Procreate, no install
Seal the file.
Procreate keeps its own record inside every file: the time-lapse, the tracked drawing time, the layers. Drop the file here and DrawDog seals that record with a signed, timestamped certificate before anyone could edit it. The file itself never leaves your device.
What this is, honestly
Sealed, not witnessed.
The desktop app records your working sessions and the time you spend, and never opens your files. Nothing can do that on an iPad; iPadOS does not let one app see another. A sealed record is the next best thing. It takes Procreate’s own account of the file and locks it at a moment in time, so it cannot be edited afterwards and a client can check it matches the file they were sent.
It cannot catch tracing over an imported image, and a Procreate document can be edited before it is sealed. The certificate says both. It is labelled Sealed record, never Certified human-made. The full method.