Midnight Sun CTF 2021 Quals ------ Backup-frank

题目给了一个pdf文档

 

 

这应该是一个PEM文本的截图，从第一行可以看出这是rsa私钥，有了私钥就好说了，直接解密就可以

但是只有部分数据可见，现在的任务应当就是恢复完整的PEM文本

 

先从图片里提取文字：

把pdf转化为图片，使用imagemagick，采用这条命令安装：sudo apt-get install imagemagick

再用tesseract-ocr提取图片中的文字，采用这条命令可以安装该软件：sudo apt-get install tesseract-ocr

convert  -density 400 ./RSA_key.pdf ./abcd.png
tesseract abcd.png abc

如果执行convert命令报错

convert: not authorized `./RSA_key.pdf' @ error/constitute.c/ReadImage/412.
convert: no images defined `./abcd.png' @ error/convert.c/ConvertImageCommand/3210.

可以尝试把/etc/ImageMagick-6/policy.xml里的 <policy domain="coder" rights="none" pattern="PDF" />

改为<policy domain="coder" rights="read|write" pattern="PDF" />，

 

我们看下生成的abc.txt的内容：

----- BEGIN RSA PRIVATE KEY—————

’C n}: \l‘J-IIJHMJ rC 'Irll(.u11CI:n-Q.-'1m‘-PHUDY

Redacted for security reasons

YZE7xr0bE94J04cqritOcE+dJ4W0mf4HumhaSE1ywcp9xN8xBucN5an1Xt8MEbj
me?udUNRvTDYHdFkv26P1K4XhesBduRpQBES/TxN4YD42td2P8PCShanOSJLWuY
chYOagwbEHBZLQLXIJne5+GUcle?X?OENRTxBHpYJgZSm/CDeUSBKHUVIyeGHK
qJGjzIAO3KvaOzjBYWi/MGKCBIeeonBURKR70P7ijGRFZDymef8r23?4uISy
RHLKFOfngnU/j8L?DWm5SGOCJOZrlkavxPAUZzLOKCAQEAwM/HdumsrXfiOfiCSA
tYLA/+JquxLxGW4nL88dgL61RVWP24PZzWPNwafWohay562+cchFMGrlquoH
Dh7A4X45WB+MBdeYTSOVzFslrlbjoPprnsLleAkCSzloOFUkmvszDfiDaLXGi
OthqsichyPGEyVHGRYv2L3UPYAhdmeYbsbcfiRuhva9tVUUMc+nFny51054vC8M
YTnyYZJqO/SOARYNtBBGBleTQowlePnthgtSka/RK+gBnKthmihi/JyngNg
T/p/FleaXfisjnTprfoXXQ]2ExC3qu/lynFltpLBpZANQ/rbc2N2NtOIBW4oJ
X2krUOKCAOBRprTEWofHJZk?A6eaL?AkthJquRlyor?Yd1wOQdaiqu020AbW
SbnblesiGGme2SIUan214ULC8?srWNBdel/WOXaZiszjrctwaZmVUQBlOu
pDer4ButBBIOthEan+pMiC93anrG/EBUBLyFZKX/CBWIBa/wa/mmCBIugtb
PCZNQIrNeaHMdoPeercjSZqXXKYIOSSXUGpMSGjEUaHiTqSZaSQJRZEsSQBOceV
squUEDPbeEhpGIUVVHarDBk2CCGiL+v0508200jmGbe1/posPNfDH1YHuthA
rVoRpE14r1M+FIXOGSGSOf12RHVBBdfFAoIBAOCUFi7Lv3+dHMJj0h4HyL23rL6y
yZXEgEjMd1EXODPQlaJSnSbSQGS+/R04051+rw5ixrquXRngtMCDtJINQuY+MiU
XFEsLtt9N4XAYOWNm/IaUGsuICBdYthBDDQ?M+FZqqulIlplFbBNpKhatfY7Yg
t226w7?B+0M18T6rIpeLOLxRSpEJmCCfojMSgVRGMFDTIHCQHpIXLJpePhlOyWOF
ywaauJBnYESIZZAZZCdefrY/unZXeWRKSZFN4d+jSmengCXwKQmXuq/pXjIaB
XqIBB+n35wDFGOX732iPBGugiDbhsQfivdoRBAtqumCTLBquXhrnuUSMMUS
————— END RSA PRIVATE KEY-----

 

不是很理想，可能因为我参数设置的有问题

还是用photoshop分离图片，再tesseract提取文字，从图片的大小和行间距离，可以估测出整个文本在50行左右，那应该就是rsa 4096位的加密，我们把隐藏的文本用0代替

----- BEGIN RSA PRIVATE KEY-----
0000000000000000000000000000000000000000000000000000000000000000
0000000000000000000000000000000000000000000000000000000000000000
0000000000000000000000000000000000000000000000000000000000000000
0000000000000000000000000000000000000000000000000000000000000000
0000000000000000000000000000000000000000000000000000000000000000
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YZE7xr0bE94J04cqr1tQCE+dJ4w0mf4vahaSE1ywcp9xN8xBucNSan1Xt8MEbj
me7udUNRvTDYHdFkv26P1K4Xhes8duRpOBES/TxN4YD42td2P8PCSthv05JLWuY
chYOa9wbEH82L91x1JneB+0Vcle7X7OENRTxBHpYJgZBm/cDeUsBKHleyeGHK
qJGjzIAO3KvaOzjGYWi/MGKCBleeonOURKR7oP7ijORFZDymef8r2374uISy
RHLKFOfWQTnO/j8L7DWm55dOcJOZr1KbDvxPAuZZLQKCAOEAwM/Hdvm8rX6Q6C8A
tYLA/+JstxLxGWAnLSBdg161RVWPz4PZzWPNwafW0hay562+cCTXFMOrlquoH
Dh7A4X45W0+MBJdeTSonFslrlbjoPprnsLleAkCSzloOFUkmvszD6DaLXOi
OthqsichyPGEyVHORYvZL3UPYAhdmeYbsbc6Ruhva9tVUUMc+nFny51054vC8M
YTnyYZJqo/SoAR7wt6806ZlsTOowlePn1hYgtSka/RK+anKyFhmihi/JyOgJNg
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Sbnb18msiGGmeZSIUan214ULCB7srWNBdel/WGXaZiszjrctwaZmVU0310u
pDer4ButBBIOt95Ean+pM1C93anrG/E8uBLyF2KX/c8WIBa/WpE/mmCSlvgtb
PCZNQLrNeaHMdOPeercj52qxxKYIoSSXUGpMSGjEUaHiTqSZaSQJR2655080ceV
squUEDPbeEhpOIUVVHarDBkZCCOiL+v0508200jmObel/posPNfDHlYHuthA
rVoRpE14r1M+FIXOOSOSOf12RHVBBdfFAoIBAOCUFi7Lv3+dHMJj0h4HyL23rL6y
yZXE9E]Md1ExoDP91aJSnSbSQGS+/R040si+rwSixrquXRngtMCDtJ1N0uY+MiU
XFEsLtt9N4XAYOWNm/1aUGsuICBdYthSDD97M+FZquqII1p1Fb3NpKhath7Yg
t226w77B+0M18T6rIpeLOLxRSpEJmCCfojM89VRGMFb7IHc9leXIJpePthyWOF
ywaauJBnYEsIzZAZZch4frY/uaneWRK82rN4d+JSmef9NCXwK9mXuq/pXjlaB
XqI86+n35wDFGOX7321PBGugiDbhsOGvdoRBAtqumCTLBquXhranSMMUS
----- END RSA PRIVATE KEY-----

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