# uname -a

Owncloud has the ability to encrypt files. You can restore your files with a complete backup/restore of your Owncloud folder, but what if you want to restore just one file from a previous backup ? Here is the code for a one-shot file decryption.

This code is the solution I posted on Github : https://github.com/owncloud/core/issues/13591

2015-08-05 : UPDATE : to make it work with binary files, use the following code instead :

<?php

// Replace these with your custom values
$datadir = '_PATH_TO_DATADIR_INCLUDING_TRAILING_SLASH_';
$filepath = 'some/directory/relative/to/datadir/myfile.txt';
$username = '_OWNCLOUD_LOGIN_';
$password = '_OWNCLOUD_PASSWORD_';

require_once '_PATH_TO_OWNCLOUD_/apps/files_encryption/lib/crypt.php';

// first get users private key and decrypt it
$encryptedUserKey = file_get_contents($datadir . $username . '/files_encryption/' . $username . '.privateKey');
$decryptedUserKey = OCA\Files_Encryption\Crypt::decryptPrivateKey($encryptedUserKey, $password);

// now we need to decrypt the file-key, therefore we use our private key and the share key
$shareKey = file_get_contents($datadir . $username . '/files_encryption/keys/' . $filepath . '/' . $username . '.shareKey');
$encryptedKeyfile = file_get_contents($datadir . $username . '/files_encryption/keys/' . $filepath . '/fileKey');
$decryptedKeyfile = OCA\Files_Encryption\Crypt::multiKeyDecrypt($encryptedKeyfile, $shareKey, $decryptedUserKey);

// finally we can use the decrypted file-key to decrypt the file
// but first, strip header block
$handle = fopen($datadir . $username . '/files/' . $filepath, 'r');

// if this block contained the header we move on to the next block
if (OCA\Files_Encryption\Crypt::isHeader($data)) {
        $header = OCA\Files_Encryption\Crypt::parseHeader($data);
        $cipher = OCA\Files_Encryption\Crypt::getCipher($header);
} else {
    die('Cannot find header');
}

// Decrypt the content block by block
$decryptedContent = '';
while ($data = fread($handle, OCA\Files_Encryption\Crypt::BLOCKSIZE)) {
    $decryptedContent .= OCA\Files_Encryption\Crypt::symmetricDecryptFileContent($data, $decryptedKeyfile, $cipher);
}
fclose($handle);

// Save the decrypted file
file_put_contents(str_replace('/', '-', $filepath), $decryptedContent);