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                <?php  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                /**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                 * @author Threema GmbH  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                 * @copyright Copyright (c) 2015-2016 Threema GmbH  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                 */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                namespace Threema\MsgApi\Tools;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                use Threema\Core\Exception;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                use Threema\Core\KeyPair;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                /**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                 * Contains static methods to do various Threema cryptography related tasks.  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                 * Support libsodium >= 0.2.0 (Namespaces)  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                 *  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                 * @package Threema\Core  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                 */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                class CryptToolSodium extends CryptTool { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	/**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @param string $data  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @param string $nonce  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @param string $senderPrivateKey  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @param string $recipientPublicKey  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @return string encrypted box  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	protected function makeBox($data, $nonce, $senderPrivateKey, $recipientPublicKey) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                		/** @noinspection PhpUndefinedNamespaceInspection @noinspection PhpUndefinedFunctionInspection */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                		$kp = \Sodium\crypto_box_keypair_from_secretkey_and_publickey($senderPrivateKey, $recipientPublicKey);  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                		/** @noinspection PhpUndefinedNamespaceInspection @noinspection PhpUndefinedFunctionInspection */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                		return \Sodium\crypto_box($data, $nonce, $kp);  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	}  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	/**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * make a secret box  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 *  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @param $data  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @param $nonce  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @param $key  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @return mixed  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	protected function makeSecretBox($data, $nonce, $key) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                		/** @noinspection PhpUndefinedNamespaceInspection @noinspection PhpUndefinedFunctionInspection */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                		return \Sodium\crypto_secretbox($data, $nonce, $key);  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	}  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	/**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @param string $box  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @param string $recipientPrivateKey  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @param string $senderPublicKey  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @param string $nonce  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @return null|string  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	protected function openBox($box, $recipientPrivateKey, $senderPublicKey, $nonce) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                		/** @noinspection PhpUndefinedNamespaceInspection @noinspection PhpUndefinedFunctionInspection */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                		$kp = \Sodium\crypto_box_keypair_from_secretkey_and_publickey($recipientPrivateKey, $senderPublicKey);  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                		/** @noinspection PhpUndefinedNamespaceInspection @noinspection PhpUndefinedFunctionInspection */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                		return \Sodium\crypto_box_open($box, $nonce, $kp);  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	}  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	/**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * decrypt a secret box  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 *  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @param string $box as binary  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @param string $nonce as binary  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @param string $key as binary  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @return string as binary  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	protected function openSecretBox($box, $nonce, $key) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                		/** @noinspection PhpUndefinedNamespaceInspection @noinspection PhpUndefinedFunctionInspection */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                		return \Sodium\crypto_secretbox_open($box, $nonce, $key);  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	}  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	/**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * Generate a new key pair.  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 *  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @return KeyPair the new key pair  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	final public function generateKeyPair() { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                		/** @noinspection PhpUndefinedNamespaceInspection @noinspection PhpUndefinedFunctionInspection */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                		$kp = \Sodium\crypto_box_keypair();  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                		/** @noinspection PhpUndefinedNamespaceInspection @noinspection PhpUndefinedFunctionInspection */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                		return new KeyPair(\Sodium\crypto_box_secretkey($kp), \Sodium\crypto_box_publickey($kp));  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	}  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	/**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @param int $size  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @return string  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	protected function createRandom($size) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                		/** @noinspection PhpUndefinedNamespaceInspection @noinspection PhpUndefinedFunctionInspection */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                		return \Sodium\randombytes_buf($size);  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	}  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	/**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * Derive the public key  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 *  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @param string $privateKey in binary  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @return string public key as binary  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	final public function derivePublicKey($privateKey) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                		/** @noinspection PhpUndefinedNamespaceInspection @noinspection PhpUndefinedFunctionInspection */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                		return \Sodium\crypto_box_publickey_from_secretkey($privateKey);  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	}  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	/**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * Converts a binary string to an hexdecimal string.  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 *  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * This is the same as PHP's bin2hex() implementation, but it is resistant to  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * timing attacks.  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 *  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @link https://paragonie.com/book/pecl-libsodium/read/03-utilities-helpers.md#bin2hex  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @param  string $binaryString The binary string to convert  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @return string  | 
            
            
                                                                                                            
                                                                
            
                                    
            
            
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                	 */  | 
            
            
                                                                        
