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LSB隐写工具对比(Stegsolve与zsteg)

LSB隐写工具对比(Stegsolve与zsteg)

 

起因

很久很久以前,有一道送分题没做出来,后来看writeup,只要zsteg就行了。

命令运行的结果

root@LAPTOP-GE0FGULA:/mnt/d# zsteg 瞅啥.bmp
[?] 2 bytes of extra data after image end (IEND), offset = 0x269b0e
extradata:0         .. ["x00" repeated 2 times]
imagedata           .. text: ["r" repeated 18 times]
b1,lsb,bY           .. <wbStego size=120, ext="x00x8ExEE", data="x1EfxDEx9ExF6xAExFAxCEx86x9E"..., even=false>
b1,msb,bY           .. text: "qwxf{you_say_chick_beautiful?}"
b2,msb,bY           .. text: "i2,C8&k0."
b2,r,lsb,xY         .. text: "UUUUUU9VUUUUUUUUUUUUUUUUUUUUUU"
b2,g,msb,xY         .. text: ["U" repeated 22 times]
b2,b,lsb,xY         .. text: ["U" repeated 10 times]
b3,g,msb,xY         .. text: "V9XDR\d@"
b4,r,lsb,xY         .. file: TIM image, Pixel at (4353,4112) Size=12850x8754
b4,g,lsb,xY         .. text: "3"""""3###33##3#UDUEEEEEDDUETEDEDDUEEDTEEEUT#!"
b4,g,msb,xY         .. text: """""""""""""""""""""DDDDDDDDDDDD""""DDDDDDDDDDDD*LD"
b4,b,lsb,xY         .. text: "gfffffvwgwfgwwfw"

b1,msb,bY读取到的flag,看的一脸懵逼,msb是啥?不是lsb隐写么?bY的b又是啥?我用stegsolve怎么没找到flag?

 

结论

两个工具的一些参数在理解上有点疑问,因此查看了源码。

Stegsolve的Data Extract功能,Bit Order选项MSBFirst和LSBFirst的区别,这个在扫描顺序中说明

zsteg不理解参数更多

-c:rgba的组合理解,r3g2b3则表示r通道的低3bit,g通道2bit,r通道3bit,如果设置为rbg不加数字的,则表示每个通道读取bit数相同,bit数有-b参数设置

-b:设置每个通道读取的bit数,从低位开始,如果不是顺序的低位开始,则可以使用掩码,比如取最低位和最高位,则可以-b 10000001或者-b 0x81

-o:设置行列的读取顺序,xy就是从上到下,从左到右,xy任意有大写的,表示倒序,不过栗子中有个bY令我费解,查看源码知道对于BMP的图片,可以不管通道,直接按字节读取,就是b的意思了,b再顺带表示x,也就是bY的顺序和xY是一样的,Yb和Yx的顺序是一样的,但是b这个的读取模式跟-c bgr -o xY好像是一样的(因为看BMP图片通道排列顺序是BGR),不太理解专门弄个这个出来干嘛。

--msb--lsb这个在组合顺序中说明

扫描顺序

行列顺序

先说下行列的扫描顺序

zsteg可以通过-o选项设置的8种组合(xy,xY,Xy,XY,yx,yX,Yx,YX),个人认为常用的就xy和xY吧

Stegsolve只有选项设置Extract By Row or Column,对应到zsteg的-o选项上就是xy和yx

字节顺序

然后是字节上的扫描顺序,因为是读取的bit再拼接数据的,那么一个字节有8bit数据,从高位开始读还是从低位开始读的顺序

Stegsolve:字节上的读取顺序与Bit Order选项有关,如果设置了MSBFirst,是从高位开始读取,LSBFirst是从低位开始读取

zsteg:只能从高位开始读,比如-b 0x81,在读取不同通道数据时,都是先读取一个字节的高位,再读取该字节的低位。对应到Stegsolve就是MSBFirst的选项。

组合顺序

对于Stegsolve和zsteg,先读取到bit数据都是先拿出来组合的,每8bit组合成一个字节,按照最先存放的Bit在低地址理解的话。

zsteg的--lsb--msb决定了组合顺序

--lsb:大端存放

--msb:小端存放

源码片段,a内存储的是读取的Bit数据,所以msb是低地址的是低位,因此是小端存放。

if a.size >= 8
  byte = 0
  if params[:bit_order] == :msb
    8.times{ |i| byte |= (a.shift<<i)}
  else
    8.times{ |i| byte |= (a.shift<<(7-i))}
  end

Stegsolve则是只有大端存放,即对应zsteg的—lsb,因为代码中有个extractBitPos变量,初始值是128,每组合1bit,就右移一次,到0后循环。
源码片段

    private void addBit(int num)
    {
        if(num!=0)
        {
           extract[extractBytePos]+=extractBitPos;
        }
        extractBitPos>>=1;
        if(extractBitPos>=1)
            return;
        extractBitPos=128;
        extractBytePos++;
        if(extractBytePos<extract.length)
            extract[extractBytePos]=0;
    }

Stegsolve

了解一下Data Extract以及不同通道存储图片的隐写

Data Extract

功能简要说明

面板

LSB隐写工具对比(Stegsolve与zsteg)

