So basically if you buy a nano ledger s, make sure you buy it from manufacturer. But with trezor, it doesn't matter? I haven't heard of cases where a compromised trezor was bought. Anyone read about that?
It is more important to buy the Trezor direct from the manufacturer. In theory, someone could load compromised firmware into a Trezor, repackage it, and sell it on to an unsuspecting buyer. Trezors come from the factory containing only a bootloader, forcing buyers to load fresh firmware. If the device arrived already loaded, then something is wrong! However, a clever hacker could still get around this protection.
The Ledger apparently performs a validity check with every use. Thus, you can have a higher confidence that the firmware isn't compromised. Plus, Ledger's use of a Secure Element (an I/O limited 2nd microcontroller) makes it a lot harder to hack.
Both devices can fall victim to social engineering. A crook could preload a seed and trick people into using it by including an "updated" manual. To protect against this, a user should spend time on the Trezor and Ledger sites and learn about how the device should work. If the received device acts differently, then something is wrong.
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The two devices have vastly different approaches to security:
Trezor has security through an open source design. This has allowed "white hats" to discover flaws. Conversely, it also allows "black hats" to create malicious clones.
Ledger has security through obscurity. Nobody outside of Ledger knows fully what is going on inside the Secure Element. But what happens when the secret leaks...