# Breaking the encryption scheme of the Moscow Internet voting system

## TL;DR

This paper demonstrates two successful attacks on the encryption scheme of Moscow's Internet voting system, revealing vulnerabilities in key size and semantic security, which were promptly fixed after disclosure.

## Contribution

It uncovers critical cryptographic vulnerabilities in the Moscow Internet voting system's encryption, highlighting the importance of proper key sizes and semantic security.

## Key findings

- Key sizes were too small, allowing private key retrieval in minutes
- The new system was not semantically secure, enabling vote counting
- Both vulnerabilities were fixed after being reported

## Abstract

In September 2019, voters for the election at the Parliament of the city of Moscow were allowed to use an Internet voting system. The source code of it had been made available for public testing. In this paper we show two successful attacks on the encryption scheme implemented in the voting system. Both attacks were sent to the developers of the system, and both issues had been fixed after that.The encryption used in this system is a variant of ElGamal over finite fields. In the first attack we show that the used key sizes are too small. We explain how to retrieve the private keys from the public keys in a matter of minutes with easily available resources.When this issue had been fixed and the new system had become available for testing, we discovered that the new implementation was not semantically secure. We demonstrate how this newly found security vulnerability can be used for counting the number of votes cast for a candidate.