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Delete rsa.py
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Daniel
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"""
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Helper functions for cryptography-related operations inside Tahoe
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using RSA public-key encryption and decryption.
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In cases where these functions happen to use and return objects that
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are documented in the `cryptography` library, code outside this module
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should only use functions from allmydata.crypto.rsa and not rely on
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features of any objects that `cryptography` documents.
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That is, the public and private keys are opaque objects; DO NOT depend
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on any of their methods.
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"""
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from __future__ import annotations
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from typing_extensions import TypeAlias
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from typing import Callable
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from functools import partial
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from cryptography.exceptions import InvalidSignature
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from cryptography.hazmat.backends import default_backend
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from cryptography.hazmat.primitives import hashes
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from cryptography.hazmat.primitives.asymmetric import rsa, padding
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from cryptography.hazmat.primitives.serialization import load_der_private_key, load_der_public_key, \
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Encoding, PrivateFormat, PublicFormat, NoEncryption
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from allmydata.crypto.error import BadSignature
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PublicKey: TypeAlias = rsa.RSAPublicKey
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PrivateKey: TypeAlias = rsa.RSAPrivateKey
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# This is the value that was used by `pycryptopp`, and we must continue to use it for
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# both backwards compatibility and interoperability.
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#
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# The docs for `cryptography` suggest to use the constant defined at
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# `cryptography.hazmat.primitives.asymmetric.padding.PSS.MAX_LENGTH`, but this causes old
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# signatures to fail to validate.
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RSA_PSS_SALT_LENGTH = 32
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RSA_PADDING = padding.PSS(
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mgf=padding.MGF1(hashes.SHA256()),
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salt_length=RSA_PSS_SALT_LENGTH,
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)
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def create_signing_keypair(key_size: int) -> tuple[PrivateKey, PublicKey]:
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"""
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Create a new RSA signing (private) keypair from scratch. Can be used with
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`sign_data` function.
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:param key_size: length of key in bits
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:returns: 2-tuple of (private_key, public_key)
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"""
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priv_key = rsa.generate_private_key(
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public_exponent=65537,
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key_size=key_size,
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backend=default_backend()
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)
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return priv_key, priv_key.public_key()
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def create_signing_keypair_from_string(private_key_der: bytes) -> tuple[PrivateKey, PublicKey]:
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"""
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Create an RSA signing (private) key from previously serialized
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private key bytes.
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:param private_key_der: blob as returned from `der_string_from_signing_keypair`
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:returns: 2-tuple of (private_key, public_key)
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"""
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_load = partial(
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load_der_private_key,
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private_key_der,
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password=None,
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backend=default_backend(),
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)
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def load_with_validation() -> PrivateKey:
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k = _load()
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assert isinstance(k, PrivateKey)
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return k
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def load_without_validation() -> PrivateKey:
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k = _load(unsafe_skip_rsa_key_validation=True)
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assert isinstance(k, PrivateKey)
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return k
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# Load it once without the potentially expensive OpenSSL validation
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# checks. These have superlinear complexity. We *will* run them just
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# below - but first we'll apply our own constant-time checks.
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load: Callable[[], PrivateKey] = load_without_validation
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try:
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unsafe_priv_key = load()
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except TypeError:
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# cryptography<39 does not support this parameter, so just load the
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# key with validation...
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unsafe_priv_key = load_with_validation()
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# But avoid *reloading* it since that will run the expensive
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# validation *again*.
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load = lambda: unsafe_priv_key
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if not isinstance(unsafe_priv_key, rsa.RSAPrivateKey):
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raise ValueError(
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"Private Key did not decode to an RSA key"
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)
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if unsafe_priv_key.key_size != 2048:
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raise ValueError(
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"Private Key must be 2048 bits"
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)
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# Now re-load it with OpenSSL's validation applied.
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safe_priv_key = load()
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return safe_priv_key, safe_priv_key.public_key()
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def der_string_from_signing_key(private_key: PrivateKey) -> bytes:
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"""
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Serializes a given RSA private key to a DER string
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:param private_key: a private key object as returned from
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`create_signing_keypair` or `create_signing_keypair_from_string`
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:returns: bytes representing `private_key`
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"""
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_validate_private_key(private_key)
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return private_key.private_bytes(
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encoding=Encoding.DER,
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format=PrivateFormat.PKCS8,
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encryption_algorithm=NoEncryption(),
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)
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def der_string_from_verifying_key(public_key: PublicKey) -> bytes:
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"""
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Serializes a given RSA public key to a DER string.
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:param public_key: a public key object as returned from
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`create_signing_keypair` or `create_signing_keypair_from_string`
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:returns: bytes representing `public_key`
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"""
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_validate_public_key(public_key)
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return public_key.public_bytes(
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encoding=Encoding.DER,
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format=PublicFormat.SubjectPublicKeyInfo,
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)
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def create_verifying_key_from_string(public_key_der: bytes) -> PublicKey:
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"""
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Create an RSA verifying key from a previously serialized public key
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:param bytes public_key_der: a blob as returned by `der_string_from_verifying_key`
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:returns: a public key object suitable for use with other
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functions in this module
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"""
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pub_key = load_der_public_key(
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public_key_der,
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backend=default_backend(),
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)
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assert isinstance(pub_key, PublicKey)
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return pub_key
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def sign_data(private_key: PrivateKey, data: bytes) -> bytes:
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"""
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:param private_key: the private part of a keypair returned from
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`create_signing_keypair_from_string` or `create_signing_keypair`
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:param data: the bytes to sign
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:returns: bytes which are a signature of the bytes given as `data`.
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"""
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_validate_private_key(private_key)
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return private_key.sign(
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data,
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RSA_PADDING,
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hashes.SHA256(),
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)
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def verify_signature(public_key: PublicKey, alleged_signature: bytes, data: bytes) -> None:
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"""
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:param public_key: a verifying key, returned from `create_verifying_key_from_string` or `create_verifying_key_from_private_key`
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:param bytes alleged_signature: the bytes of the alleged signature
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:param bytes data: the data which was allegedly signed
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"""
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_validate_public_key(public_key)
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try:
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public_key.verify(
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alleged_signature,
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data,
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RSA_PADDING,
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hashes.SHA256(),
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)
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except InvalidSignature:
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raise BadSignature()
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def _validate_public_key(public_key: PublicKey) -> None:
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"""
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Internal helper. Checks that `public_key` is a valid cryptography
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object
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"""
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if not isinstance(public_key, rsa.RSAPublicKey):
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raise ValueError(
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f"public_key must be an RSAPublicKey not {type(public_key)}"
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)
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def _validate_private_key(private_key: PrivateKey) -> None:
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"""
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Internal helper. Checks that `public_key` is a valid cryptography
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object
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"""
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if not isinstance(private_key, rsa.RSAPrivateKey):
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raise ValueError(
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f"private_key must be an RSAPrivateKey not {type(private_key)}"
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)
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