rsa_source/rsa/varblock.py

   1 # -*- coding: utf-8 -*-
   2 #
   3 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
   4 #
   5 #  Licensed under the Apache License, Version 2.0 (the "License");
   6 #  you may not use this file except in compliance with the License.
   7 #  You may obtain a copy of the License at
   8 #
   9 #      http://www.apache.org/licenses/LICENSE-2.0
  10 #
  11 #  Unless required by applicable law or agreed to in writing, software
  12 #  distributed under the License is distributed on an "AS IS" BASIS,
  13 #  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  14 #  See the License for the specific language governing permissions and
  15 #  limitations under the License.
  16 '''VARBLOCK file support
  17
  18 The VARBLOCK file format is as follows, where || denotes byte concatenation:
  19
  20     FILE := VERSION || BLOCK || BLOCK ...
  21
  22     BLOCK := LENGTH || DATA
  23
  24     LENGTH := varint-encoded length of the subsequent data. Varint comes from
  25     Google Protobuf, and encodes an integer into a variable number of bytes.
  26     Each byte uses the 7 lowest bits to encode the value. The highest bit set
  27     to 1 indicates the next byte is also part of the varint. The last byte will
  28     have this bit set to 0.
  29
  30 This file format is called the VARBLOCK format, in line with the varint format
  31 used to denote the block sizes.
  32
  33 '''
  34
  35 VARBLOCK_VERSION = 1
  36
  37 def read_varint(infile):
  38     '''Reads a varint from the file.
  39
  40     When the first byte to be read indicates EOF, (0, 0) is returned. When an
  41     EOF occurs when at least one byte has been read, an EOFError exception is
  42     raised.
  43
  44     @param infile: the file-like object to read from. It should have a read()
  45         method.
  46     @returns (varint, length), the read varint and the number of read bytes.
  47     '''
  48
  49     varint = 0
  50     read_bytes = 0
  51
  52     while True:
  53         char = infile.read(1)
  54         if len(char) == 0:
  55             if read_bytes == 0:
  56                 return (0, 0)
  57             raise EOFError('EOF while reading varint, value is %i so far' %
  58                     varint)
  59
  60         byte = ord(char)
  61         varint += (byte & 0x7F) << (7 * read_bytes)
  62
  63         read_bytes += 1
  64
  65         if not byte & 0x80:
  66             return (varint, read_bytes)
  67
  68 def write_varint(outfile, value):
  69     '''Writes a varint to a file.
  70
  71     @param outfile: the file-like object to write to. It should have a write()
  72         method.
  73     @returns the number of written bytes.
  74     '''
  75
  76     # there is a big difference between 'write the value 0' (this case) and
  77     # 'there is nothing left to write' (the false-case of the while loop)
  78
  79     if value == 0:
  80         outfile.write('\x00')
  81         return 1
  82
  83     written_bytes = 0
  84     while value > 0:
  85         to_write = value & 0x7f
  86         value = value >> 7
  87
  88         if value > 0:
  89             to_write |= 0x80
  90
  91         outfile.write(chr(to_write))
  92         written_bytes += 1
  93
  94     return written_bytes
  95
  96
  97 def yield_varblocks(infile):
  98     '''Generator, yields each block in the input file.
  99
 100     @param infile: file to read, is expected to have the VARBLOCK format as
 101         described in the module's docstring.
 102     @yields the contents of each block.
 103     '''
 104
 105     # Check the version number
 106     first_char = infile.read(1)
 107     if len(first_char) == 0:
 108         raise EOFError('Unable to read VARBLOCK version number')
 109
 110     version = ord(first_char)
 111     if version != VARBLOCK_VERSION:
 112         raise ValueError('VARBLOCK version %i not supported' % version)
 113
 114     while True:
 115         (block_size, read_bytes) = read_varint(infile)
 116
 117         # EOF at block boundary, that's fine.
 118         if read_bytes == 0 and block_size == 0:
 119             break
 120
 121         block = infile.read(block_size)
 122
 123         read_size = len(block)
 124         if read_size != block_size:
 125             raise EOFError('Block size is %i, but could read only %i bytes' %
 126                     (block_size, read_size))
 127
 128
 129         yield block
 130
 131 def yield_fixedblocks(infile, blocksize):
 132     '''Generator, yields each block of ``blocksize`` bytes in the input file.
 133
 134     :param infile: file to read and separate in blocks.
 135     :returns: a generator that yields the contents of each block
 136     '''
 137
 138     while True:
 139         block = infile.read(blocksize)
 140
 141         read_bytes = len(block)
 142         if read_bytes == 0:
 143             break
 144
 145         yield block
 146
 147         if read_bytes < blocksize:
 148             break
 149