Google CTF 2023

URL: https://capturetheflag.withgoogle.com/

Team: Kalmarunionen

Place: 4 (of 676)

Crypto: myTLS

I implemented my own mTLS. It supports forward secrecy and it should be faster than yours!

Idea:

We connect using guest credentials
Then we do dir traversal to ../../app/server-ecdhkey.pem
We fill the hmac buffer with known bytes (besides 1 byte) and bruteforce the hash recurrently
Using this we steal the server private key
The mTLS logic checks that the CA has signed client-cert (true)
Ephemeral keys are exchanged
A shared secret is generated using ECDH between server_key (which we stole) and client_cert (which we are impersonating)
A shared key is derived using above shared secret and ephemeral values which we know

Challenge files:

Solve script:

Thanks to Polly and the rest of the Kalmarunionen team for working together and solving this and a ton of challenges in Google CTF!

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