配置选项后,是通过Preview按钮进行数据的读取,因此直接跟进该按钮事件。

Bit Planes:选取通道要读取的bit位。

Bit Plane Order:一个像素值包含多个通道,不同通道的读取数据,Alpha一直是最先读的,然后会根据该项的配置决定读取顺序。

Bit Order:读取数据时,每次仅读取1Bit,该项是控制读取一个通道字节数时,读取的方向,MSBFirst表示从高位读取到低位,LSBFirst表示从低位读取到高位。因此只有当通道勾选的Bit个数大于1时,该选项才会影响返回的结果。

代码分析

文件:Extract.java

按钮事件:

    /**
     * Generate the extract and generate the preview
     * @param evt Event
     */
    private void previewButtonActionPerformed(java.awt.event.ActionEvent evt) {//GEN-FIRST:event_previewButtonActionPerformed
        generateExtract();
        generatePreview();
    }//GEN-LAST:event_previewButtonActionPerformed

跟进generateExtract(),存在内部调用,先列举了另外两个方法。

     /**
     * Retrieves the mask from the bits selected on the form
     */
     /*读取Bit Planes的配置,图片getRGB会返回一个整型,如果存在alpha,那么范围最大值就是0xffffffff,从高位至低位,每一个字节按顺序对应为 A R G B,所以getMask就是获取要获取对应Bit的掩码,存为this.mask,this.maskbits记录是全部要读取的Bit数。
     */
    private void getMask()
    {
        mask = 0;
        maskbits = 0;
        if(ab7.isSelected()) { mask += 1<<31; maskbits++;}
        if(ab6.isSelected()) { mask += 1<<30; maskbits++;}
        if(ab5.isSelected()) { mask += 1<<29; maskbits++;}
        if(ab4.isSelected()) { mask += 1<<28; maskbits++;}
        if(ab3.isSelected()) { mask += 1<<27; maskbits++;}
        if(ab2.isSelected()) { mask += 1<<26; maskbits++;}
        if(ab1.isSelected()) { mask += 1<<25; maskbits++;}
        if(ab0.isSelected()) { mask += 1<<24; maskbits++;}
        if(rb7.isSelected()) { mask += 1<<23; maskbits++;}
        if(rb6.isSelected()) { mask += 1<<22; maskbits++;}
        if(rb5.isSelected()) { mask += 1<<21; maskbits++;}
        if(rb4.isSelected()) { mask += 1<<20; maskbits++;}
        if(rb3.isSelected()) { mask += 1<<19; maskbits++;}
        if(rb2.isSelected()) { mask += 1<<18; maskbits++;}
        if(rb1.isSelected()) { mask += 1<<17; maskbits++;}
        if(rb0.isSelected()) { mask += 1<<16; maskbits++;}
        if(gb7.isSelected()) { mask += 1<<15; maskbits++;}
        if(gb6.isSelected()) { mask += 1<<14; maskbits++;}
        if(gb5.isSelected()) { mask += 1<<13; maskbits++;}
        if(gb4.isSelected()) { mask += 1<<12; maskbits++;}
        if(gb3.isSelected()) { mask += 1<<11; maskbits++;}
        if(gb2.isSelected()) { mask += 1<<10; maskbits++;}
        if(gb1.isSelected()) { mask += 1<<9; maskbits++;}
        if(gb0.isSelected()) { mask += 1<<8; maskbits++;}
        if(bb7.isSelected()) { mask += 1<<7; maskbits++;}
        if(bb6.isSelected()) { mask += 1<<6; maskbits++;}
        if(bb5.isSelected()) { mask += 1<<5; maskbits++;}
        if(bb4.isSelected()) { mask += 1<<4; maskbits++;}
        if(bb3.isSelected()) { mask += 1<<3; maskbits++;}
        if(bb2.isSelected()) { mask += 1<<2; maskbits++;}
        if(bb1.isSelected()) { mask += 1<<1; maskbits++;}
        if(bb0.isSelected()) { mask += 1; maskbits++;}
    }
    /**
     * Retrieve the ordering options from the form
     */
     /* 读取Order setting的配置,主要就是rgbOrder的不同值对应的顺序
     */
    private void getBitOrderOptions()
    {
        if(byRowButton.isSelected()) rowFirst = true;
        else rowFirst = false;
        if(LSBButton.isSelected()) lsbFirst = true;
        else lsbFirst = false;
        if(RGBButton.isSelected()) rgbOrder = 1;
        else if (RBGButton.isSelected()) rgbOrder = 2;
        else if (GRBButton.isSelected()) rgbOrder = 3;
        else if (GBRButton.isSelected()) rgbOrder = 4;
        else if (BRGButton.isSelected()) rgbOrder = 5;
        else rgbOrder = 6;
    }
     /**
     * Generates the extract from the selected options
     */
    private void generateExtract()
    {
        getMask();//获取掩码,每个像素值要获取的对应Bit的掩码,以及每个像素值获取Bit的个数。
        getBitOrderOptions();//获取Order settings
        int len = bi.getHeight() * bi.getWidth();//获取总的像素点
        len = len * maskbits; // 总的像素点*每个像素点获取的Bit数=总的Bit数
        len = (len +7)/8; // 总的Bit数转换到总的字节数,+7是没满一个字节的Bit数也对应到一个字节。(极端点比如总的Bit数就1~7Bit,也是要转为1字节,所以需要+7)
        extract = new byte[len];//存储读取到的字节数据
        extractBitPos = 128; // 每8个Bit组成一个字节数据,extractBitPos相当于权值,从128开始,因此读取的每8Bit,先读到的在高位。
        extractBytePos = 0;
        //System.out.println(bi.getHeight()+" "+bi.getWidth()+" "+len+" "+mask);
        // 根据rowFirst参数来选择读取顺序,调用extractBits读取数据
        if(rowFirst)
        {
           for(int j=0;j<bi.getHeight();j++)
              for(int i=0;i<bi.getWidth();i++)
              {
                  //System.out.println(i+" "+j+" "+extractBytePos);
                  extractBits(bi.getRGB(i, j));
              }
        }
        else
        {
           for(int i=0;i<bi.getWidth();i++)
              for(int j=0;j<bi.getHeight();j++)
                 extractBits(bi.getRGB(i, j));
        }
    }

