#!/usr/bin/env python

"""
Securely back up private keys from one Cryptech HSM to another.

This works by having the destination HSM (the one importing keys)
create an RSA keypair (the "KEKEK"), the public key of which can then
be imported into the source HSM (the one exporting keys) and used to
encrypt AES key encryption keys (KEKs) which in turn can be used to
wrap the private keys being transfered.  Transfers are encoded in
JSON; the underlying ASN.1 formats are SubjectPublicKeyInfo (KEKEK
public key) and PKCS #8 EncryptedPrivateKeyInfo (everything else).

NOTE WELL: while this process makes it POSSIBLE to back up keys
securely, it is not sufficient by itself: the operator MUST make
sure only to export keys using a KEKEK known to have been generated by
the target HSM.  See the unit tests in the source repository for
an example of how to fake this in a few lines of Python.

YOU HAVE BEEN WARNED.  Be careful out there.
"""

# Diagram of the trivial protocol we're using:
#
#    SOURCE HSM                            DESTINATION HSM
#
#                                          Generate and export KEKEK:
#                                               hal_rpc_pkey_generate_rsa()
#                                               hal_rpc_pkey_get_public_key()
#
#   Load KEKEK public        <---------    Export KEKEK public
#        hal_rpc_pkey_load()
#        hal_rpc_pkey_export()
#
#   Export PKCS #8 and KEK   ---------->   Load PKCS #8 and KEK, import key
#                                               hal_rpc_pkey_import()

import sys
import json
import uuid
import atexit
import getpass
import argparse

from cryptech.libhal import *

def main():

    parser = argparse.ArgumentParser(
        formatter_class = argparse.RawDescriptionHelpFormatter,
        description = __doc__)
    subparsers = parser.add_subparsers(
        title = "Commands (use \"--help\" after command name for help with individual commands)",
        metavar = "")
    setup_parser  = defcmd(subparsers, cmd_setup)
    export_parser = defcmd(subparsers, cmd_export)
    import_parser = defcmd(subparsers, cmd_import)
    setup_mutex_group = setup_parser.add_mutually_exclusive_group()


    parser.add_argument(
        "-p", "--pin",
        help    = "wheel PIN")


    setup_mutex_group.add_argument(
        "-n", "--new",
        action  = "store_true",
        help    = "force creation of new KEKEK")

    setup_mutex_group.add_argument(
        "-u", "--uuid",
        help    = "UUID of existing KEKEK to use")

    setup_parser.add_argument(
        "-k", "--keylen",
        type    = int,
        default = 2048,
        help    = "length of new KEKEK if we need to create one")

    setup_parser.add_argument(
        "-o", "--output",
        type    = argparse.FileType("w"),
        default = "-",
        help    = "output file")


    export_parser.add_argument(
        "-i", "--input",
        type    = argparse.FileType("r"),
        default = "-",
        help    = "input file")

    export_parser.add_argument(
        "-o", "--output",
        type    = argparse.FileType("w"),
        default = "-",
        help    = "output file")


    import_parser.add_argument(
        "-i", "--input",
        type    = argparse.FileType("r"),
        default = "-",
        help    = "input file")


    args = parser.parse_args()

    hsm = HSM()

    try:
        hsm.login(HAL_USER_WHEEL, args.pin or getpass.getpass("Wheel PIN: "))

    except HALError as e:
        sys.exit("Couldn't log into HSM: {}".format(e))

    try:
        sys.exit(args.func(args, hsm))

    finally:
        hsm.logout()


def defcmd(subparsers, func):
    assert func.__name__.startswith("cmd_")
    subparser = subparsers.add_parser(func.__name__[4:],
                                      description = func.__doc__,
                                      help = func.__doc__.strip().splitlines()[0])
    subparser.set_defaults(func = func)
    return subparser


def b64(bytes):
    return bytes.encode("base64").splitlines()

def b64join(lines):
    return "".join(lines).decode("base64")


def cmd_setup(args, hsm):
    """
    Set up backup HSM for subsequent import.
    Generates an RSA keypair with appropriate usage settings
    to use as a key-encryption-key-encryption-key (KEKEK), and
    writes the KEKEK to a JSON file for transfer to primary HSM.
    """

    result = {}

    uuids = []
    if args.uuid:
        uuids.append(args.uuid)
    elif not args.new:
        uuids.extend(hsm.pkey_match(
            type  = HAL_KEY_TYPE_RSA_PRIVATE,
            mask  = HAL_KEY_FLAG_USAGE_KEYENCIPHERMENT | HAL_KEY_FLAG_TOKEN,
            flags = HAL_KEY_FLAG_USAGE_KEYENCIPHERMENT | HAL_KEY_FLAG_TOKEN))

