A simple encryption scheme available in ResNet

Users of the ResNet app have the option to use a simple but effective encryption scheme to send messages, i.e., a subject of maximum length of 200 characters and a message text of maximum length of 3000 characters.

The encryption scheme applies the Blum-Blum-Shub algorithm [1] to generate a pseudo-random sequence $x, x^2, x^4, x^8, \ldots\pmod{n}$, where $x$ and $n$ are based on a nonce (a number only used once) and a shared secret (a password transmitted over a secure channel, e.g., at a face-to-face meeting), and that is used as a binary one-time pad XOR-ed (the ^ operator in Python) with the subject and the message text. This notebook is a Python translation of the used Kotlin code for Android devices and Swift code for iOS.

To demonstrate the strength of this encryption scheme - and also for fun 😊 - we created a PHP version of the encryption scheme available here. The message text is the private key of the Solana address CSfT8di8n1mP7KHQaHXniEGdKsWaqkyifsVVAokFbw3x and we therefore offer 133 USD for cracking this encryption scheme.

[1] Blum, L.; Blum, M.; Shub, M. (1986). "A Simple Unpredictable Pseudo-Random Number Generator". SIAM Journal on Computing. 15 (2). Society for Industrial & Applied Mathematics (SIAM): 364–383, doi:10.1137/0215025