crypto_curve25519.c

/* Copyright (c) 2012-2015, The Tor Project, Inc. */
/* See LICENSE for licensing information */

/* Wrapper code for a curve25519 implementation. */

#define CRYPTO_CURVE25519_PRIVATE
#include "orconfig.h"
#ifdef HAVE_SYS_STAT_H
#include <sys/stat.h>
#endif
#include "container.h"
#include "crypto.h"
#include "crypto_curve25519.h"
#include "util.h"
#include "torlog.h"

#include "ed25519/donna/ed25519_donna_tor.h"

/* ==============================
   Part 1: wrap a suitable curve25519 implementation as curve25519_impl
   ============================== */

#ifdef USE_CURVE25519_DONNA
int curve25519_donna(uint8_t *mypublic,
                     const uint8_t *secret, const uint8_t *basepoint);
#endif
#ifdef USE_CURVE25519_NACL
#ifdef HAVE_CRYPTO_SCALARMULT_CURVE25519_H
#include <crypto_scalarmult_curve25519.h>
#elif defined(HAVE_NACL_CRYPTO_SCALARMULT_CURVE25519_H)
#include <nacl/crypto_scalarmult_curve25519.h>
#endif
#endif

static void pick_curve25519_basepoint_impl(void);

static int curve25519_use_ed = -1;

STATIC int
curve25519_impl(uint8_t *output, const uint8_t *secret,
                const uint8_t *basepoint)
{
  uint8_t bp[CURVE25519_PUBKEY_LEN];
  int r;
  memcpy(bp, basepoint, CURVE25519_PUBKEY_LEN);
  /* Clear the high bit, in case our backend foolishly looks at it. */
  bp[31] &= 0x7f;
#ifdef USE_CURVE25519_DONNA
  r = curve25519_donna(output, secret, bp);
#elif defined(USE_CURVE25519_NACL)
  r = crypto_scalarmult_curve25519(output, secret, bp);
#else
#error "No implementation of curve25519 is available."
#endif
  memwipe(bp, 0, sizeof(bp));
  return r;
}

STATIC int
curve25519_basepoint_impl(uint8_t *output, const uint8_t *secret)
{
  int r = 0;
  if (PREDICT_UNLIKELY(curve25519_use_ed == -1)) {
    pick_curve25519_basepoint_impl();
  }

  /* TODO: Someone should benchmark curved25519_scalarmult_basepoint versus
   * an optimized NaCl build to see which should be used when compiled with
   * NaCl available.  I suspected that the ed25519 optimization always wins.
   */
  if (PREDICT_LIKELY(curve25519_use_ed == 1)) {
    curved25519_scalarmult_basepoint_donna(output, secret);
    r = 0;
  } else {
    static const uint8_t basepoint[32] = {9};
    r = curve25519_impl(output, secret, basepoint);
  }
  return r;
}

void
curve25519_set_impl_params(int use_ed)
{
  curve25519_use_ed = use_ed;
}

/* ==============================
   Part 2: Wrap curve25519_impl with some convenience types and functions.
   ============================== */

/**
 * Return true iff a curve25519_public_key_t seems valid. (It's not necessary
 * to see if the point is on the curve, since the twist is also secure, but we
 * do need to make sure that it isn't the point at infinity.) */
int
curve25519_public_key_is_ok(const curve25519_public_key_t *key)
{
  return !safe_mem_is_zero(key->public_key, CURVE25519_PUBKEY_LEN);
}

/**
 * Generate CURVE25519_SECKEY_LEN random bytes in <b>out</b>. If
 * <b>extra_strong</b> is true, this key is possibly going to get used more
 * than once, so use a better-than-usual RNG. Return 0 on success, -1 on
 * failure.
 *
 * This function does not adjust the output of the RNG at all; the will caller
 * will need to clear or set the appropriate bits to make curve25519 work.
 */
int
curve25519_rand_seckey_bytes(uint8_t *out, int extra_strong)
{
  uint8_t k_tmp[CURVE25519_SECKEY_LEN];

  if (crypto_rand((char*)out, CURVE25519_SECKEY_LEN) < 0)
    return -1;
  if (extra_strong && !crypto_strongest_rand(k_tmp, CURVE25519_SECKEY_LEN)) {
    /* If they asked for extra-strong entropy and we have some, use it as an
     * HMAC key to improve not-so-good entropy rather than using it directly,
     * just in case the extra-strong entropy is less amazing than we hoped. */
    crypto_hmac_sha256((char*) out,
                       (const char *)k_tmp, sizeof(k_tmp),
                       (const char *)out, CURVE25519_SECKEY_LEN);
  }
  memwipe(k_tmp, 0, sizeof(k_tmp));
  return 0;
}

