clevis-decrypt-fido2

#!/bin/bash

DEFAULT_TIMEOUT='120'

set -eu

[ $# -eq 1 ] && [ "$1" == "--summary" ] && exit 2

if [ -t 0 ]; then
    exec >&2
    echo
    echo "Usage: clevis decrypt fido2 < JWE > PLAINTEXT"
    echo
    exit 2
fi

get_parm () {
  elements="${1//\// --get }"
  mandatory="$2"
  #shellcheck disable=SC2086
  if ! output="$(jose fmt --json="${hdr}" ${elements} --unquote=-)" && [[ ${mandatory} -eq 1 ]]; then
    echo "${0##*/}: JWE missing '${elements##* }' header parameter!" >&2
    exit 1
  fi
  echo "${output}"
}


read -r -d . hdr64
if ! hdr="$(jose fmt --quote="$hdr64" --string --b64load --object --output=-)" ; then
    echo "${0##*/}: JWE header corrupt!" >&2
    exit 1
fi

if [ "$(get_parm /clevis/pin 1)" != 'fido2' ] ; then
    echo "${0##*/}: JWE pin mismatch!" >&2
    exit 1
fi

hmac_salt="$(get_parm /clevis/fido2/hmac_salt 1)"
rp_id="$(get_parm /clevis/fido2/rp_id 1)"
cred_id="$(get_parm /clevis/fido2/cred_id 1)"
uv="$(get_parm /clevis/fido2/uv 1)"
up="$(get_parm /clevis/fido2/up 1)"
pin="$(get_parm /clevis/fido2/pin 1)"
timeout="$(get_parm /clevis/fido2/timeout 0)"
device="$(get_parm /clevis/fido2/device 0)"

fido2_token="${FIDO2_TOKEN:-$device}"

show_msg=1
user_timeout="${TIMEOUT:-$timeout}"
timeout1="${user_timeout:-$DEFAULT_TIMEOUT}"
while [[ ! -c "${fido2_token}" ]]; do
  sleep 1
  if [[ -z "${fido2_token}" ]]; then
    fido2_tokens="$(fido2-token -L)"
    if [[ -z "${fido2_tokens}" && show_msg -eq 1 ]]; then
      echo "${0##*/}: Please insert your FIDO2 token" >&2
      show_msg=0
    fi
    fido2_token="$(echo "${fido2_tokens}" | head -n1 | cut -d':' -f1)"
    num_tokens="$(echo "${fido2_tokens}" | wc -l)"
    if ((num_tokens > 1)); then
      echo "${0##*/}: Warning: There are multiple tokens. Will use the first one (${fido2_token})." >&2
    fi
  else
    if [[ show_msg -eq 1 ]]; then
      echo "${0##*/}: Please insert your specified FIDO2 token ${fido2_token} (if available)" >&2
      show_msg=0
    fi
  fi
  # see 'sleep 1' above--we're keeping things simple here (cf. https://unix.stackexchange.com/a/156133/238272)
  timeout1=$((timeout1-1))
  if [[ ${timeout1} -lt 1 ]]; then
    if [[ -z "${fido2_token}" ]]; then
      echo "${0##*/}: Error: No FIDO2 token found within ${timeout} seconds." >&2
    else
      echo "${0##*/}: Error: specified FIDO2 token ${fido2_token} not found within ${timeout} seconds." >&2
    fi
    exit 1
  fi
done

client_hash="$(dd if=/dev/urandom bs=1 count=32 status=none | base64 -w0)"

f2a_uv="-t uv=${uv}"
if [ "${uv}" == "false" ] && ! fido2-token -I "${fido2_token}" | grep -qE "options:.* uv" ; then
  # Yubikey 5 NFC w/ libfido2 v1.14.0 does _not_ allow to explicitly specify "-t uv=true|false" below
  # (cf. https://github.com/Yubico/libfido2/issues/642#issuecomment-1303673367: "`uv` option [...] controls
  # device-native UV such as biometrics or on-authenticator PIN pad")
  f2a_uv=""
fi

#shellcheck disable=SC2086
hmac="$(printf '%s\n%s\n%s\n%s\n' "${client_hash}" "${rp_id}" "${cred_id}" "${hmac_salt}" | \
  fido2-assert -G -h ${f2a_uv} -t "up=${up}" -t "pin=${pin}" "${fido2_token}" | \
  head -n5 | tail -n1 | base64 -d | jose b64 enc -I -)"

# use the secret in a key wrapping key
jwk='{"alg":"A256GCM", "kty":"oct"}'
jwk="$(jose fmt -j "${jwk}" -q "${hmac}" -s k -Uo-)"

( printf '%s' "$jwk$hdr64." ; cat ) | exec jose jwe dec --key=- --input=-