# # Cipher/oaep.py : PKCS#1 OAEP # # =================================================================== # The contents of this file are dedicated to the public domain. To # the extent that dedication to the public domain is not available, # everyone is granted a worldwide, perpetual, royalty-free, # non-exclusive license to exercise all rights associated with the # contents of this file for any purpose whatsoever. # No rights are reserved. # # THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, # EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF # MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND # NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS # BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN # ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN # CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE # SOFTWARE. # =================================================================== from __future__ import annotations from typing import TYPE_CHECKING, Any, Optional, Protocol, Union import Crypto.Util.number from Crypto import Random from Crypto.Signature.pss import MGF1, MaskFunction, RndFunction from Crypto.Util._raw_api import is_buffer from Crypto.Util.number import bytes_to_long, ceil_div, long_to_bytes from Crypto.Util.strxor import strxor from ._pkcs1_oaep_decode import oaep_decode if TYPE_CHECKING: from Crypto.PublicKey.RSA import RsaKey class HashLikeClass(Protocol): digest_size: int def new(self, data: Optional[bytes] = ...) -> Any: ... class HashLikeModule(Protocol): digest_size: int @staticmethod def new(data: Optional[bytes] = ...) -> Any: ... HashLike = Union[HashLikeClass, HashLikeModule] Buffer = Union[bytes, bytearray, memoryview] class PKCS1OAEP_Cipher: """Cipher object for PKCS#1 OAEP. Do not create directly: use :func:`new` instead.""" def __init__( self, rsa_key: RsaKey, hashmod: HashLike, mask_func: Optional[MaskFunction], label: Buffer, rand_func: Optional[RndFunction], ) -> None: """Initialize this PKCS#1 OAEP cipher object. See :func:`new` for the parameters. """ self._key = rsa_key if not (hasattr(hashmod, "digest_size") and hasattr(hashmod, "new")): raise TypeError("A hash module or hash object is required for 'hashmod'") self._hashObj = hashmod if mask_func: self._mgf = mask_func else: self._mgf = lambda x, y: MGF1(x, y, self._hashObj) if not is_buffer(label): raise TypeError("Label must be bytes, bytearray or memoryview") self._label = bytes(label) if rand_func is None: rand_func = Random.get_random_bytes self._randfunc = rand_func def encrypt(self, message: Buffer) -> bytes: """Encrypt a message with PKCS#1 OAEP. :param message: The message to encrypt, also known as plaintext. It can be of variable length, but not longer than the RSA modulus (in bytes) minus 2, minus twice the hash output size. For instance, if you use RSA 2048 and SHA-256, the longest message you can encrypt is 190 byte long. :type message: bytes/bytearray/memoryview :returns: The ciphertext, as large as the RSA modulus. :rtype: bytes :raises ValueError: if the message is too long. """ # See 7.1.1 in RFC 8017 modBits = Crypto.Util.number.size(self._key.n) k = ceil_div(modBits, 8) # Convert from bits to bytes hLen = self._hashObj.digest_size mLen = len(message) # Step 1b ps_len = k - mLen - 2 * hLen - 2 if ps_len < 0: raise ValueError("Plaintext is too long.") # Step 2a lHash = self._hashObj.new(self._label).digest() # Step 2b ps = b"\x00" * ps_len # Step 2c db = lHash + ps + b"\x01" + bytes(message) # Step 2d ros = self._randfunc(hLen) # Step 2e dbMask = self._mgf(ros, k - hLen - 1) # Step 2f maskedDB = strxor(db, dbMask) # Step 2g seedMask = self._mgf(maskedDB, hLen) # Step 2h maskedSeed = strxor(ros, seedMask) # Step 2i em = b"\x00" + maskedSeed + maskedDB # Step 3a (OS2IP) em_int = bytes_to_long(em) # Step 3b (RSAEP) m_int = self._key._encrypt(em_int) # Step 3c (I2OSP) c = long_to_bytes(m_int, k) return c def decrypt(self, ciphertext: Buffer) -> bytes: """Decrypt a message with PKCS#1 OAEP. :param ciphertext: The encrypted message. :type ciphertext: bytes/bytearray/memoryview :returns: The original message (plaintext). :rtype: bytes :raises ValueError: if the ciphertext has the wrong length, or if decryption fails the integrity check (in which case, the decryption key is probably wrong). :raises TypeError: if the RSA key has no private half (i.e. you are trying to decrypt using a public key). """ # See 7.1.2 in RFC 8017 modBits = Crypto.Util.number.size(self._key.n) k = ceil_div(modBits, 8) # Convert from bits to bytes hLen = self._hashObj.digest_size # Step 1b and 1c if len(ciphertext) != k or k < hLen + 2: raise ValueError("Ciphertext with incorrect length.") # Step 2a (O2SIP) ct_int = bytes_to_long(ciphertext) # Step 2b (RSADP) and step 2c (I2OSP) em = self._key._decrypt_to_bytes(ct_int) # Step 3a lHash = self._hashObj.new(self._label).digest() # y must be 0, but we MUST NOT check it here in order not to # allow attacks like Manger's (http://dl.acm.org/citation.cfm?id=704143) maskedSeed = em[1 : hLen + 1] maskedDB = em[hLen + 1 :] # Step 3c seedMask = self._mgf(maskedDB, hLen) # Step 3d seed = strxor(maskedSeed, seedMask) # Step 3e dbMask = self._mgf(seed, k - hLen - 1) # Step 3f db = strxor(maskedDB, dbMask) # Step 3b + 3g res = oaep_decode(em, lHash, db) if res <= 0: raise ValueError("Incorrect decryption.") # Step 4 return db[res:] def new( rsa_key: RsaKey, *, hashmod: HashLike, mask_func: Optional[MaskFunction] = None, label: Buffer = b"", rand_func: Optional[RndFunction] = None, ) -> PKCS1OAEP_Cipher: """Return a cipher object :class:`PKCS1OAEP_Cipher` that can be used to perform PKCS#1 OAEP encryption or decryption. :param rsa_key: The key object to use to encrypt or decrypt the message. Decryption is only possible with a private RSA key. :type rsa_key: RSA key object :param hashmod: The hash function to use. This can be a module under `Crypto.Hash` or an existing hash object created from any of such modules. It must always be specified: new protocols should use SHA-256 or a stronger hash function, while `Crypto.Hash.SHA1` matches the RSAES-OAEP default defined in RFC8017, which some older protocols still require. :type hashmod: hash module or object :param mask_func: A mask generation function that accepts two parameters: a string to use as seed, and the length of the mask to generate, in bytes. If not specified, the standard MGF1 consistent with ``hashmod`` is used (a safe choice). Modify it only if you know what you are doing (for instance, for interoperability): sender and receiver must use the same one. :type mask_func: callable :param label: A label to apply to this particular encryption. If not specified, an empty string is used. Specifying a label does not improve security. :type label: bytes/bytearray/memoryview :param rand_func: A function that returns random bytes. The default is :func:`Crypto.Random.get_random_bytes`. :type rand_func: callable :raises TypeError: if ``hashmod`` is not a hash module or object. """ return PKCS1OAEP_Cipher(rsa_key, hashmod, mask_func, label, rand_func)