William Grant
128 lines
3.7 KiB
Python
128 lines
3.7 KiB
Python
# This software is provided 'as-is', without any express or implied
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# warranty. In no event will the author be held liable for any damages
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# arising from the use of this software.
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#
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# Permission is granted to anyone to use this software for any purpose,
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# including commercial applications, and to alter it and redistribute it
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# freely, subject to the following restrictions:
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#
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# 1. The origin of this software must not be misrepresented; you must not
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# claim that you wrote the original software. If you use this software
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# in a product, an acknowledgment in the product documentation would be
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# appreciated but is not required.
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# 2. Altered source versions must be plainly marked as such, and must not be
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# misrepresented as being the original software.
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# 3. This notice may not be removed or altered from any source distribution.
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#
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# Copyright (c) 2008 Greg Hewgill http://hewgill.com
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__all__ = [
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'asn1_build',
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'asn1_parse',
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'ASN1FormatError',
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'BIT_STRING',
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'INTEGER',
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'SEQUENCE',
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'OBJECT_IDENTIFIER',
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'OCTET_STRING',
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'NULL',
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]
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INTEGER = 0x02
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BIT_STRING = 0x03
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OCTET_STRING = 0x04
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NULL = 0x05
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OBJECT_IDENTIFIER = 0x06
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SEQUENCE = 0x30
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class ASN1FormatError(Exception):
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pass
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def asn1_parse(template, data):
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"""Parse a data structure according to an ASN.1 template.
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@param template: tuples comprising the ASN.1 template
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@param data: byte string data to parse
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@return: decoded structure
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"""
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r = []
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i = 0
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for t in template:
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tag = ord(data[i])
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i += 1
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if tag == t[0]:
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length = ord(data[i])
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i += 1
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if length & 0x80:
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n = length & 0x7f
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length = 0
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for j in range(n):
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length = (length << 8) | ord(data[i])
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i += 1
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if tag == INTEGER:
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n = 0
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for j in range(length):
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n = (n << 8) | ord(data[i])
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i += 1
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r.append(n)
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elif tag == BIT_STRING:
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r.append(data[i:i+length])
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i += length
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elif tag == NULL:
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assert length == 0
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r.append(None)
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elif tag == OBJECT_IDENTIFIER:
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r.append(data[i:i+length])
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i += length
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elif tag == SEQUENCE:
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r.append(asn1_parse(t[1], data[i:i+length]))
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i += length
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else:
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raise ASN1FormatError(
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"Unexpected tag in template: %02x" % tag)
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else:
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raise ASN1FormatError(
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"Unexpected tag (got %02x, expecting %02x)" % (tag, t[0]))
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return r
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def asn1_length(n):
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"""Return a string representing a field length in ASN.1 format.
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@param n: integer field length
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@return: ASN.1 field length
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"""
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assert n >= 0
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if n < 0x7f:
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return chr(n)
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r = ""
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while n > 0:
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r = chr(n & 0xff) + r
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n >>= 8
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return r
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def asn1_build(node):
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"""Build a DER-encoded ASN.1 data structure.
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@param node: (type, data) tuples comprising the ASN.1 structure
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@return: DER-encoded ASN.1 byte string
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"""
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if node[0] == OCTET_STRING:
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return chr(OCTET_STRING) + asn1_length(len(node[1])) + node[1]
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if node[0] == NULL:
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assert node[1] is None
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return chr(NULL) + asn1_length(0)
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elif node[0] == OBJECT_IDENTIFIER:
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return chr(OBJECT_IDENTIFIER) + asn1_length(len(node[1])) + node[1]
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elif node[0] == SEQUENCE:
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r = ""
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for x in node[1]:
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r += asn1_build(x)
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return chr(SEQUENCE) + asn1_length(len(r)) + r
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else:
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raise ASN1FormatError("Unexpected tag in template: %02x" % node[0])
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