Add detailed description of the current function

README.md

@@ -1,3 +1,70 @@
 # trustedboot
 
-This is a description how to enable full disk encryption where the key is managed by a tpm.
+The utility installs a Disk encryption key in the TPM and unencrypts the disk automatically during boot.
+The settings for trusted boot differ for the various Linux distros and releases.
+This version is tested for Ubuntu 20.04 LTS. It requires TPM-Tools 4.x as the parameters completely changed from 3.x
+
+## Usage
+1. Install Ubuntu 20.04 with Full Disk Encryption.
+1. Execute install.sh
+2. Reboot the machine, you will still be asked for your encryption passphrase
+3. Update the TPM PCR policy with update-luks-tpm.sh
+4. The next reboot is done automatically
+
+## Result
+- Grub is still installed, but not used (as a fallback)
+- The initial key for Disk Encryption is still valid (fallback for updates)
+- Initramfs is updated with the script to ask the TPM for the passphrase
+- A unified kernel is generated (kernel + command line parms + initramfs) as one large EFI blob
+- Therefore during Boot only PCRs 0-7 are written (GRUB uses 8-9)
+
+## Details / manual installation
+Ideas taken from 
+- https://threat.tevora.com/secure-boot-tpm-2/
+- https://medium.com/@pawitp/full-disk-encryption-on-arch-linux-backed-by-tpm-2-0-c0892cab9704
+- https://medium.com/@pawitp/its-certainly-annoying-that-tpm2-tools-like-to-change-their-command-line-parameters-d5d0f4351206
+
+I used the PCRs 0,1,4,5,7 as policy for unlocking disk encryption. The PCRs 2,3 and 6 have the same hash value and are therefore assumed as empty.
+For additional security reasons, one may consider including these registers as well (to prevent e.g. Option ROM DMA attacks).
+Furthermore I use the RNG on the TPM for secret generation and use SHA256 and ECC instead of SHA1 and RSA.
+
+All of the below instructions should be executed as root:
+1. Install required tools
+`apt install binutils tpm2-tools`
+1. Create a new secret for disk encryption:
+`tpm2_getrandom 32 -o /root/secret.bin`
+2. Add the key to disk encryption
+`cryptsetup luksAddKey /dev/nvme0n1p3 /root/secret.bin`
+3. Install the tpm-script to initramfs
+`cp -vf ./tpm2-hook.sh /etc/initramfs-tools/hooks/`
+4. The entry in /etc/crypttab should look like this:
+`dm_crypt-0 UUID=<uuid> none luks,discard,initramfs,keyscript=/usr/sbin/passphrase-from-tpm.sh`
+5. Update initramfs
+`update-initramfs -u -k all`
+6. Create the Kernel Command Line
+`echo "/vmlinuz-5.4.0-39-generic root=/dev/mapper/ubuntu--vg-ubuntu--lv ro" > /boot/kernel-command-line.txt`
+6. Create unified Kernel
+```
+mkdir -p /boot/efi/EFI/Linux
+objcopy \
+  --add-section .osrel="/usr/lib/os-release" --change-section-vma .osrel=0x20000 \
+  --add-section .cmdline="/boot/kernel-command-line.txt" --change-section-vma .cmdline=0x30000 \
+  --add-section .linux="/boot/vmlinuz-$VERSION" --change-section-vma .linux=0x40000 \
+  --add-section .initrd="/boot/initrd.img-$VERSION" --change-section-vma .initrd=0x3000000 \
+  "/usr/lib/systemd/boot/efi/linuxx64.efi.stub" "/boot/efi/EFI/Linux/Linux.efi"
+```
+7. Create a EFI Boot entry for the new unified kernel
+`efibootmgr --create --disk /dev/nvme0n1 --part 1 --label "ubuntu unified" --loader "\EFI\BOOT\Linux\Linux.efi" --verbose`
+8. Reboot the machine
+9. Store the secret key in the TPM and use the now valid PCRs as policy
+```
+tpm2_evictcontrol -C o -c 0x81000000 #evict an old passphrase before writing the new one
+tpm2_createpolicy --policy-pcr -l sha256:0,1,4,5,7 -L /root/policy.digest
+tpm2_createprimary -C e -g sha256 -G ecc256 -c /root/primary.context
+tpm2_create -g sha256 -u /root/obj.pub -r /root/obj.priv -C /root/primary.context -L /root/policy.digest -a "noda|adminwithpolicy|fixedparent|fixedtpm" -i /root/secret.bin
+tpm2_load -C /root/primary.context -u /root/obj.pub -r /root/obj.priv -c /root/load.context
+tpm2_evictcontrol -C o -c /root/load.context 0x81000000
+# tpm2_unseal -c 0x81000000 -p pcr:sha1:0,1,4,5,7 -o /root/test.bin #proof that the persistence worked
+rm -f /root/load.context /root/obj.priv /root/obj.pub /root/policy.digest /root/primary.context
+```
+10. The next reboot should work automatically