PicoCTF - john_pollard

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PicoCTF - john_pollard

tags: PicoCTF CTF Crypto

Background

openssl - rsa

Source code

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-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----

Recon

  • Hint 1: The flag is in the format picoCTF{p,q}
  • Hint 2: Try swapping p and q if it does not work 這一題就只是把certificate解出來，發現n超小，所以就直接

Exploit - openssl

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$ openssl openssl x509 -in cert -pubkey -noout > public.pem
-----BEGIN PUBLIC KEY-----
MCIwDQYJKoZIhvcNAQEBBQADEQAwDgIHEaTUUhKxfwIDAQAB
-----END PUBLIC KEY-----
$ openssl rsa -pubin -in public.pem -text
RSA Public-Key: (53 bit)
Modulus: 4966306421059967 (0x11a4d45212b17f)
Exponent: 65537 (0x10001)
writing RSA key
-----BEGIN PUBLIC KEY-----
MCIwDQYJKoZIhvcNAQEBBQADEQAwDgIHEaTUUhKxfwIDAQAB
-----END PUBLIC KEY-----

Use online tool to factor p = 67867967 q = 73176001

Flag: picoCTF{73176001,67867967}

Reference