读取数据是extractBits,nextByte是读取到的一个像素点的值,如果是lsbFirst(也就是选了Bitorder为LSBFirst,默认是MSBFirst),则是从低位从高位按顺序读取(每个通道选取2Bit以上才会有影响,如果只读取1Bit则无所谓了)。

栗子:读取alpha通道,lsbFirst,extract8Bits(nextByte,1<<24),掩码是从24位开始,依次左移1位,左移8次;msbFirst,extract8Bits(nextByte,1<<31),掩码是从31位开始,依次右移,右移8次。

    /**
     * Extract bits from the given byte taking account of
     * the options selected
     * @param nextByte the byte to extract bits from
     */
    private void extractBits(int nextByte)
    {
        if(lsbFirst)
        {
            extract8Bits(nextByte,1<<24);
            switch(rgbOrder)
            {
                case 1: //rgb
                    extract8Bits(nextByte,1<<16);
                    extract8Bits(nextByte,1<<8);
                    extract8Bits(nextByte,1);
                    break;
                case 2: //rbg
                    extract8Bits(nextByte,1<<16);
                    extract8Bits(nextByte,1);
                    extract8Bits(nextByte,1<<8);
                    break;
                case 3: //grb
                    extract8Bits(nextByte,1<<8);
                    extract8Bits(nextByte,1<<16);
                    extract8Bits(nextByte,1);
                    break;
                case 4: //gbr
                    extract8Bits(nextByte,1<<8);
                    extract8Bits(nextByte,1);
                    extract8Bits(nextByte,1<<16);
                    break;
                case 5: //brg
                    extract8Bits(nextByte,1);
                    extract8Bits(nextByte,1<<16);
                    extract8Bits(nextByte,1<<8);
                    break;
                case 6: //bgr
                    extract8Bits(nextByte,1);
                    extract8Bits(nextByte,1<<8);
                    extract8Bits(nextByte,1<<16);
                    break;
            }
        }
        else
        {
            extract8Bits(nextByte,1<<31);
            switch(rgbOrder)
            {
                case 1: //rgb
                    extract8Bits(nextByte,1<<23);
                    extract8Bits(nextByte,1<<15);
                    extract8Bits(nextByte,1<<7);
                    break;
                case 2: //rbg
                    extract8Bits(nextByte,1<<23);
                    extract8Bits(nextByte,1<<7);
                    extract8Bits(nextByte,1<<15);
                    break;
                case 3: //grb
                    extract8Bits(nextByte,1<<15);
                    extract8Bits(nextByte,1<<23);
                    extract8Bits(nextByte,1<<7);
                    break;
                case 4: //gbr
                    extract8Bits(nextByte,1<<15);
                    extract8Bits(nextByte,1<<7);
                    extract8Bits(nextByte,1<<23);
                    break;
                case 5: //brg
                    extract8Bits(nextByte,1<<7);
                    extract8Bits(nextByte,1<<23);
                    extract8Bits(nextByte,1<<15);
                    break;
                case 6: //bgr
                    extract8Bits(nextByte,1<<7);
                    extract8Bits(nextByte,1<<15);
                    extract8Bits(nextByte,1<<23);
                    break;
            }
        }
    }

extract8Bits方法,针对每个通道是要单独调用一次的,nextByte是读取的一个像素点的数据,bitMask是对应通道的掩码(根据extractBits方法的说明可知,如果是lsbFirst则是对应通道掩码的最低位,msbFirst则是对应通道掩码的最高位),在extract8Bits方法最后也有根据是lsbFirst的值选择是左移还是右移,循环8次。

bitMask循环,与this.mask与,如果不为0,说明是要读取的bit,此时就将nextByte与bitMask想与,把该bit的值存入extract

    /**
     * Examine 8 bits and check them against the mask to
     * see if any should be extracted
     * @param nextByte The byte to be examined
     * @param bitMask The bitmask to be applied
     */
    private void extract8Bits(int nextByte, int bitMask)
    {
        for(int i=0;i<8;i++)
        {
            if((mask&bitMask)!=0)
            {
                //System.out.println("call "+ mask+" "+bitMask+" "+nextByte);
                addBit(nextByte & bitMask);
            }
            if(lsbFirst)
               bitMask<<=1;
            else
               bitMask>>>=1;
        }
    }

addBit方法,num是读取的像素值与相应bit的掩码相与后的结果,如果不为0,表示那个Bit为1,否则为0,extractBitPos相当于权值,如果为1,就加extractBitPos,然后extractBitPos右移一位,如果为0就不需要加,但每次extractBitPos都是需要右移一位的,如果extractBitPos还是大于1的,说明还没循环过8次,所以就return了,如果不大于1,说明8次了,那么重置extractBitPos为128,extractBytePos+1,新的字节extract[extractBytePos]的初始值为0。

