awesome gameplay recording 01

[0:00]Hello everyone. Welcome back to the them CTF 2026 Walkthrough series. This is a five-part series of which this is the fourth part. In this video, we're going to solve the crypto challenge The Oracle. This challenge contains a Java code that uses an RSA encryption scheme to encrypt flags. Before we begin, a small recap of what we learned in the previous videos. In part one, we've solved the crypto challenge, the random number generator. In this video, we've found that the service uses a specific implementation of a cryptographically secure pseudo random number generator known as Secure Random. By abusing the seed of this random number generator, we have found the flags. In part two, we've solved the crypto challenge, the not so secure channel. In this video, we have found that the service sends encrypted messages using RSA encryption. Although the service uses proper RSA encryption, we have noticed that the service never uses private exponents larger than 2 to the power of 16. We have then abused the small private exponent to deduce the private key by using the Wiener's attack. In part three, we've solved the crypto challenge The Lucky Number. In this video, we have found that the service encrypts messages using RSA encryption, but it generates an exponent and a modulus that are not secure. Specifically, we have found that the service uses a small public exponent e and a modulus n whose factors p and q are too close together. We have then used this information to deduce the private key and decrypt the flag. In part four, we're going to solve the crypto challenge The Oracle. In this challenge, we're going to learn about the properties of RSA encryption and how it can be abused if the system contains an oracle. We're going to solve this challenge by using the Bleichenbacher's attack. Let's begin.