ParagonIE_Sodium_File::verify_core32

    public static function verify_core32($sig, $filePath, $publicKey)
    {
        /* Security checks */
        if (ParagonIE_Sodium_Core32_Ed25519::check_S_lt_L(self::substr($sig, 32, 32))) {
            throw new SodiumException('S < L - Invalid signature');
        }
        if (ParagonIE_Sodium_Core32_Ed25519::small_order($sig)) {
            throw new SodiumException('Signature is on too small of an order');
        }
        if ((self::chrToInt($sig[63]) & 224) !== 0) {
            throw new SodiumException('Invalid signature');
        }
        $d = 0;
        for ($i = 0; $i < 32; ++$i) {
            $d |= self::chrToInt($publicKey[$i]);
        }
        if ($d === 0) {
            throw new SodiumException('All zero public key');
        }

        /** @var int|bool $size */
        $size = filesize($filePath);
        if (!is_int($size)) {
            throw new SodiumException('Could not obtain the file size');
        }
        /** @var int $size */

        /** @var resource|bool $fp */
        $fp = fopen($filePath, 'rb');
        if (!is_resource($fp)) {
            throw new SodiumException('Could not open input file for reading');
        }
        /** @var resource $fp */

        /** @var bool The original value of ParagonIE_Sodium_Compat::$fastMult */
        $orig = ParagonIE_Sodium_Compat::$fastMult;

        // Set ParagonIE_Sodium_Compat::$fastMult to true to speed up verification.
        ParagonIE_Sodium_Compat::$fastMult = true;

        /** @var ParagonIE_Sodium_Core32_Curve25519_Ge_P3 $A */
        $A = ParagonIE_Sodium_Core32_Ed25519::ge_frombytes_negate_vartime($publicKey);

        $hs = hash_init('sha512');
        hash_update($hs, self::substr($sig, 0, 32));
        hash_update($hs, self::substr($publicKey, 0, 32));
        /** @var resource $hs */
        $hs = self::updateHashWithFile($hs, $fp, $size);
        /** @var string $hDigest */
        $hDigest = hash_final($hs, true);

        /** @var string $h */
        $h = ParagonIE_Sodium_Core32_Ed25519::sc_reduce($hDigest) . self::substr($hDigest, 32);

        /** @var ParagonIE_Sodium_Core32_Curve25519_Ge_P2 $R */
        $R = ParagonIE_Sodium_Core32_Ed25519::ge_double_scalarmult_vartime(
            $h,
            $A,
            self::substr($sig, 32)
        );

        /** @var string $rcheck */
        $rcheck = ParagonIE_Sodium_Core32_Ed25519::ge_tobytes($R);

        // Close the file handle
        fclose($fp);

        // Reset ParagonIE_Sodium_Compat::$fastMult to what it was before.
        ParagonIE_Sodium_Compat::$fastMult = $orig;
        return self::verify_32($rcheck, self::substr($sig, 0, 32));
    }