Source Code for Module dkim.asn1

  1  # This software is provided 'as-is', without any express or implied 
  2  # warranty.  In no event will the author be held liable for any damages 
  3  # arising from the use of this software. 
  4  # 
  5  # Permission is granted to anyone to use this software for any purpose, 
  6  # including commercial applications, and to alter it and redistribute it 
  7  # freely, subject to the following restrictions: 
  8  # 
  9  # 1. The origin of this software must not be misrepresented; you must not 
 10  #    claim that you wrote the original software. If you use this software 
 11  #    in a product, an acknowledgment in the product documentation would be 
 12  #    appreciated but is not required. 
 13  # 2. Altered source versions must be plainly marked as such, and must not be 
 14  #    misrepresented as being the original software. 
 15  # 3. This notice may not be removed or altered from any source distribution. 
 16  # 
 17  # Copyright (c) 2008 Greg Hewgill http://hewgill.com 
 18  # 
 19  # This has been modified from the original software. 
 20  # Copyright (c) 2011 William Grant <me@williamgrant.id.au> 
 21   
 22  __all__ = [ 
 23      'asn1_build', 
 24      'asn1_parse', 
 25      'ASN1FormatError', 
 26      'BIT_STRING', 
 27      'INTEGER', 
 28      'SEQUENCE', 
 29      'OBJECT_IDENTIFIER', 
 30      'OCTET_STRING', 
 31      'NULL', 
 32      ] 
 33   
 34  INTEGER = 0x02 
 35  BIT_STRING = 0x03 
 36  OCTET_STRING = 0x04 
 37  NULL = 0x05 
 38  OBJECT_IDENTIFIER = 0x06 
 39  SEQUENCE = 0x30 
 40   
 41   
 42 -class ASN1FormatError(Exception): 
 43      pass 
 44   
 45   
 46 -def asn1_parse(template, data): 
 47      """Parse a data structure according to an ASN.1 template. 
 48   
 49      @param template: tuples comprising the ASN.1 template 
 50      @param data: byte string data to parse 
 51      @return: decoded structure 
 52      """ 
 53      data = bytearray(data) 
 54      r = [] 
 55      i = 0 
 56      try: 
 57        for t in template: 
 58            tag = data[i] 
 59            i += 1 
 60            if tag == t[0]: 
 61                length = data[i] 
 62                i += 1 
 63                if length & 0x80: 
 64                    n = length & 0x7f 
 65                    length = 0 
 66                    for j in range(n): 
 67                        length = (length << 8) | data[i] 
 68                        i += 1 
 69                if tag == INTEGER: 
 70                    n = 0 
 71                    for j in range(length): 
 72                        n = (n << 8) | data[i] 
 73                        i += 1 
 74                    r.append(n) 
 75                elif tag == BIT_STRING: 
 76                    r.append(data[i:i+length]) 
 77                    i += length 
 78                elif tag == NULL: 
 79                    assert length == 0 
 80                    r.append(None) 
 81                elif tag == OBJECT_IDENTIFIER: 
 82                    r.append(data[i:i+length]) 
 83                    i += length 
 84                elif tag == SEQUENCE: 
 85                    r.append(asn1_parse(t[1], data[i:i+length])) 
 86                    i += length 
 87                else: 
 88                    raise ASN1FormatError( 
 89                        "Unexpected tag in template: %02x" % tag) 
 90            else: 
 91                raise ASN1FormatError( 
 92                    "Unexpected tag (got %02x, expecting %02x)" % (tag, t[0])) 
 93        return r 
 94      except IndexError: 
 95        raise ASN1FormatError("Data truncated at byte %d"%i) 
 96   
 97 -def asn1_length(n): 
 98      """Return a string representing a field length in ASN.1 format. 
 99   
100      @param n: integer field length 
101      @return: ASN.1 field length 
102      """ 
103      assert n >= 0 
104      if n < 0x7f: 
105          return bytearray([n]) 
106      r = bytearray() 
107      while n > 0: 
108          r.insert(n & 0xff) 
109          n >>= 8 
110      return r 
111   
112   
113 -def asn1_encode(type, data): 
114      length = asn1_length(len(data)) 
115      length.insert(0, type) 
116      length.extend(data) 
117      return length 
118   
119   
120 -def asn1_build(node): 
121      """Build a DER-encoded ASN.1 data structure. 
122   
123      @param node: (type, data) tuples comprising the ASN.1 structure 
124      @return: DER-encoded ASN.1 byte string 
125      """ 
126      if node[0] == OCTET_STRING: 
127          return asn1_encode(OCTET_STRING, node[1]) 
128      if node[0] == NULL: 
129          assert node[1] is None 
130          return asn1_encode(NULL, b'') 
131      elif node[0] == OBJECT_IDENTIFIER: 
132          return asn1_encode(OBJECT_IDENTIFIER, node[1]) 
133      elif node[0] == SEQUENCE: 
134          r = bytearray() 
135          for x in node[1]: 
136              r += asn1_build(x) 
137          return asn1_encode(SEQUENCE, r) 
138      else: 
139          raise ASN1FormatError("Unexpected tag in template: %02x" % node[0]) 
140