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ASN.1 BER decoder
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from cStringIO import StringIO | |
from struct import pack | |
from binascii import unhexlify, hexlify | |
from itertools import count, dropwhile | |
VERBOSE = True | |
class Asn1Obj: | |
'''generic frame for ASN1 fields, supports nesting''' | |
def __init__(self, klass, constructed, type, indefinite, value=None, children=[], absorbed=None): | |
self.klass = klass | |
self.constructed = constructed | |
self.type = type | |
self.indefinite = indefinite | |
self.value = value | |
self.children = children | |
self.absorbed = absorbed | |
def __repr__(self, indent=0): | |
prefix = ' '*indent | |
if self.children: | |
l = ''.join([c.__repr__(indent+2) + ',\n' for c in self.children]) | |
return '%sAsn1Obj(0x%x, %d, 0x%x, %d, children=[\n%s%s])' % (prefix, self.klass, self.constructed, self.type, self.indefinite, l, prefix) | |
else: | |
return '%sAsn1Obj(0x%x, %d, 0x%x, %d, value=%r)' % (prefix, self.klass, self.constructed, self.type, self.indefinite, self.value) | |
def push_u8(f, u): | |
f.write(pack('!B', u)) | |
class EndOfStream(Exception): pass | |
def pop_u8(f): | |
data = f.read(1) | |
if len(data) == 0: raise EndOfStream() | |
return ord(data) | |
def split_to_bits(x, n): | |
assert(x >= 0) | |
m = (1<<n)-1 | |
nimbs = [] | |
while True: | |
nimbs.append(x&m) | |
x>>=n | |
if x == 0: | |
break | |
return nimbs[::-1] | |
assert(split_to_bits(0, 8) == [0]) | |
assert(split_to_bits(1, 8) == [1]) | |
assert(split_to_bits(128, 8) == [0x80]) | |
assert(split_to_bits(255, 8) == [0xff]) | |
assert(split_to_bits(256, 8) == [1, 0]) | |
def two_complement(x): | |
assert(x < 0) | |
x = -x | |
for n in count(0, 8): | |
if pow(2, n) > x: | |
break | |
return (-x) & (pow(2, n)-1) | |
def encode_integer(x): | |
positive = (x >= 0) | |
abs_x = x | |
if not positive: | |
abs_x = two_complement(x) | |
elms = split_to_bits(abs_x, 8) | |
if positive: | |
if elms[0] & 0x80: | |
elms = [0] + elms | |
else: | |
elms = [e for e in dropwhile(lambda x: x == 255, elms)] | |
if not elms[0] & 0x80: | |
elms = [0xff] + elms | |
return ''.join(chr(c) for c in elms) | |
TESTS = ( | |
(0, '00'), | |
(127, '7f'), | |
(128, '0080'), | |
(256, '0100'), | |
(-128, '80'), | |
(-129, 'ff7f'), | |
) | |
for (v, expected) in TESTS: | |
assert(encode_integer(v) == unhexlify(expected)) | |
def encode_ber(f, obj): | |
'''encode ASN1Obj instance as Binary Encoded Representation''' | |
# encode identifier octets | |
if obj.type < 31: | |
u = (obj.klass<<6) + (obj.constructed<<5) + obj.type | |
push_u8(f, u) | |
else: | |
u = (obj.klass<<6) + (obj.constructed<<5) + 0x1f | |
push_u8(f, u) | |
type_nimbs = split_to_bits(obj.type, 7) | |
for i in range(len(type_nimbs)): | |
u = type_nimbs[i] | |
if i < len(type_nimbs)-1: | |
u |= 0x80 | |
push_u8(f, u) | |
# if obj is constructed, recursively build its value | |
if obj.constructed: | |
g = StringIO() | |
for child in obj.children: | |
encode_ber(g, child) | |
obj.value = g.getvalue() | |
# encode length octets | |
if not obj.indefinite: | |
length = len(obj.value) | |
if length <= 127: | |
push_u8(f, length) | |
else: | |
nimbs = split_to_bits(length, 8) | |
push_u8(f, 0x80 + len(nimbs)) | |
for u in nimbs: | |
push_u8(f, u) | |
else: | |
push_u8(0x80) | |
f.write(obj.value) | |
if obj.indefinite: | |
f.write('\x00\x00') | |
def decode_ber(f, level=0): | |
'''decode Binary Encoded Representation bytes to ASN1Obj''' | |
absorbed = '' | |
u = 0 | |
try: | |
u = pop_u8(f) | |
except EndOfStream: | |
return | |
absorbed += chr(u) | |
_klass = (u>>6) & 0x3 | |
_constructed = ((u>>5) & 0x1) == 1 | |
if u & 0x1f != 0x1f: | |
_type = (u & 0x1f) | |
else: | |
_type = 0 | |
while True: | |
u = pop_u8(f) | |
absorbed += chr(u) | |
_type <<= 7 | |
_type |= (u & 0x7f) | |
if u & 0x80 == 0: break | |
# at this point, _type, _klass and _constructed shall be defined | |
_length = 0 | |
u = pop_u8(f) | |
absorbed += chr(u) | |
if u <= 127: | |
_length = u | |
else: | |
nimbs = u & 0x7f | |
for i in range(nimbs): | |
u = pop_u8(f) | |
absorbed += chr(u) | |
_length <<= 8 | |
_length |= u | |
_indefinite = (_constructed == 1 and _length == 0) | |
# at this point, _length shall be defined | |
assert((_indefinite and _length == 0) or (not _indefinite and _length >= 0)) | |
_children = [] | |
_value = None | |
if not _indefinite: | |
data = f.read(_length) | |
absorbed += data | |
assert(len(data) == _length) | |
if _constructed: | |
g = StringIO(data) | |
while True: | |
child = decode_ber(g, level+1) | |
if not child: | |
break | |
_children.append(child) | |
else: | |
_value = data | |
elif _klass != 0 or _type != 0: | |
while True: | |
o = decode_ber(f, level+1) | |
_children.append(o) | |
if o.klass == 0 and o.type == 0: | |
break | |
# at this point, _children xor _value shall be defined | |
o = Asn1Obj(_klass, _constructed, _type, _indefinite, _value, _children, absorbed) | |
return o | |
if __name__ == '__main__': | |
import sys | |
f = open(sys.argv[1], 'rb') | |
o = decode_ber(f) | |
print(o) |
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