                            
            
                                    
            
            
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                	public function bin2hex($binaryString)  | 
            
            
                                                                        
                            
            
                                    
            
            
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                	{ | 
            
            
                                                                        
                            
            
                                    
            
            
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                		return \Sodium\bin2hex($binaryString);  | 
            
            
                                                                        
                            
            
                                    
            
            
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                	}  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	/**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * Converts an hexdecimal string to a binary string.  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 *  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * This is the same as PHP's hex2bin() implementation, but it is resistant to  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * timing attacks.  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 *  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @link https://paragonie.com/book/pecl-libsodium/read/03-utilities-helpers.md#hex2bin  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                	 * @param  string $hexString The hex string to convert  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    131
                 | 
                                    
                                                     | 
                
                 | 
                	 * @param  string|null $ignore	(optional) Characters to ignore  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    132
                 | 
                                    
                                                     | 
                
                 | 
                	 * @throws \Threema\Core\Exception  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    133
                 | 
                                    
                                                     | 
                
                 | 
                	 * @return string  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    134
                 | 
                                    
                                                     | 
                
                 | 
                	 */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    135
                 | 
                                    
                                                     | 
                
                 | 
                	public function hex2bin($hexString, $ignore = null)  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    136
                 | 
                                    
                                                     | 
                
                 | 
                	{ | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    137
                 | 
                                    
                                                     | 
                
                 | 
                		return \Sodium\hex2bin($hexString, $ignore);  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    138
                 | 
                                    
                                                     | 
                
                 | 
                	}  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    139
                 | 
                                    
                                                     | 
                
                 | 
                 | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    140
                 | 
                                    
                                                     | 
                
                 | 
                 | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    141
                 | 
                                    
                                                     | 
                
                 | 
                	/**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    142
                 | 
                                    
                                                     | 
                
                 | 
                	 * Compares two strings in a secure way.  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    143
                 | 
                                    
                                                     | 
                
                 | 
                	 *  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    144
                 | 
                                    
                                                     | 
                
                 | 
                	 * This is the same as PHP's strcmp() implementation, but it is resistant to  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    145
                 | 
                                    
                                                     | 
                
                 | 
                	 * timing attacks.  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    146
                 | 
                                    
                                                     | 
                
                 | 
                	 *  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    147
                 | 
                                    
                                                     | 
                
                 | 
                	 * @link https://paragonie.com/book/pecl-libsodium/read/03-utilities-helpers.md#compare  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    148
                 | 
                                    
                                                     | 
                
                 | 
                	 * @param  string $str1 The first string  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    149
                 | 
                                    
                                                     | 
                
                 | 
                	 * @param  string $str2 The second string  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    150
                 | 
                                    
                                                     | 
                
                 | 
                	 * @return bool  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    151
                 | 
                                    
                                                     | 
                
                 | 
                	 */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    152
                 | 
                                    
                                                     | 
                
                 | 
                	public function stringCompare($str1, $str2)  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    153
                 | 
                                    
                                                     | 
                
                 | 
                	{ | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    154
                 | 
                                    
                                                     | 
                
                 | 
                		// check variable type manually  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    155
                 | 
                                    
                                                     | 
                
                 | 
                		if (!is_string($str1) || !is_string($str2)) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    156
                 | 
                                    
                                                     | 
                
                 | 
                			return false;  | 
            
                            
                    | 
                        
                     | 
                     | 
                     | 
                    
                                                                                                    
                        
                         
                                                                                        
                                                                                     
                     | 
                
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    157
                 | 
                                    
                                                     | 
                
                 | 
                		}  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    158
                 | 
                                    
                                                     | 
                
                 | 
                 | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    159
                 | 
                                    
                                                     | 
                
                 | 
                		return \Sodium\memcmp($str1, $str2) === 0;  | 
            
                            
                    | 
                        
                     | 
                     | 
                     | 
                    
                                                                                                    
                        
                         
                                                                                        
                                                                                     
                     | 
                
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    160
                 | 
                                    
                                                     | 
                
                 | 
                	}  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    161
                 | 
                                    
                                                     | 
                
                 | 
                 | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    162
                 | 
                                    
                                                     | 
                
                 | 
                	/**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    163
                 | 
                                    
                                                     | 
                
                 | 
                	 * Check if implementation supported  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    164
                 | 
                                    
