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240 lines
8.7 KiB
Python
240 lines
8.7 KiB
Python
#
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# Cipher/PKCS1_OAEP.py : PKCS#1 OAEP
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#
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# ===================================================================
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# The contents of this file are dedicated to the public domain. To
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# the extent that dedication to the public domain is not available,
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# everyone is granted a worldwide, perpetual, royalty-free,
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# non-exclusive license to exercise all rights associated with the
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# contents of this file for any purpose whatsoever.
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# No rights are reserved.
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#
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# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
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# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
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# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
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# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
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# BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
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# ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
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# CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
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# SOFTWARE.
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# ===================================================================
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from hashlib import sha1
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import Crypto.Hash.SHA1
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import Crypto.Util.number
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import gmpy2
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from Crypto import Random
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from Crypto.Signature.pss import MGF1
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from Crypto.Util.number import bytes_to_long, ceil_div, long_to_bytes
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from Crypto.Util.py3compat import _copy_bytes, bord
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from Crypto.Util.strxor import strxor
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class PKCS1OAEP_Cipher:
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"""Cipher object for PKCS#1 v1.5 OAEP.
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Do not create directly: use :func:`new` instead."""
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def __init__(self, key, hashAlgo, mgfunc, label, randfunc):
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"""Initialize this PKCS#1 OAEP cipher object.
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:Parameters:
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key : an RSA key object
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If a private half is given, both encryption and decryption are possible.
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If a public half is given, only encryption is possible.
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hashAlgo : hash object
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The hash function to use. This can be a module under `Crypto.Hash`
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or an existing hash object created from any of such modules. If not specified,
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`Crypto.Hash.SHA1` is used.
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mgfunc : callable
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A mask generation function that accepts two parameters: a string to
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use as seed, and the lenth of the mask to generate, in bytes.
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If not specified, the standard MGF1 consistent with ``hashAlgo`` is used (a safe choice).
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label : bytes/bytearray/memoryview
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A label to apply to this particular encryption. If not specified,
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an empty string is used. Specifying a label does not improve
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security.
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randfunc : callable
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A function that returns random bytes.
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:attention: Modify the mask generation function only if you know what you are doing.
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Sender and receiver must use the same one.
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"""
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self._key = key
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if hashAlgo:
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self._hashObj = hashAlgo
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else:
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self._hashObj = Crypto.Hash.SHA1
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if mgfunc:
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self._mgf = mgfunc
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else:
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self._mgf = lambda x,y: MGF1(x,y,self._hashObj)
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self._label = _copy_bytes(None, None, label)
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self._randfunc = randfunc
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def can_encrypt(self):
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"""Legacy function to check if you can call :meth:`encrypt`.
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.. deprecated:: 3.0"""
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return self._key.can_encrypt()
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def can_decrypt(self):
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"""Legacy function to check if you can call :meth:`decrypt`.
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.. deprecated:: 3.0"""
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return self._key.can_decrypt()
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def encrypt(self, message):
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"""Encrypt a message with PKCS#1 OAEP.
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:param message:
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The message to encrypt, also known as plaintext. It can be of
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variable length, but not longer than the RSA modulus (in bytes)
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minus 2, minus twice the hash output size.
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For instance, if you use RSA 2048 and SHA-256, the longest message
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you can encrypt is 190 byte long.
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:type message: bytes/bytearray/memoryview
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:returns: The ciphertext, as large as the RSA modulus.
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:rtype: bytes
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:raises ValueError:
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if the message is too long.
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"""
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# See 7.1.1 in RFC3447
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modBits = Crypto.Util.number.size(self._key.n)
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k = ceil_div(modBits, 8) # Convert from bits to bytes
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hLen = self._hashObj.digest_size
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mLen = len(message)
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# Step 1b
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ps_len = k - mLen - 2 * hLen - 2
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if ps_len < 0:
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raise ValueError("Plaintext is too long.")