    /**
     * Adds another bit to the extract
     * @param num Non-zero if adding a 1-bit
     */
    private void addBit(int num)
    {
        if(num!=0)
        {
           extract[extractBytePos]+=extractBitPos;
        }
        extractBitPos>>=1;
        if(extractBitPos>=1)
            return;
        extractBitPos=128;
        extractBytePos++;
        if(extractBytePos<extract.length)
            extract[extractBytePos]=0;
    }

不同通道读取图片

功能简要说明

首先生成的图片仅是黑白图片,每个像素点的值根据读取的bit位的值,如果为1设置为白色,如果为0设置为黑色。

代码分析

打开图片后,程序主界面上的<>按钮可以获取不同通道的图片,这里仅讨论Alpha7~0,Red7~0,Green7~0,Blue7~0,也就是每个通道。

StegSolve.java中定位到按钮方法

    private void forwardButtonActionPerformed(ActionEvent evt) {
        if(bi == null) return;
        transform.forward();
        updateImage();
    }
    private void fileOpenActionPerformed(ActionEvent evt) {
        JFileChooser fileChooser = new JFileChooser(System.getProperty("user.dir"));
        FileNameExtensionFilter filter = new FileNameExtensionFilter("Images", "jpg", "jpeg", "gif", "bmp", "png");
        fileChooser.setFileFilter(filter);
        int rVal = fileChooser.showOpenDialog(this);
        System.setProperty("user.dir", fileChooser.getCurrentDirectory().getAbsolutePath());
        if(rVal == JFileChooser.APPROVE_OPTION)
        {
            sfile = fileChooser.getSelectedFile();
            try
            {
                bi = ImageIO.read(sfile);
                transform = new Transform(bi);
                newImage();
            }
            catch (Exception e)
            {
                JOptionPane.showMessageDialog(this, "Failed to load file: " +e.toString());
            }
        }
    }

主要方法定位到了Transform类,打开文件时初始化,参数是图片的数据。

Transform.java

构造函数,originalImage记录原始图片数据,transform是转换后的数据,先初始化为原始图片数据,transNum的值对应不同的操作。

    /*
 * transforms
 * 0 - none
 * 1 - inversion
 * 2-9 - alpha planes
 * 10-17 - r planes
 * 18-25 - g planes
 * 26-33 - b planes
 * 34 full alpha
 * 35 full red
 * 36 full green
 * 37 full blue
 * 38 random color1
 * 39 random color2
 * 40 random color3
 * 41 gray bits
 */
    Transform(BufferedImage bi)
    {
        originalImage = bi;
        transform = originalImage;
        transNum=0;
    }

forward方法,,每次点击一次按钮,为加一次transNum,然后根据transNum的值去执行对应的操作。transNum值对应的操作除了注释中的说明,也可以从getText方法中获取,栗子:Alpha plane 0对应的transNum值为9

    public void forward()
    {
        transNum++;
        if(transNum>MAXTRANS) transNum=0;
        calcTrans();
    }
    public String getText()
    {
        switch(transNum)
        {
            case 0:
              return "Normal Image";
            case 1:
              return "Colour Inversion (Xor)";
            case 2:
            case 3:
            case 4:
            case 5:
            case 6:
            case 7:
            case 8:
            case 9:
              return "Alpha plane " + (9 - transNum);
            case 10:
            case 11:
            case 12:
            case 13:
            case 14:
            case 15:
            case 16:
            case 17:
              return "Red plane " + (17 - transNum);
            case 18:
            case 19:
            case 20:
            case 21:
            case 22:
            case 23:
            case 24:
            case 25:
              return "Green plane " + (25 - transNum);
            case 26:
            case 27:
            case 28:
            case 29:
            case 30:
            case 31:
            case 32:
            case 33:
              return "Blue plane " + (33 - transNum);
            case 34:
              return "Full alpha";
            case 35:
              return "Full red";
            case 36:
              return "Full green";
            case 37:
              return "Full blue";
            case 38:
              return "Random colour map 1";
            case 39:
              return "Random colour map 2";
            case 40:
              return "Random colour map 3";
            case 41:
              return "Gray bits";
            default:
              return "";
        }
    }

calcTrans方法,是一个switch方法,根据transNum的值调用方法,而我关心的不同通道获取的图片都是调用transfrombit方法,这里仅截取关心的

    private void calcTrans()
    {
        switch(transNum)
        {
            case 2:
                transfrombit(31);
                return;
            case 3:
                transfrombit(30);
                return;
            case 4:
                transfrombit(29);
                return;
            case 5:
                transfrombit(28);
                return;
            case 6:
                transfrombit(27);
                return;
            case 7:
                transfrombit(26);
                return;
            case 8:
                transfrombit(25);
                return;
            case 9:
                transfrombit(24);
                return;
            case 10:
                transfrombit(23);
                return;
            case 11:
                transfrombit(22);
                return;
            case 12:
                transfrombit(21);
                return;
            case 13:
                transfrombit(20);
                return;
            case 14:
                transfrombit(19);
                return;
            case 15:
                transfrombit(18);
                return;
            case 16:
                transfrombit(17);
                return;
            case 17:
                transfrombit(16);
                return;
            case 18:
                transfrombit(15);
                return;
            case 19:
                transfrombit(14);
                return;
            case 20:
                transfrombit(13);
                return;
            case 21:
                transfrombit(12);
                return;
            case 22:
                transfrombit(11);
                return;
            case 23:
                transfrombit(10);
                return;
            case 24:
                transfrombit(9);
                return;
            case 25:
                transfrombit(8);
                return;
            case 26:
                transfrombit(7);
                return;
            case 27:
                transfrombit(6);
                return;
            case 28:
                transfrombit(5);
                return;
            case 29:
                transfrombit(4);
                return;
            case 30:
                transfrombit(3);
                return;
            case 31:
                transfrombit(2);
                return;
            case 32:
                transfrombit(1);
                return;
            case 33:
                transfrombit(0);
                return;
            default:
               transform = originalImage;
               return;
        }
    }

