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import hashlib
from typing import Optional
import argon2
import hkdf
[docs]def stretch_argon2_secret(email: str, password: str, salt: Optional[bytes] = None) -> bytes:
"""
Compute argon2id based secret from user email and password only. This uses Argon2id
for stretching a potentially weak user password/PIN and subsequent HKDF based key
extending to derive private key material (PKM) for different usage contexts.
The Argon2 parameters used are the following:
* kdf ``argon2id-13``
* time cost ``4096``
* memory cost ``512``
* parallelism ``1``
See `draft-irtf-cfrg-argon2 <https://datatracker.ietf.org/doc/draft-irtf-cfrg-argon2/>`__ and
`argon2-cffi <https://argon2-cffi.readthedocs.io/en/stable/>`__.
:param email: User email.
:param password: User password.
:param salt: Optional salt to use (must be 16 bytes long). If none is given, compute salt
from email as ``salt = SHA256(email)[:16]``.
:return: The computed private key material (256b, 32 octets).
"""
if not salt:
m = hashlib.sha256()
m.update(email.encode('utf8'))
salt = m.digest()[:16]
assert len(salt) == 16
pkm = argon2.low_level.hash_secret_raw(
secret=password.encode('utf8'),
salt=salt,
time_cost=4096,
memory_cost=512,
parallelism=1,
hash_len=32,
type=argon2.low_level.Type.ID,
version=19,
)
return pkm
[docs]def expand_argon2_secret(pkm: bytes, context: bytes, salt: Optional[bytes] = None) -> bytes:
"""
Expand ``pkm`` and ``context`` into a key of length ``bytes`` using
HKDF's expand function based on HMAC SHA-512). See the HKDF draft RFC and paper for usage notes.
:param pkm:
:param context:
:param salt:
:return:
"""
kdf = hkdf.Hkdf(salt=salt, input_key_material=pkm, hash=hashlib.sha512)
key = kdf.expand(info=context, length=32)
return key
[docs]def pkm_from_argon2_secret(email: str, password: str, context: str, salt: Optional[bytes] = None) -> bytes:
"""
:param email:
:param password:
:param context:
:param salt:
:return:
"""
if not salt:
m = hashlib.sha256()
m.update(email.encode('utf8'))
salt = m.digest()[:16]
assert len(salt) == 16
context = context.encode('utf8')
pkm = stretch_argon2_secret(email=email, password=password, salt=salt)
key = expand_argon2_secret(pkm=pkm, context=context, salt=salt)
return key