Secure messaging is its own (increasingly well-defined) cryptographic problem. Group secure messaging is a distinct problem with its own cryptography. Backups are their own problem with their own constraints. Package signatures are their own problem.<p>There isn't one cryptosystem that does all these things. For messaging you want a carefully-defined notion of forward secrecy and post-compromise recovery, along with a careful decision about repudiation/non-repudation (1990s cryptography doctrine would suggest you just always want nonrepudiabilty, which turns out to be disastrous for privacy). For backup, you want deduplication and content addressing. For packages, you want transparency logs. You almost never want long-lived keys, but you often want identify binding.<p>PGP solves basically none of these problems, and offers instead a 1998 API (I'm not kidding, I thought about what year to pin it at) to basic cryptography primitives.