
                               uint8_t *key_out, size_t key_out_len)
{
  int i, r = -1;
  uint8_t *cp, *tmp = tor_malloc(key_in_len+1);
  uint8_t digest[DIGEST_LEN];

  /* If we try to get more than this amount of key data, we'll repeat blocks.*/
  tor_assert(key_out_len <= DIGEST_LEN*256);

  memcpy(tmp, key_in, key_in_len);
  for (cp = key_out, i=0; cp < key_out+key_out_len;
       ++i, cp += DIGEST_LEN) {
    tmp[key_in_len] = i;
    if (crypto_digest((char*)digest, (const char *)tmp, key_in_len+1) < 0)
      goto exit;
    memcpy(cp, digest, MIN(DIGEST_LEN, key_out_len-(cp-key_out)));
  }

  r = 0;
 exit:
  memwipe(tmp, 0, key_in_len+1);
  tor_free(tmp);
  memwipe(digest, 0, sizeof(digest));
  return r;
}

/** Expand some secret key material according to RFC5869, using SHA256 as the
 * underlying hash.  The <b>key_in_len</b> bytes at <b>key_in</b> are the
 * secret key material; the <b>salt_in_len</b> bytes at <b>salt_in</b> and the
 * <b>info_in_len</b> bytes in <b>info_in_len</b> are the algorithm's "salt"
 * and "info" parameters respectively.  On success, write <b>key_out_len</b>
 * bytes to <b>key_out</b> and return 0.  Assert on failure.
 */
int
crypto_expand_key_material_rfc5869_sha256(
                                    const uint8_t *key_in, size_t key_in_len,
                                    const uint8_t *salt_in, size_t salt_in_len,
                                    const uint8_t *info_in, size_t info_in_len,
                                    uint8_t *key_out, size_t key_out_len)
{
  uint8_t prk[DIGEST256_LEN];
  uint8_t tmp[DIGEST256_LEN + 128 + 1];
  uint8_t mac[DIGEST256_LEN];
  int i;
  uint8_t *outp;
  size_t tmp_len;

  crypto_hmac_sha256((char*)prk,
                     (const char*)salt_in, salt_in_len,
                     (const char*)key_in, key_in_len);

  /* If we try to get more than this amount of key data, we'll repeat blocks.*/
  tor_assert(key_out_len <= DIGEST256_LEN * 256);
  tor_assert(info_in_len <= 128);
  memset(tmp, 0, sizeof(tmp));
  outp = key_out;
  i = 1;

  while (key_out_len) {
    size_t n;
    if (i > 1) {
      memcpy(tmp, mac, DIGEST256_LEN);
      memcpy(tmp+DIGEST256_LEN, info_in, info_in_len);
      tmp[DIGEST256_LEN+info_in_len] = i;
      tmp_len = DIGEST256_LEN + info_in_len + 1;
    } else {
      memcpy(tmp, info_in, info_in_len);
      tmp[info_in_len] = i;
      tmp_len = info_in_len + 1;
    }
    crypto_hmac_sha256((char*)mac,
                       (const char*)prk, DIGEST256_LEN,
                       (const char*)tmp, tmp_len);
    n = key_out_len < DIGEST256_LEN ? key_out_len : DIGEST256_LEN;
    memcpy(outp, mac, n);
    key_out_len -= n;
    outp += n;
    ++i;
  }

  memwipe(tmp, 0, sizeof(tmp));
  memwipe(mac, 0, sizeof(mac));
  return 0;
}

/** 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);
}

/* random numbers */

/** How many bytes of entropy we add at once.
 *
 * This is how much entropy OpenSSL likes to add right now, so maybe it will
 * work for us too. */
#define ADD_ENTROPY 32

/** Set the seed of the weak RNG to a random value. */
void
crypto_seed_weak_rng(tor_weak_rng_t *rng)
{
  unsigned seed;
  crypto_rand((void*)&seed, sizeof(seed));
  tor_init_weak_random(rng, seed);
}

#ifdef TOR_UNIT_TESTS
int break_strongest_rng_syscall = 0;
int break_strongest_rng_fallback = 0;
#endif