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# Step 2a
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lHash = sha1(self._label).digest()
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# Step 2b
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ps = b'\x00' * ps_len
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# Step 2c
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db = lHash + ps + b'\x01' + _copy_bytes(None, None, message)
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# Step 2d
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ros = self._randfunc(hLen)
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# Step 2e
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dbMask = self._mgf(ros, k-hLen-1)
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# Step 2f
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maskedDB = strxor(db, dbMask)
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# Step 2g
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seedMask = self._mgf(maskedDB, hLen)
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# Step 2h
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maskedSeed = strxor(ros, seedMask)
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# Step 2i
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em = b'\x00' + maskedSeed + maskedDB
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# Step 3a (OS2IP)
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em_int = bytes_to_long(em)
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# Step 3b (RSAEP)
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m_int = gmpy2.powmod(em_int, self._key.e, self._key.n)
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# Step 3c (I2OSP)
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c = long_to_bytes(m_int, k)
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return c
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def decrypt(self, ciphertext):
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"""Decrypt a message with PKCS#1 OAEP.
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:param ciphertext: The encrypted message.
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:type ciphertext: bytes/bytearray/memoryview
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:returns: The original message (plaintext).
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:rtype: bytes
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:raises ValueError:
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if the ciphertext has the wrong length, or if decryption
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fails the integrity check (in which case, the decryption
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key is probably wrong).
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:raises TypeError:
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if the RSA key has no private half (i.e. you are trying
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to decrypt using a public key).
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"""
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# See 7.1.2 in RFC3447
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modBits = Crypto.Util.number.size(self._key.n)
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k = ceil_div(modBits,8) # Convert from bits to bytes
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hLen = self._hashObj.digest_size
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# Step 1b and 1c
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if len(ciphertext) != k or k<hLen+2:
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raise ValueError("Ciphertext with incorrect length.")
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# Step 2a (O2SIP)
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ct_int = bytes_to_long(ciphertext)
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# Step 2b (RSADP)
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# m_int = self._key._decrypt(ct_int)
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m_int = gmpy2.powmod(ct_int, self._key.d, self._key.n)
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# Complete step 2c (I2OSP)
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em = long_to_bytes(m_int, k)
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# Step 3a
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lHash = sha1(self._label).digest()
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# Step 3b
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y = em[0]
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# y must be 0, but we MUST NOT check it here in order not to
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# allow attacks like Manger's (http://dl.acm.org/citation.cfm?id=704143)
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maskedSeed = em[1:hLen+1]
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maskedDB = em[hLen+1:]
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# Step 3c
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seedMask = self._mgf(maskedDB, hLen)
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# Step 3d
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seed = strxor(maskedSeed, seedMask)
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# Step 3e
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dbMask = self._mgf(seed, k-hLen-1)
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# Step 3f
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db = strxor(maskedDB, dbMask)
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# Step 3g
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one_pos = hLen + db[hLen:].find(b'\x01')
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lHash1 = db[:hLen]
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invalid = bord(y) | int(one_pos < hLen)
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hash_compare = strxor(lHash1, lHash)
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for x in hash_compare:
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invalid |= bord(x)
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for x in db[hLen:one_pos]:
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invalid |= bord(x)
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if invalid != 0:
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raise ValueError("Incorrect decryption.")
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# Step 4
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return db[one_pos + 1:]
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def new(key, hashAlgo=None, mgfunc=None, label=b'', randfunc=None):
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"""Return a cipher object :class:`PKCS1OAEP_Cipher` that can be used to perform PKCS#1 OAEP encryption or decryption.
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:param key:
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The key object to use to encrypt or decrypt the message.
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Decryption is only possible with a private RSA key.
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:type key: RSA key object
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:param hashAlgo:
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The hash function to use. This can be a module under `Crypto.Hash`
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or an existing hash object created from any of such modules.
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If not specified, `Crypto.Hash.SHA1` is used.
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:type hashAlgo: hash object
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:param mgfunc:
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A mask generation function that accepts two parameters: a string to
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use as seed, and the lenth of the mask to generate, in bytes.
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If not specified, the standard MGF1 consistent with ``hashAlgo`` is used (a safe choice).
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:type mgfunc: callable
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:param label:
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A label to apply to this particular encryption. If not specified,
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an empty string is used. Specifying a label does not improve
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security.
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:type label: bytes/bytearray/memoryview
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:param randfunc:
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A function that returns random bytes.
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The default is `Random.get_random_bytes`.
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:type randfunc: callable
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"""
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if randfunc is None:
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randfunc = Random.get_random_bytes
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return PKCS1OAEP_Cipher(key, hashAlgo, mgfunc, label, randfunc)
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