<?php
/*************************************************************
# 076 License

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 */
namespace Std\Lib\Crypto;

/**
 * X25519（RFC 7748）の鍵交換
 *
 * 例：
 *   [$priv, $pub] = X25519::generate();
 *   $shared = X25519::sharedSecret($priv, $peerPub);
 */
class X25519 {
  private static ?\GMP $p = null;

  /**
   * 新しい鍵の組
   *
   * @return array  [秘密鍵（32バイト）, 公開鍵（32バイト）]
   */
  public static function generate(): array {
    $priv = random_bytes(32);
    return [$priv, self::publicKey($priv)];
  }

  /** 秘密鍵から公開鍵 */
  public static function publicKey(string $priv): string {
    return self::ladder($priv, "\x09".str_repeat("\0", 31));
  }

  /**
   * 共有の秘密
   *
   * @param string $priv  自分の秘密鍵
   * @param string $pub  相手の公開鍵
   * @return string  32バイト
   * @throws \InvalidArgumentException  長さが違う、又は結果が全部0（小さい位数の点）の場合
   */
  public static function sharedSecret(string $priv, string $pub): string {
    $s = self::ladder($priv, $pub);
    if ($s === str_repeat("\0", 32)) throw new \InvalidArgumentException('X25519：不正な公開鍵');
    return $s;
  }

  /** モンゴメリ・ラダー（RFC 7748 5） */
  private static function ladder(string $k, string $u): string {
    if (strlen($k) !== 32 || strlen($u) !== 32) throw new \InvalidArgumentException('X25519：鍵は32バイト');
    $p = self::$p ??= gmp_sub(gmp_pow(2, 255), 19);

    $k[0] = chr(ord($k[0]) & 248);
    $k[31] = chr((ord($k[31]) & 127) | 64);
    $u[31] = chr(ord($u[31]) & 127);
    $scalar = gmp_import(strrev($k));
    $x1 = gmp_mod(gmp_import(strrev($u)), $p);

    $x2 = gmp_init(1);
    $z2 = gmp_init(0);
    $x3 = $x1;
    $z3 = gmp_init(1);
    $swap = 0;

    for ($t = 254; $t >= 0; $t--) {
      $kt = gmp_testbit($scalar, $t) ? 1 : 0;
      if ($swap ^ $kt) {
        [$x2, $x3] = [$x3, $x2];
        [$z2, $z3] = [$z3, $z2];
      }
      $swap = $kt;

      $a = gmp_add($x2, $z2);
      $aa = gmp_mod(gmp_mul($a, $a), $p);
      $b = gmp_sub($x2, $z2);
      $bb = gmp_mod(gmp_mul($b, $b), $p);
      $e = gmp_sub($aa, $bb);
      $c = gmp_add($x3, $z3);
      $d = gmp_sub($x3, $z3);
      $da = gmp_mul($d, $a);
      $cb = gmp_mul($c, $b);
      $x3 = gmp_mod(gmp_pow(gmp_add($da, $cb), 2), $p);
      $z3 = gmp_mod(gmp_mul($x1, gmp_pow(gmp_sub($da, $cb), 2)), $p);
      $x2 = gmp_mod(gmp_mul($aa, $bb), $p);
      $z2 = gmp_mod(gmp_mul($e, gmp_add($aa, gmp_mul(121665, $e))), $p);
    }
    if ($swap) {
      [$x2, $x3] = [$x3, $x2];
      [$z2, $z3] = [$z3, $z2];
    }

    $r = gmp_mod(gmp_mul($x2, gmp_powm($z2, gmp_sub($p, 2), $p)), $p);
    return strrev(str_pad(gmp_export($r), 32, "\0", STR_PAD_LEFT));
  }
}