/** Try to get <b>out_len</b> bytes of the strongest entropy we can generate,
 * via system calls, storing it into <b>out</b>. Return 0 on success, -1 on
 * failure.  A maximum request size of 256 bytes is imposed.
 */
static int
crypto_strongest_rand_syscall(uint8_t *out, size_t out_len)
{
  tor_assert(out_len <= MAX_STRONGEST_RAND_SIZE);

  /* We only log at notice-level here because in the case that this function
   * fails the crypto_strongest_rand_raw() caller will log with a warning-level
   * message and let crypto_strongest_rand() error out and finally terminating
   * Tor with an assertion error.
   */

#ifdef TOR_UNIT_TESTS
  if (break_strongest_rng_syscall)
    return -1;
#endif

#if defined(_WIN32)
  static int provider_set = 0;
  static HCRYPTPROV provider;

  if (!provider_set) {
    if (!CryptAcquireContext(&provider, NULL, NULL, PROV_RSA_FULL,
                             CRYPT_VERIFYCONTEXT)) {
      log_notice(LD_CRYPTO, "Unable to set Windows CryptoAPI provider [1].");
      return -1;
    }
    provider_set = 1;
  }
  if (!CryptGenRandom(provider, out_len, out)) {
    log_notice(LD_CRYPTO, "Unable get entropy from the Windows CryptoAPI.");
    return -1;
  }

  return 0;
#elif defined(__linux__) && defined(SYS_getrandom)
  static int getrandom_works = 1; /* Be optimistic about our chances... */

  /* getrandom() isn't as straightforward as getentropy(), and has
   * no glibc wrapper.
   *
   * As far as I can tell from getrandom(2) and the source code, the
   * requests we issue will always succeed (though it will block on the
   * call if /dev/urandom isn't seeded yet), since we are NOT specifying
   * GRND_NONBLOCK and the request is <= 256 bytes.
   *
   * The manpage is unclear on what happens if a signal interrupts the call
   * while the request is blocked due to lack of entropy....
   *
   * We optimistically assume that getrandom() is available and functional
   * because it is the way of the future, and 2 branch mispredicts pale in
   * comparison to the overheads involved with failing to open
   * /dev/srandom followed by opening and reading from /dev/urandom.
   */
  if (PREDICT_LIKELY(getrandom_works)) {
    long ret;
    /* A flag of '0' here means to read from '/dev/urandom', and to
     * block if insufficient entropy is available to service the
     * request.
     */
    const unsigned int flags = 0;
    do {
      ret = syscall(SYS_getrandom, out, out_len, flags);
    } while (ret == -1 && ((errno == EINTR) ||(errno == EAGAIN)));

    if (PREDICT_UNLIKELY(ret == -1)) {
      /* LCOV_EXCL_START we can't actually make the syscall fail in testing. */
      tor_assert(errno != EAGAIN);
      tor_assert(errno != EINTR);

      /* Useful log message for errno. */
      if (errno == ENOSYS) {
        log_notice(LD_CRYPTO, "Can't get entropy from getrandom()."
                   " You are running a version of Tor built to support"
                   " getrandom(), but the kernel doesn't implement this"
                   " function--probably because it is too old?"
                   " Trying fallback method instead.");
      } else {
        log_notice(LD_CRYPTO, "Can't get entropy from getrandom(): %s."
                              " Trying fallback method instead.",
                   strerror(errno));
      }

      getrandom_works = 0; /* Don't bother trying again. */
      return -1;
      /* LCOV_EXCL_STOP */
    }

    tor_assert(ret == (long)out_len);
    return 0;
  }

  return -1; /* getrandom() previously failed unexpectedly. */
#elif defined(HAVE_GETENTROPY)
  /* getentropy() is what Linux's getrandom() wants to be when it grows up.
   * the only gotcha is that requests are limited to 256 bytes.
   */
  return getentropy(out, out_len);
#else
  (void) out;
#endif /* defined(_WIN32) || ... */

  /* This platform doesn't have a supported syscall based random. */
  return -1;
}

/** Try to get <b>out_len</b> bytes of the strongest entropy we can generate,
 * via the per-platform fallback mechanism, storing it into <b>out</b>.
 * Return 0 on success, -1 on failure.  A maximum request size of 256 bytes
 * is imposed.
 */
static int
crypto_strongest_rand_fallback(uint8_t *out, size_t out_len)
{
#ifdef TOR_UNIT_TESTS
  if (break_strongest_rng_fallback)
    return -1;
#endif

