libsignal-service-rs doesn't sanity check plaintext envelopes are not sanity-checked

Vulnerability Description

libsignal-service-rs is a Rust version of the libsignal-service-java library which implements the core functionality to communicate with Signal servers. Prior to commit 82d70f6720e762898f34ae76b0894b0297d9b2f8, plaintext content envelopes could be injected by a server or a malicious client, and may have been able to bypass the end-to-end encryption and authentication. The vulnerability is fixed per 82d70f6720e762898f34ae76b0894b0297d9b2f8. The `Metadata` struct contains an additional `was_encrypted` field, which breaks the API, but should be easily resolvable. No known workarounds are available.

References

• https://github.com/whisperfish/libsignal-service-rs/security/advisories/GHSA-hrrc-wpfw-5hj2
• https://github.com/whisperfish/libsignal-service-rs/commit/82d70f6720e762898f34ae76b0894b0297d9b2f8