    for uuid in uuids:
        with hsm.pkey_open(uuid) as kekek:
            if kekek.key_type != HAL_KEY_TYPE_RSA_PRIVATE:
                sys.stderr.write("Key {} is not an RSA private key\n".format(uuid))
            elif (kekek.key_flags & HAL_KEY_FLAG_USAGE_KEYENCIPHERMENT) == 0:
                sys.stderr.write("Key {} does not allow key encipherment\n".format(uuid))
            else:
                result.update(kekek_uuid   = str(kekek.uuid),
                              kekek_pubkey = b64(kekek.public_key))
                break

    if not result and not args.uuid:
        with hsm.pkey_generate_rsa(
                keylen = args.keylen,
                flags = HAL_KEY_FLAG_USAGE_KEYENCIPHERMENT | HAL_KEY_FLAG_TOKEN) as kekek:
            result.update(kekek_uuid   = str(kekek.uuid),
                          kekek_pubkey = b64(kekek.public_key))
    if not result:
        sys.exit("Could not find suitable KEKEK")

    result.update(comment = "KEKEK public key")
    json.dump(result, args.output, indent = 4, sort_keys = True)
    args.output.write("\n")


def key_flag_names(flags):
    names = dict(digitalsignature = HAL_KEY_FLAG_USAGE_DIGITALSIGNATURE,
                 keyencipherment  = HAL_KEY_FLAG_USAGE_KEYENCIPHERMENT,
                 dataencipherment = HAL_KEY_FLAG_USAGE_DATAENCIPHERMENT,
                 token            = HAL_KEY_FLAG_TOKEN,
                 public           = HAL_KEY_FLAG_PUBLIC,
                 exportable       = HAL_KEY_FLAG_EXPORTABLE)
    return ", ".join(sorted(k for k, v in names.iteritems() if (flags & v) != 0))


def cmd_export(args, hsm):
    """
    Export encrypted keys from primary HSM.
    Takes a JSON file containing KEKEK (generated by running this
    script's "setup" command against the backup HSM), installs that
    key on the primary HSM, and backs up keys encrypted to the KEKEK
    by writing them to another JSON file for transfer to the backup HSM.
    """

    db = json.load(args.input)

    result = []

    kekek = None
    try:
        kekek = hsm.pkey_load(der   = b64join(db["kekek_pubkey"]),
                              flags = HAL_KEY_FLAG_USAGE_KEYENCIPHERMENT)

        for uuid in hsm.pkey_match(mask  = HAL_KEY_FLAG_EXPORTABLE,
                                   flags = HAL_KEY_FLAG_EXPORTABLE):
            with hsm.pkey_open(uuid) as pkey:

                if pkey.key_type in (HAL_KEY_TYPE_RSA_PRIVATE, HAL_KEY_TYPE_EC_PRIVATE):
                    pkcs8, kek = kekek.export_pkey(pkey)
                    result.append(dict(
                        comment = "Encrypted private key",
                        pkcs8   = b64(pkcs8),
                        kek     = b64(kek),
                        uuid    = str(pkey.uuid),
                        flags   = pkey.key_flags))

                elif pkey.key_type in (HAL_KEY_TYPE_RSA_PUBLIC, HAL_KEY_TYPE_EC_PUBLIC):
                    result.append(dict(
                        comment = "Public key",
                        spki    = b64(pkey.public_key),
                        uuid    = str(pkey.uuid),
                        flags   = pkey.key_flags))

    finally:
        if kekek is not None:
            kekek.delete()

    db.update(comment = "Cryptech Alpha encrypted key backup",
              keys    = result)
    json.dump(db, args.output, indent = 4, sort_keys = True)
    args.output.write("\n")


def cmd_import(args, hsm):
    """
    Import encrypted keys into backup HSM.
    Takes a JSON file containing a key backup (generated by running
    this script's "export" command against the primary HSM) and imports
    keys into the backup HSM.
    """

    db = json.load(args.input)
    with hsm.pkey_open(uuid.UUID(db["kekek_uuid"]).bytes) as kekek:
        for k in db["keys"]:
            pkcs8 = b64join(k.get("pkcs8", ""))
            spki  = b64join(k.get("spki",  ""))
            kek   = b64join(k.get("kek",   ""))
            flags =         k.get("flags",  0)
            if pkcs8 and kek:
                with kekek.import_pkey(pkcs8 = pkcs8, kek = kek, flags = flags) as pkey:
                    print "Imported {} as {}".format(k["uuid"], pkey.uuid)
            elif spki:
                with hsm.pkey_load(der = spki, flags = flags) as pkey:
                    print "Loaded {} as {}".format(k["uuid"], pkey.uuid)


if __name__ == "__main__":
    main()