#!/usr/bin/env bash
#
# Seal a password into the TPM 2.0 and persist it at a handle. Release of
# the password is bound to a PCR policy: unsealing only succeeds if the
# current PCR values match those recorded at seal time.
# Requires: tpm2-tools, access to /dev/tpmrm0 (tss group or root).

set -euo pipefail

HANDLE="0x81010001"
PCR_LIST="sha256:7"
FORCE=0

usage() {
	cat <<-EOF
	Usage: $(basename "$0") [-H handle] [-l pcr-list] [-f]

	  -H handle     Persistent TPM handle to store the sealed password at
	                (default: ${HANDLE})
	  -l pcr-list   PCR bank and indices to bind release to, in
	                tpm2_createpolicy --policy-pcr -l format
	                (default: ${PCR_LIST})
	  -f            Evict an existing object at that handle before sealing
	EOF
}

while getopts "H:l:fh" opt; do
	case "$opt" in
	H) HANDLE="$OPTARG" ;;
	l) PCR_LIST="$OPTARG" ;;
	f) FORCE=1 ;;
	h) usage; exit 0 ;;
	*) usage; exit 1 ;;
	esac
done

command -v tpm2_createprimary >/dev/null || {
	echo "tpm2-tools is required (apt install tpm2-tools)" >&2
	exit 1
}

if tpm2_readpublic -c "$HANDLE" >/dev/null 2>&1; then
	if [ "$FORCE" -eq 1 ]; then
		tpm2_evictcontrol -C o -c "$HANDLE" >/dev/null
	else
		echo "A sealed object already exists at ${HANDLE}; rerun with -f to replace it" >&2
		exit 1
	fi
fi

WORKDIR="$(mktemp -d)"
trap 'rm -rf "$WORKDIR"' EXIT
umask 077

read -r -s -p "Password to seal: " PASSWORD
echo >&2
read -r -s -p "Confirm password: " PASSWORD_CONFIRM
echo >&2

if [ "$PASSWORD" != "$PASSWORD_CONFIRM" ]; then
	echo "Passwords do not match" >&2
	exit 1
fi

tpm2_createprimary -C o -c "$WORKDIR/primary.ctx" -Q

tpm2_createpolicy -Q \
	--policy-pcr \
	-l "$PCR_LIST" \
	-L "$WORKDIR/policy.digest"

printf '%s' "$PASSWORD" | tpm2_create \
	-C "$WORKDIR/primary.ctx" \
	-L "$WORKDIR/policy.digest" \
	-i - \
	-u "$WORKDIR/seal.pub" \
	-r "$WORKDIR/seal.priv" \
	-Q

unset PASSWORD PASSWORD_CONFIRM

tpm2_load \
	-C "$WORKDIR/primary.ctx" \
	-u "$WORKDIR/seal.pub" \
	-r "$WORKDIR/seal.priv" \
	-c "$WORKDIR/seal.ctx" \
	-Q

tpm2_evictcontrol -C o -c "$WORKDIR/seal.ctx" "$HANDLE" >/dev/null

echo "Password sealed and persisted at handle ${HANDLE}, bound to PCRs ${PCR_LIST}"