題目連結

https://learn.cylabacademy.org/library/460

題目解法

  1. 下載檔案後發現是 .png 檔，使用 strings 指令得到下面這段文字，是一首藏頭詩，內容是 CHECKLSB。
    LSB 指的是 Least Significant Bit，也就是將資料藏在每個像素中最不明顯的一位元，用肉眼無法察覺。

    webshell:~$ strings red.png 
IHDR
tEXtPoem
Crimson heart, vibrant and bold,
Hearts flutter at your sight.
Evenings glow softly red,
Cherries burst with sweet life.
Kisses linger with your warmth.
Love deep as merlot.
Scarlet leaves falling softly,
Bold in every stroke.x
IDATx
IEND

  2. 要找到使用 LSB 隱藏的資料，可以使用 zsteg 這個工具。

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    webshell:~$ zsteg red.png 
meta Poem           .. text: "Crimson heart, vibrant and bold,\nHearts flutter at your sight.\nEvenings glow softly red,\nCherries burst with sweet life.\nKisses linger with your warmth.\nLove deep as merlot.\nScarlet leaves falling softly,\nBold in every stroke."
b1,rgba,lsb,xy      .. text: "cGljb0NURntyM2RfMXNfdGgzX3VsdDFtNHQzX2N1cjNfZjByXzU0ZG4zNTVffQ==cGljb0NURntyM2RfMXNfdGgzX3VsdDFtNHQzX2N1cjNfZjByXzU0ZG4zNTVffQ==cGljb0NURntyM2RfMXNfdGgzX3VsdDFtNHQzX2N1cjNfZjByXzU0ZG4zNTVffQ==cGljb0NURntyM2RfMXNfdGgzX3VsdDFtNHQzX2N1cjNfZjByXzU0ZG4zNTVffQ=="
b1,rgba,msb,xy      .. file: OpenPGP Public Key
b2,g,lsb,xy         .. text: "ET@UETPETUUT@TUUTD@PDUDDDPE"
b2,rgb,lsb,xy       .. file: OpenPGP Secret Key
b2,bgr,msb,xy       .. file: OpenPGP Public Key
b2,rgba,lsb,xy      .. file: OpenPGP Secret Key
b2,rgba,msb,xy      .. text: "CIkiiiII"
b2,abgr,lsb,xy      .. file: OpenPGP Secret Key
b2,abgr,msb,xy      .. text: "iiiaakikk"
b3,rgba,msb,xy      .. text: "#wb#wp#7p"
b3,abgr,msb,xy      .. text: "7r'wb#7p"
b4,b,lsb,xy         .. file: 0421 Alliant compact executable not stripped

  3. 可以發現輸出結果中有一段亂碼明顯是使用 base64 編碼後的文字，將其解碼即可得到 flag。

    cGljb0NURntyM2RfMXNfdGgzX3VsdDFtNHQzX2N1cjNfZjByXzU0ZG4zNTVffQ==
picoCTF{r3d-------------------------------55_}