#!/bin/sh
# Sign with a key held in an HSM, through PKCS#11.
#
#   hsm-sign detached IN OUT        a detached signature of IN (RSA PKCS#1 v1.5 over
#                                   SHA-256), written to OUT
#   hsm-sign apk IN.apk OUT.apk     sign an APK (signature schemes v2 and v3), then verify
#   hsm-sign make-cert SUBJECT      one-time setup for apk: a self-signed certificate for
#                                   the key, written to the token beside it, and printed
#                                   as PEM
#   hsm-sign list                   what Java sees on the token, for diagnosing an alias
#
# Environment:
#   PKCS11_PIN         the user PIN (required)
#   PKCS11_MODULE      the PKCS#11 library; default: opensc's
#   PKCS11_SLOT_INDEX  which slot, by position in the slot list; default 0
#   KEY_ID             the key's CKA_ID in hex; default 03
#   KEY_LABEL          the label make-cert gives the certificate; default apk-cert
#   KEY_ALIAS          which key apksigner uses, when the token holds more than one
#
# Why apk needs a certificate on the token: an APK signature embeds the signer's
# certificate, and Java's PKCS#11 key store only offers a private key that has a
# certificate with the same CKA_ID. A key used only for detached signatures has none
# until make-cert writes one. Android pins an app to its signing certificate for good,
# so make-cert runs once and its certificate is kept.
set -eu

die() {
  echo "hsm-sign: $*" >&2
  exit 1
}

[ -n "${PKCS11_PIN:-}" ] || die "PKCS11_PIN is not set"
module="${PKCS11_MODULE:-$(find /usr/lib -name opensc-pkcs11.so | head -n 1)}"
[ -f "$module" ] || die "no PKCS#11 module at '$module'"
slot_index="${PKCS11_SLOT_INDEX:-0}"
key_id="${KEY_ID:-03}"
work="$(mktemp -d)"
trap 'rm -rf "$work"' EXIT
chmod 700 "$work"

# SunPKCS11's configuration: which library, which slot.
cat > "$work/pkcs11.cfg" <<EOF
name = HSM
library = $module
slotListIndex = $slot_index
EOF

# What apksigner and keytool need to reach the token through Java. The PIN goes by
# environment variable name, never on a command line.
java_store() {
  printf '%s\n' -storetype PKCS11 -providerClass sun.security.pkcs11.SunPKCS11 \
    -providerArg "$work/pkcs11.cfg"
}

# apksigner's --provider-class instantiates the provider by reflection, which the module
# system refuses for SunPKCS11 on Java 9 and later. The JDK's own list of providers
# already names SunPKCS11, unconfigured; this points that same entry at the token, so
# the provider is installed by the JVM and apksigner only has to ask for a PKCS11 store.
java_security() {
  conf="$(dirname "$(dirname "$(readlink -f "$(command -v java)")")")/conf/security/java.security"
  entry="$(grep -o '^security\.provider\.[0-9]*=SunPKCS11' "$conf" | cut -d= -f1)" ||
    die "no SunPKCS11 entry in $conf"
  printf '%s=SunPKCS11 %s\n' "$entry" "$work/pkcs11.cfg" > "$work/java.security"
  echo "-Djava.security.properties=$work/java.security"
}

case "${1:-}" in
  detached)
    [ $# -eq 3 ] || die "usage: hsm-sign detached IN OUT"
    # The signature action-hsm-sign has always made, so signatures verify (and match
    # sigstore entries) the same across versions: PKCS#1 v1.5 is deterministic.
    pkcs11-tool --module "$module" --slot-index "$slot_index" --login --pin env:PKCS11_PIN \
      --sign --id "$key_id" --mechanism SHA256-RSA-PKCS --input-file "$2" --output-file "$3"
    ;;

  apk)
    [ $# -eq 3 ] || die "usage: hsm-sign apk IN.apk OUT.apk"
    set -- "$2" "$3"
    alias_args=""
    [ -n "${KEY_ALIAS:-}" ] && alias_args="--ks-key-alias ${KEY_ALIAS}"
    # shellcheck disable=SC2086 # alias_args is two words or none, by construction
    JAVA_TOOL_OPTIONS="$(java_security)" apksigner sign \
      --ks NONE --ks-type PKCS11 --ks-pass env:PKCS11_PIN \
      $alias_args \
      --in "$1" --out "$2"
    # Proof, in the log, of what was signed and by whom.
    apksigner verify --verbose --print-certs "$2"
    ;;

  make-cert)
    [ $# -eq 2 ] || die "usage: hsm-sign make-cert '/CN=...'"
    subject="$2"
    # OpenSSL's PKCS#11 engine, pointed at the same module, signs the certificate with
    # the HSM's key. The PIN is in a file only this process can read, not an argument.
    cat > "$work/openssl.cnf" <<EOF
openssl_conf = conf
[conf]
engines = engines
[engines]
pkcs11 = p11
[p11]
engine_id = pkcs11
MODULE_PATH = $module
PIN = $PKCS11_PIN
init = 0
[req]
distinguished_name = dn
[dn]
EOF
    uri_id="$(printf '%s' "$key_id" | sed 's/../%&/g')"
    # Thirty years: the certificate's dates are not checked for APK signatures, and
    # replacing it would mean a new app identity on every device.
    OPENSSL_CONF="$work/openssl.cnf" openssl req -new -x509 -sha256 -days 10950 \
      -subj "$subject" -engine pkcs11 -keyform engine \
      -key "pkcs11:id=${uri_id};type=private" -out "$work/cert.pem"
    openssl x509 -in "$work/cert.pem" -outform DER -out "$work/cert.der"
    pkcs11-tool --module "$module" --slot-index "$slot_index" --login --pin env:PKCS11_PIN \
      --write-object "$work/cert.der" --type cert --id "$key_id" --label "${KEY_LABEL:-apk-cert}"
    cat "$work/cert.pem"
    ;;

  list)
    # shellcheck disable=SC2046 # java_store prints one argument per line
    keytool -list -v -storepass:env PKCS11_PIN -keystore NONE $(java_store)
    ;;

  *)
    die "usage: hsm-sign detached IN OUT | apk IN OUT | make-cert SUBJECT | list"
    ;;
esac