Full solution of EZDSA challenge from MidnightSun CTF Quals 2019.

tl;dr retrieving key using Euler’s Criterion to break signature authentication

tl;dr solving RSA Digital Signature using it’s homomorphic property:

1. Calculate the signature of factors of message M to be signed, separately
2. Combine them by multiplication over modulus to get the signature of M

tl;dr Coppersmith’s Attack to recover RSA primes

tl;dr

1. Find roots in a polynomial ring over $ mod;p $
2. Use Hensel’s Lifting lemma to find roots over $ mod;p^k $

tl;dr Known plaintext attack on a multithreaded AES-CTR

tl;dr

1. Extract data from given images using binwalk
2. Tranform given diophantine equation into a cubic curve and retrieve EC parameters
3. Solve ECDLP given in extracted data using Pohlig Hellman Algorithm

tl;dr

1. CTR Bit Flipping
2. Break GHASH to get authentication key H (unintended approach)
3. Bypass authentication

Writeup from Junior 35c3CTF’18

Junior 35c3CTF 2018: pretty-linear

Category: Crypto
Challenge Points: 158
Solves: 28

Description:

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The NSA gave us these packets, they said it should be just enough to break this crypto.

surveillance.pcap server.py

Difficulty estimate: Medium-Hard