● 400 Points 
● crypto
● By: JP Smith

nc challenge.uiuc.tf 11345

throwback to when the aztecs sacked mitlan


Taking a look at the provided python, it looks like we somehow need to forge a signature for the message ‘right below’. Luckily for me, some of the T&C helped me immediately recognize the specific vulnerability they’re going for. Specifically:

    “we must use the latest versions of all libraries”,
    “we must use 2048-bit keys with e = 3”, and
    “DATED: 2015-07-29”

tell me that the intended solution uses Bleichenbacher’s signature forgery on e=3 and PKCS#1 v1.5, which python-rsa was vulnerable to until early 2016. I was at 33c3 a few months ago where one of my crypto role models, Filippo Valsorda, gave a session on implementing this specific attack against python-rsa. I didn’t find out about the session until after it happened, but I did end up reading his excellent article on the attack, which includes an example implementation.

Using Filippo’s example, we just have to change the target message to ‘right below’ and we have a fully functional forgery generator. Using it is straightforward:

I have no idea what “aztecs sacked mitlan” refers to, though. A really obtuse hint towards ASN.1?

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