transfrombit方法,参数d基本就是读取第dbit的数据,根据之前的说明 Alpha 7是getRGB的数据的最高位,第31bit,根据getText方法可以知道Aplpha 7对应的transNum值为2,再看calcTrans的case2就是调用transfrombit(31)。

    private void transfrombit(int d)
    {
        transform = new BufferedImage(originalImage.getWidth(), originalImage.getHeight(), BufferedImage.TYPE_INT_RGB);
        for(int i=0;i<originalImage.getWidth();i++)
            for(int j=0;j<originalImage.getHeight();j++)
            {
                int col=0;
                int fcol = originalImage.getRGB(i,j);
                if(((fcol>>>d)&1)>0)//右移d个bit位,再取最低位,如果大于0表示对应Bit位为1,那么就设置对应像素值为0xffffff,也就是(255,255,255),对应白色,如果Bit位为0,则是设置为(0,0,0),对应为黑色
                   col=0xffffff;
                transform.setRGB(i, j, col);
             }
    }

 

zsteg

跟进一下代码执行流程,了解各个参数的意义。

入口

程序执行流程的文件

/bin/zsteg

/lib/zsteg.rb run方法

/lib/zsteg/cli/cli.rb run方法,这里会对参数解析,这里截取一些之后需要用到的参数,完整的自行看源码吧,解析完参数后,主要是最后的动态方法调用,@actions=[‘check’],因此动态调用check方法

def run
@actions = []
@options = {

 :verbose => 0,
 :limit => Checker::DEFAULT_LIMIT,
 :order => Checker::DEFAULT_ORDER
}
optparser = OptionParser.new do |opts|

 opts.banner = "Usage: zsteg [options] filename.png [param_string]"
 opts.separator ""

 opts.on("-c", "--channels X", /[rgba,1-8]+/,
         "channels (R/G/B/A) or any combination, comma separated",
         "valid values: r,g,b,a,rg,bgr,rgba,r3g2b3,..."
 ) do |x|
   @options[:channels] = x.split(',')
   # specifying channels on command line disables extra checks
   @options[:extra_checks] = false
 end

 opts.on("-b", "--bits N", "number of bits, single int value or '1,3,5' or range '1-8'",
         "advanced: specify individual bits like '00001110' or '0x88'"
 ) do |x|
   a = []
   x = '1-8' if x == 'all'
   x.split(',').each do |x1|
     if x1['-']
       t = x1.split('-')
       a << Range.new(parse_bits(t[0]), parse_bits(t[1])).to_a
     else
       a << parse_bits(x1)
     end
   end
   @options[:bits] = a.flatten.uniq
   # specifying bits on command line disables extra checks
   @options[:extra_checks] = false
 end

 opts.on "--lsb", "least significant BIT comes first" do
   @options[:bit_order] = :lsb
 end
 opts.on "--msb", "most significant BIT comes first" do
   @options[:bit_order] = :msb
 end

 opts.on("-o", "--order X", /all|auto|[bxy,]+/i,
         "pixel iteration order (default: '#{@options[:order]}')",
         "valid values: ALL,xy,yx,XY,YX,xY,Xy,bY,...",
 ){ |x| @options[:order] = x.split(',') }
  if (argv = optparser.parse(@argv)).empty?
    puts optparser.help
    return
  end

  @actions = DEFAULT_ACTIONS if @actions.empty?

  argv.each do |arg|
    if arg[','] && !File.exist?(arg)
      @options.merge!(decode_param_string(arg))
      argv.delete arg
    end
  end

  argv.each_with_index do |fname,idx|
    if argv.size > 1 && @options[:verbose] >= 0
      puts if idx > 0
      puts "[.] #{fname}".green
    end
    next unless @img=load_image(@fname=fname)

    @actions.each do |action|
      if action.is_a?(Array)
        self.send(*action) if self.respond_to?(action.first)
      else
        self.send(action) if self.respond_to?(action)
      end
    end
  end
rescue Errno::EPIPE
  # output interrupt, f.ex. when piping output to a 'head' command
  # prevents a 'Broken pipe - <STDOUT> (Errno::EPIPE)' message
end

/lib/zsteg/cli/cli.rb check方法

def check Checker.new(@img, @options).check end

/lib/zsteg/checker.rb initialize方法,初始化一些成员变量,@extractor也是传入了图像数据的,通道判断了图片属性是否有alpha通道。

 def initialize image, params = {}
   @params = params
   @cache = {}; @wastitles = Set.new
   @image = image.is_a?(ZPNG::Image) ? image : ZPNG::Image.load(image)
   @extractor = Extractor.new(@image, params)
   @channels = params[:channels] ||
     if @image.alpha_used?
       %w'r g b a rgb bgr rgba abgr'
     else
       %w'r g b rgb bgr'
     end
   @verbose = params[:verbose] || -2
   @file_cmd = FileCmd.new
   @results = []

