More-sophisticated criminals, on the other hand, will immediately pop out any SIM card and only turn on the device well out of range of open wireless hotspots, gaining all the time in the world to find their way into your personal data. In this case, your choice of passcode must be commensurate with the amount of time that you want the information on the device to remain inaccessible.
For example, a six-digit numeric passcode will likely give you enough time to reset all your critical passwords, like those you use to access your email accounts and banking websites.
If, on the other hand, you keep confidential data on the device that could remain "hot" for years--legal documents, for example--a long alphanumeric passcode may be necessary to ensure that you've at least tried to apply a good standard of care in keeping that information secure.
Finding the right balance between convenience and safety is not that hard once you understand how passcodes work; Apple can likely be counted on to fix whatever new vulnerabilities are found in its operating system in a timely manner, but your devices are only going to be as secure as the locking codes you pick.
After all, as nuclear weapon designers have discovered, the most sophisticated security mechanism is easily undone by the kind of combination a character in a science-fiction satire would keep on his luggage.
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