cross-posted from: https://community.hackliberty.org/post/77
An entity controlling MEGA’s core infrastructure can tamper with the encrypted RSA private key and deceive the client into leaking information about one of the prime factors of the RSA modulus during the session ID exchange. More specifically, the session ID that the client decrypts with the mauled private key and sends to the server will reveal whether the prime is smaller or greater than an adversarially chosen value. This information enables a binary search for the prime factor, with one comparison per client login attempt, allowing the adversary to recover the private RSA key after 1023 client logins. Using lattice cryptanalysis, the number of login attempts required for the attack can be reduced to 512.