Tiniest keyboard
Recently received my Tomu from crowdsupply. The Tomu is an opensource hardware/software project by Sutajio Ko-Usagi. The entire circuit board fits inside the USB connector. The only part that sticks out are the two touch sensitive contacts and a pair of LEDs.The are several different firmware examples including a U2F security key. There is sample code that emulates a HID keyboard.
The Tomu comes with a clear plastic housing. The board is so thin that it would not stay securely in a USB port without the case.
The bottom of the board has all the circuitry. There isn't much more than the ARM chip and a few resistors/capacitors. There is one red and one green LED.
Next to a U2F Zero.
Very little of it sticks out of the port.
There is also the Fomu, which is a FPGA development board the same size as the Tomu.
The Somu is a Tomu size FIDO2 key from the maker of the U2F Zero, and Solokey.