All posts by Alkanor

[Sharif University CTF 2016] [Crypto 100 – Universal re-encryption] Write Up

Description

Universal Re-Encryption
Let p be a prime, and g be an element of ℤ/pℤ of prime order q.
Let x ∈ ℤ/qℤ be the private key, and h = g^x (mod p) be the public key.

To encrypt a message m ∈ ℤ/pℤ, pick two random values r, s ∈ ℤ/qℤ, and compute the ciphertext as follows:
(a, b, c, d) = (g^r, h^r, g^s, mh^s).

Download a valid ciphertext σ = (a, b, c, d) below, and compute another valid ciphertext σ′ = (a′, b′, c′, d′) such that:
σ and σ′ decrypt to the same message;
a ≠ a′ and b ≠ b′ and c ≠ c′ and d ≠ d′.

Continue reading [Sharif University CTF 2016] [Crypto 100 – Universal re-encryption] Write Up

[HackIM 2016] [Crypto 500 – Crypto question 1] Write Up

Description

Crypto Question 1 500 pts

You are in this GAME. A critical mission, and you are surrounded by the beauties, ready to shed their slik gowns on your beck. On onside your feelings are pulling you apart and another side you are called by the duty. The biggiest question is seX OR success? The signals of subconcious mind are not clear, cryptic. You also have the message of heart which is clear and cryptic. You just need to use three of them and find whats the clear message of your Mind… What you must do?

Continue reading [HackIM 2016] [Crypto 500 – Crypto question 1] Write Up

[HackIM 2016] [Crypto 500 – Crypto question 5] Write Up

Description

Crypto Question 5 500 pts

Now you are one step away from knowing who is that WARRIOR. The Fighter who will decide the fate of war between the 2 countries. The Pride of One and Envey of the Other… You have got the secrete file which has the crucial information to identify the fighter. But the file is encrypted with a RSA-Private key. Good news you have its corresponding public key in a file. Bad news there are 49 other keys. Whos is the Fighter.Now

Continue reading [HackIM 2016] [Crypto 500 – Crypto question 5] Write Up