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iOS file protection includes four FileProtectionType levels: complete (most restrictive, file inaccessible when device locked), completeUnlessOpen (accessible if already open when locked), completeUntilFirstUserAuthentication (accessible after first unlock), and none (no encryption). Choose based on data sensitivity and background access requirements.

In Swift, set file protection using FileManager attributes with the fileProtectionType key when creating files, or apply it retroactively to existing files. Specify the appropriate FileProtectionType level matching your security and accessibility needs before writing sensitive data.

Background access depends on the protection level chosen. Complete protection blocks background access entirely; completeUnlessOpen allows access if the file was already open; completeUntilFirstUserAuthentication and none permit background access after first device unlock.

Use completeUntilFirstUserAuthentication for health data requiring background access. It encrypts files at rest and becomes accessible after the user first unlocks the device, balancing security with background execution requirements.

Access errors after lock occur when file protection level restricts access in the device's locked state. Verify your protection level matches your app's needs; complete protection always blocks locked access, while other levels have specific unlock or background-access conditions.

File protection accessibility varies by level and device state: complete blocks all locked access; completeUnlessOpen depends on pre-lock file state; completeUntilFirstUserAuthentication permits post-unlock access; none provides no encryption. Map your app's operational scenarios to matching protection levels.