By default, when you touch a Yubikey it types an OTP code. These codes looks like:
cccjgjgkhcbbirdrfdnlnghhfgrtnnlgedjlftrbdeut
Theoretically they are used for Yubico’s proprietry authentication, but in reality they are mostly annoying. This is especially true for Yubikey Nano, which is impossible to remove without touching it and triggering the OTP.
Disabling the OTP is possible using the Yubikey Manager, and does not affect any other functionality of the Yubikey.
$ sudo apt install yubikey-manager
$ ykman config usb --disable otp
Disable OTP.
Configure USB interface? [y/N]: y
Note for Yubikey 4: the above command will not work and fail withe the following error:
Error: Configuring applications is not supported on this YubiKey. Use the `mode` command to configure USB interfaces.
Instead use the following command:
$ ykman mode FIDO+CCID
Set mode of YubiKey to FIDO+CCID? [y/N]: y
Mode set! You must remove and re-insert your YubiKey for this change to take effect.