private function getChecksumsMiddleware()
    {
        return function (callable $handler) {
            return function (
                CommandInterface $command,
                RequestInterface $request = null
            ) use ($handler) {
                // Accept "ContentSHA256" with a lowercase "c" to match other Glacier params.
                if (!$command['ContentSHA256'] && $command['contentSHA256']) {
                    $command['ContentSHA256'] = $command['contentSHA256'];
                    unset($command['contentSHA256']);
                }

                // If uploading, then make sure checksums are added.
                $name = $command->getName();
                if (($name === 'UploadArchive' || $name === 'UploadMultipartPart')
                    && (!$command['checksum'] || !$command['ContentSHA256'])
                ) {
                    $body = $request->getBody();
                    if (!$body->isSeekable()) {
                        throw new CouldNotCreateChecksumException('sha256');
                    }

                    // Add a tree hash if not provided.
                    if (!$command['checksum']) {
                        $body = new HashingStream(
                            $body, new TreeHash(),
                            function ($result) use ($command, &$request) {
                                $request = $request->withHeader(
                                    'x-amz-sha256-tree-hash',
                                    bin2hex($result)
                                );
                            }
                        );
                    }

                    // Add a linear content hash if not provided.
                    if (!$command['ContentSHA256']) {
                        $body = new HashingStream(
                            $body, new PhpHash('sha256'),
                            function ($result) use ($command) {
                                $command['ContentSHA256'] = bin2hex($result);
                            }
                        );
                    }

                    // Read the stream in order to calculate the hashes.
                    while (!$body->eof()) {
                        $body->read(1048576);
                    }
                    $body->seek(0);
                }

                // Set the content hash header if a value is in the command.
                if ($command['ContentSHA256']) {
                    $request = $request->withHeader(
                        'x-amz-content-sha256',
                        $command['ContentSHA256']
                    );
                }

                return $handler($command, $request);
            };
        };
    }