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Read part 1: RC5 block cipher implement in PHP and Javascript

Application of this block cipher method for stream cipher:
There are many methods, but we can provide the method below:
Split your string into many block with 64 bits (8 characters), then use this block cipher method. We can use the method Null-padding for the case the string length is odd.
The key must have 16 characters (128 bits), so, we must use one Key-scheduling algorithm (KSA) to convert the key to the new key 128 bits.

PHP:

    private static function _nullpadding($str,$length=16){
        if(strlen($str)%$length != 0){
            $str .= str_repeat(chr(0),$length-strlen($str)%$length);
        }
        return $str;
    }
 
    private static function key($key){
        for ($s = array(), $i = 0; $i < 256; $i++)
            $s[$i] = $i;
        for ($j = 0, $i = 0; $i < 256; $i++)
        {
            $j = ($j + $s[$i] + ord($key[$i % strlen($key)])) % 256;
            $x = $s[$i];
            $s[$i] = $s[$j];
            $s[$j] = $x;
        }
         
        return $s;
    }
    
    // convert 32bits interger to 4characters string
    private static function _32bits2str($block){
        return pack('N',$block & 0xffffffff);
    }
    // convert 4characters string to 32bits interger
    private static function _str232bits($block){
        return unpack('N*',$block)[1];
    }
    
    //Stream cipher encrypt use RC5
    public static function RC5enc($str,$key,$mode='ECB'){
        $str = self::_nullpadding($str, 8);//var_dump($str);die();
        $enc = '';
        $k = self::key($key); //var_dump($k);die();
        self::rc5_init($k);
        for($i=0;$i<strlen($str)/8;$i++){
            $block = substr($str,$i*8,8);
            $chr = array(self::_str232bits(substr($block,0,4)),self::_str232bits(substr($block,4,4)));  
            $e = self::rc5_encrypt($chr);
            $enc .= self::_32bits2str($e[0]).self::_32bits2str($e[1]);
        }
        return $enc;
    }
     
    //Stream cipher decrypt use RC5
    public static function RC5dec($str,$key,$mode='ECB'){
        $enc = '';
        $k = self::key($key); 
        self::rc5_init($k); 
        for($i=0;$i<strlen($str)/8;$i++){
            $block = substr($str,$i*8,8);
            $chr = array(self::_str232bits(substr($block,0,4)),self::_str232bits(substr($block,4,4))); 
            $e = self::rc5_decrypt($chr);
            $enc .= self::_32bits2str($e[0]).self::_32bits2str($e[1]);
        }
        return rtrim($enc,chr(0));
    }

Here, we use ECB mode, you can use CBC, PCBC, CFB, OFB, CTR mode.

Javascript:

Example: