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Instead, they're made for camping and hiking, where security isn't an issue. Most travelers use packs that are easy targets for thieves and pickpockets.
A 100% theft-proof backpack doesn't exist. It's just not possible. Look for a lockable bag that will deter casual theft. If your bag is locked and the next person's bag isn't, a potential thief will skip yours in favor of the easier target.
Your goal should be to avoid being the easiest mark. A few simple safety measures will go a long way towards keeping your stuff safe.
Using a secure backpack designed for travel is the safest choice.
A lockable backpack offers the security features you need. If you want to keep your belongings safe, they need to be locked securely in a backpack designed for travel. You need to know that your backpack's zippers are designed to be lockable. Zipper pulls with holes in the ends don't count.
Backpacks that are intended for camping, hiking, or mountaineering don't have safety features. Their zippers can't be locked and usually only have a single zipper pull. Even worse, many adventure backpacks don't use zippers at all -- they use drawstrings that can't be locked and don't provide any protection from crooks.
All Tortuga travel backpacks use the world's highest-quality brand of zippers, YKK. These zippers are easily lockable because of special metal loops specifically designed for TSA-approved locks (see above). Every zipper on a Tortuga's main compartment can be locked.
You don't need to rig a system together just to lock up your bag.
Using a secure backpack designed for travel, like a Tortuga, is a much better and safer choice than using an unsafe backpack designed for hiking or camping.
Compare travel backpacks here and find the right bag for your trip.