Fetches the opaque container and the ETag; a 304 skips both the download and the 4-second derivation, which is the whole point of keeping the tag between calls. Redirects are not followed - one to another host would send the bearer token there.
Local testing goes through adb reverse tcp:8080 tcp:80, which makes the phone reach the PC as 127.0.0.1 and satisfies the endpoint's loopback exemption. That needs usesCleartextTraffic in the manifest, marked for removal before shipping.
generateSecret takes a KeySpec; passing a JObject made the bridge emit a signature no method matches, and the call failed at runtime. Swallowed as 'unavailable', which hid my own mistake for a full test round - the failure now carries its exception message.
Measured on device: the Pascal PBKDF2 takes 6651 ms, unusable per unlock. AES-GCM through JNI works, plaintext and hash match the vector, so the container format is validated end to end.
The Pascal loop takes 1736 ms on a desktop CPU, so several seconds on a phone - too slow for every unlock. SecretKeyFactory is native and roughly an order of magnitude faster, but PBEKeySpec takes a char[] and the provider picks the encoding, which was the reason for avoiding it.
The non-ASCII vector settles it on a real device: same key means the provider uses UTF-8 and we adopt it; a different key means the Pascal implementation stays. DecryptContainer keeps using the Pascal one until that measurement says otherwise.
Two stages tested separately, because a global failure says nothing about where: PBKDF2 against the known derived key - which also runs on Win64, so that stage validates in seconds without deploying to a phone - then AES-GCM against the plaintext hash, Android only.
The vector is embedded as a unit rather than read from a file: no network, no deployment, so a first-run failure can only come from the decryption itself. Work runs on a thread; 600k HMAC rounds on the UI thread would have Android kill the app for not responding.
r-zakarya