#ifdef _WIN32
  /* Windows exclusively uses crypto_strongest_rand_syscall(). */
  (void)out;
  (void)out_len;
  return -1;
#else /* !(defined(_WIN32)) */
  static const char *filenames[] = {
    "/dev/srandom", "/dev/urandom", "/dev/random", NULL
  };
  int fd, i;
  size_t n;

  for (i = 0; filenames[i]; ++i) {
    log_debug(LD_FS, "Considering %s as entropy source", filenames[i]);
    fd = open(sandbox_intern_string(filenames[i]), O_RDONLY, 0);
    if (fd<0) continue;
    log_info(LD_CRYPTO, "Reading entropy from \"%s\"", filenames[i]);
    n = read_all(fd, (char*)out, out_len, 0);
    close(fd);
    if (n != out_len) {
      /* LCOV_EXCL_START
       * We can't make /dev/foorandom actually fail. */
      log_notice(LD_CRYPTO,
                 "Error reading from entropy source %s (read only %lu bytes).",
                 filenames[i],
                 (unsigned long)n);
      return -1;
      /* LCOV_EXCL_STOP */
    }

    return 0;
  }

  return -1;
#endif /* defined(_WIN32) */
}

/** Try to get <b>out_len</b> bytes of the strongest entropy we can generate,
 * storing it into <b>out</b>. Return 0 on success, -1 on failure.  A maximum
 * request size of 256 bytes is imposed.
 */
STATIC int
crypto_strongest_rand_raw(uint8_t *out, size_t out_len)
{
  static const size_t sanity_min_size = 16;
  static const int max_attempts = 3;
  tor_assert(out_len <= MAX_STRONGEST_RAND_SIZE);

  /* For buffers >= 16 bytes (128 bits), we sanity check the output by
   * zero filling the buffer and ensuring that it actually was at least
   * partially modified.
   *
   * Checking that any individual byte is non-zero seems like it would
   * fail too often (p = out_len * 1/256) for comfort, but this is an
   * "adjust according to taste" sort of check.
   */
  memwipe(out, 0, out_len);
  for (int i = 0; i < max_attempts; i++) {
    /* Try to use the syscall/OS favored mechanism to get strong entropy. */
    if (crypto_strongest_rand_syscall(out, out_len) != 0) {
      /* Try to use the less-favored mechanism to get strong entropy. */
      if (crypto_strongest_rand_fallback(out, out_len) != 0) {
        /* Welp, we tried.  Hopefully the calling code terminates the process
         * since we're basically boned without good entropy.
         */
        log_warn(LD_CRYPTO,
                 "Cannot get strong entropy: no entropy source found.");
        return -1;
      }
    }

    if ((out_len < sanity_min_size) || !tor_mem_is_zero((char*)out, out_len))
      return 0;
  }

  /* LCOV_EXCL_START
   *
   * We tried max_attempts times to fill a buffer >= 128 bits long,
   * and each time it returned all '0's.  Either the system entropy
   * source is busted, or the user should go out and buy a ticket to
   * every lottery on the planet.
   */
  log_warn(LD_CRYPTO, "Strong OS entropy returned all zero buffer.");

  return -1;
  /* LCOV_EXCL_STOP */
}

/** Try to get <b>out_len</b> bytes of the strongest entropy we can generate,
 * storing it into <b>out</b>.
 */
void
crypto_strongest_rand(uint8_t *out, size_t out_len)
{
#define DLEN SHA512_DIGEST_LENGTH
  /* We're going to hash DLEN bytes from the system RNG together with some
   * bytes from the openssl PRNG, in order to yield DLEN bytes.
   */
  uint8_t inp[DLEN*2];
  uint8_t tmp[DLEN];
  tor_assert(out);
  while (out_len) {
    crypto_rand((char*) inp, DLEN);
    if (crypto_strongest_rand_raw(inp+DLEN, DLEN) < 0) {
      // LCOV_EXCL_START
      log_err(LD_CRYPTO, "Failed to load strong entropy when generating an "
              "important key. Exiting.");
      /* Die with an assertion so we get a stack trace. */
      tor_assert(0);
      // LCOV_EXCL_STOP
    }
    if (out_len >= DLEN) {
      SHA512(inp, sizeof(inp), out);
      out += DLEN;
      out_len -= DLEN;
    } else {
      SHA512(inp, sizeof(inp), tmp);
      memcpy(out, tmp, out_len);
      break;
    }
  }
  memwipe(tmp, 0, sizeof(tmp));
  memwipe(inp, 0, sizeof(inp));
#undef DLEN
}

