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# This file is part of the sbk project
# https://github.com/mbarkhau/sbk
#
# Copyright (c) 2019-2021 Manuel Barkhau (mbarkhau@gmail.com) - MIT License
# SPDX-License-Identifier: MIT
"""Parameter encoding/decoding/initialization.
Data layout for reference.
| Field | Size | Info |
| ------------------- | ------ | -------------------------------------------------------- |
| `f_version` | 4 bit | ... |
| `f_threshold` | 4 bit | minimum shares required for recovery |
| | | max: 1..2**4 = 1..16 |
| `f_share_no` | 8 bit | For shares, the x-coordinate > 0 |
| `f_kdf_parallelism` | 4 bit | `ceil(2 ** n) = kdf_parallelism` in number of threads |
| `f_kdf_mem_cost` | 6 bit | `ceil(1.25 ** n) = kdf_mem_cost` in MiB |
| `f_kdf_time_cost` | 6 bit | `ceil(1.5 ** n) = kdf_time_cost` in iterations |
0 3 4 7 8 11 12 17 18 23 24 31
[ ver ] [thres] [kdf_p] [ kdf_mem ] [ kdf_time] [ share_no ]
4bit 4bit 4bit 6bit 6bit 8bit
"""
import os
import struct
import typing as typ
from . import kdf
from . import primes
SBK_VERSION_V1 = 1
DEFAULT_RAW_SALT_LEN = 13
DEFAULT_BRAINKEY_LEN = 6
ENV_RAW_SALT_LEN = os.getenv('SBK_DEBUG_RAW_SALT_LEN')
ENV_BRAINKEY_LEN = os.getenv('SBK_DEBUG_BRAINKEY_LEN')
RAW_SALT_LEN = int(ENV_RAW_SALT_LEN) if ENV_RAW_SALT_LEN else DEFAULT_RAW_SALT_LEN
BRAINKEY_LEN = int(ENV_BRAINKEY_LEN) if ENV_BRAINKEY_LEN else DEFAULT_BRAINKEY_LEN
# linear fit evaluated with: python -m sbk.ui_common
RAW_SALT_MIN_ENTROPY = RAW_SALT_LEN * 0.19 + 0.3
BRAINKEY_MIN_ENTROPY = BRAINKEY_LEN * 0.19 + 0.3
PARAM_CFG_LEN = 3
SHARE_X_COORD_LEN = 1
SALT_LEN = PARAM_CFG_LEN + RAW_SALT_LEN
MASTER_KEY_LEN = RAW_SALT_LEN + BRAINKEY_LEN
SHARE_LEN = PARAM_CFG_LEN + SHARE_X_COORD_LEN + RAW_SALT_LEN + BRAINKEY_LEN
MIN_ENTROPY = int(os.getenv('SBK_MIN_ENTROPY' , "16"))
MAX_ENTROPY_WAIT = int(os.getenv('SBK_MAX_ENTROPY_WAIT', "10"))
DEFAULT_KDF_TARGET_DURATION = int(os.getenv('SBK_KDF_TARGET_DURATION', "90"))
DEFAULT_THRESHOLD = int(os.getenv('SBK_THRESHOLD' , "3"))
DEFAULT_NUM_SHARES = int(os.getenv('SBK_NUM_SHARES', "5"))
# constrained by f_threshold (4bits)
MAX_THRESHOLD = 16
class ParamConfig(typ.NamedTuple):
version : int
threshold : int
num_shares: int
kdf_params: kdf.KDFParams
@property
def prime(self) -> int:
master_key_bits = MASTER_KEY_LEN * 8
return primes.get_pow2prime(master_key_bits)
def init_param_config(
kdf_params: kdf.KDFParams,
threshold : int,
num_shares: typ.Optional[int] = None,
) -> ParamConfig:
_num_shares = threshold if num_shares is None else num_shares
if threshold > _num_shares:
errmsg = f"threshold must be <= num_shares, got {threshold} > {_num_shares}"
raise ValueError(errmsg)
if not 1 <= threshold <= MAX_THRESHOLD:
errmsg = f"Invalid threshold {threshold}"
raise ValueError(errmsg)
param_cfg = ParamConfig(
version=SBK_VERSION_V1,
threshold=threshold,
num_shares=_num_shares,
kdf_params=kdf_params,
)
return param_cfg
def bytes2param_cfg(data: bytes) -> ParamConfig:
"""Deserialize ParamConfig from the Salt or a Share."""
if len(data) < 3:
errmsg = f"Invalid params len={len(data)}"
raise ValueError(errmsg)
# B: Unsigned Char (1 byte)
# H: Unsigned Short (2 bytes)
fields_01, fields_234 = struct.unpack("!BH", data[:3])
# We don't include the share_no in the ParamConfig, it
# is decoded separately for each share.
# share_no = _fields_5
version = (fields_01 >> 4) & 0xF
f_threshold = (fields_01 >> 0) & 0xF
if version != SBK_VERSION_V1:
raise ValueError(f"Unsupported Version {version}")
threshold = f_threshold + 1
# The param_cfg encoding doesn't include num_shares as it's
# only required when originally generating the shares. The
# minimum value is threshold, so that is what we set it to.
num_shares = threshold
kdf_params = kdf.KDFParams.decode(fields_234)
return ParamConfig(version, threshold, num_shares, kdf_params)
def param_cfg2bytes(param_cfg: ParamConfig) -> bytes:
"""Serialize ParamConfig.
Since these fields are part of the salt, we try
to keep the serialized param_cfg small and leave
more room for randomness, hence the bit twiddling.
"""
f_threshold = param_cfg.threshold - 1
fields_01 = 0
fields_01 |= param_cfg.version << 4
fields_01 |= f_threshold
fields_234 = param_cfg.kdf_params.encode()
param_cfg_data = struct.pack("!BH", fields_01, fields_234)
return param_cfg_data
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