For the record: - The *cryptography* in Signal is probably fine; a practical attack would be a big surprise. - Signal lacks specific features required for classified systems, such as security labels, certified identities, revocation, etc. - Signal runs on uncontrolled, insecure platforms connected to the Internet, rendering it unsuitable for classified even if it had the above features. - Adding classified features to Signal would make it unusable for most purposes for which it's intended.