ASIS CTF Quals 2018: Crypto Write-ups

Write-ups of 2 Crypto challenges from ASIS CTF Quals 2018- Uncle Sam(123 points) and OPEC (207 points)


Pragyan CTF- RSA’s Quest

Points: 200 Description: Rivest comes up with an encryption, and Shamir creates a service for decrypting any cipher text encrypted using Rivests’s encryption. Adleman is asked to decrypt a specific ciphertext, but he is not able to do so directly through Shamir’s service. Help him out. This CTF had some okay-ish crypto challenges(didn't like the first... Continue Reading →

CNVService- AceBear CTF 2018 Writeup

This challenge was by far the most interesting and tricky Bit Flipping Attack I had come across, and I couldn't resist but share the write-up of this challenge. The challenge is one of the best illustrations of Bit Flipping Attack on Chained Block Cipher modes so, it is highly recommended for people who want to get... Continue Reading →

DLP and Baby Step Giant Step Algorithm

Topics covered: Cyclic Groups Discrete Logarithm Problem Applications Attacks on DLP Brute Force Attack on DLP Baby Step Giant Step Algorithm CTF Challenges based on DLP Prerequisites: Mathematical Structures Cyclic Groups Abelian Groups in which all the elements can be produced using a single element in the group. This element is known as a generator... Continue Reading →

Finite Fields- Number Theory

This blog post covers one of the most important Mathematical Structures for Cryptography- Fields. It is used in both Symmetric and Asymmetric key Cryptography. This blog post gives a basic introduction to Finite Fields and arithmetic operations on it, but I hope the purpose of it is served- to make people, who don't have basic knowledge... Continue Reading →

ASIS Finals CTF 2017- Gracias Writeup

  Points: 287    Solves: 9    Category: Crypto Description: Some people think that combination of cryptographic systems will definitely improve the security. That's your turn to prove them wrong. This challenge is a bit simpler than it looks like. It is a multi-prime RSA challenge where we are given an encryption script, which has two functions, one... Continue Reading →

Blinding Attack on RSA Digital Signatures

This blog primarily focuses on Blinding Attack- an elementary vulnerability in RSA cryptosystem used to forge Digital Signatures. The working and properties of Digital Signatures will be described before directly jumping onto the attack. In the end, we discuss ways to prevent this attack. Digital Signature using RSA RSA is a kind of "Trapdoor One-way... Continue Reading →

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