"How many TOTP secrets can be stored on YubiKeys and Nitrokeys?":
– Nitrokeys: up to 16
– YubiKeys: up to 32
(Values may differ due to different firmware, or hardware variants.)
In both cases, you need additional software to use TOTP since these tokens don't come with their own internal clock. A clock would require an energy source, but these tokens don't contain batteries.
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