coast

coast

• Category: Crypto
• Score: 465/500
• Solves: 21

Description

Isogeny cryptography seems so fun, so I build a new cryptosystem based on CSIDH.

Solution

First, due to how the group action is implemented the sign of es in private key does not matter. Second, the order of the auxiliary point G leaks the degree of isogeny. (i.e.g G has order p+1 but becomes (p+1)//3 after a 3-isogeny)

So computing the order of G in public key reveals the secret isogeny, and we can compute the shared secret to decrypt the flag. See solve.sage for details.