'Decrypt a PKCS#1 v1.5 ciphertext.
This function is named ``RSAES-PKCS1-V1_5-DECRYPT``, and is specified in
section 7.2.2 of RFC3447.
For a complete example see `Crypto.Cipher.PKCS1_v1_5`.
:Parameters:
ct : byte string
The ciphertext that contains the message to recover.
sentinel : any type
The object to return to indicate that an error was detected during decryption.
:Return: A byte string. It is either the original message or the ``sentinel`` (in case of an error).
:Raise ValueError:
If the ciphertext length is incorrect
:Raise TypeError:
If the RSA key has no private half.
:attention:
You should **never** let the party who submitted the ciphertext know that
this function returned the ``sentinel`` value.
Armed with such knowledge (for a fair amount of carefully crafted but invalid ciphertexts),
an attacker is able to recontruct the plaintext of any other encryption that were carried out
with the same RSA public key (see `Bleichenbacher\'s`__ attack).
In general, it should not be possible for the other party to distinguish
whether processing at the server side failed because the value returned
was a ``sentinel`` as opposed to a random, invalid message.
In fact, the second option is not that unlikely: encryption done according to PKCS#1 v1.5
embeds no good integrity check. There is roughly one chance
in 2^16 for a random ciphertext to be returned as a valid message
(although random looking).
It is therefore advisabled to:
1. Select as ``sentinel`` a value that resembles a plausable random, invalid message.
2. Not report back an error as soon as you detect a ``sentinel`` value.
Put differently, you should not explicitly check if the returned value is the ``sentinel`` or not.
3. Cover all possible errors with a single, generic error indicator.
4. Embed into the definition of ``message`` (at the protocol level) a digest (e.g. ``SHA-1``).
It is recommended for it to be the rightmost part ``message``.
5. Where possible, monitor the number of errors due to ciphertexts originating from the same party,
and slow down the rate of the requests from such party (or even blacklist it altogether).
**If you are designing a new protocol, consider using the more robust PKCS#1 OAEP.**
.. __: http://www.bell-labs.com/user/bleichen/papers/pkcs.ps'

def decrypt(self, ct, sentinel):

    modBits = Crypto.Util.number.size(self._key.n)
    k = ceil_div(modBits, 8)
    if (len(ct) != k):
        raise ValueError('Ciphertext    with    incorrect    length.')
    m = self._key.decrypt(ct)
    em = ((bchr(0) * (k - len(m))) + m)
    sep = em.find(bchr(0), 2)
    if ((not em.startswith(b('\x00\x02'))) or (sep < 10)):
        return sentinel
    return em[(sep + 1):]