                                                     | 
                
                 | 
                	 * @return bool  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    165
                 | 
                                    
                                                     | 
                
                 | 
                	 */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    166
                 | 
                                    
                                                     | 
                
                 | 
                	public function isSupported() { | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    167
                 | 
                                    
                                                     | 
                
                 | 
                		return true === extension_loaded('libsodium') | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    168
                 | 
                                    
                                                     | 
                
                 | 
                			&& false === method_exists('Sodium', 'sodium_version_string'); | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    169
                 | 
                                    
                                                     | 
                
                 | 
                	}  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    170
                 | 
                                    
                                                     | 
                
                 | 
                 | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    171
                 | 
                                    
                                                     | 
                
                 | 
                	/**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    172
                 | 
                                    
                                                     | 
                
                 | 
                	 * Validate crypt tool  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    173
                 | 
                                    
                                                     | 
                
                 | 
                	 *  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    174
                 | 
                                    
                                                     | 
                
                 | 
                	 * @return bool  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    175
                 | 
                                    
                                                     | 
                
                 | 
                	 * @throws Exception  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    176
                 | 
                                    
                                                     | 
                
                 | 
                	 */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    177
                 | 
                                    
                                                     | 
                
                 | 
                	public function validate() { | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    178
                 | 
                                    
                                                     | 
                
                 | 
                		if(false === $this->isSupported()) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    179
                 | 
                                    
                                                     | 
                
                 | 
                			throw new Exception('Sodium implementation not supported'); | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    180
                 | 
                                    
                                                     | 
                
                 | 
                		}  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    181
                 | 
                                    
                                                     | 
                
                 | 
                		return true;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    182
                 | 
                                    
                                                     | 
                
                 | 
                	}  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    183
                 | 
                                    
                                                     | 
                
                 | 
                 | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    184
                 | 
                                    
                                                     | 
                
                 | 
                	/**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    185
                 | 
                                    
                                                     | 
                
                 | 
                	 * @return string  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    186
                 | 
                                    
                                                     | 
                
                 | 
                	 */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    187
                 | 
                                    
                                                     | 
                
                 | 
                	public function getName() { | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    188
                 | 
                                    
                                                     | 
                
                 | 
                		return 'sodium';  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    189
                 | 
                                    
                                                     | 
                
                 | 
                	}  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    190
                 | 
                                    
                                                     | 
                
                 | 
                 | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    191
                 | 
                                    
                                                     | 
                
                 | 
                	/**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    192
                 | 
                                    
                                                     | 
                
                 | 
                	 * Description of the CryptTool  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    193
                 | 
                                    
                                                     | 
                
                 | 
                	 * @return string  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    194
                 | 
                                    
                                                     | 
                
                 | 
                	 */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    195
                 | 
                                    
                                                     | 
                
                 | 
                	public function getDescription() { | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    196
                 | 
                                    
                                                     | 
                
                 | 
                		/** @noinspection PhpUndefinedNamespaceInspection @noinspection PhpUndefinedFunctionInspection */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    197
                 | 
                                    
                                                     | 
                
                 | 
                		return 'Sodium implementation '.\Sodium\version_string();  | 
            
            
                                                                                                            
                            
            
                                    
            
            
                | 
                    198
                 | 
                                    
                                                     | 
                
                 | 
                	}  | 
            
            
                                                                                                            
                                                                
            
                                    
            
            
                | 
                    199
                 | 
                                    
                                                     | 
                
                 | 
                }  | 
            
            
                                                        
            
                                    
            
            
                | 
                    200
                 | 
                                    
                                                     | 
                
                 | 
                 | 
            
            
                        
If you return a value from a function or method, it should be a sub-type of the type that is given by the parent type f.e. an interface, or abstract method. This is more formally defined by the Lizkov substitution principle, and guarantees that classes that depend on the parent type can use any instance of a child type interchangably. This principle also belongs to the SOLID principles for object oriented design.
Let’s take a look at an example:
Our function
my_functionexpects aPostobject, and outputs the author of the post. The base classPostreturns a simple string and outputting a simple string will work just fine. However, the child classBlogPostwhich is a sub-type ofPostinstead decided to return anobject, and is therefore violating the SOLID principles. If aBlogPostwere passed tomy_function, PHP would not complain, but ultimately fail when executing thestrtouppercall in its body.