/** Seed OpenSSL's random number generator with bytes from the operating
 * system.  Return 0 on success, -1 on failure.
 */
int
crypto_seed_rng(void)
{
  int rand_poll_ok = 0, load_entropy_ok = 0;
  uint8_t buf[ADD_ENTROPY];

  /* OpenSSL has a RAND_poll function that knows about more kinds of
   * entropy than we do.  We'll try calling that, *and* calling our own entropy
   * functions.  If one succeeds, we'll accept the RNG as seeded. */
  rand_poll_ok = RAND_poll();
  if (rand_poll_ok == 0)
    log_warn(LD_CRYPTO, "RAND_poll() failed."); // LCOV_EXCL_LINE

  load_entropy_ok = !crypto_strongest_rand_raw(buf, sizeof(buf));
  if (load_entropy_ok) {
    RAND_seed(buf, sizeof(buf));
  }

  memwipe(buf, 0, sizeof(buf));

  if ((rand_poll_ok || load_entropy_ok) && RAND_status() == 1)
    return 0;
  else
    return -1;
}

/** Write <b>n</b> bytes of strong random data to <b>to</b>. Supports mocking
 * for unit tests.
 *
 * This function is not allowed to fail; if it would fail to generate strong
 * entropy, it must terminate the process instead.
 */
MOCK_IMPL(void,
crypto_rand, (char *to, size_t n))
{
  crypto_rand_unmocked(to, n);
}

/** Write <b>n</b> bytes of strong random data to <b>to</b>.  Most callers
 * will want crypto_rand instead.
 *
 * This function is not allowed to fail; if it would fail to generate strong
 * entropy, it must terminate the process instead.
 */
void
crypto_rand_unmocked(char *to, size_t n)
{
  int r;
  if (n == 0)
    return;

  tor_assert(n < INT_MAX);
  tor_assert(to);
  r = RAND_bytes((unsigned char*)to, (int)n);
  /* We consider a PRNG failure non-survivable. Let's assert so that we get a
   * stack trace about where it happened.
   */
  tor_assert(r >= 0);
}

/** Return a pseudorandom integer, chosen uniformly from the values
 * between 0 and <b>max</b>-1 inclusive.  <b>max</b> must be between 1 and
 * INT_MAX+1, inclusive. */
int
crypto_rand_int(unsigned int max)
{
  unsigned int val;
  unsigned int cutoff;
  tor_assert(max <= ((unsigned int)INT_MAX)+1);
  tor_assert(max > 0); /* don't div by 0 */

  /* We ignore any values that are >= 'cutoff,' to avoid biasing the
   * distribution with clipping at the upper end of unsigned int's
   * range.
   */
  cutoff = UINT_MAX - (UINT_MAX%max);
  while (1) {
    crypto_rand((char*)&val, sizeof(val));
    if (val < cutoff)
      return val % max;
  }
}

/** Return a pseudorandom integer, chosen uniformly from the values i such
 * that min <= i < max.
 *
 * <b>min</b> MUST be in range [0, <b>max</b>).
 * <b>max</b> MUST be in range (min, INT_MAX].
 */
int
crypto_rand_int_range(unsigned int min, unsigned int max)
{
  tor_assert(min < max);
  tor_assert(max <= INT_MAX);

  /* The overflow is avoided here because crypto_rand_int() returns a value
   * between 0 and (max - min) inclusive. */
  return min + crypto_rand_int(max - min);
}

/** As crypto_rand_int_range, but supports uint64_t. */
uint64_t
crypto_rand_uint64_range(uint64_t min, uint64_t max)
{
  tor_assert(min < max);
  return min + crypto_rand_uint64(max - min);
}

/** As crypto_rand_int_range, but supports time_t. */
time_t
crypto_rand_time_range(time_t min, time_t max)
{
  tor_assert(min < max);
  return min + (time_t)crypto_rand_uint64(max - min);
}

