1<?php
2// This file is part of Moodle - http://moodle.org/
3//
4// Moodle is free software: you can redistribute it and/or modify
5// it under the terms of the GNU General Public License as published by
6// the Free Software Foundation, either version 3 of the License, or
7// (at your option) any later version.
8//
9// Moodle is distributed in the hope that it will be useful,
10// but WITHOUT ANY WARRANTY; without even the implied warranty of
11// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
12// GNU General Public License for more details.
13//
14// You should have received a copy of the GNU General Public License
15// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
16
17/**
18 * Baking badges library.
19 *
20 * @package    core
21 * @subpackage badges
22 * @copyright  2012 onwards Totara Learning Solutions Ltd {@link http://www.totaralms.com/}
23 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
24 * @author     Yuliya Bozhko <yuliya.bozhko@totaralms.com>
25 */
26
27defined('MOODLE_INTERNAL') || die();
28
29/**
30 * Information on PNG file chunks can be found at http://www.w3.org/TR/PNG/#11Chunks
31 * Some other info on PNG that I used http://garethrees.org/2007/11/14/pngcrush/
32 *
33 * Example of use:
34 * $png = new PNG_MetaDataHandler('file.png');
35 *
36 * if ($png->check_chunks("tEXt", "openbadge")) {
37 *     $newcontents = $png->add_chunks("tEXt", "openbadge", 'http://some.public.url/to.your.assertion.file');
38 * }
39 *
40 * file_put_contents('file.png', $newcontents);
41 */
42
43class PNG_MetaDataHandler
44{
45    /** @var string File content as a string */
46    private $_contents;
47    /** @var int Length of the image file */
48    private $_size;
49    /** @var array Variable for storing parsed chunks */
50    private $_chunks;
51
52    /**
53     * Prepares file for handling metadata.
54     * Verifies that this file is a valid PNG file.
55     * Unpacks file chunks and reads them into an array.
56     *
57     * @param string $contents File content as a string
58     */
59    public function __construct($contents) {
60        $this->_contents = $contents;
61        $png_signature = pack("C8", 137, 80, 78, 71, 13, 10, 26, 10);
62
63        // Read 8 bytes of PNG header and verify.
64        $header = substr($this->_contents, 0, 8);
65
66        if ($header != $png_signature) {
67            debugging('This is not a valid PNG image');
68        }
69
70        $this->_size = strlen($this->_contents);
71
72        $this->_chunks = array();
73
74        // Skip 8 bytes of IHDR image header.
75        $position = 8;
76        do {
77            $chunk = @unpack('Nsize/a4type', substr($this->_contents, $position, 8));
78            $this->_chunks[$chunk['type']][] = substr($this->_contents, $position + 8, $chunk['size']);
79
80            // Skip 12 bytes chunk overhead.
81            $position += $chunk['size'] + 12;
82        } while ($position < $this->_size);
83    }
84
85    /**
86     * Checks if a key already exists in the chunk of said type.
87     * We need to avoid writing same keyword into file chunks.
88     *
89     * @param string $type Chunk type, like iTXt, tEXt, etc.
90     * @param string $check Keyword that needs to be checked.
91     *
92     * @return boolean (true|false) True if file is safe to write this keyword, false otherwise.
93     */
94    public function check_chunks($type, $check) {
95        if (array_key_exists($type, $this->_chunks)) {
96            foreach (array_keys($this->_chunks[$type]) as $typekey) {
97                list($key, $data) = explode("\0", $this->_chunks[$type][$typekey]);
98
99                if (strcmp($key, $check) == 0) {
100                    debugging('Key "' . $check . '" already exists in "' . $type . '" chunk.');
101                    return false;
102                }
103            }
104        }
105        return true;
106    }
107
108    /**
109     * Adds a chunk with keyword and data to the file content.
110     * Chunk is added to the end of the file, before IEND image trailer.
111     *
112     * @param string $type Chunk type, like iTXt, tEXt, etc.
113     * @param string $key Keyword that needs to be added.
114     * @param string $value Currently an assertion URL that is added to an image metadata.
115     *
116     * @return string $result File content with a new chunk as a string. Can be used in file_put_contents() to write to a file.
117     * @throws \moodle_exception when unsupported chunk type is defined.
118     */
119    public function add_chunks($type, $key, $value) {
120        if (strlen($key) > 79) {
121            debugging('Key is too big');
122        }
123
124        $dataparts = [];
125        if ($type === 'iTXt') {
126            // International textual data (iTXt).
127            // Keyword:             1-79 bytes (character string).
128            $dataparts[] = $key;
129            // Null separator:      1 byte.
130            $dataparts[] = "\x00";
131            // Compression flag:    1 byte
132            // A value of 0 means no compression.
133            $dataparts[] = "\x00";
134            // Compression method:  1 byte
135            // If compression is disabled, the method should also be 0.
136            $dataparts[] = "\x00";
137            // Language tag:        0 or more bytes (character string)
138            // When there is no language specified leave empty.
139
140            // Null separator:      1 byte.
141            $dataparts[] = "\x00";
142            // Translated keyword:  0 or more bytes
143            // When there is no translation specified, leave empty.
144
145            // Null separator:      1 byte.
146            $dataparts[] = "\x00";
147            // Text:                0 or more bytes.
148            $dataparts[] = $value;
149        } else if ($type === 'tEXt') {
150            // Textual data (tEXt).
151            // Keyword:             1-79 bytes (character string).
152            $dataparts[] = $key;
153            // Null separator:      1 byte.
154            $dataparts[] = "\0";
155            // Text:                n bytes (character string).
156            $dataparts[] = $value;
157        } else {
158            throw new \moodle_exception('Unsupported chunk type: ' . $type);
159        }
160
161        $data = implode($dataparts);
162
163        $crc = pack("N", crc32($type . $data));
164        $len = pack("N", strlen($data));
165
166        // Chunk format: length + type + data + CRC.
167        // CRC is a CRC-32 computed over the chunk type and chunk data.
168        $newchunk = $len . $type . $data . $crc;
169        $this->_chunks[$type] = $data;
170
171        $result = substr($this->_contents, 0, $this->_size - 12)
172                . $newchunk
173                . substr($this->_contents, $this->_size - 12, 12);
174
175        return $result;
176    }
177}
178