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File name : clevis-decrypt-tang
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#!/bin/bash -e
# vim: set tabstop=8 shiftwidth=4 softtabstop=4 expandtab smarttab colorcolumn=80:
#
# Copyright (c) 2017 Red Hat, Inc.
# Author: Nathaniel McCallum <npmccallum@redhat.com>
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.
#

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

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

read -r -d . hdr

if ! jhd="$(jose b64 dec -i- <<< "$hdr")"; then
    echo "Error decoding JWE protected header!" >&2
    exit 1
fi

if [ "$(jose fmt -j- -Og clevis -g pin -u- <<< "$jhd")" != "tang" ]; then
    echo "JWE pin mismatch!" >&2
    exit 1
fi

if ! clt="$(jose fmt -j- -Og epk -Oo- <<< "$jhd")"; then
    echo "JWE missing required 'epk' header parameter!" >&2
    exit 1
fi

if ! kid="$(jose fmt -j- -Og kid -Su- <<< "$jhd")"; then
    echo "JWE missing required 'kid' header parameter!" >&2
    exit 1
fi

if ! keys="$(jose fmt -j- -Og clevis -g tang -g adv -Oo- <<< "${jhd}")"; then
    echo "JWE missing required 'clevis.tang.adv' header parameter!" >&2
    exit 1
fi

# Check if the thumbprint we have in `kid' is in the advertised keys.
CLEVIS_DEFAULT_THP_ALG=S1        # SHA-1
CLEVIS_ALTERNATIVE_THP_ALGS=S256 # SHA-256

if ! srv="$(jose jwk thp -i- -f "${kid}" -a "${CLEVIS_DEFAULT_THP_ALG}" \
            <<< "${keys}")"; then
    # `kid' thumprint not in the advertised keys, but it's possible it was
    # generated using a different algorithm than the default one.
    # Let us try the alternative supported algorithms to make sure `kid'
    # really is not part of the advertised keys.
    for alg in ${CLEVIS_ALTERNATIVE_THP_ALGS}; do
        srv="$(jose jwk thp -i- -f "$kid" -a "${alg}" <<< "${keys}")" && break
    done
    if [ -z "${srv}" ]; then
        echo "JWE header validation of 'clevis.tang.adv' failed: key thumbprint does not match" >&2
        exit 1
    fi
fi

if ! url="$(jose fmt -j- -Og clevis -g tang -g url -Su- <<< "$jhd")"; then
    echo "JWE missing required 'clevis.tang.url' header parameter!" >&2
    exit 1
fi

if ! crv="$(jose fmt -j- -Og crv -Su- <<< "$clt")"; then
    echo "Unable to determine EPK's curve!" >&2
    exit 1
fi

if ! eph="$(jose jwk gen -i "{\"alg\":\"ECMR\",\"crv\":\"$crv\"}")"; then
    echo "Error generating ephemeral key!" >&2
    exit 1
fi

xfr="$(jose jwk exc -i '{"alg":"ECMR"}' -l- -r- <<< "$clt$eph")"

rec_url="$url/rec/$kid"
ct="Content-Type: application/jwk+json"
if ! rep="$(curl -sfg -X POST -H "$ct" --data-binary @- "$rec_url" <<< "$xfr")"; then
    echo "Error communicating with the server $url" >&2
    exit 1
fi

if ! rep="$(jose fmt -j- -Og kty -q EC -EUUg crv -q "$crv" -EUUo- <<< "$rep")"; then
    echo "Received invalid server reply!" >&2
    exit 1
fi

tmp="$(jose jwk exc -i '{"alg":"ECMR"}' -l- -r- <<< "$eph$srv")"
rep="$(jose jwk pub -i- <<< "$rep")"
jwk="$(jose jwk exc -l- -r- <<< "$rep$tmp")"
exec jose jwe dec -k- -i- < <(echo -n "$jwk$hdr."; /bin/cat)