I wonder, how can this end-to-end encryption mechanism be marketed as an unbreakable system? How are the keys distributed, and how is secure synchronization between devices achieved? Especially, how are offline devices handled during synchronization?
How does End-to-End encryption work?
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End-to-End Encryption (E2EE) is a method of encryption designed to ensure that data can only be read by the communicating parties. The core principle is that the message is encrypted on the sender's device and decrypted only on the recipient's device. Since intermediary servers cannot see the content of the data during this process, the system is marketed as "unbreakable." However, it's important to note that if E2EE is not correctly implemented at the application level (e.g., weak key management or backdoors), it can lead to serious security vulnerabilities. While organizations like NIST acknowledge that E2EE is mathematically secure, implementation errors or side-channel attacks can weaken the system.
Key distribution is one of the most critical components of E2EE. In most modern systems, a pair of keys (public and private) is generated for each user. Symmetric keys shared between users are securely exchanged using protocols like Diffie-Hellman. During this process, "Public Key Infrastructure" (PKI) or decentralized systems (e.g., the Signal Protocol in the Signal app) are often used. Key exchange between devices for secure synchronization is typically signed with the device's own private key to ensure authentication.
Offline synchronization is crucial in E2EE systems. For example, apps like WhatsApp store messages on the sender's device while waiting for the recipient's device to come online, then synchronize when the target device is back online. In this scenario, sent messages remain encrypted with the recipient's public key and can only be decrypted on the device with the corresponding private key. While some systems facilitate offline data access through methods like "backup keys," these can introduce additional security risks.
Finally, the security level of E2EE depends on the protocol and system architecture it is implemented in. Application errors, malicious actors, or physical attacks (e.g., extracting keys from memory) can cause a theoretically "secure" system to be vulnerable in practice. For this reason, it is important for users to review both the application's security documentation and independent audit reports.
I also had the same questions when developing an end-to-end encryption (E2EE) mechanism in a mobile app before. In our project, we based the encryption of users' messages on the Signal Protocol. The most critical part was the private keys generated specifically for each user's device and never leaving the device. How we securely distributed these keys was the most challenging part for me.
In the solution we developed for offline devices, we used a holdover process to perform synchronization when the devices came back online. This way, offline devices automatically received new keys and their encrypted data when they went online. However, in this process, it's important not to forget to add a series of verification steps to ensure the accuracy of the incoming data. This way, we ensured secure synchronization while maintaining the integrity of the data.