RPi OTP Derive Key

Small utility for deriving decryption keys from a key in OTP on a Raspberry Pi.

Intended for use with cryptsetup.

For device-unique keys, you can optionally use the board serial as salt.

Syntax: rpiotpdk -h | [-s] [-H] [-c count] <-p|-u> <-l label>
Where:
  -h    Show this help
  -s    Use the board serial as salt (zero salt otherwise)
  -H    Output derived key in hex format rather than raw binary
  -c  <count> Derive a key of length <count> bytes (default 32)
  -p    Use the device Private key in OTP as key (not always supported)
  -u    Use the User OTP data as key
  -l <label>  Derive key for label <label>

Examples:

Derive a key for the label "rootfs" which is common across devices as long as the same key has been loaded into the user OTP area:

rpiotpdk -u -l rootfs

Corresponding OpenSSL commandline (substitute hexkey value as necessary):

openssl kdf -digest sha256 -keylen 32 -kdfopt hexkey:0000000000000000000000000000000000000000000000000000000000000000 -kdfopt info:rootfs -kdfopt hexsalt:0000000000000000 -binary hkdf

Derive a key for the label "localfs" which is unique (but predictable) to the device:

rpiotdk -u -s -l localfs

Corresponding OpenSSL commandline (substitute hexsalt with the serial number from /proc/cpuinfo, in little-endian order):

openssl kdf -digest sha256 -keylen 32 -kdfopt hexkey:0000000000000000000000000000000000000000000000000000000000000000 -kdfopt info:localfs -kdfopt hexsalt:a8948ead00000010 hkdf -binary

I.e. for the above example, grep Serial /proc/cpuinfo | awk '{print $3}' gives 10000000ad8e94a8. An automatic way of reversing it correctly is via

grep Serial /proc/cpuinfo  | awk '{print $3}' | grep -o .. | tac | paste -sd ''