AlexCTF 2017 - CR4 Poor RSACategory: writeups
Tags: alexctf-2017 crypto
CR4 Poor RSA
Crypto - 200 Points
This time Fady decided to go for modern cryptography implementations, He is fascinated with choosing his own prime numbers, so he picked up RSA once more. Yet he was unlucky again!
-----BEGIN PUBLIC KEY----- ME0wDQYJKoZIhvcNAQEBBQADPAAwOQIyUqmeJJ7nzzwMv5Y6AJZhdyvJzfbh4/v8 bkSgel4PiURXqfgcOuEyrFaD01soulwyQkMCAwEAAQ== -----END PUBLIC KEY-----
Just like CR3, this challenge was based on RSA.
Looking with OpenSSL at the public key we can see the public exponent
e and the modulus
openssl rsa -pubin -inform PEM -text -noout < key.pub Public-Key: (399 bit) Modulus: 52:a9:9e:24:9e:e7:cf:3c:0c:bf:96:3a:00:96:61: 77:2b:c9:cd:f6:e1:e3:fb:fc:6e:44:a0:7a:5e:0f: 89:44:57:a9:f8:1c:3a:e1:32:ac:56:83:d3:5b:28: ba:5c:32:42:43 Exponent: 65537 (0x10001)
n wasn’t very big so we searched on factordb.
Bingo. factordb entry for our modulus.
At this moment, some other teams already solved this challenge before us, so thank you if you posted the factored modulus on factordb!
The same magic as with CR3
The flag is