A first Pithos app for phones. It runs the browser app in a WebView and
answers the page's own files from the APK under the gateway's origin, so
the page stays the same-origin client it was designed as and the files
are still encrypted and decrypted in the page.
What the phone adds: "Save to Pithos" in other apps' share menus (the
files wait in a bar until a folder is open), downloads written to the
Downloads folder, the system file picker for uploads, and outside links
opened in the phone's browser.
The browser app learns the two hooks (window.PithosAndroid) and says
plainly when a browser is too old to sign in, instead of a bare
"NotSupportedError".
A page served by the gateway that signs in with the Pithos key file
(Ed25519 in Web Crypto) or an Export JSON S3 key, then browses, views,
downloads, uploads, renames and deletes, encrypting and decrypting in the
page with the same formats as the desktop. Checked against a local
gateway and real s3d: files the desktop encrypted open byte-identical in
the browser, and browser uploads open in the desktop client; the server
only ever holds encrypted names. Remember on this device keeps the key
sealed with a passphrase in IndexedDB.
The browser app must read and write the same encrypted files as the
desktop Pithos. web/lib/crypt.js reimplements core/crypt.js on Web
Crypto (AES-SIV and CMAC built from AES-CBC with a zero IV and AES-CTR,
since Web Crypto has neither), and web/lib/sigv4.js the SigV4 signing.
Cross-tests: the RFC 5297 vector, identical drive keys and names, bodies
encrypted on one side and opened on the other (empty, chunk-edge and
multi-chunk files, ranges), tamper and wrong-path rejection, and equal
SigV4 headers and presigned URLs.
