text: /* Copyright (c) 2019, Google Inc.
   *
   * Permission to use, copy, modify, and/or distribute this software for any
   * purpose with or without fee is hereby granted, provided that the above
   * copyright notice and this permission notice appear in all copies.
   *
   * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
   * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
   * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
   * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
   * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
   * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
   * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
  
  #include <openssl_grpc/ec_key.h>
  
  #include <string.h>
  
  #include <openssl_grpc/buf.h>
  #include <openssl_grpc/ec.h>
  #include <openssl_grpc/err.h>
  #include <openssl_grpc/digest.h>
  #include <openssl_grpc/hkdf.h>
  #include <openssl_grpc/mem.h>
  
  #include "../fipsmodule/ec/internal.h"
  
  
  EC_KEY *EC_KEY_derive_from_secret(const EC_GROUP *group, const uint8_t *secret,
                                    size_t secret_len) {
  #define EC_KEY_DERIVE_MAX_NAME_LEN 16
    const char *name = EC_curve_nid2nist(EC_GROUP_get_curve_name(group));
    if (name == NULL || strlen(name) > EC_KEY_DERIVE_MAX_NAME_LEN) {
      OPENSSL_PUT_ERROR(EC, EC_R_UNKNOWN_GROUP);
      return NULL;
    }
  
    // Assemble a label string to provide some key separation in case |secret| is
    // misused, but ultimately it's on the caller to ensure |secret| is suitably
    // separated.
    static const char kLabel[] = "derive EC key ";
    char info[sizeof(kLabel) + EC_KEY_DERIVE_MAX_NAME_LEN];
    BUF_strlcpy(info, kLabel, sizeof(info));
    BUF_strlcat(info, name, sizeof(info));
  
    // Generate 128 bits beyond the group order so the bias is at most 2^-128.
  #define EC_KEY_DERIVE_EXTRA_BITS 128
  #define EC_KEY_DERIVE_EXTRA_BYTES (EC_KEY_DERIVE_EXTRA_BITS / 8)
  
    if (EC_GROUP_order_bits(group) <= EC_KEY_DERIVE_EXTRA_BITS + 8) {
      // The reduction strategy below requires the group order be large enough.
      // (The actual bound is a bit tighter, but our curves are much larger than
      // 128-bit.)
      OPENSSL_PUT_ERROR(EC, ERR_R_INTERNAL_ERROR);
      return NULL;
    }
  
    uint8_t derived[EC_KEY_DERIVE_EXTRA_BYTES + EC_MAX_BYTES];
    size_t derived_len = BN_num_bytes(&group->order) + EC_KEY_DERIVE_EXTRA_BYTES;
    assert(derived_len <= sizeof(derived));
    if (!HKDF(derived, derived_len, EVP_sha256(), secret, secret_len,
              /*salt=*/NULL, /*salt_len=*/0, (const uint8_t *)info,
              strlen(info))) {
      return NULL;
    }
  
    EC_KEY *key = EC_KEY_new();
    BN_CTX *ctx = BN_CTX_new();
    BIGNUM *priv = BN_bin2bn(derived, derived_len, NULL);
    EC_POINT *pub = EC_POINT_new(group);
    if (key == NULL || ctx == NULL || priv == NULL || pub == NULL ||
        // Reduce |priv| with Montgomery reduction. First, convert "from"
        // Montgomery form to compute |priv| * R^-1 mod |order|. This requires
        // |priv| be under order * R, which is true if the group order is large
        // enough. 2^(num_bytes(order)) < 2^8 * order, so:
        //
        //    priv < 2^8 * order * 2^128 < order * order < order * R
        !BN_from_montgomery(priv, priv, group->order_mont, ctx) ||
        // Multiply by R^2 and do another Montgomery reduction to compute
        // priv * R^-1 * R^2 * R^-1 = priv mod order.
        !BN_to_montgomery(priv, priv, group->order_mont, ctx) ||
        !EC_POINT_mul(group, pub, priv, NULL, NULL, ctx) ||
        !EC_KEY_set_group(key, group) || !EC_KEY_set_public_key(key, pub) ||
        !EC_KEY_set_private_key(key, priv)) {
      EC_KEY_free(key);
      key = NULL;
      goto err;
    }
  
  err:
    OPENSSL_cleanse(derived, sizeof(derived));
    BN_CTX_free(ctx);
    BN_free(priv);
    EC_POINT_free(pub);
    return key;
  }
  