NAME

       packet.derunpack - DER decoding module

DESCRIPTION

       Decode  using  ASN.1  DER  (Distinguished  Encoding Representation) ASN.1: Abstract Syntax
       Notation 1

       This module does not completely decode all DER data types, the  following  is  a  list  of
       supported data types in this implementation:
           INTEGER,
           BIT_STRING,
           NULL,
           OBJECT_IDENTIFIER,
           GeneralizedTime,
           Strings (OCTET STRING, PrintableString, etc.)
           SEQUENCE OF,
           SEQUENCE,

CLASSES

   class DERunpack(packet.unpack.Unpack)
       DER unpack object

       Usage:
           from packet.derunpack import DERunpack

           x = DERunpack(buffer)

           # Get the decoded object structure for the stream bytes in buffer
           obj = x.get_item()

           Where obj is of the form:
               obj = {
                   application = {
                       context-tag0 = int|list|dictionary,
                       context-tag1 = int|list|dictionary,
                       ...
                       context-tagN = int|list|dictionary,
                   }
               }

           Example:
             For the following ASN.1 definition:
               TEST ::= [APPLICATION 10] SEQUENCE {
                   id       [0] INTEGER,
                   numbers  [1] SEQUENCE OF INTEGER,
                   data     [2] SEQUENCE {
                       -- NOTE: first tag is [1], not [0]
                       type   [1] INTEGER,
                       value  [2] PrintableString,
                   },
               }

             Using the streamed bytes of the above ASN.1 definition,
             the following is returned by get_item():
               obj = {
                   10 = {              # Application 10
                       0: 53,          # id: context-tag=0, value=53
                       1: [1,2,3],     # numbers: context-tag=1, value=[1,2,3]
                       2: {            # data: context-tag=1, value=structure
                           1: 2,       # id: context-tag=1, value=2
                           2: "test",  # id: context-tag=2, value="test"
                       }
                   }
               }

       Methods defined here:
       ---------------------

       der_date(self, size)
       Return a date time of type GeneralizedTime
       Type GeneralizedTime takes values of the year, month, day, hour,
       minute, second, and second fraction in any of following three forms:

       Local time: "YYYYMMDDHH[MM[SS[.fff]]]"
       Universal time (UTC): "YYYYMMDDHH[MM[SS[.fff]]]Z"
       Difference between local and UTC times" "YYYYMMDDHH[MM[SS[.fff]]]+|-HHMM".

       Where the optional fff is accurate to three decimal places

       der_integer(self, size=None, unsigned=False)
       Return an integer given the size of the integer in bytes

              size:  Number of bytes for the integer, if this option is not given
                     the method get_size() is used to get the size of the integer

              unsigned:
                     Usually an unsigned integer is encoded with a leading byte
                     of all zeros but when decoding data of BIT_STRING type all
                     decoded bytes must be unsigned so they can be concatenated
                     correctly

       der_oid(self, size)
       Return an object identifier (OID)

       get_item(self, oid=None)
       Get item from the byte stream using TLV
       This is a recursive function where the tag and length are decoded
       and then this function is called to get the value if tag is one of
       primitive or non-constructed types.

       Calling this method right after instantiation of the object will
       decode the whole ASN.1 representation

       get_size(self)
       Get the size of element (length in TLV)

       Short form: bit8=0, one octet, length given by bits 7-1 (0-127)
       Long form:  bit8=1, 2-127 octet, bits 7-1 give number of length
                   objects

       Example:
         Short form (bit8=0):
           0x0f (0b00001111): length is 0x0f (15)
         Long form (bit8=1 of first byte):
           0x820123 (0b100000100000000100100011):
           length is given by the next 2 bytes (first 7-1 bits 0x02)
           Next two bytes gives the length 0x0123 = 291

       get_tag(self)
       Get the tag along with the tag class and form or P/C bit

       The first byte(s) of the TLV (Type, Length, Value) is the type
       which has the following format:
         First byte:
           bits 8-7: tag class
           bit 6:    form or P/C (Constructed if bit is set)
           bits 5-1: tag number (0-30)
                     if all bits are 1's (decimal 31) then one or more
                     bytes are required for the tag

         If bits 5-1 are all 1's in the first byte, the tag is given
         in the following bytes:
         Extra byes for tag:
           bit 8:    next byte is part of tag
           bits 7-1: tag bits

       Examples:
         0xa1 (0b10100001): Short form
           tag class: 0b10 = 2 (CONTEXT)
           P/C:       0b0  = 0 (not constructed)

         0x1f8107 (0b000111111000000100000111): Long form
           tag class: 0b00 = 0 (UNIVERSAL -- standard tag)
           P/C:       0b0  = 0 (not constructed)
           tag:   0b11111 = 31 (tag is given in following bytes)
           First extra byte: 0x81 (0b10000001)
             bit8=1  : there is an extra byte after this
             bits 7-1: 0b0000001 (0x01 most significant 7 bits of tag)
           Second extra byte: 0x07 (0b00000111)
             bit8=1  : this is the last byte
             bits 7-1: 0b0000111 (0x07 least significant 7 bits of tag)
           Tag number: big-endian bits from extra bytes (7 bits each)
                      14 bits: 0x0087 (0x01 << 7 + 0x07) = 135

SEE ALSO

       packet.unpack(3)

BUGS

       No known bugs.

AUTHOR

       Jorge Mora (mora@netapp.com)