'Decrypt a single block'

def decryptBlock(self, encryptedBlock):

    if (self.decryptBlockCount == 0):
        if (self.iv == None):
            self.prior_CT_block = encryptedBlock
            return ''
        else:
            assert (len(self.iv) == self.blockSize), 'Bad    IV    size    on    CBC    decryption'
            self.prior_CT_block = self.iv
    dct = self.baseCipher.decryptBlock(encryptedBlock)
    '    XOR    the    prior    decrypted    CT    with    the    prior    CT    '
    dct_XOR_priorCT = xor(self.prior_CT_block, dct)
    self.prior_CT_block = encryptedBlock
    return dct_XOR_priorCT
