YubiKey: Switch between U2F and OTP modes

It’s best to follow these steps as a root, to avoid issues with device recognition after switching modes:

su - root

Install the Yubikey manager:

$ guix package -i python-yubikey-manager

Verify your device is recognized:

$ ykman list
YubiKey 5 Nano [FIDO] Serial: 109*****

Determine current connection mode:

$ ykman mode
Current connection mode is: FIDO
Supported USB interfaces are: OTP, FIDO, CCID

# for more information
$ ykman info
Device type: YubiKey 5 Nano
Serial number: 109*****
Firmware version: 5.2.4
Form factor: Nano (USB-A)
Enabled USB interfaces: FIDO

OTP             Disabled
FIDO U2F        Enabled 
OpenPGP         Disabled
PIV             Disabled
OATH            Disabled
FIDO2           Enabled

Switch to OTP

$ ykman mode otp
Set mode of YubiKey to OTP? [y/N]: y

When you touch your YubiKey now, you should get a OTP input (cccccdligrgcdtoqkjsnahtnhfvbhicjbgasdiujesc), to whatever field is selected. Depending on whether you’re using Slot 1 or 2, this happens immideately, or after a 3 seconds delay.

To switch back to FIDO

ykman mode FIDO
Set mode of YubiKey to FIDO? [y/N]: y

Unless not supported, it’s best to stick to FIDO U2F for ease and security.