ParagonIE_Sodium_File::verify

    public static function verify($sig, $filePath, $publicKey)
    {
        /* Type checks: */
        if (!is_string($sig)) {
            throw new TypeError('Argument 1 must be a string, ' . gettype($sig) . ' given.');
        }
        if (!is_string($filePath)) {
            throw new TypeError('Argument 2 must be a string, ' . gettype($filePath) . ' given.');
        }
        if (!is_string($publicKey)) {
            throw new TypeError('Argument 3 must be a string, ' . gettype($publicKey) . ' given.');
        }

        /* Input validation: */
        if (self::strlen($sig) !== ParagonIE_Sodium_Compat::CRYPTO_SIGN_BYTES) {
            throw new TypeError('Argument 1 must be CRYPTO_SIGN_BYTES bytes');
        }
        if (self::strlen($publicKey) !== ParagonIE_Sodium_Compat::CRYPTO_SIGN_PUBLICKEYBYTES) {
            throw new TypeError('Argument 3 must be CRYPTO_SIGN_PUBLICKEYBYTES bytes');
        }
        if (self::strlen($sig) < 64) {
            throw new SodiumException('Signature is too short');
        }

        if (PHP_INT_SIZE === 4) {
            return self::verify_core32($sig, $filePath, $publicKey);
        }

        /* Security checks */
        if (
            (ParagonIE_Sodium_Core_Ed25519::chrToInt($sig[63]) & 240)
                &&
            ParagonIE_Sodium_Core_Ed25519::check_S_lt_L(self::substr($sig, 32, 32))
        ) {
            throw new SodiumException('S < L - Invalid signature');
        }
        if (ParagonIE_Sodium_Core_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 $size */
        $size = filesize($filePath);
        if (!is_int($size)) {
            throw new SodiumException('Could not obtain the file size');
        }

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

        /** @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_Core_Curve25519_Ge_P3 $A */
        $A = ParagonIE_Sodium_Core_Ed25519::ge_frombytes_negate_vartime($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 */
        $h = ParagonIE_Sodium_Core_Ed25519::sc_reduce($hDigest) . self::substr($hDigest, 32);

        /** @var ParagonIE_Sodium_Core_Curve25519_Ge_P2 $R */
        $R = ParagonIE_Sodium_Core_Ed25519::ge_double_scalarmult_vartime(
            $h,
            $A,
            self::substr($sig, 32)
        );

        /** @var string $rcheck */
        $rcheck = ParagonIE_Sodium_Core_Ed25519::ge_tobytes($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));
    }