docs: Add USB U2F key device documentation

Add USB U2F key device documentation:
- USB U2F key device
- Building
- Using u2f-emulated
- Using u2f-passthru
- Libu2f-emu

Signed-off-by: César Belley <cesar.belley@lse.epita.fr>
Message-id: 20200826114209.28821-3-cesar.belley@lse.epita.fr
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

docs/u2f.txt

@@ -0,0 +1,101 @@
+QEMU U2F Key Device Documentation.
+
+Contents
+1. USB U2F key device
+2. Building
+3. Using u2f-emulated
+4. Using u2f-passthru
+5. Libu2f-emu
+
+1. USB U2F key device
+
+U2F is an open authentication standard that enables relying parties
+exposed to the internet to offer a strong second factor option for end
+user authentication.
+
+The standard brings many advantages to both parties, client and server,
+allowing to reduce over-reliance on passwords, it increases authentication
+security and simplifies passwords.
+
+The second factor is materialized by a device implementing the U2F
+protocol. In case of a USB U2F security key, it is a USB HID device
+that implements the U2F protocol.
+
+In Qemu, the USB U2F key device offers a dedicated support of U2F, allowing
+guest USB FIDO/U2F security keys operating in two possible modes:
+pass-through and emulated.
+
+The pass-through mode consists of passing all requests made from the guest
+to the physical security key connected to the host machine and vice versa.
+In addition, the dedicated pass-through allows to have a U2F security key
+shared on several guests which is not possible with a simple host device
+assignment pass-through.
+
+The emulated mode consists of completely emulating the behavior of an
+U2F device through software part. Libu2f-emu is used for that.
+
+
+2. Building
+
+To ensure the build of the u2f-emulated device variant which depends
+on libu2f-emu: configuring and building:
+
+    ./configure --enable-u2f && make
+
+
+3. Using u2f-emulated
+
+To work, an emulated U2F device must have four elements:
+ * ec x509 certificate
+ * ec private key
+ * counter (four bytes value)
+ * 48 bytes of entropy (random bits)
+
+To use this type of device, this one has to be configured, and these
+four elements must be passed one way or another.
+
+Assuming that you have a working libu2f-emu installed on the host.
+There are three possible ways of configurations:
+ * ephemeral
+ * setup directory
+ * manual
+
+Ephemeral is the simplest way to configure, it lets the device generate
+all the elements it needs for a single use of the lifetime of the device.
+
+    qemu -usb -device u2f-emulated
+
+Setup directory allows to configure the device from a directory containing
+four files:
+ * certificate.pem: ec x509 certificate
+ * private-key.pem: ec private key
+ * counter: counter value
+ * entropy: 48 bytes of entropy
+
+    qemu -usb -device u2f-emulated,dir=$dir
+
+Manual allows to configure the device more finely by specifying each
+of the elements necessary for the device:
+ * cert
+ * priv
+ * counter
+ * entropy
+
+    qemu -usb -device u2f-emulated,cert=$DIR1/$FILE1,priv=$DIR2/$FILE2,counter=$DIR3/$FILE3,entropy=$DIR4/$FILE4
+
+
+4. Using u2f-passthru
+
+On the host specify the u2f-passthru device with a suitable hidraw:
+
+    qemu -usb -device u2f-passthru,hidraw=/dev/hidraw0
+
+
+5. Libu2f-emu
+
+The u2f-emulated device uses libu2f-emu for the U2F key emulation. Libu2f-emu
+implements completely the U2F protocol device part for all specified
+transport given by the FIDO Alliance.
+
+For more information about libu2f-emu see this page:
+https://github.com/MattGorko/libu2f-emu.