   @params[:bits]  ||= DEFAULT_BITS
   @params[:order] ||= DEFAULT_ORDER
   @params[:limit] ||= DEFAULT_LIMIT

   if @params[:min_str_len]
     @min_str_len = @min_wholetext_len = @params[:min_str_len]
   else
     @min_str_len = DEFAULT_MIN_STR_LEN
     @min_wholetext_len = @min_str_len - 2
   end
   @strings_re = /[x20-x7ernt]{#@min_str_len,}/

   @extra_checks = params.fetch(:extra_checks, DEFAULT_EXTRA_CHECKS)
 end

/lib/zsteg/checker.rb check方法,截取部分,会判断图片是否是bmp的,只有bmp的-o选项内才有b,如果设置为all也只是多了bY的选项,但是通过之后代码分析是可以by yb Yb的。判断order中是否有b用的是正则,因此大小写一样。接着数据读取就到check_channels方法了。

def check
  @found_anything = false
  @file_cmd.start!

  if @image.format == :bmp
    case params[:order].to_s.downcase
    when /all/
      params[:order] = %w'bY xY xy yx XY YX Xy yX Yx'
    when /auto/
      params[:order] = %w'bY xY'
    end
  else
    case params[:order].to_s.downcase
    when /all/
      params[:order] = %w'xy yx XY YX Xy yX xY Yx'
    when /auto/
      params[:order] = 'xy'
    end
  end

  Array(params[:order]).uniq.each do |order|
    (params[:prime] == :all ? [false,true] : [params[:prime]]).each do |prime|
      Array(params[:bits]).uniq.each do |bits|
        p1 = @params.merge :bits => bits, :order => order, :prime => prime
        if order[/b/i]
          # byte iterator does not need channels
          check_channels nil, p1
        else
          channels.each{ |c| check_channels c, p1 }
        end
      end
    end
  end

  if @found_anything
    print "r" + " "*20 + "r" if @need_cr
  else
    puts "r[=] nothing :(" + " "*20 # line cleanup
  end

  if @extra_checks
    Analyzer.new(@image).analyze!
  end

  # return everything found if this method was called from some code
  @results
ensure
  @file_cmd.stop!
end

/lib/zsteg/checker.rb check_channels方法,首先判断是否设置了bit_order,没设置则两个都测试,之后就是区分两种模式了,channels有值的,最后是去的color_extractor.rb,没有值的去的byte_extractor.rb。

color_extractor模式,还要判断channels指定的模式,是就rgb还是会单独指定每个通道读取多少Bit的。确定过每个像素读取多少bit,然后乘以总的像素点除以8确认读取字节数。

byte)extractor模式,nbits是-b参数指定的读取bit数,乘以一行的字节数,再乘以高/8。

show_title title输出当前模式

data = @extractor.extract p1读取数据

    def check_channels channels, params
      unless params[:bit_order]
        check_channels(channels, params.merge(:bit_order => :lsb))
        check_channels(channels, params.merge(:bit_order => :msb))
        return
      end

      p1 = params.clone

      # number of bits
      # equals to params[:bits] if in range 1..8
      # otherwise equals to number of 1's, like 0b1000_0001
      nbits = p1[:bits] <= 8 ? p1[:bits] : (p1[:bits]&0xff).to_s(2).count("1")

      show_bits = true
      # channels is a String
      if channels
        p1[:channels] =
          if channels[1] && channels[1] =~ /AdZ/
            # 'r3g2b3'
            a=[]
            cbits = 0
            (channels.size/2).times do |i|
              a << (t=channels[i*2,2])
              cbits += t[1].to_i
            end
            show_bits = false
            @max_hidden_size = cbits * @image.width
            a
          else
            # 'rgb'
            a = channels.chars.to_a
            @max_hidden_size = a.size * @image.width * nbits
            a
          end
        # p1[:channels] is an Array
      elsif params[:order] =~ /b/i
        # byte extractor
        @max_hidden_size = @image.scanlines[0].decoded_bytes.size * nbits
      else
        raise "invalid params #{params.inspect}"
      end
      @max_hidden_size *= @image.height/8

      bits_tag =
        if show_bits
          if params[:bits] > 0x100
            if params[:bits].to_s(2) =~ /(1{1,8})$/
              # mask => number of bits
              "b#{$1.size}"
            else
              # mask
              "b#{(params[:bits]&0xff).to_s(2)}"
            end
          else
            # number of bits
            "b#{params[:bits]}"
          end
        end

      title = [
        bits_tag,
        channels,
        params[:bit_order],
        params[:order],
        params[:prime] ? 'prime' : nil
      ].compact.join(',')

      return if @wastitles.include?(title)
      @wastitles << title

      show_title title

      p1[:title] = title
      data = @extractor.extract p1

      if p1[:invert]
        data.size.times{ |i| data.setbyte(i, data.getbyte(i)^0xff) }
      end

      @need_cr = !process_result(data, p1) # carriage return needed?
      @found_anything ||= !@need_cr
    end

/lib/zsteg/extractor.rb 根据-o选项中是否包含b选择不同模式

 def extract params = {}
   @limit = params[:limit].to_i
   @limit = 2**32 if @limit <= 0

   if params[:order] =~ /b/i
     byte_extract params
   else
     color_extract params
   end
 end

