path: root/cryptech_backup

#!/usr/bin/env python3

"""
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.

We also implement a software-based variant on this backup mechanism,
for cases where there is no second HSM.  The protocol is much the
same, but the KEKEK is generated in software and encrypted using a
symmetric key derived from a passphrase using PBKDF2.  This requires
the PyCryptodome library, and is only as secure as memory on the machine
where you're running it (so it's theoretically vulnerable to root or
anybody with access to /dev/mem).  Don't use this mode unless you
understand the risks, and see the "NOTE WELL" above.

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 base64
import struct
import getpass
import argparse
import binascii

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_mutex_group.add_argument(
        "-s", "--soft-backup",
        action = "store_true",
        help   = "software-based backup, see warnings")

    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(der):
    return base64.b64encode(der).decode().splitlines()

def b64join(lines):
    return base64.b64decode("".join(lines).encode())


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.soft_backup:
        SoftKEKEK.generate(args, result)
    elif 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")

    if args.soft_backup:
        result.update(comment = "KEKEK software keypair")
    else:
        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.items() 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)

    soft_key = SoftKEKEK.is_soft_key(db)

    with (hsm.pkey_load(SoftKEKEK.recover(db), HAL_KEY_FLAG_USAGE_KEYENCIPHERMENT)
          if soft_key else
          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 soft_key:
            kekek.delete()


class AESKeyWrapWithPadding(object):
    """
    Implementation of AES Key Wrap With Padding from RFC 5649.
    """

    class UnwrapError(Exception):
        "Something went wrong during unwrap."

    def __init__(self, key):
        from Cryptodome.Cipher import AES
        self.ctx = AES.new(key, AES.MODE_ECB)

    def _encrypt(self, b1, b2):
        aes_block = self.ctx.encrypt(b1 + b2)
        return aes_block[:8], aes_block[8:]

    def _decrypt(self, b1, b2):
        aes_block = self.ctx.decrypt(b1 + b2)
        return aes_block[:8], aes_block[8:]

    @staticmethod
    def _start_stop(start, stop):               # Syntactic sugar
        step = -1 if start > stop else 1
        return range(start, stop + step, step)

    @staticmethod
    def _xor(R0, t):
        return struct.pack(">Q", struct.unpack(">Q", R0)[0] ^ t)

    def wrap(self, Q):
        "RFC 5649 section 4.1."
        m = len(Q)                              # Plaintext length
        if m % 8 != 0:                          # Pad Q if needed
            Q += b"\x00" * (8 - (m % 8))
        R = [struct.pack(">LL", 0xa65959a6, m)] # Magic MSB(32,A), build LSB(32,A)
        R.extend(Q[i : i + 8]                   # Append Q
                 for i in range(0, len(Q), 8))
        n = len(R) - 1
        if n == 1:
            R[0], R[1] = self._encrypt(R[0], R[1])
        else:
            # RFC 3394 section 2.2.1
            for j in self._start_stop(0, 5):
                for i in self._start_stop(1, n):
                    R[0], R[i] = self._encrypt(R[0], R[i])
                    R[0] = self._xor(R[0], n * j + i)
        assert len(R) == (n + 1) and all(len(r) == 8 for r in R)
        return b"".join(R)