/** Return a pseudorandom 64-bit integer, chosen uniformly from the values
 * between 0 and <b>max</b>-1 inclusive. */
uint64_t
crypto_rand_uint64(uint64_t max)
{
  uint64_t val;
  uint64_t cutoff;
  tor_assert(max < UINT64_MAX);
  tor_assert(max > 0); /* don't div by 0 */

  /* We ignore any values that are >= 'cutoff,' to avoid biasing the
   * distribution with clipping at the upper end of unsigned int's
   * range.
   */
  cutoff = UINT64_MAX - (UINT64_MAX%max);
  while (1) {
    crypto_rand((char*)&val, sizeof(val));
    if (val < cutoff)
      return val % max;
  }
}

/** Return a pseudorandom double d, chosen uniformly from the range
 * 0.0 <= d < 1.0.
 */
double
crypto_rand_double(void)
{
  /* We just use an unsigned int here; we don't really care about getting
   * more than 32 bits of resolution */
  unsigned int u;
  crypto_rand((char*)&u, sizeof(u));
#if SIZEOF_INT == 4
#define UINT_MAX_AS_DOUBLE 4294967296.0
#elif SIZEOF_INT == 8
#define UINT_MAX_AS_DOUBLE 1.8446744073709552e+19
#else
#error SIZEOF_INT is neither 4 nor 8
#endif /* SIZEOF_INT == 4 || ... */
  return ((double)u) / UINT_MAX_AS_DOUBLE;
}

/** Generate and return a new random hostname starting with <b>prefix</b>,
 * ending with <b>suffix</b>, and containing no fewer than
 * <b>min_rand_len</b> and no more than <b>max_rand_len</b> random base32
 * characters. Does not check for failure.
 *
 * Clip <b>max_rand_len</b> to MAX_DNS_LABEL_SIZE.
 **/
char *
crypto_random_hostname(int min_rand_len, int max_rand_len, const char *prefix,
                       const char *suffix)
{
  char *result, *rand_bytes;
  int randlen, rand_bytes_len;
  size_t resultlen, prefixlen;

  if (max_rand_len > MAX_DNS_LABEL_SIZE)
    max_rand_len = MAX_DNS_LABEL_SIZE;
  if (min_rand_len > max_rand_len)
    min_rand_len = max_rand_len;

  randlen = crypto_rand_int_range(min_rand_len, max_rand_len+1);

  prefixlen = strlen(prefix);
  resultlen = prefixlen + strlen(suffix) + randlen + 16;

  rand_bytes_len = ((randlen*5)+7)/8;
  if (rand_bytes_len % 5)
    rand_bytes_len += 5 - (rand_bytes_len%5);
  rand_bytes = tor_malloc(rand_bytes_len);
  crypto_rand(rand_bytes, rand_bytes_len);

  result = tor_malloc(resultlen);
  memcpy(result, prefix, prefixlen);
  base32_encode(result+prefixlen, resultlen-prefixlen,
                rand_bytes, rand_bytes_len);
  tor_free(rand_bytes);
  strlcpy(result+prefixlen+randlen, suffix, resultlen-(prefixlen+randlen));

  return result;
}

/** Return a randomly chosen element of <b>sl</b>; or NULL if <b>sl</b>
 * is empty. */
void *
smartlist_choose(const smartlist_t *sl)
{
  int len = smartlist_len(sl);
  if (len)
    return smartlist_get(sl,crypto_rand_int(len));
  return NULL; /* no elements to choose from */
}

/** Scramble the elements of <b>sl</b> into a random order. */
void
smartlist_shuffle(smartlist_t *sl)
{
  int i;
  /* From the end of the list to the front, choose at random from the
     positions we haven't looked at yet, and swap that position into the
     current position.  Remember to give "no swap" the same probability as
     any other swap. */
  for (i = smartlist_len(sl)-1; i > 0; --i) {
    int j = crypto_rand_int(i+1);
    smartlist_swap(sl, i, j);
  }
}