在分类说明两个模式的时候,先将一个方法拿出来做个说明,bit_indexes

bit_indexes

通过代码可以知道,在扫描一个字节的时候,zsteg是固定的从高位扫描至低位的

 def bit_indexes bits
   if (1..8).include?(bits)
     # number of bits
     # 1 => [0]
     # ...
     # 8 => [7,6,5,4,3,2,1,0]
     bits.times.to_a.reverse
   else
     # mask
     mask = bits & 0xff
     r = []
     8.times do |i|
       r << i if mask[i] == 1
     end
     r.reverse
   end
 end

byte_extract

/lib/zsteg/extractor/byte_extractor.rb data列表是用于存储字节数据,a是用于存储bit数据。

通过byte_iterator方法遍历每个字节,会根据order参数是否有小写b,决定x方向的正序还是倒序,是否有小写y决定y方向的正序还是倒序。

根据x,y的值读取到对应字节,然后根据bit_indexes获取的bidx(注定只能高位至低位)去读取对应Bit值

当a.size为8时,就会组成一个字节,根据bit_order的值决定a中的8bit数据是大端还是小端

msb是小端,lsb是大端。

module ZSteg
class Extractor
 # ByteExtractor extracts bits from each scanline bytes
 # actual for BMP+wbStego combination
 module ByteExtractor

   def byte_extract params = {}
     bidxs = bit_indexes params[:bits]

     if params[:prime]
       pregenerate_primes(
         :max   => @image.scanlines[0].size * @image.height,
         :count => (@limit*8.0/bidxs.size).ceil
       )
     end

     data = ''.force_encoding('binary')
     a = [0]*params[:shift].to_i        # prepend :shift zero bits
     byte_iterator(params) do |x,y|
       sl = @image.scanlines[y]

       value = sl.decoded_bytes.getbyte(x)
       bidxs.each do |bidx|
         a << value[bidx]
       end

       if a.size >= 8
         byte = 0
         if params[:bit_order] == :msb
           8.times{ |i| byte |= (a.shift<<i)}
         else
           8.times{ |i| byte |= (a.shift<<(7-i))}
         end
         #printf "[d] %02x %08bn", byte, byte
         data << byte.chr
         if data.size >= @limit
           print "[limit #@limit]".gray if @verbose > 1
           break
         end
       end
     end
     if params[:strip_tail_zeroes] != false && data[-1,1] == "x00"
       oldsz = data.size
       data.sub!(/x00+Z/,'')
       print "[zerotail #{oldsz-data.size}]".gray if @verbose > 1
     end
     data
   end

   # 'xy': b=0,y=0; b=1,y=0; b=2,y=0; ...
   # 'yx': b=0,y=0; b=0,y=1; b=0,y=2; ...
   # ...
   # 'xY': b=0,  y=MAX; b=1,    y=MAX; b=2,    y=MAX; ...
   # 'XY': b=MAX,y=MAX; b=MAX-1,y=MAX; b=MAX-2,y=MAX; ...
   def byte_iterator params
     type = params[:order]
     if type.nil? || type == 'auto'
       type = @image.format == :bmp ? 'bY' : 'by'
     end
     raise "invalid iterator type #{type}" unless type =~ /A(by|yb)Z/i

     sl0 = @image.scanlines.first

     # XXX don't try to run it on interlaced PNGs!
     x0,x1,xstep =
       if type.index('b')
         [0, sl0.decoded_bytes.size-1, 1]
       else
         [sl0.decoded_bytes.size-1, 0, -1]
       end

     y0,y1,ystep =
       if type.index('y')
         [0, @image.height-1, 1]
       else
         [@image.height-1, 0, -1]
       end

     # cannot join these lines from ByteExtractor and ColorExtractor into
     # one method for performance reason:
     #   it will require additional yield() for EACH BYTE iterated

     if type[0,1].downcase == 'b'
       # ROW iterator
       if params[:prime]
         idx = 0
         y0.step(y1,ystep){ |y| x0.step(x1,xstep){ |x|
           yield(x,y) if @primes.include?(idx)
           idx += 1
         }}
       else
         y0.step(y1,ystep){ |y| x0.step(x1,xstep){ |x| yield(x,y) }}
       end
     else
       # COLUMN iterator
       if params[:prime]
         idx = 0
         x0.step(x1,xstep){ |x| y0.step(y1,ystep){ |y|
           yield(x,y) if @primes.include?(idx)
           idx += 1
         }}
       else
         x0.step(x1,xstep){ |x| y0.step(y1,ystep){ |y| yield(x,y) }}
       end
     end
   end
 end
end
end

color_extractor

/lib/zsteg/extractor/color_extractor.rb data列表是用于存储字节数据,a是用于存储bit数据。

通过coord_iterator方法遍历每个字节,会根据order参数是否有小写x,决定x方向的正序还是倒序,是否有小写y决定y方向的正序还是倒序。

根据x,y的值读取到对应字节,然后根据bit_indexes获取的ch_masks(注定只能高位至低位)去读取对应Bit值,只是还要根据channel的值,如果是单个字符,表示读取的bit数是通过-b设置的,因此传入params[:bits],否则就是2个字符,读取第2个字符表示读取的bit数。

当a.size为8时,就会组成一个字节,根据bit_order的值决定a中的8bit数据是大端还是小端 msb是小端,lsb是大端。

module ZSteg
  class Extractor
    # ColorExtractor extracts bits from each pixel's color
    module ColorExtractor

      def color_extract params = {}
        channels = Array(params[:channels])
        #pixel_align = params[:pixel_align]

        ch_masks = []
        case channels.first.size
        when 1
          # ['r', 'g', 'b']
          channels.each{ |c| ch_masks << [c[0], bit_indexes(params[:bits])] }
        when 2
          # ['r3', 'g2', 'b3']
          channels.each{ |c| ch_masks << [c[0], bit_indexes(c[1].to_i)] }
        else
          raise "invalid channels: #{channels.inspect}" if channels.size != 1
          t = channels.first
          if t =~ /A[rgba]+Z/
            return color_extract(params.merge(:channels => t.split('')))
          end
          raise "invalid channels: #{channels.inspect}"
        end