    def unwrap(self, C):
        "RFC 5649 section 4.2."
        if len(C) % 8 != 0:
            raise self.UnwrapError("Ciphertext length {} is not an integral number of blocks"
                                   .format(len(C)))
        n = (len(C) // 8) - 1
        R = [C[i : i + 8] for i in range(0, len(C), 8)]
        if n == 1:
            R[0], R[1] = self._decrypt(R[0], R[1])
        else:
            # RFC 3394 section 2.2.2 steps (1), (2), and part of (3)
            for j in self._start_stop(5, 0):
                for i in self._start_stop(n, 1):
                    R[0] = self._xor(R[0], n * j + i)
                    R[0], R[i] = self._decrypt(R[0], R[i])
        magic, m = struct.unpack(">LL", R[0])
        if magic != 0xa65959a6:
            raise self.UnwrapError("Magic value in AIV should have been 0xa65959a6, was 0x{:08x}"
                                   .format(magic))
        if m <= 8 * (n - 1) or m > 8 * n:
            raise self.UnwrapError("Length encoded in AIV out of range: m {}, n {}".format(m, n))
        R = b"".join(R[1:])
        assert len(R) ==  8 * n
        if R[m:].strip(b"\x00"):
            raise self.UnwrapError("Nonzero trailing bytes 0x{}".format(binascii.hexlify(R[m:]).decode("ascii")))
        return R[:m]


class SoftKEKEK(object):
    """
    Wrapper around all the goo we need to implement soft backups.
    Requires PyCrypto on about every other line.
    """

    oid_aesKeyWrap = b"\x60\x86\x48\x01\x65\x03\x04\x01\x30"

    # This is sick, but probably harmless, and lets us keep using
    # PyCrypto for a little longer.

    try:
        import time
        time.clock
    except AttributeError:
        time.clock = time.process_time

    def parse_EncryptedPrivateKeyInfo(self, der):
        from Cryptodome.Util.asn1 import DerObject, DerSequence, DerOctetString, DerObjectId
        encryptedPrivateKeyInfo = DerSequence()
        encryptedPrivateKeyInfo.decode(der)
        encryptionAlgorithm = DerSequence()
        algorithm = DerObjectId()
        encryptedData = DerOctetString()
        encryptionAlgorithm.decode(encryptedPrivateKeyInfo[0])
        DerObject.decode(algorithm, encryptionAlgorithm[0])
        DerObject.decode(encryptedData, encryptedPrivateKeyInfo[1])
        if algorithm.payload != self.oid_aesKeyWrap:
            raise ValueError
        return encryptedData.payload

    def encode_EncryptedPrivateKeyInfo(self, der):
        from Cryptodome.Util.asn1 import DerSequence, DerOctetString
        return DerSequence([
            DerSequence([
                struct.pack("BB", 0x06, len(self.oid_aesKeyWrap)) + self.oid_aesKeyWrap
            ]).encode(),
            DerOctetString(der).encode()
        ]).encode()

    def gen_salt(self, bytes = 16):
        from Cryptodome import Random
        return Random.new().read(bytes)

    def wrapper(self, salt, keylen = 256, iterations = 8000):
        from Cryptodome.Protocol.KDF import PBKDF2
        from Cryptodome.Hash         import SHA256, HMAC
        return AESKeyWrapWithPadding(PBKDF2(
            password = getpass.getpass("KEKEK Passphrase: "),
            salt     = salt,
            dkLen    = keylen//8,
            count    = iterations,
            prf      = lambda p, s: HMAC.new(p, s, SHA256).digest()))

    @classmethod
    def is_soft_key(cls, db):
        return all(k in db for k in ("kekek_pkcs8", "kekek_salt"))

    @classmethod
    def generate(cls, args, result):
        from Cryptodome.PublicKey import RSA
        self = cls()
        k = RSA.generate(args.keylen)
        salt  = self.gen_salt()
        spki  = k.publickey().exportKey(format = "DER")
        pkcs8 = self.encode_EncryptedPrivateKeyInfo(self.wrapper(salt).wrap(
            k.exportKey(format = "DER", pkcs = 8)))
        result.update(kekek_salt   = b64(salt),
                      kekek_pkcs8  = b64(pkcs8),
                      kekek_pubkey = b64(spki))

    @classmethod
    def recover(cls, db):
        self = cls()
        return self.wrapper(b64join(db["kekek_salt"])).unwrap(
            self.parse_EncryptedPrivateKeyInfo(b64join(db["kekek_pkcs8"])))


if __name__ == "__main__":
    main()