Quantum processors used in cyberdecks for hacking and code-breaking can shred almost any known cryptography, given enough time; but they are not omnipotent. If you really, really need a secure communications channel, and are prepared to jump through hoops for it, there is something you can do.
Ancient cryptographers would sometimes use one-time pads to encode messages in a way that was nearly unbreakable. The 207x equivalent is a symmetric entropy pool. Two parties share a massive pool of carefully generated, thoroughly randomised data. The pool is used to encrypt all communications between them before they are sent via the public Matrix. The other party uses their matching copy of the pool to decrypt the message again. In theory, this cannot be decrypted by attackers, as it appears to be completely random noise.
Shared entropy pools come with some significant downsides, however:
Most users of entropy pools use specialised hardware to store and work with them. This is a credstick-sized device that plugs into a standard dataport on a commlink or cyberdeck. The commlink sends the encrypted data stream into the device, and decrypted data comes back. The main advantage of this is that when being used normally the entropy pool cannot be hacked even if you hack the commlink, as the commlink cannot read the pool directly.
However, by flicking a recessed switch, the device can be put into setup mode; this is designed to allow pools to be created, synchonised, and copied. Hence, with physical access to any device for a minute or two, an attacker can easily duplicate the pool.