        # total number of bits = sum of all channels bits
        nbits = ch_masks.map{ |x| x[1].size }.inject(&:+)

        if params[:prime]
          pregenerate_primes(
            :max   => @image.width * @image.height,
            :count => (@limit*8.0/nbits/channels.size).ceil
          )
        end

        data = ''.force_encoding('binary')
        a = [0]*params[:shift].to_i        # prepend :shift zero bits
        catch :limit do
          coord_iterator(params) do |x,y|
            color = @image[x,y]

            ch_masks.each do |c,bidxs|
              value = color.send(c)
              bidxs.each do |bidx|
                a << value[bidx]
              end
            end
            #p [x,y,a.size,a]

            while a.size >= 8
              byte = 0
              #puts a.join
              if params[:bit_order] == :msb
                8.times{ |i| byte |= (a.shift<<i)}
              else
                8.times{ |i| byte |= (a.shift<<(7-i))}
              end
              #printf "[d] %02x %08bn", byte, byte
              data << byte.chr
              if data.size >= @limit
                print "[limit #@limit]".gray if @verbose > 1
                throw :limit
              end
              #a.clear if pixel_align
            end
          end
        end
        if params[:strip_tail_zeroes] != false && data[-1,1] == "x00"
          oldsz = data.size
          data.sub!(/x00+Z/,'')
          print "[zerotail #{oldsz-data.size}]".gray if @verbose > 1
        end
        data
      end

      # 'xy': x=0,y=0; x=1,y=0; x=2,y=0; ...
      # 'yx': x=0,y=0; x=0,y=1; x=0,y=2; ...
      # ...
      # 'xY': x=0,  y=MAX; x=1,    y=MAX; x=2,    y=MAX; ...
      # 'XY': x=MAX,y=MAX; x=MAX-1,y=MAX; x=MAX-2,y=MAX; ...
      def coord_iterator params
        type = params[:order]
        if type.nil? || type == 'auto'
          type = @image.format == :bmp ? 'xY' : 'xy'
        end
        raise "invalid iterator type #{type}" unless type =~ /A(xy|yx)Z/i

        x0,x1,xstep =
          if type.index('x')
            [0, @image.width-1, 1]
          else
            [@image.width-1, 0, -1]
          end

        y0,y1,ystep =
          if type.index('y')
            [0, @image.height-1, 1]
          else
            [@image.height-1, 0, -1]
          end

        # cannot join these lines from ByteExtractor and ColorExtractor into
        # one method for performance reason:
        #   it will require additional yield() for EACH BYTE iterated

        if type[0,1].downcase == 'x'
          # ROW iterator
          if params[:prime]
            idx = 0
            y0.step(y1,ystep){ |y| x0.step(x1,xstep){ |x|
              yield(x,y) if @primes.include?(idx)
              idx += 1
            }}
          else
            y0.step(y1,ystep){ |y| x0.step(x1,xstep){ |x| yield(x,y) }}
          end
        else
          # COLUMN iterator
          if params[:prime]
            idx = 0
            x0.step(x1,xstep){ |x| y0.step(y1,ystep){ |y|
              yield(x,y) if @primes.include?(idx)
              idx += 1
            }}
          else
            x0.step(x1,xstep){ |x| y0.step(y1,ystep){ |y| yield(x,y) }}
          end
        end
      end
    end
  end
end

 

结果

起因里的zsteg的参数现在都解释过了,而用stegsolve没有看到flag是因为8bit数据是按照大端模式组成的字节,而flag是需要以小端模式组成,所以当我选择stegsolve来做题时,注定是拿不到flag了,都是时辰的错。

然后bY其实和xY的结果是一样的,只是要确定通道的排列方式,bmp按顺序存的通道顺序是bgr。

root@LAPTOP-GE0FGULA:/mnt/d# zsteg -c bgr -o xY --msb -b1 瞅啥.bmp
[?] 2 bytes of extra data after image end (IEND), offset = 0x269b0e
b1,bgr,msb,xY       .. text: "qwxf{you_say_chick_beautiful?}"

再来看下stegsolve,首先知道-o是Y,因此图片需要倒一下,所以手动修改bmp的高度为原值的负值,图片就倒过来了。

LSB隐写工具对比(Stegsolve与zsteg)

选中的序列和flag的值,生成二进制序列对比一下,应是每8个bit都是倒序的。

#encoding:utf-8
from binascii import b2a_hex,a2b_hex

flag = "qwxf{you_say_chick_beautiful?}"
stegsolve = "8eee1e66de9ef6aeface869efac61696c6d6fa46a686ae2e9666ae36fcbe"
flag = bin(int(b2a_hex(flag),16))[2:]
stegsolve = bin(int(stegsolve,16))[2:]

def show(a,b):
    if len(a) % 2 != 0:
        a = '0'+a
    if len(b) % 2 != 0:
        b = '0'+b
    for i in xrange(0,len(a),8):
        print a[i:i+8]+"  "+b[i:i+8]

show(flag,stegsolve)

自己看下结果吧。

原文链接:https://www.anquanke.com/post/id/189154

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