/** Generate a new keypair and return the secret key.  If <b>extra_strong</b>
 * is true, this key is possibly going to get used more than once, so
 * use a better-than-usual RNG. Return 0 on success, -1 on failure. */
int
curve25519_secret_key_generate(curve25519_secret_key_t *key_out,
                               int extra_strong)
{
  if (curve25519_rand_seckey_bytes(key_out->secret_key, extra_strong) < 0)
    return -1;

  key_out->secret_key[0] &= 248;
  key_out->secret_key[31] &= 127;
  key_out->secret_key[31] |= 64;

  return 0;
}

void
curve25519_public_key_generate(curve25519_public_key_t *key_out,
                               const curve25519_secret_key_t *seckey)
{
  curve25519_basepoint_impl(key_out->public_key, seckey->secret_key);
}

int
curve25519_keypair_generate(curve25519_keypair_t *keypair_out,
                            int extra_strong)
{
  if (curve25519_secret_key_generate(&keypair_out->seckey, extra_strong) < 0)
    return -1;
  curve25519_public_key_generate(&keypair_out->pubkey, &keypair_out->seckey);
  return 0;
}

/** Write the <b>datalen</b> bytes from <b>data</b> to the file named
 * <b>fname</b> in the tagged-data format.  This format contains a
 * 32-byte header, followed by the data itself.  The header is the
 * NUL-padded string "== <b>typestring</b>: <b>tag</b> ==".  The length
 * of <b>typestring</b> and <b>tag</b> must therefore be no more than
 * 24.
 **/
int
crypto_write_tagged_contents_to_file(const char *fname,
                                     const char *typestring,
                                     const char *tag,
                                     const uint8_t *data,
                                     size_t datalen)
{
  char header[32];
  smartlist_t *chunks = smartlist_new();
  sized_chunk_t ch0, ch1;
  int r = -1;

  memset(header, 0, sizeof(header));
  if (tor_snprintf(header, sizeof(header),
                   "== %s: %s ==", typestring, tag) < 0)
    goto end;
  ch0.bytes = header;
  ch0.len = 32;
  ch1.bytes = (const char*) data;
  ch1.len = datalen;
  smartlist_add(chunks, &ch0);
  smartlist_add(chunks, &ch1);

  r = write_chunks_to_file(fname, chunks, 1, 0);

 end:
  smartlist_free(chunks);
  return r;
}

/** Read a tagged-data file from <b>fname</b> into the
 * <b>data_out_len</b>-byte buffer in <b>data_out</b>. Check that the
 * typestring matches <b>typestring</b>; store the tag into a newly allocated
 * string in <b>tag_out</b>. Return -1 on failure, and the number of bytes of
 * data on success.  Preserves the errno from reading the file. */
ssize_t
crypto_read_tagged_contents_from_file(const char *fname,
                                      const char *typestring,
                                      char **tag_out,
                                      uint8_t *data_out,
                                      ssize_t data_out_len)
{
  char prefix[33];
  char *content = NULL;
  struct stat st;
  ssize_t r = -1;
  size_t st_size = 0;
  int saved_errno = 0;

  *tag_out = NULL;
  st.st_size = 0;
  content = read_file_to_str(fname, RFTS_BIN|RFTS_IGNORE_MISSING, &st);
  if (! content) {
    saved_errno = errno;
    goto end;
  }
  if (st.st_size < 32 || st.st_size > 32 + data_out_len) {
    saved_errno = EINVAL;
    goto end;
  }
  st_size = (size_t)st.st_size;

  memcpy(prefix, content, 32);
  prefix[32] = 0;
  /* Check type, extract tag. */
  if (strcmpstart(prefix, "== ") || strcmpend(prefix, " ==") ||
      ! tor_mem_is_zero(prefix+strlen(prefix), 32-strlen(prefix))) {
    saved_errno = EINVAL;
    goto end;
  }

  if (strcmpstart(prefix+3, typestring) ||
      3+strlen(typestring) >= 32 ||
      strcmpstart(prefix+3+strlen(typestring), ": ")) {
    saved_errno = EINVAL;
    goto end;
  }

  *tag_out = tor_strndup(prefix+5+strlen(typestring),
                         strlen(prefix)-8-strlen(typestring));

  memcpy(data_out, content+32, st_size-32);
  r = st_size - 32;

 end:
  if (content)
    memwipe(content, 0, st_size);
  tor_free(content);
  if (saved_errno)
    errno = saved_errno;
  return r;
}

/** DOCDOC */
int
curve25519_keypair_write_to_file(const curve25519_keypair_t *keypair,
                                 const char *fname,
                                 const char *tag)
{
  uint8_t contents[CURVE25519_SECKEY_LEN + CURVE25519_PUBKEY_LEN];
  int r;

  memcpy(contents, keypair->seckey.secret_key, CURVE25519_SECKEY_LEN);
  memcpy(contents+CURVE25519_SECKEY_LEN,
         keypair->pubkey.public_key, CURVE25519_PUBKEY_LEN);

  r = crypto_write_tagged_contents_to_file(fname,
                                           "c25519v1",
                                           tag,
                                           contents,
                                           sizeof(contents));

  memwipe(contents, 0, sizeof(contents));
  return r;
}

/** DOCDOC */
int
curve25519_keypair_read_from_file(curve25519_keypair_t *keypair_out,
                                  char **tag_out,
                                  const char *fname)
{
  uint8_t content[CURVE25519_SECKEY_LEN + CURVE25519_PUBKEY_LEN];
  ssize_t len;
  int r = -1;

  len = crypto_read_tagged_contents_from_file(fname, "c25519v1", tag_out,
                                              content, sizeof(content));
  if (len != sizeof(content))
    goto end;

  memcpy(keypair_out->seckey.secret_key, content, CURVE25519_SECKEY_LEN);
  curve25519_public_key_generate(&keypair_out->pubkey, &keypair_out->seckey);
  if (tor_memneq(keypair_out->pubkey.public_key,
                 content + CURVE25519_SECKEY_LEN,
                 CURVE25519_PUBKEY_LEN))
    goto end;

  r = 0;

 end:
  memwipe(content, 0, sizeof(content));
  if (r != 0) {
    memset(keypair_out, 0, sizeof(*keypair_out));
    tor_free(*tag_out);
  }
  return r;
}

/** Perform the curve25519 ECDH handshake with <b>skey</b> and <b>pkey</b>,
 * writing CURVE25519_OUTPUT_LEN bytes of output into <b>output</b>. */
void
curve25519_handshake(uint8_t *output,
                     const curve25519_secret_key_t *skey,
                     const curve25519_public_key_t *pkey)
{
  curve25519_impl(output, skey->secret_key, pkey->public_key);
}

/** Check whether the ed25519-based curve25519 basepoint optimization seems to
 * be working. If so, return 0; otherwise return -1. */
static int
curve25519_basepoint_spot_check(void)
{
  static const uint8_t alicesk[32] = {
    0x77,0x07,0x6d,0x0a,0x73,0x18,0xa5,0x7d,
    0x3c,0x16,0xc1,0x72,0x51,0xb2,0x66,0x45,
    0xdf,0x4c,0x2f,0x87,0xeb,0xc0,0x99,0x2a,
    0xb1,0x77,0xfb,0xa5,0x1d,0xb9,0x2c,0x2a
  };
  static const uint8_t alicepk[32] = {
    0x85,0x20,0xf0,0x09,0x89,0x30,0xa7,0x54,
    0x74,0x8b,0x7d,0xdc,0xb4,0x3e,0xf7,0x5a,
    0x0d,0xbf,0x3a,0x0d,0x26,0x38,0x1a,0xf4,
    0xeb,0xa4,0xa9,0x8e,0xaa,0x9b,0x4e,0x6a
  };
  const int loop_max=200;
  int save_use_ed = curve25519_use_ed;
  unsigned char e1[32] = { 5 };
  unsigned char e2[32] = { 5 };
  unsigned char x[32],y[32];
  int i;
  int r=0;

  /* Check the most basic possible sanity via the test secret/public key pair
   * used in "Cryptography in NaCl - 2. Secret keys and public keys".  This
   * may catch catastrophic failures on systems where Curve25519 is expensive,
   * without requiring a ton of key generation.
   */
  curve25519_use_ed = 1;
  r |= curve25519_basepoint_impl(x, alicesk);
  if (fast_memneq(x, alicepk, 32))
    goto fail;

  /* Ok, the optimization appears to produce passable results, try a few more
   * values, maybe there's something subtle wrong.
   */
  for (i = 0; i < loop_max; ++i) {
    curve25519_use_ed = 0;
    r |= curve25519_basepoint_impl(x, e1);
    curve25519_use_ed = 1;
    r |= curve25519_basepoint_impl(y, e2);
    if (fast_memneq(x,y,32))
      goto fail;
    memcpy(e1, x, 32);
    memcpy(e2, x, 32);
  }

  goto end;
 fail:
  r = -1;
 end:
  curve25519_use_ed = save_use_ed;
  return r;
}

/** Choose whether to use the ed25519-based curve25519-basepoint
 * implementation. */
static void
pick_curve25519_basepoint_impl(void)
{
  curve25519_use_ed = 1;

  if (curve25519_basepoint_spot_check() == 0)
    return;

  log_warn(LD_CRYPTO, "The ed25519-based curve25519 basepoint "
           "multiplication seems broken; using the curve25519 "
           "implementation.");
  curve25519_use_ed = 0;
}

/** Initialize the curve25519 implementations. This is necessary if you're
 * going to use them in a multithreaded setting, and not otherwise. */
void
curve25519_init(void)
{
  pick_curve25519_basepoint_impl();
}