/**
 * Destroy the <b>sz</b> bytes of data stored at <b>mem</b>, setting them to
 * the value <b>byte</b>.
 * If <b>mem</b> is NULL or <b>sz</b> is zero, nothing happens.
 *
 * This function is preferable to memset, since many compilers will happily
 * optimize out memset() when they can convince themselves that the data being
 * cleared will never be read.
 *
 * Right now, our convention is to use this function when we are wiping data
 * that's about to become inaccessible, such as stack buffers that are about
 * to go out of scope or structures that are about to get freed.  (In
 * practice, it appears that the compilers we're currently using will optimize
 * out the memset()s for stack-allocated buffers, but not those for
 * about-to-be-freed structures. That could change, though, so we're being
 * wary.)  If there are live reads for the data, then you can just use
 * memset().
 */
void
memwipe(void *mem, uint8_t byte, size_t sz)
{
  if (sz == 0) {
    return;
  }
  /* If sz is nonzero, then mem must not be NULL. */
  tor_assert(mem != NULL);

  /* Data this large is likely to be an underflow. */
  tor_assert(sz < SIZE_T_CEILING);

  /* Because whole-program-optimization exists, we may not be able to just
   * have this function call "memset".  A smart compiler could inline it, then
   * eliminate dead memsets, and declare itself to be clever. */

#if defined(SecureZeroMemory) || defined(HAVE_SECUREZEROMEMORY)
  /* Here's what you do on windows. */
  SecureZeroMemory(mem,sz);
#elif defined(HAVE_RTLSECUREZEROMEMORY)
  RtlSecureZeroMemory(mem,sz);
#elif defined(HAVE_EXPLICIT_BZERO)
  /* The BSDs provide this. */
  explicit_bzero(mem, sz);
#elif defined(HAVE_MEMSET_S)
  /* This is in the C99 standard. */
  memset_s(mem, sz, 0, sz);
#else
  /* This is a slow and ugly function from OpenSSL that fills 'mem' with junk
   * based on the pointer value, then uses that junk to update a global
   * variable.  It's an elaborate ruse to trick the compiler into not
   * optimizing out the "wipe this memory" code.  Read it if you like zany
   * programming tricks! In later versions of Tor, we should look for better
   * not-optimized-out memory wiping stuff...
   *
   * ...or maybe not.  In practice, there are pure-asm implementations of
   * OPENSSL_cleanse() on most platforms, which ought to do the job.
   **/

  OPENSSL_cleanse(mem, sz);
#endif /* defined(SecureZeroMemory) || defined(HAVE_SECUREZEROMEMORY) || ... */

  /* Just in case some caller of memwipe() is relying on getting a buffer
   * filled with a particular value, fill the buffer.
   *
   * If this function gets inlined, this memset might get eliminated, but
   * that's okay: We only care about this particular memset in the case where
   * the caller should have been using memset(), and the memset() wouldn't get
   * eliminated.  In other words, this is here so that we won't break anything
   * if somebody accidentally calls memwipe() instead of memset().
   **/
  memset(mem, byte, sz);
}

/** @{ */
/** Uninitialize the crypto library. Return 0 on success. Does not detect
 * failure.
 */
int
crypto_global_cleanup(void)
{
#if OPENSSL_VERSION_NUMBER < OPENSSL_V_SERIES(1,1,0)
  EVP_cleanup();
#endif
#ifndef NEW_THREAD_API
  ERR_remove_thread_state(NULL);
#endif
#if OPENSSL_VERSION_NUMBER < OPENSSL_V_SERIES(1,1,0)
  ERR_free_strings();
#endif

  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;

#ifndef DISABLE_ENGINES
#if OPENSSL_VERSION_NUMBER < OPENSSL_V_SERIES(1,1,0)
  ENGINE_cleanup();
#endif
#endif

  CONF_modules_unload(1);
#if OPENSSL_VERSION_NUMBER < OPENSSL_V_SERIES(1,1,0)
  CRYPTO_cleanup_all_ex_data();
#endif

  crypto_openssl_free_all();

  crypto_early_initialized_ = 0;
  crypto_global_initialized_ = 0;
  have_seeded_siphash = 0;
  siphash_unset_global_key();

  return 0;
}

/** @} */

#ifdef USE_DMALLOC
/** Tell the crypto library to use Tor's allocation functions rather than
 * calling libc's allocation functions directly. Return 0 on success, -1
 * on failure. */
int
crypto_use_tor_alloc_functions(void)
{
  int r = CRYPTO_set_mem_ex_functions(tor_malloc_, tor_realloc_, tor_free_);
  return r ? 0 : -1;
}
#endif /* defined(USE_DMALLOC) */