The principal block and character devices related to kernel security are /dev/random and /dev/urandom. These devices...
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allow the retrieval of random data for use in generating cryptographic keys and other applications that require secure random numbers. For example, they are used when generating SSH keys.
The /dev/random is a high entropy device that requires user-generated input for output randomness, like hitting the keyboard. If it can't get sufficient input, it will block until sufficient input is available. In comparison, if /dev/urandom hasn't got sufficient input, it will return a strong hash of what input it has. This is allegedly slightly less secure than the /dev/random model, but any attack that exploited it, while feasible, would be hard to initiate.
You can read about these devices in more detail here.
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